Study on forced air convection cooling for electronic assemblies
Institute of Scientific and Technical Information of China (English)
2008-01-01
The slotted fin concept was employed to improve the air cooling performance of plate-fin in heat sinks.Numerical simulations of laminar heat transfer and flow pressure drop were conducted for the integral plate fin,discrete plate fin and discrete slotted fin heat sinks.It is found that the performance of the discrete plate fin is better than that of the integral continuum plate fin and the performance of slotted fin is better than that of the discrete plate fin at the same pumping power of the fan.A new type of heat sink characterized by discrete and slotted fin surfaces with thinner fins and smaller spaces between fins is then proposed.Preliminary computation shows that this type of heat sink may be useful for the next generation of higher thermal load CPUs.The limit of cooling capacity for air-cooling techniques was also addressed.
Experimental StudyHigh Altitude Forced Convective Cooling of Electromechanical Actuation Systems
2016-01-01
within industry, so as to keep its applicability to what might be used to cool EMAS. Therefore, a parallel plate sink was chosen. This type of heat ...cooling 3 Approved for public release; distribution unlimited. o Convection, liquid to air heat exchanger o Convection, air cooling with fan...surface or heat exchanger to remove the thermal energy away from critical components and convect it to an airstream. The most common source of the air
Sundarraj, Pradeepkumar; Taylor, Robert A.; Banerjee, Debosmita; Maity, Dipak; Sinha Roy, Susanta
2017-01-01
Hybrid solar thermoelectric generators (HSTEGs) have garnered significant research attention recently due to their potential ability to cogenerate heat and electricity. In this paper, theoretical and experimental investigations of the electrical and thermal performance of a HSTEG system are reported. In order to validate the theoretical model, a laboratory scale HSTEG system (based on forced convection cooling) is developed. The HSTEG consists of six thermoelectric generator modules, an electrical heater, and a stainless steel cooling block. Our experimental analysis shows that the HSTEG is capable of producing a maximum electrical power output of 4.7 W, an electrical efficiency of 1.2% and thermal efficiency of 61% for an average temperature difference of 92 °C across the TEG modules with a heater power input of 382 W. These experimental results of the HSTEG system are found to be in good agreement with the theoretical prediction. This experimental/theoretical analysis can also serve as a guide for evaluating the performance of the HSTEG system with forced convection cooling.
Forced Convective Air Cooling from Electronic Component Arrays in a Parallel Plate Channel
Institute of Scientific and Technical Information of China (English)
D.Y.Cai; Y.P.Gan; 等
1994-01-01
This paper discusses air forced convection heat transfer from inline protruding elements arranged in eight rows.The streamwise and spanwise spacings between elements were varied using a splitter plate that can be positioned at three different modular configurations.A set of empirical formulas was presented to correlate the experimental data for the design of air cooling systems.Arrays of componets with one odd-size module have been tested also.Experimental results show that blocks near the entrance and behind the odd-size module have improved performance compared with uniform arrangements.Accordingly,temperature sensitive components are suggested to be arranged in these locations.
Experimental study of an upward sub-cooled forced convection in a rectangular channel
Kouidri, A.; Madani, B.; Roubi, B.; Hamadouche, A.
2016-07-01
The upward sub-cooled forced convection in a rectangular channel is investigated experimentally. The aim of the present work is the studying of the local heat transfer phenomena. Concerning the experimentation: the n-pentane is used as a working fluid, the independent variables are: the velocity in the range from 0.04 to 0.086 m/s and heat flux density with values between 1.8 and 7.36 W/cm2. The results show that the local Nusselt number distribution is not uniform along the channel; however, uniformity is observed in the mean Nusselt number for Reynolds under 1600. On the other hand, a new correlation to predict the local fluid temperature is established as a function of local wall temperature. The wall's heat is dissipated under the common effect of the sub-cooled regime; therefore, the local heat transfer coefficient is increased. The study of the thermal equilibrium showed that for Reynolds less than 1500; almost all of the heat flux generated by the heater cartridges is absorbed by the fluid.
Convective cooling of photovoltaics
Energy Technology Data Exchange (ETDEWEB)
Hodge, E.; Gibbons, C. [Energy Engineering Group, Mechanical Engineering Department, Cork Institute of Technology, Bishopstown, Cork (Ireland)
2004-07-01
Most solar cells presently on the market are based on silicon wafers, the so-called first generation technology. As this technology has matured costs have become increasingly dominated by material costs. In the last ten years, continuous work has brought the efficiency of standard cells to the 25% region. A switch to second generation or thin film technology cells now seems imminent. Thin film technology eliminates the silicon wafer and offer the prospect of reducing material and manufacturing costs, but they exhibit lower efficiencies of around 10% for a commercial device. Third generation or tandem cells are currently at a 'proof of concept' research level, with a theoretical conversion rate of 86.8% being asserted Whatever the material construction and manufacturing method of cells, the thermal effect of overheating will prevail in the semiconductor and it is accepted that a lowered temperature will bring about an increase in conversion efficiency. The aim of this project is to improve the efficiency of PV electrical output, by convectively cooling the cells through perforations in them. As the cells heat up they lose efficiency. As the panel heats up a loss in efficiency of 0.5% per C increase in temperature has been recorded. (orig.)
Freche, John C; Schum, Eugene F
1951-01-01
Blade-to-coolant convective heat-transfer coefficients were obtained on a forced-convection water-cooled single-stage turbine over a large laminar flow range and over a portion of the transition range between laminar and turbulent flow. The convective coefficients were correlated by the general relation for forced-convection heat transfer with laminar flow. Natural-convection heat transfer was negligible for this turbine over the Grashof number range investigated. Comparison of turbine data with stationary tube data for the laminar flow of heated liquids showed good agreement. Calculated average midspan blade temperatures using theoretical gas-to-blade coefficients and blade-to-coolant coefficients from stationary-tube data resulted in close agreement with experimental data.
Thornton, E. A.; Wieting, A. R.
1979-01-01
Conventional versus upwind convective finite elements, and lumped versus consistent formulations for practical conduction/forced convection analysis are evaluated on the basis of numerical studies, with finite element and finite difference lumped-parameter temperatures compared to closed-form analytical solutions for convection problems. Attention is given to two practical combined conduction and forced convection applications, stressing that the finite element method, showing superior accuracy, is competitive with the finite difference lumped-parameter method. Also considered are the computational time savings offered by the zero capacitance nodes procedure and comparative finite element and finite difference lumped-parameter computer times. The present study has reference to the design of actively cooled engine and airframe structures for hypersonic flight.
Kozlova, Sofya V; Ryzhkov, Ilya I
2014-09-01
In this paper, laminar convective heat transfer of water-alumina nanofluid in a circular tube with uniform heat flux at the tube wall is investigated. The investigation is performed numerically on the basis of two-component model, which takes into account nanoparticle transport by diffusion and thermophoresis. Two thermal regimes at the tube wall, heating and cooling, are considered and the influence of nanoparticle migration on the heat transfer is analyzed comparatively. The intensity of thermophoresis is characterized by a new empirical model for thermophoretic mobility. It is shown that the nanoparticle volume fraction decreases (increases) in the boundary layer near the wall under heating (cooling) due to thermophoresis. The corresponding variations of nanofluid properties and flow characteristics are presented and discussed. The intensity of heat transfer for the model with thermophoresis in comparison to the model without thermophoresis is studied by plotting the dependence of the heat transfer coefficient on the Peclet number. The effectiveness of water-alumina nanofluid is analyzed by plotting the average heat transfer coefficient against the required pumping power. The analysis of the results reveals that the water-alumina nanofluid shows better performance in the heating regime than in the cooling regime due to thermophoretic effect.
Cho, Jae Hyun; Batta, A.; Casamassima, V.; Cheng, X.; Choi, Yong Joon; Hwang, Il Soon; Lim, Jun; Meloni, P.; Nitti, F. S.; Dedul, V.; Kuznetsov, V.; Komlev, O.; Jaeger, W.; Sedov, A.; Kim, Ji Hak; Puspitarini, D.
2011-08-01
As highly promising coolant for new generation nuclear reactors, liquid Lead-Bismuth Eutectic has been extensively worldwide investigated. With high expectation about this advanced coolant, a multi-national systematic study on LBE was proposed in 2007, which covers benchmarking of thermal hydraulic prediction models for Lead-Alloy Cooled Advanced Nuclear Energy System (LACANES). This international collaboration has been organized by OECD/NEA, and nine organizations - ENEA, ERSE, GIDROPRESS, IAEA, IPPE, KIT/IKET, KIT/INR, NUTRECK, and RRC KI - contribute their efforts to LACANES benchmarking. To produce experimental data for LACANES benchmarking, thermal-hydraulic tests were conducted by using a 12-m tall LBE integral test facility, named as Heavy Eutectic liquid metal loop for integral test of Operability and Safety of PEACER (HELIOS) which has been constructed in 2005 at the Seoul National University in the Republic of Korea. LACANES benchmark campaigns consist of a forced convection (phase-I) and a natural circulation (phase-II). In the forced convection case, the predictions of pressure losses based on handbook correlations and that obtained by Computational Fluid Dynamics code simulation were compared with the measured data for various components of the HELIOS test facility. Based on comparative analyses of the predictions and the measured data, recommendations for the prediction methods of a pressure loss in LACANES were obtained. In this paper, results for the forced convection case (phase-I) of LACANES benchmarking are described.
Fundamental Research on Convective Heat Transfer in Electronic Cooling Technology
Institute of Scientific and Technical Information of China (English)
C.F.Ma; Y.P.Gan; 等
1992-01-01
During the past six years comprehensive research programs have been conducted at the Beijing Polytechnic University to provide a better understanding of heat transfer characteristics of existing and condidate cooling techniques for electronic and microelestanding of heat transfer characteristics of existing and condidate cooling techniques for electronic and microleectronic devices.This paper provides a review and summary of the programs with emphasis on direct liquid cooling.Included in this review are the heat transfer investigations related to the following cooling modes:liquid free,mixed and forced convection.liquid jet impingement,flowing liquid film cooling,pool boiling,spray cooling,foreign gas jet impingement in liquid pool,and forced convection air-cooling.
Thermal computations for electronics conductive, radiative, and convective air cooling
Ellison, Gordon
2010-01-01
IntroductionPrimary mechanisms of heat flowConductionApplication example: Silicon chip resistance calculationConvectionApplication example: Chassis panel cooled by natural convectionRadiationApplication example: Chassis panel cooled only by radiation 7Illustrative example: Simple thermal network model for a heat sinked power transistorIllustrative example: Thermal network circuit for a printed circuit boardCompact component modelsIllustrative example: Pressure and thermal circuits for a forced air cooled enclosureIllustrative example: A single chip package on a printed circuit board-the proble
Directory of Open Access Journals (Sweden)
Rasoul Nikbakhti
2016-03-01
Full Text Available This paper deals with a numerical investigation of double-diffusive natural convective heat and mass transfer in a cavity filled with Newtonian fluid. The active parts of two vertical walls of the cavity are maintained at fixed but different temperatures and concentrations, while the other two walls, as well as inactive areas of the sidewalls, are considered to be adiabatic and impermeable to mass transfer. The length of the thermally active part equals half of the height. The non-dimensional forms of governing transport equations that describe double-diffusive natural convection for two-dimensional incompressible flow are functions of temperature or energy, concentration, vorticity, and stream-function. The coupled differential equations are discretized via FDM (Finite Difference Method. The Successive-Over-Relaxation (SOR method is used in the solution of the stream function equation. The analysis has been done for an enclosure with different aspect ratios ranging from 0.5 to 11 for three different combinations of partially active sections. The results are presented graphically in terms of streamlines, isotherms and isoconcentrations. In addition, the heat and mass transfer rate in the cavity is measured in terms of the average Nusselt and Sherwood numbers for various parameters including thermal Grashof number, Lewis number, buoyancy ratio and aspect ratio. It is revealed that the placement order of partially thermally active walls and the buoyancy ratio influence significantly the flow pattern and the corresponding heat and mass transfer performance in the cavity.
Indian Academy of Sciences (India)
Ankur Kumar; Jyeshtharaj B Joshi; Arun K Nayak; Pallippattu K Vijayan
2015-05-01
In this paper, a review is presented on the experimental investigations and the numerical simulations performed to analyze the thermal-hydraulic performance of the air-cooled heat exchangers. The air-cooled heat exchangers mostly consist of the finned-tube bundles. The primary role of the extended surfaces (fins) is to provide more heat transfer area to enhance the rate of heat transfer on the air side. The secondary role of the fins is to generate vortices, which help in enhancing the mixing and the heat transfer coefficient. In this study, the annular and plate fins are considered, the annular fins are further divided into four categories: (1) plane annular fins, (2) serrated fins, (3) crimped spiral fins, (4) perforated fins, and similarly for the plate fins, the fin types are: (1) plain plate fins, (2) wavy plate fins, (3) plate fins with DWP, and (4) slit and strip fins. In Section 4, the performance of the various types of fins is presented with respect to the parameters: (1) Reynolds number, (2) fin pitch, (3) fin height, (4) fin thickness, (5) tube diameter, (6) tube pitch, (7) tube type, (8) number of tube rows, and (9) effect of dehumidifying conditions. In Section 5, the conclusions and the recommendations for the future work have been given.
Convective cores in galactic cooling flows
Kritsuk, A G; Müller, E
2000-01-01
We use hydrodynamic simulations with adaptive grid refinement to study the dependence of hot gas flows in X-ray luminous giant elliptical galaxies on the efficiency of heat supply to the gas. We consider a number of potential heating mechanisms including Type Ia supernovae and sporadic nuclear activity of a central supermassive black hole. As a starting point for this research we use an equilibrium hydrostatic recycling model (Kritsuk 1996). We show that a compact cooling inflow develops, if the heating is slightly insufficient to counterbalance radiative cooling of the hot gas in the central few kiloparsecs. An excessive heating in the centre, instead, drives a convectively unstable outflow. We model the onset of the instability and a quasi-steady convective regime in the core of the galaxy in two-dimensions assuming axial symmetry. Provided the power of net energy supply in the core is not too high, the convection remains subsonic. The convective pattern is dominated by buoyancy driven large-scale mushroom-...
Directory of Open Access Journals (Sweden)
Sundus Hussein Abd
2012-01-01
Full Text Available In this research, an experimental study was conducted to high light the impact of the exterior shape of a cylindrical body on the forced and free convection heat transfer coefficients when the body is hold in the entrance of an air duct. The impact of changing the body location within the air duct and the air speed are also demonstrated. The cylinders were manufactured with circular, triangular and square sections of copper for its high thermal conductivity with appropriate dimensions, while maintaining the surface area of all shapes to be the same. Each cylinder was heated to a certain temperature and put inside the duct at certain locations. The temperature of the cylinder was then monitored. The heat transfer coefficient were then calculated for forced convection for several Reynolds number (4555-18222.The study covered free convection impact for values of Rayleigh number ranging between (1069-3321. Imperical relationships were obtained for all cases of forced and free convection and compared with equations of circular cylindrical shapes found in literature. These imperical equations were found to be in good comparison with that of other sources.
Optimal convection cooling flows in general geometries
Alben, Silas
2016-01-01
We generalize a recent method for computing optimal 2D convection cooling flows in a horizontal layer to a wide range of geometries, including those relevant for technological applications. We write the problem in a conformal pair of coordinates which are the pure conduction temperature and its harmonic conjugate. We find optimal flows for cooling a cylinder in an annular domain, a hot plate embedded in a cold surface, and a channel with hot interior and cold exterior. With a constraint of fixed kinetic energy, the optimal flows are all essentially the same in the conformal coordinates. In the physical coordinates, they consist of vortices ranging in size from the length of the hot surface to a small cutoff length at the interface of the hot and cold surfaces. With the constraint of fixed enstrophy (or fixed rate of viscous dissipation), a geometry-dependent metric factor appears in the equations. The conformal coordinates are useful here because they map the problems to a rectangular domain, facilitating num...
Study of Natural Convection Passive Cooling System for Nuclear Reactors
Abdillah, Habibi; Saputra, Geby; Novitrian; Permana, Sidik
2017-07-01
Fukushima nuclear reactor accident occurred due to the reactor cooling pumps and followed by all emergencies cooling systems could not work. Therefore, the system which has a passive safety system that rely on natural laws such as natural convection passive cooling system. In natural convection, the cooling material can flow due to the different density of the material due to the temperature difference. To analyze such investigation, a simple apparatus was set up and explains the study of natural convection in a vertical closed-loop system. It was set up that, in the closed loop, there is a heater at the bottom which is representing heat source system from the reactor core and cooler at the top which is showing the cooling system performance in room temperature to make a temperature difference for convection process. The study aims to find some loop configurations and some natural convection performances that can produce an optimum flow of cooling process. The study was done and focused on experimental approach and simulation. The obtained results are showing and analyzing in temperature profile data and the speed of coolant flow at some point on the closed-loop system.
Gokoglu, S. A.; Rosner, D. E.
1984-01-01
A cooled object (heat exchanger tube or turbine blade) is considered to be immersed in a hot fluid stream containing trace amounts of suspended vapors and/or small particles. Numerical prediction calculations were done for self-similar laminar boundary layers and law-of-the-wall turbulent boundary layers. Correlations are presented for the effect of thermophoresis in the absence of transpiration cooling and viscous dissipation; the effect of real suction and blowing in the absence of thermophoresis; the effect of viscous dissipation on thermophoresis in the absence of transpiration cooling; and the combined effect of viscous dissipation and transpiration cooling on thermophoresis. The final correlation, St/St-sub-zero, is insensitive to particle properties, Euler number, and local mainstream temperature.
Théard, M A; Tempelhoff, R; Crowder, C M; Cheng, M A; Todorov, A; Dacey, R G
1997-07-01
Hypothermia for cerebral protection is usually achieved by administration of intravenous fluids at room temperature, cooling ambient air, ice packs, and a temperature-adjustable circulating water mattress. We compared cooling by conduction by using a water mattress to cool by convection by using a forced-air cooling device. Twenty patients were prospectively randomized to two groups: 10 patients cooled by convection (CC) and 10 patients cooled by traditional methods (TC). Two patients in the CC group were withdrawn from the study and excluded from the analysis; one patient for failure to cool despite the use of both techniques, and the other for the abrupt onset of arrhythmias and myocardial depression during hypothermia. Temperature was measured at the tympanic membrane, pulmonary artery, and esophageal probe sites and recorded every 15 min. The time required to reach the target temperature range of 33-34 degrees C was recorded. We found no differences in the temperatures measured at the three sites during cooling and rewarming. Baseline temperatures recorded from the pulmonary artery catheter before beginning "active cooling" were similar in both groups (TC, 35.0 +/- 0.2 degrees C vs. CC, 35.3 +/- 0.1 degrees C). We found no difference in the time to target temperature between TC and CC (TC, 178 +/- 25 min vs. CC, 142 +/- 21 min). One patient had some arrhythmias on cooling in the convective group, but her preoperative condition may have been responsible. In conclusion, cooling by convection appears to be a safe alternative to conduction cooling.
Forced Convection Heat Transfer in Circular Pipes
Tosun, Ismail
2007-01-01
One of the pitfalls of engineering education is to lose the physical insight of the problem while tackling the mathematical part. Forced convection heat transfer (the Graetz-Nusselt problem) certainly falls into this category. The equation of energy together with the equation of motion leads to a partial differential equation subject to various…
Convection-Dominated Accretion Flows with Radiative Cooling
Institute of Scientific and Technical Information of China (English)
LI Shuang-Liang; XUE Li; LU Ju-Fu
2007-01-01
@@ By numerically solving the set of basic equations describing black hole accretion flows with low accretion rates,we show that although the dynamical structure of these flows is essentially unaffected by radiative processes in comparison with the case in which the radiation is not considered, the radiative cooling can be more important than the advective cooling in the flow's convection-dominated zone, and this result may have implications to distinguish observationally convection-dominated accretion flows from advection-dominated accretion flows.
MR-6 type fuel elements cooling in natural convection conditions after the reactor shut down
Energy Technology Data Exchange (ETDEWEB)
Pytel, K.; Bykowski, W.; Moldysz, A. [Institute of Atomic Energy, Otwock Swierk (Poland)
2002-07-01
Natural cooling conditions of the nuclear fuel in the channel type reactor after its shut down are commonly determined with relatively high uncertainty. This is not only to he lack of adequate measurements of thermal parameters i.e. the residual power generation, the coolant flow and temperatures, but also due to indeterminate model of convection mechanism. The numerical simulation of natural convection in multitube fuel assembly in the fuel channel leads to various convection modes including evidently chaotic behaviour. To determine the real cooling conditions in the MARIA research reactor a series of experiments has been performed with fuel assembly equipped with a set of thermocouples. After some forced cooling period (the shortest was half an hour after the reactor shut down) the reactor was left with the only natural convection. Two completely different cooling modes have been observed. The MARIA core consists of series of individual fuel channel and so called bypasses, maintaining the hydraulic properties of the fuel channel, connected in parallel. Initially, the convection cells were established trough few so-called bypasses providing a very effective mode of cooling. In this mode the flow charts were identical to those existing in forced cooling mode. After certain period the system switched on the second cooling mode with natural circulation within the individual fuel cells. Higher temperatures and temperature fluctuations were characteristic for this mode approaching 30 deg in amplitude. In almost all the cases the system was switching few times between modes, but eventually remained in the second mode. The switching times were not regular and the process has a chaotic behaviour. (author)
Thornton, E. A.
1979-01-01
Three practical problems in conduction/forced convection heat transfer are analyzed using a simplified engineering formulation of convective finite elements. Upwind and conventional finite element solutions are compared for steady-state and transient applications.
Transition between free, mixed and forced convection
Jaeger, W.; Trimborn, F.; Niemann, M.; Saini, V.; Hering, W.; Stieglitz, R.; Pritz, B.; Fröhlich, J.; Gabi, M.
2017-07-01
In this contribution, numerical methods are discussed to predict the heat transfer to liquid metal flowing in rectangular flow channels. A correct representation of the thermo-hydraulic behaviour is necessary, because these numerical methods are used to perform design and safety studies of components with rectangular channels. Hence, it must be proven that simulation results are an adequate representation of the real conditions. Up to now, the majority of simulations are related to forced convection of liquid metals flowing in circular pipes or rod bundle, because these geometries represent most of the components in process engineering (e.g. piping, heat exchanger). Open questions related to liquid metal heat transfer, among others, is the behaviour during the transition of the heat transfer regimes. Therefore, this contribution aims to provide useful information related to the transition from forced to mixed and free convection, with the focus on a rectangular flow channel. The assessment of the thermo-hydraulic behaviour under transitional heat transfer regimes is pursued by means of system code simulations, RANS CFD simulations, LES and DNS, and experimental investigations. Thereby, each of the results will compared to the others. The comparison of external experimental data, DNS data, RANS data and system code simulation results shows that the global heat transfer can be consistently represented for forced convection in rectangular flow channels by these means. Furthermore, LES data is in agreement with RANS CFD results for different Richardson numbers with respect to temperature and velocity distribution. The agreement of the simulation results among each other and the hopefully successful validation by means of experimental data will fosters the confidence in the predicting capabilities of numerical methods, which can be applied to engineering application.
Temporal response of laser power standards with natural convective cooling.
Xu, Tao; Gan, Haiyong; Yu, Jing; Zang, Erjun
2016-01-25
Laser power detectors with natural convective cooling are convenient to use and hence widely applicable in a power range below 150 W. However, the temporal response characteristics of the laser power detectors need to be studied in detail for accurate measurement. The temporal response based on the absolute laser power standards with natural convective cooling is studied through theoretical analysis, numerical simulations, and experimental verifications. Our results show that the response deviates from a single exponential function and that an ultimate response balance is difficult to achieve because the temperature rise of the heat sink leads to continuous increase of the response. To determine the measurement values, an equal time reading method is proposed and validated by the laser power calibrations.
Institute of Scientific and Technical Information of China (English)
Pei－XueJiang; Ze－PeiRen; 等
1995-01-01
Forced and mixed convection heat and mass transfer are studied numerically for water containing metallic corrosion products in a heated or cooled vertical tube with variable thermophysical properties at super-citical pressures.the fouling mechanisms and fouling models are presented.The influence of variable properties at super-critical pressures on forced or mixed convection has been analyzed.The differences between heat and mass transfer under heating and cooling conditions are discussed.It is found that variable properties,especially buoyancy,greatly influence the fluid flow and heat mass fransfer.
The convection cooling system of the Yakutsk permafrost seed repository
Institute of Scientific and Technical Information of China (English)
Vladimir N.Panin; Georgii P.Kuzmin
2014-01-01
Temperature is critical to maintaining seed viability under long term storage conditions. It has been common practice to use refrigeration systems to maintain required storage temperatures. A seed repository constructed in permafrost in Ya kutsk, Russia is the first seed storage facility that relies solely on natural cold. This paper describes the design and per formance of the cooling system of the repository. An innovative aspect of the cooling system is that it utilizes the patterns of temperature wave propagation in permafrost. Predicted and measured ground temperatures for the first year of operation are presented and analyzed. Results indicate that convection air cooling systems can be used to control the temperature regime in underground facilities in permafrost.
Cooling by convection vs cooling by conduction for treatment of fever in critically ill adults.
Creechan, T; Vollman, K; Kravutske, M E
2001-01-01
Cooling with water-flow blankets, which are difficult to manipulate and interfere with patients' care, may be ineffective in controlling fever. To compare the effectiveness of cooling via convective airflow blankets with cooling via conductive water-flow blankets for treatment of fever in critically ill adults. A 2-group experimental design was used to compare cooling via convection (n = 20) with cooling via conduction (n = 17) in critically ill adults with an infection-related fever of 38.5 degrees C or greater. Esophageal temperature was measured every 15 minutes until a temperature of 38.0 degrees C was reached or 8 hours had elapsed. Alternative cooling measures were withheld unless the temperature increased to more than 40.0 degrees C. Data on nurses' satisfaction were collected, and complications related to each cooling method were examined. Temperatures decreased more rapidly in the airflow group (mean decrease, 0.377 degree C/h) than in the water-flow group (mean decrease, 0.163 degree C/h). A temperature of 38.0 degrees C was achieved more often in the airflow group (75% vs 47.1%). Fever (temperature > 38.5 degrees C) recurred sooner in the water-flow group (6.6 hours) than in the airflow group (22.2 hours). Both methods were easy to use. Compared with the water-flow blanket, the airflow blanket was recommended for future use twice as often and interfered less with patients' care. In critically ill adults with an infection or a suspected infection, cooling with an airflow blanket is more effective and more preferred for cooling than is cooling with a water-flow blanket.
Turbulence convective heat transfer for cooling the photovoltaic cells
Arianmehr, Iman
Solar PV (photovoltaic) is a rapidly advancing renewable energy technology which converts sunlight directly into electricity. One of the outstanding challenges of the current PV technology is the reduction in its conversion efficiency with increasing PV panel temperature, which is closely associated with the increase in solar intensity and the ambient temperature surrounding the PV panels. To more effectively capture the available energy when the sun is most intense, significant efforts have been invested in active and passive cooling research over the last few years. While integrated cooling systems can lead to the highest total efficiencies, they are usually neither the most feasible nor the most cost effective solutions. This work examines some simple passive means of manipulating the prevailing wind turbulence to enhance convective heat transfer over a heated plate in a wind tunnel.
Directory of Open Access Journals (Sweden)
Tirivanhu Chinyoka
2015-01-01
Full Text Available This article examines the combined effects of buoyancy force and asymmetrical convective cooling on unsteady MHD channel flow and heat transfer characteristics of an incompressible, reactive, variable viscosity and electrically conducting third grade fluid. The chemical kinetics in the flow system is exothermic and the asymmetric convective heat transfers at the channel walls follow the Newton’s law of cooling. The coupled nonlinear partial differential equations governing the problem are derived and solved numerically using a semi-implicit finite difference scheme. Graphical results are presented and physical aspects of the problem are discussed with respect to various parameters embedded in the system.
Latent cooling and microphysics effects in deep convection
Fernández-González, S.; Wang, P. K.; Gascón, E.; Valero, F.; Sánchez, J. L.
2016-11-01
Water phase changes within a storm are responsible for the enhancement of convection and therefore the elongation of its lifespan. Specifically, latent cooling absorbed during evaporation, melting and sublimation is considered the main cause of the intensification of downdrafts. In order to know more accurately the consequences of latent cooling caused by each of these processes (together with microphysical effects that they induce), four simulations were developed with the Wisconsin Dynamical and Microphysical Model (WISCDYMM): one with all the microphysical processes; other without sublimation; melting was suppressed in the third simulation; and evaporation was disabled in the fourth. The results show that sublimation cooling is not essential to maintain the vertical currents of the storm. This is demonstrated by the fact that in the simulation without sublimation, maximum updrafts are in the same range as in the control simulation, and the storm lifespan is similar or even longer. However, melting was of vital importance. The storm in the simulation without melting dissipated prematurely, demonstrating that melting is indispensable to the enhancement of downdrafts below the freezing level and for avoiding the collapse of low level updrafts. Perhaps the most important finding is the crucial influence of evaporative cooling above the freezing level that maintains and enhances mid-level downdrafts in the storm. It is believed that this latent cooling comes from the evaporation of supercooled liquid water connected with the Bergeron-Findeisen process. Therefore, besides its influence at low levels (which was already well known), this evaporative cooling is essential to strengthen mid-level downdrafts and ultimately achieve a quasi-steady state.
Mixed time integration schemes for transient conduction forced-convection analysis
Liu, W. K.; Lin, J. I.
1983-01-01
A partition procedure for forced-convection conduction transient problems is presented. Mixed time partitions are defined wherein coupled conduction force-matrix equations are discretized using an implicit integration method, followed by derivation of a mixed time integration technique. Explicit-implicit and explicit-explicit partitions are performed for a stability analysis for transient conditions, e.g., those found in an actively air-cooled engine and airframe structure.
Natural Convection and Boiling for Cooling SRP Reactors During Loss of Circulation Conditions
Energy Technology Data Exchange (ETDEWEB)
Buckner, M.R.
2001-06-26
This study investigated natural convection and boiling as a means of cooling SRP reactors in the event of a loss of circulation accident. These studies show that single phase natural convection cooling of SRP reactors in shutdown conditions with the present piping geometry is probably not feasible.
A numerical study of momentum and forced convection heat transfer ...
African Journals Online (AJOL)
temperature fields, axial velocity profiles, local and average Nusselt numbers, and skin frictions were ... Key words: Finite volume method - Turbulent flow - Forced convection - Waved baffles. .... numerical simulations are conducted in a two-.
Design of force-cooled conductors for large fusion magnets
Energy Technology Data Exchange (ETDEWEB)
Dresner, L.; Lue, J.W.
1977-01-01
Conductors cooled by supercritical helium in forced convection are under active consideration for large toroidal fusion magnets. One of the central problems in designing such force cooled conductors is to maintain an adequate stability margin while keeping the pumping power tolerably low. A method has been developed for minimizing the pumping power for fixed stability by optimally choosing the matrix-to-superconductor and the metal-to-helium ratios. Such optimized conductors reduce pumping power requirements for fusion size magnets to acceptable limits. Furthermore, the mass flow and hence pumping losses can be varied through a magnet according to the local magnetic field and magnitude of desired stability margin. Force cooled conductors give flexibility in operation, permitting, for example, higher fields to be obtained than originally intended by lowering the bath temperature or increasing the pumping power or both. This flexibility is only available if the pumping power is low to begin with. Scaling laws for the pumping requirement and stability margin as functions of operating current density, number of strands and such physical parameters as stabilizer resistivity and critical current density, have been proved. Numerical examples will be given for design of conductors intended for use in large toroidal fusion magnet systems.
Observation of dendritic growth under the influence of forced convection
Roshchupkina, O.; Shevchenko, N.; Eckert, S.
2015-06-01
The directional solidification of Ga-25wt%In alloys within a Hele-Shaw cell was visualized by X-ray radioscopy. The investigations are focused on the impact of melt convection on the dendritic growth. Natural convection occurs during a bottom up solidification because lighter solute is rejected during crystallization. Forced convection was produced by a specific electromagnetic pump. The direction of forced melt flow is almost horizontal at the solidification front. Melt flow induces various effects on grain morphology primarily caused by convective transport of solute, such as a facilitation of the growth of primary trunks or lateral branches, dendrite remelting, fragmentation or freckle formation depending on the dendrite orientation, the flow direction and intensity. Forced flow eliminates solutal plumes and damps local fluctuations of solute. A preferential growth of the secondary arms occurs at the upstream side of the dendrites, whereas high solute concentration at the downstream side inhibits the formation of secondary branches.
Analytical solution of conjugate turbulent forced convection boundary layer flow over plates
Directory of Open Access Journals (Sweden)
Joneydi Shariatzadeh Omid
2016-01-01
Full Text Available A conjugate (coupled forced convection heat transfer from a heated conducting plate under turbulent boundary layer flow is considered. A heated plate of finite thickness is cooled under turbulent forced convection boundary layer flow. Because the conduction and convection boundary layer flow is coupled (conjugated in the problem, a semi-analytical solution based on Differential Transform Method (DTM is presented for solving the non-linear integro-differential equation occurring in the problem. The main conclusion is that in the conjugate heat transfer case the temperature distribution of the plate is flatter than the one in the non-conjugate case. This feature is more pronounced under turbulent flow when compared with the laminar flow.
Finite-Element Analysis of Forced Convection and Conduction
Wieting, A. R.
1982-01-01
TAP2 thermal-analysis program was developed as part of research on finite element methodology for thermal analysis of convectively cooled structures, such as scramjet engines and hypersonic aircraft. Program simplifies computations when both structural and thermal analyses are required and is suited for thermal analysis of nuclear reactors and solar-panel heating systems.
Convective cooling in a pool-type research reactor
Sipaun, Susan; Usman, Shoaib
2016-01-01
A reactor produces heat arising from fission reactions in the nuclear core. In the Missouri University of Science and Technology research reactor (MSTR), this heat is removed by natural convection where the coolant/moderator is demineralised water. Heat energy is transferred from the core into the coolant, and the heated water eventually evaporates from the open pool surface. A secondary cooling system was installed to actively remove excess heat arising from prolonged reactor operations. The nuclear core consists of uranium silicide aluminium dispersion fuel (U3Si2Al) in the form of rectangular plates. Gaps between the plates allow coolant to pass through and carry away heat. A study was carried out to map out heat flow as well as to predict the system's performance via STAR-CCM+ simulation. The core was approximated as porous media with porosity of 0.7027. The reactor is rated 200kW and total heat density is approximately 1.07+E7 Wm-3. An MSTR model consisting of 20% of MSTR's nuclear core in a third of the reactor pool was developed. At 35% pump capacity, the simulation results for the MSTR model showed that water is drawn out of the pool at a rate 1.28 kg s-1 from the 4" pipe, and predicted pool surface temperature not exceeding 30°C.
Convective cooling in a pool-type research reactor
Energy Technology Data Exchange (ETDEWEB)
Sipaun, Susan, E-mail: susan@nm.gov.my [Malaysian Nuclear Agency, Industrial Technology Division, Blok 29T, Bangi 43200, Selangor (Malaysia); Usman, Shoaib, E-mail: usmans@mst.edu [Missouri University of Science and Technology, Nuclear Engineering, 222 Fulton Hall 301 W.14th St., Rolla 64509 MO (United States)
2016-01-22
A reactor produces heat arising from fission reactions in the nuclear core. In the Missouri University of Science and Technology research reactor (MSTR), this heat is removed by natural convection where the coolant/moderator is demineralised water. Heat energy is transferred from the core into the coolant, and the heated water eventually evaporates from the open pool surface. A secondary cooling system was installed to actively remove excess heat arising from prolonged reactor operations. The nuclear core consists of uranium silicide aluminium dispersion fuel (U{sub 3}Si{sub 2}Al) in the form of rectangular plates. Gaps between the plates allow coolant to pass through and carry away heat. A study was carried out to map out heat flow as well as to predict the system’s performance via STAR-CCM+ simulation. The core was approximated as porous media with porosity of 0.7027. The reactor is rated 200kW and total heat density is approximately 1.07+E7 Wm{sup −3}. An MSTR model consisting of 20% of MSTR’s nuclear core in a third of the reactor pool was developed. At 35% pump capacity, the simulation results for the MSTR model showed that water is drawn out of the pool at a rate 1.28 kg s{sup −1} from the 4” pipe, and predicted pool surface temperature not exceeding 30°C.
Forced Gravity Waves and the Tropospheric Response to Convection
Halliday, Oliver; Parker, Doug; Griffiths, Stephen; Stirling, Alison
2017-04-01
It has been known for some time that gravity waves facilitate atmospheric adjustment to convective heating. Further, convectively forced gravity waves condition the neighbouring atmosphere for the initiation and / or suppression of convection. Despite this, the radiation of gravity waves in macro-scale models (which are typically forced at the grid-scale, by existing parameterization schemes) is not well understood. We present here theoretical and numerical work directed toward improving our understanding of convectively forced gravity wave effects at the mesoscale. Using the linear hydrostatic equations of motion for an incompressible (but non-Boussinesq) fluid with vertically varying buoyancy frequency, we find a radiating solution to prescribed sensible heating. We then interrogate the spatial and temporal sensitivity of the vertical velocity and potential temperature response to different heating functions, considering the remote and near-field forced response both to steady and pulsed heating. We find that the meso-scale tropospheric response to convection is significantly dependent on the upward radiation characteristics of the gravity waves, which are in turn dependent upon the temporal and spatial structure of the source, and stratification of the domain. Moving from a trapped to upwardly-radiating solution there is a 50% reduction in tropospherically averaged vertical velocity, but significant perturbations persist for up to 4 hours in the far-field. Furthermore, we find the tropospheric adjustment to be sensitive to the horizontal length scale of the heating, observing a 20% reduction in vertical velocity when comparing the response from a 10 km to a 100 km heat source. We assess the implications for parameterization of convection in coarse-grained models in the light of these findings and argue that an idealized 'full-physics' nonlinear simulation of deep convection in the MetUM is qualitatively described by the linear solution: departures are quantified
Use of local convective and radiant cooling at warm environment
DEFF Research Database (Denmark)
Melikov, Arsen Krikor; Krejcirikova, Barbora; Kaczmarczyk, Jan
2012-01-01
with one panel equipped with small fans. A reference condition without cooling was tested as well. The response of the subjects to the exposed conditions was collected by computerized questionnaires. The cooling devices significantly (p.... The acceptability of the thermal environment was similar for all cooling devices. The acceptability of air movement and PAQ increased when the local cooling methods were used. The best results were achieved with personalized ventilation and cooling fan. The minimal improvement in PAQ was reported when the radiant...
Cooling of Water in a Flask: Convection Currents in a Fluid with a Density Maximum
Velasco, S.; White, J. A.; Roman, F. L.
2010-01-01
The effect of density inversion on the convective flow of water in a spherical glass flask cooled with the help of an ice-water bath is shown. The experiment was carried out by temperature measurements (cooling curves) taken at three different heights along the vertical diameter of the flask. Flows inside the flask are visualized by seeding the…
Cooling of Water in a Flask: Convection Currents in a Fluid with a Density Maximum
Velasco, S.; White, J. A.; Roman, F. L.
2010-01-01
The effect of density inversion on the convective flow of water in a spherical glass flask cooled with the help of an ice-water bath is shown. The experiment was carried out by temperature measurements (cooling curves) taken at three different heights along the vertical diameter of the flask. Flows inside the flask are visualized by seeding the…
Dag, Yusuf
Forced convection over traditional surfaces such as flat plate, cylinder and sphere have been well researched and documented. Data on forced convection over airfoil surfaces, however, remain very scanty in literature. High altitude vehicles that employ airfoils as lifting surfaces often suffer leading edge ice accretions which have tremendous negative consequences on the lifting capabilities and stability of the vehicle. One of the ways of mitigating the effect of ice accretion involves judicious leading edge convective cooling technique which in turn depends on the accuracy of convective heat transfer coefficient used in the analysis. In this study empirical investigation of convective heat transfer measurements on asymmetric airfoil is presented at different angle of attacks ranging from 0° to 20° under subsonic flow regime. The top and bottom surface temperatures are measured at given points using Senflex hot film sensors (Tao System Inc.) and used to determine heat transfer characteristics of the airfoils. The model surfaces are subjected to constant heat fluxes using KP Kapton flexible heating pads. The monitored temperature data are then utilized to determine the heat convection coefficients modelled empirically as the Nusselt Number on the surface of the airfoil. The experimental work is conducted in an open circuit-Eiffel type wind tunnel, powered by a 37 kW electrical motor that is able to generate subsonic air velocities up to around 41 m/s in the 24 square-inch test section. The heat transfer experiments have been carried out under constant heat flux supply to the asymmetric airfoil. The convective heat transfer coefficients are determined from measured surface temperature and free stream temperature and investigated in the form of Nusselt number. The variation of Nusselt number is shown with Reynolds number at various angles of attacks. It is concluded that Nusselt number increases with increasing Reynolds number and increase in angle of attack from 0
NANOFLUID PROPERTIES FOR FORCED CONVECTION HEAT TRANSFER: AN OVERVIEW
Directory of Open Access Journals (Sweden)
W.H.Azmi
2013-06-01
Full Text Available Nanoﬂuids offer a significant advantage over conventional heat transfer ﬂuids and consequently, they have attracted much attention in recent years. The engineered suspension of nano-sized particles in a base liquid alters the properties of these nanofluids. Many researchers have measured and modeled the thermal conductivity and viscosity of nanofluids. The estimation of forced convective heat transfer coefficients is done through experiments with either metal or nonmetal solid particles dispersed in water. Regression equations are developed for the determination of the thermal conductivity and viscosity of nanofluids. The parameters influencing the decrease in convection heat transfer, observed by certain investigators, is explained.
Operational experience with forced cooled superconducting magnets
Energy Technology Data Exchange (ETDEWEB)
Ivanov, D.P., E-mail: denis.ivanov30@mail.ru [National Research Center “Kurchatov Institute”, Moscow 123182 (Russian Federation); Kolbasov, B.N., E-mail: kolbasov@nfi.kiae.ru [National Research Center “Kurchatov Institute”, Moscow 123182 (Russian Federation); Anashkin, I.O.; Khvostenko, P.P. [National Research Center “Kurchatov Institute”, Moscow 123182 (Russian Federation); Pan, W.J. [Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei, Anhui 230031 (China); Pradhan, S.; Sharma, A.N. [Institute for Plasma Research, Bhat, Gandhinagar, Gujarat 382428 (India); Song, Y.T.; Weng, P.D. [Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei, Anhui 230031 (China)
2013-10-15
Highlights: ► Seventeen breakdowns happened in the fusion facilities with forced cooled superconducting magnets (FCSMs). ► The breakdowns always began on the electric, cryogenic and diagnostic communications (ECDCs) and never on the coils. ► In all the FCSMs the ECDCs were always insulated worse than the coils. ► For reliable operation of ITER organization team should essentially improve the ECDC insulation. ► Use of stainless steel grounded casings filled up with solid insulation over all the ECDCs is the best way to get reliable insulation. -- Abstract: Force-cooled concept has been chosen for ITER superconducting magnet to get reliable coil insulation using vacuum-pressure impregnation (VPI) technology. However 17 breakdowns occurred during operation of six magnets of this type or their single coil tests at operating voltage < 3 kV, while ITER needs 12 kV. All the breakdowns started on electric, cryogenic and diagnostic communications (ECDCs) by the high voltage induced at fast current variations in magnets concurrently with vacuum deterioration, but never on the coils, though sometimes the latter were damaged too. It suggests that simple wrap insulation currently employed on ECDCs and planned to be used in ITER is unacceptable. Upgrade of the ECDC insulation to the same level as on the coils is evidently needed. This could be done by covering each one from ECDCs with vacuum-tight grounded stainless steel casings filled up with solid insulator using VPI-technology. Such an insulation will be insensitive to in-cryostat conditions, excluding helium leaks and considerably simplifying the tests thus allowing saving time and cost. However it is not accepted in ITER design yet. So guarantee of breakdown prevention is not available.
Plates of the dinosaur stegosaurus: forced convection heat loss fins?
Farlow, J O; Thompson, C V; Rosner, D E
1976-06-11
It is suggested that the plates along the arched back and tail of Stegosaurus served an important thermoregulatory function as forced convection "fins." Wind tunnel experiments on finned models, internal heat conduction calculations, and direct observations of the morphology and internal structure of stegosaur plates support this hypothesis, demonstrating the comparative effectiveness of the plates as heat dissipaters, controllable through input blood flow rate, temperature, and body orientation (with respect to wind).
DEFF Research Database (Denmark)
Le Dreau, Jerome; Heiselberg, Per; Jensen, Rasmus Lund
2013-01-01
Night-time ventilation is a promising approach to reduce the energy needed for cooling buildings without reducing thermal comfort. Nevertheless actual building simulation tools have showed their limits in predicting accurately the efficiency of night-time ventilation, mainly due to inappropriate...... air and the room, and floor emissivities. This extensive experimental study enabled a detailed analysis of the convective and radiative flow at the different surfaces of the room. The experimentally derived convective heat transfer coefficients (CHTC) have been compared to existing correlations...... ventilation, the convective heat transfer was well predicted by existing correlations. Nevertheless the change of floor emissivity influenced the CHTC at the surface of interest....
Regimes of Axisymmetric Flow and Scaling Laws in a Rotating Annulus with Local Convective Forcing
Directory of Open Access Journals (Sweden)
Susie Wright
2017-07-01
Full Text Available We present a numerical study of axisymmetric flow in a rotating annulus in which local thermal forcing, via a heated annular ring on the outside of the base and a cooled circular disk in the centre of the top surface, drives convection. This new configuration is a variant of the classical thermally-driven annulus, where uniform heating and cooling are applied through the outer and inner sidewalls respectively. The annulus provides an analogue to a planetary circulation and the new configuration, with its more relaxed vertical thermal boundary conditions, is expected to better emulate vigorous convection in the tropics and polar regions as well as baroclinic instability in the mid-latitude baroclinic zone. Using the Met Office/Oxford Rotating Annulus Laboratory (MORALS code, we have investigated a series of equilibrated, two dimensional axisymmetric flows across a large region of parameter space. These are characterized in terms of their velocity and temperature fields. When rotation is applied several distinct flow regimes may be identified for different rotation rates and strengths of differential heating. These regimes are defined as a function of the ratio of the horizontal Ekman layer thickness to the non-rotating thermal boundary layer thickness and are found to be similar to those identified in previous annulus experiments. Convection without rotation is also considered and the scaling of the heat transport with Rayleigh number is calculated. This is then compared with existing work on the classical annulus as well as horizontal and Rayleigh-Bénard convection. As with previous studies on both rotating and non-rotating convection the system’s behaviour is found to be aspect ratio dependent. This dependence is seen in the scaling of the non-rotating Nusselt number and in transitions between regimes in the rotating case although further investigation is required to fully explain these observations.
Thermal Conditions in a Simulated Office Environment with Convective and Radiant Cooling Systems
DEFF Research Database (Denmark)
Mustakallio, Panu; Bolashikov, Zhecho Dimitrov; Kostov, Kalin
2013-01-01
ventilation (MVRC). CB was based on convection cooling while the remaining three systems (CBR, CCMV and MVRC) on combined radiant and convective cooling. Measurements were performed in design (64 W/m2) and usual (38 W/m2) cooling conditions. Air temperature, operative temperature, radiant asymmetry, air......The thermal conditions in a two person office room were measured with four air conditioning systems: chilled beam (CB), chilled beam with radiant panel (CBR), chilled ceiling with ceiling installed mixing ventilation (CCMV) and four desk partition mounted local radiant cooling panels with mixing...... velocity and turbulent intensity were measured and draft rate levels calculated in the room. Manikin-based equivalent temperature (MBET) was determined by two thermal manikins to identify the impact of the local thermal conditions generated by the studied systems on occupants’ thermal comfort. The results...
Various aspects of magnetic field influence on forced convection
Directory of Open Access Journals (Sweden)
Pleskacz Lukasz
2016-01-01
Full Text Available Flows in the channels of various geometry can be found everywhere in industrial or daily life applications. They are used to deliver media to certain locations or they are the place where heat may be exchanged. For Authors both points of view are interesting. The enhancement methods for heat transfer during the forced convection are demanded due to a technological development and tendency to miniaturization. At the same time it is also worth to find mechanisms that would help to avoid negative effects like pressure losses or sedimentation in the channel flows. This paper shows and discuss various aspects of magnetic field influence on forced convection. A mathematical model consisted of the mass, momentum and energy conservation equations. In the momentum conservation equation magnetic force term was included. In order to calculate this magnetic force Biot-Savart’s law was utilized. Numerical analysis was performed with the usage of commonly applied software. However, userdefined functions were implemented. The results revealed that both temperature and velocity fields were influenced by the strong magnetic field.
Three dimensional free convection couette flow with transpiration cooling
Institute of Scientific and Technical Information of China (English)
无
2006-01-01
Free convection flow between two vertical parallel plates with transverse sinusoidal injection of the fluid at the stationary plate and its corresponding removal by constant suction through the plate in uniform motion has been analyzed. Due to this type of injection velocity, the flow becomes three-dimensional. Analytical expressions for the velocity, temperature, skin friction and rate of heat transfer were obtained. The important characteristics of the problem, namely the skin friction and the rate of heat transfer are discussed in detail with the help of graphs.
Thermal interaction between free convection and forced convection along a vertical conducting wall
Shu, Jian-Jun
2015-01-01
A theoretical study is presented in this paper to investigate the conjugate heat transfer across a vertical finite wall separating two forced and free convection flows at different temperatures. The heat conduction in the wall is in the transversal direction and countercurrent boundary layers are formed on the both sides of the wall. The governing equations of this problem and their corresponding boundary conditions are all cast into a dimensionless form by using a non-similarity transformation. These resultant equations with multiple singular points are solved numerically using a very efficient singular perturbation method. The effects of the resistance parameters and Prandtl numbers on heat transfer characteristics are investigated.
Single phase channel flow forced convection heat transfer
Energy Technology Data Exchange (ETDEWEB)
Hartnett, J.P.
1999-04-01
A review of the current knowledge of single phase forced convection channel flow of liquids (Pr > 5) is presented. Two basic channel geometries are considered, the circular tube and the rectangular duct. Both laminar flow and turbulent flow are covered. The review begins with a brief overview of the heat transfer behavior of Newtonian fluids followed by a more detailed presentation of the behavior of purely viscous and viscoelastic Non-Newtonian fluids. Recent developments dealing with aqueous solutions of high molecular weight polymers and aqueous solutions of surfactants are discussed. The review concludes by citing a number of challenging research opportunities.
Operating temperatures for a convectively cooled recessed incandescent light fixture
Energy Technology Data Exchange (ETDEWEB)
Yarbrough, D.W.; Toor, I.
1980-12-01
Test results are given for the operation of a recessed incandescent light fixture intended for residential use. The fixture is labeled for use in direct contact with attic thermal insulation. Temperature control of the powered fixture is provided by convective heat transfer from the ceiling side of the fixture. The fixture was operated at power levels up to two times the rated power of 75 watts and under thermal insulations up to R-40. In all operating configurations tested the fixture surface in contact with attic insulation was found to be less than 175/sup 0/C. The observed surface temperatures are judged to be safe for operation in contact with loose-fill or batt-type insulations. It was observed that the power leads inside one fixture configuration are exposed to temperatures as high as 168/sup 0/C. The electrical insulation could, therefore, have a limited life. The properties of the internal fixture wiring were not, however, studied in detail.
Evaluation of immersion coil designs for natural convection-driven batch cooling or heating in tanks
Energy Technology Data Exchange (ETDEWEB)
Joye, Donald D.; Smith, Michael A.
2000-09-01
Various designs of immersion coils for batch cooling or heating of liquids in tanks were investigated to determine which design was most efficient. This equipment has direct utility in chemical batch processing operations and, particularly, wort cooling - a critical step in small-scale batch brewing of beer. This operation does not use a stirrer or agitation for a variety of reasons, hence it is dependent on natural-convection mechanisms for heat transfer, yet quick cooling is desired. Four basic coil designs with several permutations were evaluated. The results fell into four groups - best, good, mediocre, and worst performers. We found that coil placement and shape were design parameters that had a critical effect on the rate of cooling. Coil spacing also had a significant effect on the cooling rate when it was less than about 2 diameters. (Author)
Numerical study of forced convective heat transfer around airships
Dai, Qiumin; Fang, Xiande
2016-02-01
Forced convective heat transfer is an important factor that affects the thermal characteristics of airships. In this paper, the steady state forced convective heat transfer around an ellipsoid is numerically investigated. The numerical simulation is carried out by commercial computational fluid dynamic (CFD) software over the extended Re range from 20 to 108 and the aspect ratio from 2 to 4. Based on the regression and optimization with software, a new piecewise correlation of the Nusselt number at constant wall temperature for ellipsoid is proposed, which is suitable for applications to airships and other ellipse shaped bodies such as elliptical balloons. The thermal characteristics of a stratospheric airship in midsummer located in the north hemisphere are numerical studied. The helium temperature predicated using the new correlation is compared to those predicted by correlations applicable for spheres and flat plates. The results show that the helium temperature obtained using the new correlation at noon is about 5.4 K lower than that using the correlation of spheres and about 2.1 K higher than that of flat plates.
Modeling of steady-state convective cooling of cylindrical Li-ion cells
Shah, K.; Drake, S. J.; Wetz, D. A.; Ostanek, J. K.; Miller, S. P.; Heinzel, J. M.; Jain, A.
2014-07-01
While Lithium-ion batteries have the potential to serve as an excellent means of energy storage, they suffer from several operational safety concerns. Temperature excursion beyond a specified limit for a Lithium-ion battery triggers a sequence of decomposition and release, which can preclude thermal runaway events and catastrophic failure. To optimize liquid or air-based convective cooling approaches, it is important to accurately model the thermal response of Lithium-ion cells to convective cooling, particularly in high-rate discharge applications where significant heat generation is expected. This paper presents closed-form analytical solutions for the steady-state temperature profile in a convectively cooled cylindrical Lithium-ion cell. These models account for the strongly anisotropic thermal conductivity of cylindrical Lithium-ion batteries due to the spirally wound electrode assembly. Model results are in excellent agreement with experimentally measured temperature rise in a thermal test cell. Results indicate that improvements in radial thermal conductivity and axial convective heat transfer coefficient may result in significant peak temperature reduction. Battery sizing optimization using the analytical model is discussed, indicating the dependence of thermal performance of the cell on its size and aspect ratio. Results presented in this paper may aid in accurate thermal design and thermal management of Lithium-ion batteries.
Fingering convection and cloudless models for cool brown dwarf atmospheres
Tremblin, P; Mourier, P; Baraffe, I; Chabrier, G; Drummond, B; Homeier, D; Venot, O
2015-01-01
This work aims to improve the current understanding of the atmospheres of brown dwarfs, especially cold ones with spectral type T and Y, whose modeling is a current challenge. Silicate and iron clouds are believed to disappear at the photosphere at the L/T transition, but cloudless models fail to reproduce correctly the spectra of T dwarfs, advocating for the addition of more physics, e.g. other types of clouds or internal energy transport mechanisms. We use a one-dimensional (1D) radiative/convective equilibrium code ATMO to investigate this issue. This code includes both equilibrium and out-of-equilibrium chemistry and solves consistently the PT structure. Included opacity sources are H2-H2, H2-He, H2O, CO, CO2, CH4, NH3, K, Na, and TiO, VO if they are present in the atmosphere. We show that the spectra of Y dwarfs can be accurately reproduced with a cloudless model if vertical mixing and NH3 quenching are taken into account. T dwarf spectra still have some reddening in e.g. J - H compared to cloudless mode...
Analytical model of transient thermal effect on convectional cooled end-pumped laser rod
Indian Academy of Sciences (India)
Khalid S Shibib; Mohammad A Munshid; Kadim A Hubiter
2013-10-01
The transient analytical solutions of temperature distribution, stress, strain and optical path difference in convectional cooled end-pumped laser rod are derived. The results are compared with other works and good agreements are found. The effects of increasing the edge cooling and face cooling are studied. It is found that an increase in the edge cooling has significant effect on reducing the maximum temperature that can be reached in the laser rod but it has no effect on the value of optical path difference. It is also found that increasing this type of cooling significantly reduces the time required to reach the thermal equilibrium with a slight increase in the max. tensile hoop stress that can be reached as the cooling increases. On the other hand, increase in face cooling reduces the response time, optical path difference and the maximum temperature that can be reached in the laser rod but a significant increase in the max. tensile hoop stress is observed. A matching between the advantages of these two type of cooling may be useful for a designer.
Energetic dynamics of a rotating horizontal convection model of an ocean basin with wind forcing
Zemskova, Varvara; White, Brian; Scotti, Alberto
2016-11-01
We analyze the energetic dynamics in a rotating horizontal convection model, where flow is driven by a differential buoyancy forcing along a horizontal surface. This model is used to quantify the influence of surface heating and cooling and surface wind stress on the Meridional Overturning Circulation. We study a model of the Southern Ocean in a rectangular basin with surface cooling on one end (the South pole) and surface warming on the other end (mid-latitudes). Free-slip boundary conditions are imposed in the closed box, while zonally periodic boundary conditions are enforced in the reentrant channel. Wind stress and differential buoyancy forcing are applied at the top boundary. The problem is solved numerically using a 3D DNS model based on a finite-volume AMR solver for the Boussinesq Navier-Stokes equations with rotation. The overall dynamics, including large-scale overturning, baroclinic eddying, turbulent mixing, and resulting energy cascades are investigated using the local Available Potential Energy framework introduced in. We study the relative contributions of surface buoyancy and wind forcing along with the effects of bottom topography to the energetic balance of this dynamic model. This research is part of the Blue Waters sustained-petascale computing project, supported by the NSF (awards OCI-0725070, ACI-1238993 and ACI-14-44747) and the state of Illinois.
Large scale three-dimensional topology optimisation of heat sinks cooled by natural convection
DEFF Research Database (Denmark)
Alexandersen, Joe; Sigmund, Ole; Aage, Niels
2016-01-01
This work presents the application of density-based topology optimisation to the design of three-dimensional heat sinks cooled by natural convection. The governing equations are the steady-state incompressible Navier-Stokes equations coupled to the thermal convection-diffusion equation through...... and several optimised designs are presented for Grashof numbers between 103 and 106. Interestingly, it is observed that the number of branches in the optimised design increases with increasing Grashof numbers, which is opposite to two-dimensional topology optimised designs. Furthermore, the obtained...... topologies verify prior conclusions regarding fin length/thickness ratios and Biot numbers, but also indicate that carefully tailored and complex geometries may improve cooling behaviour considerably compared to simple heat fin geometries. (C) 2016 Elsevier Ltd. All rights reserved....
Natural air convection for cooling of particle accelerator electromagnets: case studies at CERN
Moreira, Mariana
Cooling by natural convection is investigated on two air-cooled corrector magnets at CERN. The heat transfer coefficient (HTC) for each magnet is estimated analytically using established empirical correlations for certain geometries. The HTC is also estimated numerically with three-dimensional steady-state finite element simulations. Air convection around the respective coils as well as heat conduction inside the coils are modelled. Different formulas for the HTC are tested in the post-processing of the simulation results. The HTC for each magnet is then determined experimentally by measuring surface temperatures on the coils through time at constant currents. A method to extract the HTC from these temperature curves is developed, in which the curves are fitted to a function that is derived from a thermodynamical analysis of the heating process. Some plausible ranges for the HTC for each magnet for different currents are obtained. The results of the different estimation methods are compared with the experimen...
Introduction to light forces, atom cooling, and atom trapping
Savage, Craig,
1995-01-01
This paper introduces and reviews light forces, atom cooling and atom trapping. The emphasis is on the physics of the basic processes. In discussing conservative forces the semi-classical dressed states are used rather than the usual quantized field dressed states.
Evaporation of a binary liquid film by forced convection
Directory of Open Access Journals (Sweden)
Nasr Abdelaziz
2011-01-01
Full Text Available This paper deals with a numerical analysis of the evaporation of a thin binary liquid film by forced convection inside a channel constituted by two parallel plates. The first plate is externally insulated and wetted by a thin water ethylene glycol film while the second is dry and isothermal. The liquid mixture consists of water (the more volatile component and ethylene glycol while the gas mixture has three components: dry air, water vapour and ethylene-glycol vapour. The set of non linear and coupled equations expressing the conservation of mass, momentum, energy and species in the liquid and gas mixtures is solved numerically using a finite difference method. Results concerns with the effects of inlet ambience conditions and the inlet liquid concentration of ethylene glycol on the distribution of the temperature, concentrations profiles and the axial variation of the evaporation rate of species i.
The coupling of conduction with forced convection in Graetz problems
Energy Technology Data Exchange (ETDEWEB)
Pozzi, A.; Lupo, M. (Univ. of Naples (Italy))
1990-05-01
This paper presents an analytical solution of the energy equation for a coupled conduction-forced convection heat transfer problem in ducts. In order to compare the thermal field in plane and circular ducts, the solution is obtained in a form describing both flows. The method is based on an asymptotic expansion of the Laplace transform of the temperature and on an application of stationary-phase method; it enabled the authors to write the solution, for any value of the coupling parameter p, in terms of confluent hypergeometric functions. Some simple and accurate expressions of the interface temperature, temperature at the axis, bulk temperature, and Nusselt number are given for small and high values of p. The accuracy of the results is proved by a comparison with those obtained through an expansion in terms of 120 eigenfunctions.
Analysis of flow instabilities in forced-convection steam generator
Institute of Scientific and Technical Information of China (English)
无
2006-01-01
Because of the practical importance of two-phase instabilities, substantial efforts have been made to date to understand the physical phenomena governing such instabilities and to develop computational tools to model the dynamics. The purpose of this study is to present a numerical model for the analysis of flow-induced instabilities in forced-convection steam generator. The model is based on the assumption of homogeneous two-phase flow and thermodynamic equilibrium of the phases. The thermal capacity of the heater wall has been included in the analysis. The model is used to analyze the flow instabilities in the steam generator and to study the effects of system pressure, mass flux, inlet temperature and inlet/outlet restriction, gap size, the ratio of do /di, and the ratio of qi/qo on the system behavior.
Ergodicity in randomly forced Rayleigh-Bénard convection
Földes, J.; Glatt-Holtz, N. E.; Richards, G.; Whitehead, J. P.
2016-11-01
We consider the Boussinesq approximation for Rayleigh-Bénard convection perturbed by an additive noise and with boundary conditions corresponding to heating from below. In two space dimensions, with sufficient stochastic forcing in the temperature component and large Prandtl number Pr > 0, we establish the existence of a unique ergodic invariant measure. In three space dimensions, we prove the existence of a statistically invariant state, and establish unique ergodicity for the infinite Prandtl Boussinesq system. Throughout this work we provide streamlined proofs of unique ergodicity which invoke an asymptotic coupling argument, a delicate usage of the maximum principle, and exponential martingale inequalities. Lastly, we show that the background method of Constantin and Doering (1996 Nonlinearity 9 1049-60) can be applied in our stochastic setting, and prove bounds on the Nusselt number relative to the unique invariant measure.
Mechanistic Multidimensional Modeling of Forced Convection Boiling Heat Transfer
Directory of Open Access Journals (Sweden)
Michael Z. Podowski
2009-01-01
Full Text Available Due to the importance of boiling heat transfer in general, and boiling crisis in particular, for the analysis of operation and safety of both nuclear reactors and conventional thermal power systems, extensive efforts have been made in the past to develop a variety of methods and tools to evaluate the boiling heat transfer coefficient and to assess the onset of temperature excursion and critical heat flux (CHF at various operating conditions of boiling channels. The objective of this paper is to present mathematical modeling concepts behind the development of mechanistic multidimensional models of low-quality forced convection boiling, including the mechanisms leading to temperature excursion and the onset of CHF.
Soybean drying characteristics in microwave rotary dryer with forced convection
Institute of Scientific and Technical Information of China (English)
Ruifang WANG; Zhanyong LI; Yanhua LI; Jingsheng YE
2009-01-01
A new hybrid drying technique by combining microwave and forced convection drying within a rotary drum, i.e., microwave rotary drying, was developed with the purpose to improve the uniformity of microwave drying. In a laboratory microwave rotary dryer, rewetted soybean was utilized as experimental material to study the effects of drum rotating speed, ventilation flow rate, and specific microwave power on the drying kinetics and cracking ratio of soybean. It was found that, with rotation, the cracking ratio can be lowered but without distinct improvement in the drying rate. Increasing ventilation flow rate and specific microwave power can improve the drying rate, but the cracking ratio also increases as a negative result. The cracking ratio lower than 10% can be attained for ventilation flow rate lower than 2.0 m3·h-1 or specific microwave energy lower than 0.4 kW·kg-1 in the present experiments.
Free and forced convective-diffusion solutions by finite element methods
Energy Technology Data Exchange (ETDEWEB)
Gartling, D.K.; Nickell, R.E.
1976-01-01
Several free and forced convective-diffusion examples are solved and compared to either laboratory experiment or closed-form analysis. The problems solved illustrate the application of finite element methods to both strongly-coupled and weakly-coupled velocity and temperature fields governed by the steady-state momentum and energy equations. Special attention is given to internal forced convection with temperature-dependent viscosity and free convection within an enclosure.
Single phase forced convection cooling of high power leds
Ozdemir, M.Z.; Chestakov, D.; Frijns, A.J.H.
2011-01-01
LEDs are strong candidates for future illumination applications dueto their much lower consumption of energy compared to conventional lighting options. One of key problems in development of LEDs is successful thermal management during illumination. Therefore, current research ongoing related to high
Forced-Convection, Liquid-Cooled, Microchannel Heat Sinks
1988-01-07
as electrical insulator) Flip chip over Clean chip (T,A,M,W/S,W,M,I) and dry Mount chip with beeswax to a glass slide (be sure wax coats the entire...resistor side of chip) Soak in BHF for 3 min Wash with Dl water and dry Demount chip from glass slide Mount chip with beeswax to a polishing...T,A,M,W/S,W,M,I) and dry Pre-etch the surface with full-strength potassium ferricyanide for 5 min (do not use solvents after this step to insure
Sympathetic laser cooling of graphene with Casimir-Polder forces
Ribeiro, Sofia; Terças, Hugo
2016-10-01
We propose a scheme to actively cool the fundamental flexural (out-of-plane) mode of a graphene sheet via vacuum forces. Our setup consists of a cold-atom cloud placed close to a graphene sheet at distances of a few micrometers. The atoms couple to the graphene membrane via Casimir-Polder forces. By deriving a self-consistent set of equations governing the dynamics of the atomic gas and the flexural modes of the graphene, we show it is possible to cool graphene from room temperatures by actively (laser) cooling an atomic gas. By choosing the right set of experimental parameters we are able to cool a graphene sheet down to ˜60 μ K .
Directory of Open Access Journals (Sweden)
Kovalenko A. V.
2015-01-01
Full Text Available In the article, we have suggested a general mathematical model of non-stationary and non-isothermal process of a binary electrolyte transfer in dilute solutions in an electro-membrane system (EMS, taking into account the joint action of gravitational convection, forced convection and electro convection in potential dynamic mode. This model is a boundary problem for a system of two-dimensional quasi-linear Navier-Stokes equation and Nernst-Planck-Poisson in partial derivatives equation. We have developed a theory of similarity of the process of heat and mass transfer in electro-membrane systems, specifically, in a desalting channel of electro dialysis apparatus, taking into account joint actions of concentration polarization, space charge, gravity convection, forced convection and electro convection. It is shown that the criterion of electro convection does not directly depend on the initial concentration, and, therefore, electro convection occurs at any initial concentration. At the same time, the criterion of concentration convection linearly dependents on the initial concentration, and, therefore, at high concentrations, concentration convection prevails, while at lower concentrations, the role of gravitational convection begins to fall whereas the role of electro convection increases. The theory of similarity of the process of heat and mass transfer in the desalting channel of electro dialysis apparatus built in this work taking into account the joint action of concentration polarization, space charge, gravity convection, forced convection and electro convection is important for engineering calculations, for scaling the results of experiments in an electro-membrane cell for industrial electro dialysis water desalting apparatus
Improving the optomechanical entanglement and cooling by photothermal force
Abdi, Mehdi
2012-01-01
Cooling and Entanglement in optomechanical systems coupled through radiation pressure and photothermal force is studied. To develop the photothermal model, we derive an expression for deformation constant of the force. Exploiting linearized quantum Langevin equations we investigate dynamics of such systems. According to our analysis, in addition to separate action of radiation pressure and photothermal force, their cross correlation effect plays an important role in dynamics of the system. We also achieve an exact relation for the phonon number of the mechanical resonator in such systems, and then we derive an analytical expression for it at weak coupling limit. At strong coupling regime, we show that utilizing the photothermal pressure makes the ground state cooling more approachable. The effect of photothermal force on the optomechanical entanglement is investigated in detail. According to our exact numerical and approximate analytical studies, even though the photothermal force is naturally a dissipative f...
Precise convective cooling simulation of electronic equipment under various g-conditions
Adam, Johannes; Stuempel, Dieter; Rath, Michael
1991-12-01
Using the thermohydraulic code 'THEBES' a three dimensional flow analysis of a Spacelab rack under forced convection and a combined convective, conductive and radiative analysis of a closed electronic box under various g conditions are presented. The capabilities and features of THEBES are described. The motivation to extend the thermal analysis cycle for better treatment of fluid flow and solid to air heat transfer is discussed. It is proposed to include THEBES in a thermal analysis toolsat and present a concept for integration of THEBES with ESABASE.
Development of a mechanistic model for forced convection subcooled boiling
Shaver, Dillon R.
The focus of this work is on the formulation, implementation, and testing of a mechanistic model of subcooled boiling. Subcooled boiling is the process of vapor generation on a heated wall when the bulk liquid temperature is still below saturation. This is part of a larger effort by the US DoE's CASL project to apply advanced computational tools to the simulation of light water reactors. To support this effort, the formulation of the dispersed field model is described and a complete model of interfacial forces is formulated. The model has been implemented in the NPHASE-CMFD computer code with a K-epsilon model of turbulence. The interfacial force models are built on extensive work by other authors, and include novel formulations of the turbulent dispersion and lift forces. The complete model of interfacial forces is compared to experiments for adiabatic bubbly flows, including both steady-state and unsteady conditions. The same model is then applied to a transient gas/liquid flow in a complex geometry of fuel channels in a sodium fast reactor. Building on the foundation of the interfacial force model, a mechanistic model of forced-convection subcooled boiling is proposed. This model uses the heat flux partitioning concept and accounts for condensation of bubbles attached to the wall. This allows the model to capture the enhanced heat transfer associated with boiling before the point of net generation of vapor, a phenomenon consistent with existing experimental observations. The model is compared to four different experiments encompassing flows of light water, heavy water, and R12 at different pressures, in cylindrical channels, an internally heated annulus, and a rectangular channel. The experimental data includes axial and radial profiles of both liquid temperature and vapor volume fraction, and the agreement can be considered quite good. The complete model is then applied to simulations of subcooled boiling in nuclear reactor subchannels consistent with the
Hydrothermal convection and mordenite precipitation in the cooling Bishop Tuff, California, USA
Randolph-Flagg, N. G.; Breen, S. J.; Hernandez, A.; Self, S.; Manga, M.
2014-12-01
We present field observations of erosional columns in the Bishop Tuff and then use laboratory results and numerical models to argue that these columns are evidence of relict convection in a cooling ignimbrite. Many square kilometers of the Bishop Tuff have evenly-spaced, vertical to semi-vertical erosional columns, a result of hydrothermal alteration. These altered regions are more competent than the surrounding tuff, are 0.1-0.7 m in diameter, are separated by ~ 1 m, and in some cases are more than 8 m in height. JE Bailey (U. of Hawaii, dissertation, 2005) suggested that similar columns in the Bandelier Tuff were formed when slumping allowed water to pool at the surface of the still-cooling ignimbrite. As water percolated downward it boiled generating evenly spaced convection cells similar to heat pipes. We quantify this conceptual model and apply it the Bishop Tuff to understand the physics within ignimbrite-borne hydrothermal systems. We use thin sections to measure changing porosity and use scanning electron microscope (SEM) and x-ray diffraction (XRD) analyses to show that pore spaces in the columns are cemented by the mineral mordenite, a low temperature zeolite that precipitates between 120-200 oC (Bish et al., 1982), also found in the Bandelier Tuff example. We then use scaling to show 1) that water percolating into the cooling Bishop Tuff would convect and 2) that the geometry and spacing of the columns is predicted by the ignimbrite temperature and permeability. We use the computer program HYDROTHERM (Hayba and Ingebritsen, 1994; Kipp et al., 2008) to model 2-phase convection in the Bishop Tuff. By systematically changing permeability, initial temperature, and topography we can identify the pattern of flows that develop when the ignimbrite is cooled by water from above. Hydrothermally altered columns in ignimbrite are the natural product of coupled heat, mass, and chemical transport and have similarities to other geothermal systems, economic ore deposits
Directory of Open Access Journals (Sweden)
Xie Yong-Hui
2016-01-01
Full Text Available Gas turbine blade trailing edge is easy to burn out under the exposure of high-temperature gas due to its thin shape. The cooling of this area is an important task in gas turbine blade design. The structure design and analysis of trailing edge is critical because of the complexity of geometry, arrangement of cooling channels, design requirement of strength, and the working condition of high heat flux. In the present paper, a 3-D model of the trailing edge cooling channel is constructed and both structures with and without land are numerically investigated at different blowing ratio. The distributions of film cooling effectiveness and convective heat transfer coefficient on cutback and land surface are analyzed, respectively. According to the results, it is obtained that the distributions of film cooling effectiveness and convective heat transfer coefficient both show the symmetrical characteristics as a result of the periodic structure of the trailing edge. The increase of blowing ratio significantly improves the film cooling effectiveness and convective heat transfer coefficient on the cutback surface, which is beneficial to the cooling of trailing edge. It is also found that the land structure is advantageous for enhancing the streamwise film cooling effectiveness of the trailing edge surface while the film cooling effectiveness on the land surface remains at a low level. Convective heat transfer coefficient exhibits a strong dependency with the blowing ratio, which suggests that film cooling effectiveness and convective heat transfer coefficient must be both considered and analyzed in the design of trailing edge cooling structure.
DEFF Research Database (Denmark)
Melikov, Arsen Krikor; Duszyk, Marcin; Krejcirikova, Barbora
2012-01-01
The effect of four local cooling devices (convective, radiant and combined) on thermal comfort and perceived air quality reported by 24 subjects at 28 ˚C and 50% RH was studied. The devices studied were: (1) desk cooling fan, (2) personalized ventilation providing clean air, (3) two radiant panels...
Directory of Open Access Journals (Sweden)
Federico VÃƒÂ¡zquez
2008-12-01
Full Text Available The heat transfer problem of a zero-mean oscillatory flow of a Maxwell fluid between infinite parallel plates with boundary conditions of the third kind is considered. With these conditions, the amount of heat entering or leaving the system depends on the external temperature as well as on the convective heat transfer coefficient. The local and global time-averaged entropy production are computed, and the consequences of convective cooling of the plates are also assessed. It is found that the global entropy production is a minimum for certain suitable combination of the physical parameters. For a discrete set of values of the oscillatory Reynolds number, the extracted heat from one of the plates shows maxima.
Large scale three-dimensional topology optimisation of heat sinks cooled by natural convection
Alexandersen, Joe; Aage, Niels
2015-01-01
This work presents the application of density-based topology optimisation to the design of three-dimensional heat sinks cooled by natural convection. The governing equations are the steady-state incompressible Navier-Stokes equations coupled to the thermal convection-diffusion equation through the Bousinessq approximation. The fully coupled non-linear multiphysics system is solved using stabilised trilinear equal-order finite elements in a parallel framework allowing for the optimisation of large scale problems with order of 40-330 million state degrees of freedom. The flow is assumed to be laminar and several optimised designs are presented for Grashof numbers between $10^3$ and $10^6$. Interestingly, it is observed that the number of branches in the optimised design increases with increasing Grashof numbers, which is opposite to two-dimensional optimised designs.
Core cooling by subsolidus mantle convection. [thermal evolution model of earth
Schubert, G.; Cassen, P.; Young, R. E.
1979-01-01
Although vigorous mantle convection early in the thermal history of the earth is shown to be capable of removing several times the latent heat content of the core, a thermal evolution model of the earth in which the core does not solidify can be constructed. The large amount of energy removed from the model earth's core by mantle convection is supplied by the internal energy of the core which is assumed to cool from an initial high temperature given by the silicate melting temperature at the core-mantle boundary. For the smaller terrestrial planets, the iron and silicate melting temperatures at the core-mantle boundaries are more comparable than for the earth; the models incorporate temperature-dependent mantle viscosity and radiogenic heat sources in the mantle. The earth models are constrained by the present surface heat flux and mantle viscosity and internal heat sources produce only about 55% of the earth model's present surface heat flow.
Critical heat flux in natural convection cooled TRIGA reactors with hexagonal bundle
Energy Technology Data Exchange (ETDEWEB)
Yang, J.; Avery, M.; De Angelis, M.; Anderson, M.; Corradini, M. [Univ. of Wisconsin-Madison, 1500 Engineering Drive, Madison, WI 53706 (United States); Feldman, E. E.; Dunn, F. E.; Matos, J. E. [Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439 (United States)
2012-07-01
A three-rod bundle Critical Heat Flux (CHF) study at low flow, low pressure, and natural convection condition has been conducted, simulating TRIGA reactors with the hexagonally configured core. The test section is a custom-made trefoil shape tube with three identical fuel pin heater rods located symmetrically inside. The full scale fuel rod is electrically heated with a chopped-cosine axial power profile. CHF experiments were carried out with the following conditions: inlet water subcooling from 30 K to 95 K; pressure from 110 kPa to 230 kPa; mass flux up to 150 kg/m{sup 2}s. About 50 CHF data points were collected and compared with a few existing CHF correlations whose application ranges are close to the testing conditions. Some tests were performed with the forced convection to identify the potential difference between the CHF under the natural convection and forced convection. The relevance of the CHF to test parameters is investigated. (authors)
Cooling force on ions in a magnetized electron plasma
Nersisyan, H B
2011-01-01
Electron cooling is a well-established method to improve the phase space quality of ion beams in storage rings. In the common rest frame of the ion and the electron beam the ion is subjected to a drag force and it experiences a loss or a gain of energy which eventually reduces the energy spread of the ion beam. A calculation of this process is complicated as the electron velocity distribution is anisotropic and the cooling process takes place in a magnetic field which guides the electrons. In this paper the cooling force is calculated in a model of binary collisions (BC) between ions and magnetized electrons, in which the Coulomb interaction is treated up to second order as a perturbation to the helical motion of the electrons. The calculations are done with the help of an improved BC theory which is uniformly valid for any strength of the magnetic field and where the second-order two-body forces are treated in the interaction in Fourier space without specifying the interaction potential. The cooling force is...
Steady Thermal Field Simulation of Forced Air-cooled Column-type Air-core Reactor
Institute of Scientific and Technical Information of China (English)
DENG Qiu; LI Zhenbiao; YIN Xiaogen; YUAN Zhao
2013-01-01
Modeling the steady thermal field of the column-type air-core reactor,and further analyzing its distribution regularity,will help optimizing reactor design as well as improving its quality.The operation mechanism and inner insulation structure of a novel current limiting column-type air-core reactor is introduced in this paper.The finite element model of five encapsulation forced air-cooled column type air-core reactor is constructed using Fluent.Most importantly,this paper present a new method that,the steady thermal field of reactor working under forced air-cooled condition is simulated without arbitrarily defining the convection heat transfer coefficient for the initial condition; The result of the thermal field distribution shows that,the maximum steady temperature rise of forced air-cooled columntype air-core reactor happens approximately 5％ to its top.The law of temperature distribution indicates:In the 1/3part of the reactor to its bottom,the temperature will rise rapidly to the increasing of height,yet the gradient rate is gradually decreasing; In the 5 ％ part of the reactor to its top,the temperature will drop rapidly to the increasing of height; In the part between,the temperature will rise slowly to the increasing of height.The conclusion draws that more thermal withstand capacity should be considered at the 5 ％ part of the reactor to its top to achieve optimal design solution.
Institute of Scientific and Technical Information of China (English)
Pei－XueJiang; Ze－PeiRen; 等
1993-01-01
A numerical study is reported of laminar natural convective heat and mass transfer on a vertical cooled plate for water containing metal corrosion products at super-critical pressures.The influence of variable properties at super-critical pressures on natural convertion has been analyzed.The difference between heat and mass transfer under cooling or heating conditions is also discussed and some correlations for heat and mass transfer under cooling conditions are recommended.
Indian Academy of Sciences (India)
M H Mkwizu; O D Makinde; Yaw Nkansah-Gyekye
2015-10-01
This work investigates the effects of convective cooling on entropy generation in a transient generalized Couette flow of water-based nanofluids containing Copper (Cu) and Alumina (Al2O3) as nanoparticles. Both First and Second Laws of thermodynamics are utilised to analyse the problem. The model partial differential equations for momentum and energy balance are tackled numerically using a semidiscretization finite difference method together with Runge–Kutta Fehlberg integration scheme. Graphical results on the effects of parameter variation on velocity, temperature, skin friction, Nusselt number, entropy generation rate, irreversibility ratio and Bejan number are presented and discussed.
DEFF Research Database (Denmark)
Le Dréau, J.; Heiselberg, P.
2014-01-01
on both radiation and convection. In order to characterise the advantages and drawbacks of the different terminals, steady-state simulations of a typical office room have been performed using four types of terminals (active chilled beam, radiant floor, wall and ceiling). A sensitivity analysis has been...... conducted to determine the parameters influencing their thermal performance the most. The air change rate, the outdoor temperature and the air temperature stratification have the largest effect on the cooling need (maintaining a constant operative temperature). For air change rates higher than 0.5 ACH...
Directory of Open Access Journals (Sweden)
Ahmada Omar Ali
2015-01-01
Full Text Available This paper investigates numerically the effects of variable viscosity on unsteady generalized Couette flow of a water base nanofluid with convective cooling at the moving surface. The Buongiorno model utilized for the nanofluid incorporates the effects of Brownian motion and thermophoresis. The nonlinear governing equations of continuity, momentum, energy and nanoparticles concentration are tackled numerically using a semi discretization finite difference method together with Runge-Kutta Fehlberg integration scheme. Numerical results for velocity, temperature, and nanoparticles concentration profiles together with skin friction and Nusselt number are obtained graphically and discussed quantitatively.
Makinde, O. D.; Chinyoka, T.
2010-12-01
This present study consists of a numerical investigation of transient heat transfer in channel flow of an electrically conducting variable viscosity Boussinesq fluid in the presence of a magnetic field and thermal radiation. The temperature dependent nature of viscosity is assumed to follow an exponentially model and the system exchanges heat with the ambient following Newton's law of cooling. The governing nonlinear equations of momentum and energy transport are solved numerically using a semi-implicit finite difference method. Solutions are presented in graphical form and given in terms of fluid velocity, fluid temperature, skin friction and heat transfer rate for various parametric values. Our results reveal that combined effect of thermal radiation, magnetic field, viscosity variation and convective cooling have significant impact in controlling the rate of heat transfer in the boundary layer region.
Computational Modelling of Couette Flow of Nanofluids with Viscous Heating and Convective Cooling
Directory of Open Access Journals (Sweden)
Oluwole Daniel Makinde
2014-01-01
Full Text Available The combined effect of viscous heating and convective cooling on Couette flow and heat transfer characteristics of water base nanofluids containing Copper Oxide (CuO and Alumina (Al2O3 as nanoparticles is investigated. It is assumed that the nanofluid flows in a channel between two parallel plates with the channel’s upper plate accelerating and exchange heat with the ambient surrounding following the Newton’s law of cooling, while the lower plate is stationary and maintained at a constant temperature. Using appropriate similarity transformation, the governing Navier-Stokes and the energy equations are reduced to a set of nonlinear ordinary differential equations. These equations are solved analytically by regular perturbation method with series improvement technique and numerically by an efficient Runge-Kutta-Fehlberg integration technique coupled with shooting method. The effects of the governing parameters on the dimensionless velocity, temperature, skin friction, pressure drop and Nusselt number are presented graphically, and discussed quantitatively.
Bouchenafa, Rachid; Saim, Rachid; Abboudi, Said
2015-09-01
Forced convection is a phenomenon associated with the heat transfer fluid flows. The presence of convection affects simultaneously the thermal and hydrodynamic fields, the problem is thus coupled. This form of heat transfer inside ducts occurs in many practical applications such as solar collectors, heat exchangers, cooling of electronic components as well as chemical and nuclear. In this work, we are interested primarily for a numerical study of thermo-hydraulic performances of an incompressible turbulent flow of air through a heat sink composed of several rows of bars of square section. Profiles and the axial velocity fields, as well as profiles and the distribution of the Nusselt number are plotted for all the geometry considered and chosen for different sections. The effects of geometrical parameters of the model and the operating parameters on the dynamic and thermal behavior of the air are analyzed.
Experimental characterization of self-sensing SMA actuators under controlled convective cooling
Lewis, N.; York, A.; Seelecke, S.
2013-09-01
Shape memory alloy (SMA) wires are attractive for actuation systems due to their high energy density, light weight and silent operation. In addition, they feature self-sensing capabilities by relating electrical resistance measurements to strain changes. In real world applications SMAs typically operate in non-ambient air and it is imperative to understand an actuator’s behavior under varying convective cooling conditions, especially for smaller diameter wires, where convective effects are amplified. This paper shows that the multi-functionality of SMA actuators can be further extended by related heating power to convective air speed. It investigates the relationship between the normalized excess power needed and corresponding airspeed under controlled, laminar airflow patterns in a small-scale wind tunnel. For each experiment, airflow through the wind tunnel, strain in the SMA wire, and power supplied to the SMA wire were controlled, while the stress and resistance of the wire were measured. The ability to understand and predict an SMA wire’s behavior under various external airflows will aid in the design and understanding of future SMA actuated structures, such as micro-air vehicles, and shows that SMAs can function as self-sensing actuators and airspeed sensors.
Directory of Open Access Journals (Sweden)
Kathrin Wapler
2015-04-01
Full Text Available Two severe summer-time convective events in Germany are investigated which can be classified by the prevailing synoptic conditions into a strong and a weak forcing case. The strong forcing case exhibits a larger scale precipitation pattern caused by frontal ascent whereas scattered convection is dominating the convective activity in the weak forcing case. Other distinguished differences between the cases are faster movement of convective cells and larger regions with significant loss mainly due to severe gusts in the strong forcing case. A comprehensive set of various observations is used to characterise the two different events. The observations include measurements from a lightning detection network, precipitation radar, geostationary satellite and weather stations, as well as information from an automated cell detection algorithm based on radar reflectivity which is combined with severe weather reports, and damage data from insurances. Forecast performance at various time scales is analysed ranging from nowcasting and warning to short-range forecasting. Various methods and models are examined, including human warnings, observation-based nowcasting algorithms and high-resolution ensemble prediction systems. The analysis shows the advantages of a multi-sensor and multi-source approach in characterising convective events and their impacts. Using data from various sources allows to combine the different strengths of observational data sets, especially in terms of spatial coverage or data accuracy, e.g. damage data from insurances provide good spatial coverage with little meteorological information while measurements at weather stations provide accurate but pointwise observations. Furthermore, using data from multiple sources allow for a better understanding of the convective life cycle. Several parameters from different instruments are shown to have a predictive skill for convective development, these include satellite-based cloud-top cooling
Analysis of sublimation-cooled coated mirrors in convective and radiative environments.
Howe, J. T.; Green, M. J.
1973-01-01
Analytical solutions were obtained for the thermal response of a transpiration- or sublimation-cooled spherical mirror coating exposed to convective and radiative heating. The solutions allow unlimited spectral detail to be accounted for. Results indicate that transpiration-cooled thick coatings (1 cm) may withstand up to 10 kW/sq cm on a steady basis without excessive temperature rise for quartzlike materials with an internal absorption coefficient of 0.01 per cm. On a transient basis, fluxes up to 20 kW/sq cm can be accommodated for a second (cW laser exposure time), 4 kW/sq cm for 5 sec (planetary entry heating time), and of the order of MW/sq cm for millisecond times (short-duration laser bursts) without transpiration cooling for a material with an absorption coefficient of 0.1 per cm. Proportionately higher fluxes can be accommodated with lower absorption coefficients. Thermal stresses produced by the heat pulse are found to be high but within the strength of the materials. The regime in which meaningful solutions may be obtained is mapped in detail.
Convective Heat Transfer Coefficients of the Human Body under Forced Convection from Ceiling
DEFF Research Database (Denmark)
Kurazumi, Yoshihito; Rezgals, Lauris; Melikov, Arsen Krikor
2014-01-01
The average convective heat transfer coefficient for a seated human body exposed to downward flow from above was determined. Thermal manikin with complex body shape and size of an average Scandinavian female was used. The surface temperature distribution of the manikin’s body was as the skin...... of the convective heat transfer coefficient of the whole body (hc [W/(m2•K)]) was proposed: hc=4.088+6.592V1.715 for a seated naked body at 20ºC and hc=2.874+7.427V1.345 for a seated naked body at 26ºC. Differences in the convective heat transfer coefficient of the whole body in low air velocity range, V
Trujillo, Abraham Gerardo
In the past decades, interest in developing hydrocarbon-fueled rocket engines for deep spaceflight missions has continued to grow. In particular, liquid methane (LCH4) has been of interest due to the weight efficiency, storage, and handling advantages it offers over several currently used propellants. Deep space exploration requires reusable, long life rocket engines. Due to the high temperatures reached during combustion, the life of an engine is significantly impacted by the cooling system's efficiency. Regenerative (regen) cooling is presented as a viable alternative to common cooling methods such as film and dump cooling since it provides improved engine efficiency. Due to limited availability of experimental sub-critical liquid methane cooling data for regen engine design, there has been an interest in studying the heat transfer characteristics of the propellant. For this reason, recent experimental studies at the Center for Space Exploration Technology Research (cSETR) at the University of Texas at El Paso (UTEP) have focused on investigating the heat transfer characteristics of sub-critical CH4 flowing through sub-scale cooling channels. To conduct the experiments, the csETR developed a High Heat Flux Test Facility (HHFTF) where all the channels are heated using a conduction-based thermal concentrator. In this study, two smooth channels with cross sectional geometries of 1.8 mm x 4.1 mm and 3.2 mm x 3.2 mm were tested. In addition, three roughened channels all with a 3.2 mm x 3.2 mm square cross section were also tested. For the rectangular smooth channel, Reynolds numbers ranged between 68,000 and 131,000, while the Nusselt numbers were between 40 and 325. For the rough channels, Reynolds numbers ranged from 82,000 to 131,000, and Nusselt numbers were between 65 and 810. Sub-cooled film-boiling phenomena were confirmed for all the channels presented in this work. Film-boiling onset at Critical Heat Flux (CHF) was correlated to a Boiling Number (Bo) of
Directory of Open Access Journals (Sweden)
Hanafi Abdalla S.
2008-01-01
Full Text Available This paper presents experimental and numerical studies for the case of turbulent forced and mixed convection flow of water through narrow vertical rectangular channel. The channel is composed of two parallel plates which are heated at a uniform heat flux, whereas, the other two sides of the channel are thermally insulated. The plates are of 64 mm in width, 800 mm in height, and separated from each other at a narrow gap of 2.7 mm. The Nusselt number distribution along the flow direction normalized by the Nusselt number for the case of turbulent forced convection flow is obtained experimentally with a comparison with the numerical results obtained from a commercial computer code. The quantitative determination of the nor- malized Nusselt number with respect to the dimension-less number Z = (Gr/Re21/8Pr0.5 is presented with a comparison with previous experimental results. Qualitative results are presented for the normalized temperature and velocity profiles in the transverse direction with a comparison between the forced and mixed convection flow for both the cases of upward and downward flow directions. The effect of the axial locations and the parameter Gr/Re on the variation of the normalized temperature profiles in the transverse direction for both the regions of forced and mixed convection and for both of the upward and downward flow directions are obtained. The normalized velocity profiles in the transverse directions are also determined at different inlet velocity and heat fluxes for the previous cases. It is found that the normalized Nusselt number is greater than one in the mixed convection region for both the cases of upward and downward flow and correlated well with the dimension-less parameter Z for both of the forced and mixed convection regions. The temperature profiles increase with increasing the axial location along the flow direction or the parameter Gr/Re for both of the forced and mixed convection regions, but this increase is
Asghar, S.; Hussain, Q.; Hayat, T.; Alsaedi, A.
2015-07-01
This article addresses the heat transfer in a peristaltic flow of a reactive combustible viscous fluid through a porous saturated medium. The flow here is induced because of travelling waves along the channel walls. It is assumed that exothermic chemical reactions take place within the channel under the Arrhenius kinetics and the convective heat exchange with the ambient medium at the surfaces of the channel walls follows Newton's law of cooling. The analysis is carried out in the presence of viscous dissipation and without consumption of the material. The governing equations are formulated by employing the long-wavelength approximation. Closed-form solutions for the stream function, axial velocity, and axial pressure gradient are obtained. It is found that the temperature decreases at high Biot numbers, and the Nusselt number increases with increasing reaction parameter. The Biot number and reaction parameter produce the opposite effects on the Nusselt number.
A Study of Nucleate Boiling with Forced Convection in Microgravity
Merte, Herman, Jr.
1999-01-01
The ultimate objective of basic studies of flow boiling in microgravity is to improve the understanding of the processes involved, as manifested by the ability to predict its behavior. This is not yet the case for boiling heat transfer even in earth gravity, despite the considerable research activity over the past 30 years. The elements that constitute the nucleate boiling process - nucleation, growth, motion, and collapse of the vapor bubbles (if the bulk liquid is subcooled) - are common to both pool and flow boiling. It is well known that the imposition of bulk liquid motion affects the vapor bubble behavior relative to pool boiling, but does not appear to significantly influence the heat transfer. Indeed, it has been recommended in the past that empirical correlations or experimental data of pool boiling be used for design purposes with forced convection nucleate boiling. It is anticipated that such will most certainly not be possible for boiling in microgravity, based on observations made with pool boiling in microgravity. In earth gravity buoyancy will act to remove the vapor bubbles from the vicinity of the heater surface regardless of how much the imposed bulk velocity is reduced, depending, of course, on the geometry of the system. Vapor bubbles have been observed to dramatically increase in size in pool boiling in microgravity, and the heat flux at which dryout took place was reduced considerably below what is generally termed the critical heat flux (CHF) in earth gravity, depending on the bulk liquid subcooling. However, at heat flux levels below dryout, the nucleate pool boiling process was enhanced considerably over that in earth gravity, in spite of the large vapor bubbles formed in microgravity and perhaps as a consequence. These large vapor bubbles tended to remain in the vicinity of the heater surface, and the enhanced heat transfer appeared to be associated with the presence of what variously has been referred to as a liquid microlayer between the
DEFF Research Database (Denmark)
Le Dreau, Jerome; Heiselberg, Per; Jensen, Rasmus Lund
2015-01-01
The complexity and diversity of airflow in buildings make the accurate definition of convective heat transfer coefficients (CHTCs) difficult. In a full-scale test facility, the convective heat transfer of two cooling systems (active chilled beam and radiant wall) has been investigated under steady......-state and dynamic conditions. With the air-based cooling system, a dependency of the convective heat transfer on the air jet trajectory has been observed. New correlations have been developed, introducing a modified Archimedes number to account for the air flow pattern. The accuracy of the new correlations has been...... evaluated to±15%. Besides the study with an air-based cooling system, the convective heat transfer with a radiant cooling system has also been investigated. The convective flow at the activated surface is mainly driven by natural convection. For other surfaces, the complexity of the flow and the large...
Maldonado, Jaime J.
1994-01-01
Hypersonic vehicles are exposed to extreme thermal conditions compared to subsonic aircraft; therefore, some level of thermal management is required to protect the materials used. Normally, hypersonic vehicles experience the highest temperatures in the nozzle throat, and aircraft and propulsion system leading edges. Convective heat transfer augmentation techniques can be used in the thermal management system to increase heat transfer of the cooling channels in those areas. The techniques studied in this report are pin-fin, offset-fin, ribbed and straight roughened channel. A smooth straight channel is used as the baseline for comparing the techniques. SINDA '85, a lumped parameter finite difference thermal analyzer, is used to model the channels. Subroutines are added to model the fluid flow assuming steady one dimensional compressible flow with heat addition and friction. Correlations for convective heat transfer and friction are used in conjunction with the fluid flow analysis mentioned. As expected, the pin-fin arrangement has the highest heat transfer coefficient and the largest pressure drop. All the other devices fall in between the pin-fin and smooth straight channel. The selection of the best heat augmentation method depends on the design requirements. A good approach may be a channel using a combination of the techniques. For instance, several rows of pin-fins may be located at the region of highest heat flux, surrounded by some of the other techniques. Thus, the heat transfer coefficient is maximized at the region of highest heat flux while the pressure drop is not excessive.
Evaluation of finite element formulations for transient conduction forced-convection analysis
Thornton, E. A.; Wieting, A. R.
1979-01-01
Numerical studies clarifying the advantages and disavantages of conventional versus upwind convective finite elements are presented along with lumped versus consistent formulations for practical conduction forced-convection analysis. A finite-element procedure for treatment of negligible capacitance fluid nodes is presented. The procedure is based on procedures used in finite-element structural dynamics to treat nodes with negligible structural mass. Two finite-element programs and a finite-difference lumped-parameter program used in the studies are discussed. Evaluation studies utilizing three convection and two combined conduction-convection problems are then presented and discussed. Additionally, the computational time saving offered by the finite element procedure is considered for a practical combined conduction-convection problem.
Forced Convection Heat Transfer in Plate Channels Filled with Packed Beds or Sintered Porous Media
Institute of Scientific and Technical Information of China (English)
姜培学; 李勐; 任泽霈
2002-01-01
In the present work, forced convection heat transfer in plate channels filled with metallic or non-metallic particles (packed beds) or sintered porous media is simulated numerically using a thermal non-equilibrium model. The numerical simulation results are compared with experimental data. The difference between convection heat transfer in packed beds and in sintered porous media and the effects of the boundary condition assumptions are investigated. The results show that the numerical simulation of convection heat transfer of air or water in packed beds using the local thermal non-equilibrium model and the variable porosity model agrees well with the experimental data. The convection heat transfer coefficient in sintered porous media is much higher than that in packed beds. In the numerical simulation of convection heat transfer in sintered porous media, the boundary conditions on the wall should be that the particle temperatures are equal to the fluid temperature.
Development of a Forced-Convection Liquid-Fluoride-Salt Test Loop
Energy Technology Data Exchange (ETDEWEB)
Yoder Jr, Graydon L [ORNL; Wilson, Dane F [ORNL; Peretz, Fred J [ORNL; Wilgen, John B [ORNL; Romanoski, Glenn R [ORNL; Kisner, Roger A [ORNL; Holcomb, David Eugene [ORNL; Heatherly, Dennis Wayne [ORNL; Aaron, Adam M [ORNL
2010-01-01
A small forced-convection molten-fluoride-salt loop is being constructed at Oak Ridge National Laboratory to examine the heat transfer behavior of FLiNaK salt in a heated pebble bed. Objectives of the experiment include reestablishing infrastructure needed for fluoride-salt loop testing, developing a unique inductive heating technique for performing heat transfer (or other) experiments, measuring heat transfer characteristics in a liquid-fluoride-salt-cooled pebble bed, and demonstrating the use of silicon carbide (SiC) as a structural component for salt systems. The salt loop will consist of an Inconel 600 piping system, a sump-type pump, a SiC test section, and an air-cooled heat exchanger, as well as auxiliary systems needed to pre-heat the loop, transport salt into and out of the loop, and maintain an inert cover gas over the salt. A 30,000 Hz inductive heating system will be used to provide up to 250 kW of power to a 15 cm diameter SiC test section containing a packed bed of 3 cm graphite spheres. A SiC-to-Inconel 600 joint will use a conventional nickel/grafoil spiral wound gasket sandwiched between SiC and Inconel flanges. The loop system can provide up to 4.5 kg/s of salt flow at a head of 0.125 MPa and operate at a pressure just above atmospheric. Pebble Reynolds numbers of up to 2600 are possible with this configuration. A sump system is provided to drain and store the salt when not in use. Instrumentation on the loop will include pressure, temperature, and flow measurements, while the test section will be instrumented to provide pebble and FLiNaK temperatures.
Heat transfer in forced convection boiling of oil-non-azeotropic binary refrigerant mixtures
Energy Technology Data Exchange (ETDEWEB)
Sami, S.M.; Tulej, P.; Fang, L. (Moncton Univ., NB (Canada))
1993-12-01
In this paper, the results of the heat transfer, forced convection, boiling characteristics of non-azeotropic refrigerant mixtures and oil are presented. This includes heat transfer coefficients for pure and binary mixtures under boiling conditions outside enhanced surface tubing. Local convective heat transfer coefficients have been determined using a modified Wilson-plot technique. Heat transfer correlations were established as a function of the binary mixture mass flow rate, and oil concentration, as well as key flow parameters. (author)
Maki, Syou; Tanaka, Keito; Morimoto, Shotaro
2017-02-01
We examined, by three-dimensional numerical computations, the magnetothermal convection of air (a paramagnetic substance) enclosed in a cylindrical vessel with a Rayleigh-Benard model under the application of an axisymmetric magnetic force at the center of a solenoidal superconducting magnet. Axisymmetric steady convective flows were induced when the magnitude of the radial component of the magnetic force (fmR) was 1.0 and 5.0 times that of the gravitational force at the vessel sidewall; e.g., the hot air was concentrated at the vessel center and the cold air was driven to the vicinity of the vessel sidewall. This flow pattern was similar to the case of water (a diamagnetic substance), although the axisymmetric arrangements of hot and cold water were the reverse of the present convection of air. When fmR was 0.5 times that of the gravitational force, the axisymmetric flows appeared only in the vicinity of the vessel sidewall. Unsteady convective rolls simultaneously occurred in the vessel center, and they repeatedly combined and separated from each other. When fmR was 0.1 times that of the gravitational force, there were barely any axisymmetric flows in the close vicinity of the vessel sidewall, while the initial convective flows remained in most other parts of the vessel. Thus, we varied the magnitude of fmR and clarified the transitional processes of isothermal and velocity distributions of magnetothermal convection. We discuss those convective flows with the magnitude and direction of fmR.
Convection heat transfer from discrete heat sources in a liquid cooled rectangular channel
Energy Technology Data Exchange (ETDEWEB)
Bhowmik, H. [Department of Mechanical Engineering, Dhaka University of Engineering and Technology, Gazipur 1700 (Bangladesh); Tso, C.P. [Faculty of Engineering and Technology, Multimedia University, Jalan Ayer Keroh Lama, 75450 Melaka (Malaysia); Tou, K.W.; Tan, F.L. [School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 (Singapore)
2005-11-01
Steady-state experiments are performed to study general convective heat transfer from an in-line four simulated electronic chips in a vertical rectangular channel using water as the working fluid. The experimental data covers a wide range for laminar flow under natural, mixed and forced convection conditions with Reynolds number based on channel hydraulic diameter ranging from 40 to 2220 and Reynolds number based on heat source length ranging from 50 to 2775, respectively. The heat flux ranges from 0.1W/cm{sup 2} to 0.6W/cm{sup 2}. The inlet water temperature is at 24{sup o}C. The effect of heat fluxes, flow rates and geometrical parameters such as chip number are investigated. The experimental results indicate that the heat transfer coefficient is strongly affected by Reynolds number and fully-developed values of heat transfer coefficient are reached before the first chip. Empirical correlations are developed for relations using Nusselt number, Reynolds number and Grashof number, based on channel hydraulic diameter. (author)
Multi-crystalline silicon solidification under controlled forced convection
Cablea, M.; Zaidat, K.; Gagnoud, A.; Nouri, A.; Chichignoud, G.; Delannoy, Y.
2015-05-01
Multi-crystalline silicon wafers have a lower production cost compared to mono-crystalline wafers. This comes at the price of reduced quality in terms of electrical properties and as a result the solar cells made from such materials have a reduced efficiency. The presence of different impurities in the bulk material plays an important role during the solidification process. The impurities are related to different defects (dislocations, grain boundaries) encountered in multi-crystalline wafers. Applying an alternative magnetic field during the solidification process has various benefits. Impurities concentration in the final ingot could be reduced, especially metallic species, due to a convective term added in the liquid that reduces the concentration of impurities in the solute boundary layer. Another aspect is the solidification interface shape that is influenced by the electromagnetic stirring. A vertical Bridgman type furnace was used in order to study the solidification process of Si under the influence of a travelling magnetic field able to induce a convective flow in the liquid. The furnace was equipped with a Bitter type three-phase electromagnet that provides the required magnetic field. A numerical model of the furnace was developed in ANSYS Fluent commercial software. This paper presents experimental and numerical results of this approach, where interface markings were performed.
Uddin, Md Jashim; Khan, Waqar A; Ismail, A I Md
2013-01-01
A two-dimensional steady forced convective flow of a Newtonian fluid past a convectively heated permeable vertically moving plate in the presence of a variable magnetic field and radiation effect has been investigated numerically. The plate moves either in assisting or opposing direction to the free stream. The plate and free stream velocities are considered to be proportional to x(m) whilst the magnetic field and mass transfer velocity are taken to be proportional to x((m-1)/2) where x is the distance along the plate from the leading edge of the plate. Instead of using existing similarity transformations, we use a linear group of transformations to transform the governing equations into similarity equations with relevant boundary conditions. Numerical solutions of the similarity equations are presented to show the effects of the controlling parameters on the dimensionless velocity, temperature and concentration profiles as well as on the friction factor, rate of heat and mass transfer. It is found that the rate of heat transfer elevates with the mass transfer velocity, convective heat transfer, Prandtl number, velocity ratio and the magnetic field parameters. It is also found that the rate of mass transfer enhances with the mass transfer velocity, velocity ratio, power law index and the Schmidt number, whilst it suppresses with the magnetic field parameter. Our results are compared with the results existing in the open literature. The comparisons are satisfactory.
Convection forced by a descending dry layer and low-level moist convergence
Russell, Andrew; Vaughan, Geraint; Norton, Emily G.; Ricketts, Hugo M. A.; Morcrette, Cyril J.; Hewison, Tim J.; Browning, Keith. A.; Blyth, Alan M.
2009-03-01
A narrow line of convective showers was observed over southern England on 18 July 2005 during the Convective Storm Initiation Project (CSIP). The showers formed behind a cold front (CF), beneath two apparently descending dry layers (i.e. sloping so that they descended relative to the instruments observing them). The lowermost dry layer was associated with a tropopause fold from a depression, which formed 2 d earlier from a breaking Rossby wave, located northwest of the UK. The uppermost dry layer had fragmented from the original streamer due to rotation around the depression (This rotation was also responsible for the observations of apparent descent-ascent would otherwise be seen behind a CF). The lowermost dry layer descended over the UK and overran higher θw air beneath it, resulting in potential instability. Combined with a surface convergence line (which triggered the convection but had less impact on the convective available potential energy than the potential instability), convection was forced up to 5.5 km where the uppermost dry layer capped it. The period when convection was possible was very short, thus explaining the narrowness of the shower band. Convective Storm Initiation Project observations and model data are presented to illustrate the unique processes in this case.
Energy Technology Data Exchange (ETDEWEB)
Ben Amara, Sami; Laguerre, Onrawee [Cemagref - Refrigeration Processes Engineering Research Unit, parc de Tourvoie, BP 44, 92163 cedex, Antony (France); Flick, Denis [National Agronomic Institute - INAPG, 16 rue Claude Bernard, 75231 cedex 05, Paris (France)
2004-12-01
During cooling with low air velocity (u{<=}0.2 m.s{sup -1}) of a stack of foodstuffs (a few centimeters dimension), the radiation and conduction between products can be of the same order of magnitude as convection. A method was developed to quantify these various transfer modes. The experiment was carried out using an in-line spherical arrangement; however, the same methodology can be applied to other product shapes. The results confirm that the heat transfers by radiation and conduction cannot be neglected. In addition, the convective heat transfer coefficient varies not only with air velocity but also with the product position in the stack. (authors)
Study on Forced Convective Heat Transfer of Non-Newtonian Nanofluids
Institute of Scientific and Technical Information of China (English)
Yurong He; Yubin Men; Xing Liu; Huilin Lu; Haisheng Chen; Yulong Ding
2009-01-01
This paper is concerned with the forced convective heat transfer of dilute liquid suspensions of nanoparticles (nanofluids) flowing through a straight pipe under laminar conditions. Stable nanofluids are formulated by using the high shear mixing and ultrasonication methods. They are then characterised for their size, surface charge, thermal and rheological properties and tested for their convective heat transfer behaviour. Mathematical model-ling is performed to simulate the convective heat transfer of nanofluids using a single phase flow model and con-sidering nanofluids as both Newtonian and non-Newtonian fluid. Both experiments and mathematical modelling show that nanofluids can substantially enhance the convective heat transfer. Analyses of the results suggest that the non-Newtonian character of nanofluids influences the overall enhancement, especially for nanofluids with an obvious non-Newtonian character.
Variability of radiatively forced diurnal cycle of intense convection in the tropical west pacific
Energy Technology Data Exchange (ETDEWEB)
Gray, W.M.; Sheaffer, J.D.; Thorson, W.B. [Colorado State Univ., Fort Collins, CO (United States)
1996-04-01
Strong differences occur in daytime versus nighttime (DVN) net radiative cooling in clear versus cloudy areas of the tropical atmosphere. Daytime average cooling is approximately -0.7{degrees}C/day, whereas nighttime net tropospheric cooling rates are about -1.5{degrees}C/day, an approximately two-to-one difference. The comparatively strong nocturnal cooling in clear areas gives rise to a diurnally varying vertical circulation and horizontal convergence cycle. Various manifestations of this cyclic process include the observed early morning heavy rainfall maxima over the tropical oceans. The radiatively driven DVN circulation appears to strongly modulate the resulting diurnal cycle of intense convection which creates the highest, coldest cloudiness over maritime tropical areas and is likely a fundamental mechanism governing both small and large scale dynamics over much of the tropical environment.
Energy Technology Data Exchange (ETDEWEB)
Silva, Alice Cunha da; Su, Jian, E-mail: alicecs@poli.ufrj.br, E-mail: sujian@nuclear.ufrj.br [Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, RJ (Brazil)
2013-07-01
The High Temperature Gas cooled Reactor (HTGR) is a fourth generation thermal nuclear reactor, graphite-moderated and helium cooled. The HTGRs have important characteristics making essential the study of these reactors, as well as its fuel element. Examples of these are: high thermal efficiency,low operating costs and construction, passive safety attributes that allow implication of the respective plants. The Pebble Bed Modular Reactor (PBMR) is a HTGR with spherical fuel elements that named the reactor. This fuel element is composed by a particulate region with spherical inclusions, the fuel UO2 particles, dispersed in a graphite matrix and a convective heat transfer by Helium happens on the outer surface of the fuel element. In this work, the transient heat conduction in a spherical fuel element of a pebble-bed high temperature reactor was studied in a transient situation of combined convective and radiative cooling. Improved lumped parameter model was developed for the transient heat conduction in the two-layer composite sphere subjected to combined convective and radiative cooling. The improved lumped model was obtained through two-point Hermite approximations for integrals. Transient combined convective and radiative cooling of the two-layer spherical fuel element was analyzed to illustrate the applicability of the proposed lumped model, with respect to die rent values of the Biot number, the radiation-conduction parameter, the dimensionless thermal contact resistance, the dimensionless inner diameter and coating thickness, and the dimensionless thermal conductivity. It was shown by comparison with numerical solution of the original distributed parameter model that the improved lumped model, with H2,1/H1,1/H0,0 approximation yielded significant improvement of average temperature prediction over the classical lumped model. (author)
Minder, J. R.; Smith, R. B.; Nugent, A. D.; Kirshbaum, D. J.
2011-12-01
Shallow convection is a pervasive feature of orographic precipitation, but its detailed role remains poorly understood. The mountainous Caribbean island of Dominica is a natural laboratory for isolating the role of shallow convection in orographic rainfall. It lies in a region of persistent easterly trade wind flow, and receives much of its rainfall from shallow convection that is forced mechanically by ascent of easterly flow over the Dominican terrain. The Dominica Experiment (DOMEX) has focused on convective orographic precipitation over the island from 2007-2011. The first phase of the project developed a climatology of rainfall and theories to explain the observed enhancement over the terrain. The second phase of the project (Apr-May 2011) has provided a detailed view of 20 individual rainfall events with data from: surface gauges, time-lapse photography, operational radar scans, a mountaintop weather station, and both in situ and remote observations from the University of Wyoming King Air research aircraft. Focusing on ascent--forced convection during DOMEX has revealed a number of the key processes that control the rainfall. Upwind of the island, clouds and water vapor anomalies exist that appear to play a crucial role in seeding the convection over the terrain and determining its vigor. Over the windward slopes the air is readily lifted with little flow deflection. Strong convective cells rapidly develop to produce large rainfall rates. Over the lee slopes of the terrain there is an abrupt transition to a deep and turbulent plunging flow that quickly eliminates convective clouds, but allows for the spillover of rainfall. The air that passes over the island is transformed such that low-levels are dried, warmed and decelerated, and the downwind wake becomes less hospitable to trade wind cumuli.
Core flows and heat transfer induced by inhomogeneous cooling with sub- and supercritical convection
Dietrich, Wieland; Wicht, Johannes
2016-01-01
The amount and spatial pattern of heat extracted from cores of terrestrial planets is ultimately controlled by the thermal structure of the lower rocky mantle. Using the most common model to tackle this problem, a rapidly rotating and differentially cooled spherical shell containing an incompressible and viscous liquid is numerically investigated. To gain the physical basics, we consider a simple, equatorial symmetric perturbation of the CMB heat flux shaped as a spherical harmonic $Y_{11}$. The thermodynamic properties of the induced flows mainly depend on the degree of nonlinearity parametrised by a horizontal Rayleigh number $Ra_h=q^\\ast Ra$, where $q^\\ast$ is the relative CMB heat flux anomaly amplitude and $Ra$ is the Rayleigh number which controls radial buoyancy-driven convection. Depending on $Ra_h$ we characterise three flow regimes through their spatial patterns, heat transport and flow speed scalings: in the linear conductive regime the radial inward flow is found to be phase shifted $90^\\circ$ eas...
Study of natural convection cooling of a nanofluid subjected to a magnetic field
Mahmoudi, Ahmed; Mejri, Imen; Omri, Ahmed
2016-06-01
This paper presents a numerical study of natural convection cooling of water-Al2O3 nanofluid by two heat sinks vertically attached to the horizontal walls of a cavity subjected to a magnetic field. The left wall is hot, the right wall is cold, while the horizontal walls are insulated. Lattice Boltzmann method (LBM) is applied to solve the coupled equations of flow and temperature fields. This study has been carried out for the pertinent parameters in the following ranges: Rayleigh number of the base fluid, Ra =103 to 105, Hartmann number varied from Ha = 0 to 60 and the solid volume fraction of nanoparticles between ϕ = 0 and 6%. In order to investigate the effect of heat sinks location, three different configurations of heat sinks are considered. The effects of Rayleigh numbers, Hartmann number and heat sinks location on the streamlines, isotherms, Nusselt number are investigated. Results show that the heat transfer rate decreases with the increase of Hartmann number and increases with the rise of Rayleigh number. In addition it is observed that the average Nusselt number increases linearly with the increase of the nanoparticles solid volume fraction. Also, results show that the heat sinks positions greatly influence the heat transfer rate depending on the Hartmann number, Rayleigh number and nanoparticle solid volume fraction.
Directory of Open Access Journals (Sweden)
Chen An
2017-01-01
Full Text Available In this work, we studied the transient combined convection and radiation of multilayer spherical media with volumetric heat generation, extending the previous work on the particular case of a spherical body subjected to radiative cooling. The proposed lumped models were obtained through two-point Hermite approximations for the average temperature and heat flux in each layer. For the average temperature, the plain trapezoidal rule (H0,0 approximation was employed in all layers, except for the innermost layer, where the second-order two-side corrected trapezoidal rule (H2,1 approximation was utilized. For the heat flux, the plain trapezoidal rule (H0,0 approximation was employed for all the layers. The transient heat conduction in a TRISO-coated fuel particle being composed of five layers (namely, fuel kernel, buffer of porous carbon, inner pyrocarbon, silicon carbide, and outer pyrocarbon was analyzed using the proposed lumped models, the results of which were verified by comparison with the finite difference solution of the original distributed parameter model. Parametric studies were conducted to examine the effects of the dimensionless heat generation rate, the radiation-conduction parameter, and the Biot number on the temporal variations of the average temperatures.
Additional force field in cooling process of cellular Al alloy
Institute of Scientific and Technical Information of China (English)
郑明军; 何德坪; 戴戈
2002-01-01
The foaming process of Al alloy is similar to that of Al, but there is a solid-liquid state zone in the solidification process of cellular Al alloy which does not exist in the case of Al. In the unidirectional solidification of cellular Al alloy, the proportion of the solid phase gradually reduces from the solid front to the liquid front. This will introduce a force and result in a serious quick shrinkage. By the mathematic and physical mode, the solidification of the cellular Al alloy is studied. The data measured by experiment are close to the result calculated by the mode. This kind of shrinkage can be solved by suitable cooling method in appropriate growth stage. The compressive strength of the cellular Al alloy made by this way is 40% higher than that of cellular Al.
Rotating turbulent Rayleigh–Bénard convection subject to harmonically forced flow reversals
Geurts, Bernard J.; Kunnen, Rudie P.J.
2014-01-01
The characteristics of turbulent flow in a cylindrical Rayleigh–Bénard convection cell which can be modified considerably in case rotation is included in the dynamics. By incorporating the additional effects of an Euler force, i.e., effects induced by non-constant rotation rates, a remarkably strong
Analytical Solution of Forced-Convective Boundary-Layer Flow over a Flat Plate
DEFF Research Database (Denmark)
Mirgolbabaei, H.; Barari, Amin; Ibsen, Lars Bo
2010-01-01
In this letter, the problem of forced convection heat transfer over a horizontal flat plate is investigated by employing the Adomian Decomposition Method (ADM). The series solution of the nonlinear differential equations governing on the problem is developed. Comparison between results obtained...
Heat transfer in a gray tube with forced convection, internal radiation and axial wall conduction
Chung, B. T. F.; Thompson, J. E.
1983-01-01
A method of successive approximations is employed to solve the problem of heat transfer to a transparent gas flowing through a radiating-conducting tube with turbulent forced convection between the tube wall and the gas, and with energy generation in the wall. Emphasis is given to the effect of emissivity of the wall to the tube and gas temperature profiles.
Analytical Solution of Forced-Convective Boundary-Layer Flow over a Flat Plate
DEFF Research Database (Denmark)
Mirgolbabaei, H.; Barari, Amin; Ibsen, Lars Bo;
2010-01-01
In this letter, the problem of forced convection heat transfer over a horizontal flat plate is investigated by employing the Adomian Decomposition Method (ADM). The series solution of the nonlinear differential equations governing on the problem is developed. Comparison between results obtained...
Laboratory grey cast iron continuous casting line with electromagnetic forced convection support
2010-01-01
The article describes the construction of a 20 mm diameter grey cast iron ingots continuous casting laboratory line. This line is made ofthree main units: melting unit (induction furnace), casting unit and the pulling unit. In order to improve the homogeneity of themicrostructure of ingots (by applying forced convection of liquid metal during the crystallization process) in this case a crystallizer system generating the forced movement of liquid metal based on a system of electrical power win...
Energy Technology Data Exchange (ETDEWEB)
Huh, Seon Jeong; Lee, Hee Joon [Kookmin University, Seoul (Korea, Republic of); Kim, Myoung Jun; Moon, Joo Hyung; Bae, Youngmin; Kim, Young-In [Korea Atomic Energy Research Institute, Daejeon (Korea, Republic of)
2016-10-15
Recently emergency cooldown tank(ECT) is a great concern of passive cooling system for the safety of nuclear reactor. After the operation of a conventional passive cooling system for an extended period, however, the water level falls as a result of the evaporation from the ECT, as steam is emitted from the open top of the tank. In this study, the effect of heat transfer area at the air cooled condensing heat exchanger was investigated by changing 5×5 tube banks into 4×4 and 3×3. Moreover, each of air-side natural convective heat transfer coefficient of tube banks was compared to existing correlations. This study presents the effect of heat transfer area at air-cooled condensing heat exchanger. As heat transfer area decreased, the temperature of outlet increased. In other words, the cooling performance got lower with the decrease of heat transfer area. In addition, the average natural convective heat transfer coefficient was 15.3 W/m{sup 2}/K from the 4×4 tube banks, and 4.92 W/m{sup 2}/K from the 3×3 tube banks, which had quite a large error more than 46% especially with the value of 4×4 tube banks compared to the value from correlation equation. Therefore, according to this result, it is needed to measure the local heat transfer coefficient of vertical cylinder more elaborately in further study.
Forced Convective Heat Transfer in a Porous Plate Channel
Institute of Scientific and Technical Information of China (English)
PeixueJiang; ZhanWang; 等
1997-01-01
Fored convective heat transfer in a plate channel filled with metallic spherical particales was investigated experimentally and numerically.The test section ,58mm×80mm×50mm in size,was heated by a 0.4mm thick plate electrical heater,The coolant water flow rate ranged from 0.015 to 0.833 kg/s.The local wall temperature distribution was measured along with the inlet and outlet fliud temperatures and pressures.The results illustrate the heat transfer augmentation and increased pressure drop caused by the porous medium.The heat transfer coefficient was increased 5-12 times by the porous media although the hydraulic resistance was increased even more.The Nusselt number and the heat transfer coefficient increased with decreasing particle diameter,while the pressure drop decreased as the particle diameter increased.It was found that,for the conditions studied(metallic packed bed),the effect of thermal dispersion did not need to be considered in the physical model,as opposed to a non-metallic packed bed,where thermal dispersion is important.
Diaguila, Anthony J; Freche, John C
1951-01-01
Blade-to-coolant heat-transfer data and operating data were obtained with a natural-convection water-cooled turbine over range of turbine speeds and inlet-gas temperatures. The convective coefficients were correlated by the general relation for natural-convection heat transfer. The turbine data were displaced from a theoretical equation for natural convection heat transfer in the turbulent region and from natural-convection data obtained with vertical cylinders and plates; possible disruption of natural convection circulation within the blade coolant passages was thus indicated. Comparison of non dimensional temperature-ratio parameters for the blade leading edge, midchord, and trailing edge indicated that the blade cooling effectiveness is greatest at the midchord and least at the trailing edge.
Numerical analysis of laminar forced convection in a spherical annulus
Energy Technology Data Exchange (ETDEWEB)
Tuft, D.B.
1980-07-21
Calculations of steady laminar incompressible fluid-flow and heat transfer in a spherical annulus are presented. Steady pressures, temperatures, velocities, and heat transfer coefficients are calculated for an insulated outer sphere and a 0/sup 0/C isothermal inner sphere with 50/sup 0/C heated water flowing in the annulus. The inner sphere radius is 13.97 cm, the outer sphere radius is 16.83 cm and the radius ratio is 1.2. The transient axisymmetric equations of heat, mass, and momentum conservation are solved numerically in spherical coordinates. The transient solution is carried out in time until steady state is achieved. A variable mesh is used to improve resolution near the inner sphere where temperature and velocity gradients are steep. It is believed that this is the first fully two-dimensional analysis of forced flow in a spherical annulus. Local and bulk Nusselt numbers are presented for Reynolds numbers from 4.4 to 440. Computed bulk Nusselt numbers ranged from 2 to 50 and are compared to experimental results from the literature. Inlet flow jetting off the inner sphere and flow separation are predicted by the analysis. The location of wall jet separation was found to be a function of Reynolds number, indicating the location of separation depends upon the ratio of inertia to viscous forces. Wall jet separation has a pronounced effect on the distribution of local heat flux. The area between inlet and separation was found to be the most significant area for heat transfer. Radial distributions of azimuthal velocity and temperature are presented for various angles beginning at the inlet. Inner sphere pressure distribution is presented and the effect on flow separation is discussed.
Convectively Forced Gravity Waves and their Sensitivity to Heating Profile and Atmospheric Structure
Halliday, Oliver; Parker, Douglas; Griffiths, Stephen; Vosper, Simon; Stirling, Alison
2016-04-01
It has been known for some time that convective heating is communicated to its environment by gravity waves. Despite this, the radiation of gravity waves in macro-scale models, which are typically forced at the grid-scale by meso-scale parameterization schemes, is not well understood. We present here theoretical work directed toward improving our fundamental understanding of convectively forced gravity wave effects at the meso-scale, in order to begin to address this problem. Starting with the hydrostatic, non-rotating, 2D, Boussinesq equations in a slab geometry, we find a radiating, analytical solution to prescribed sensible heat forcing for both the vertical velocity and potential temperature response. Both Steady and pulsed heating with adjustable horizontal structure is considered. From these solutions we construct a simple model capable of interrogating the spatial and temporal sensitivity to chosen heating functions of the remote forced response in particular. By varying the assumed buoyancy frequency, the influence of the model stratosphere on the upward radiation of gravity waves, and in turn, on the tropospheric response can be understood. Further, we find that the macro-scale response to convection is highly dependent on the radiation characteristics of gravity waves, which are in turn dependent upon the temporal and spatial structure of the source, and upper boundary condition of the domain.
Finite element methodology for transient conduction/forced-convection thermal analysis
Thornton, E. A.; Wieting, A. R.
1979-01-01
Finite element methodology for steady state thermal analysis of convectively cooled structures has been extended for transient analysis. The finite elements are based on representing the fluid passages by fluid bulk-temperature nodes and fluid-solid interface nodes. The formulation of the finite element equations for a typical flow passage is based on the weighted residual method with upwind weighting functions. Computer implementation of the convective finite element methodology using explicit and implicit time integration algorithms is described. Accuracy and efficiency of the methodology is evaluated by comparisons with analytical solutions and finite-difference lumped-parameter analyses. The comparative analyses demonstrate that finite element conduction/conduction methodology may be used to predict transient temperatures with an accuracy equal or superior to the lumped-parameter finite-difference method.
Uhlig, Ralf; Frantz, Cathy; Fritsch, Andreas
2016-05-01
External receiver configurations are directly exposed to ambient wind. Therefore, a precise determination of the convective losses is a key factor in the prediction and evaluation of the efficiency of the solar absorbers. Based on several studies, the forced convective losses of external receivers are modeled using correlations for a roughened cylinder in a cross-flow of air. However at high wind velocities, the thermal efficiency measured during the Solar Two experiment was considerably lower than the efficiency predicted by these correlations. A detailed review of the available literature on the convective losses of external receivers has been made. Three CFD models of different level of detail have been developed to analyze the influence of the actual shape of the receiver and tower configuration, of the receiver shape and of the absorber panels on the forced convective heat transfer coefficients. The heat transfer coefficients deduced from the correlations have been compared to the results of the CFD simulations. In a final step the influence of both modeling approaches on the thermal efficiency of an external tubular receiver has been studied in a thermal FE model of the Solar Two receiver.
Natural versus forced convection in laminar starting plumes
Rogers, Michael C
2009-01-01
A starting plume or jet has a well-defined, evolving head that is driven through the surrounding quiescent fluid by a localized flux of either buoyancy or momentum, or both. We studied the scaling and morphology of starting plumes produced by a constant flux of buoyant fluid from a small, submerged outlet. The plumes were laminar and spanned a wide range of plume Richardson numbers Ri. Ri is the dimensionless ratio of the buoyancy forces to inertial effects, and is thus our measurements crossed over the transition between buoyancy-driven plumes and momentum-driven jets. We found that the ascent velocity of the plume, nondimensionalized by Ri, exhibits a power law relationship with Re, the Reynolds number of the injected fluid in the outlet pipe. We also found that as the threshold between buoyancy-driven and momentum-driven flow was crossed, two distinct types of plume head mophologies existed: confined heads, produced in the Ri > 1 regime, and dispersed heads, which are found in the Ri < 1 regime. Head di...
Energy Technology Data Exchange (ETDEWEB)
Thomas, L P; Marino, B M [Facultad de Ciencias Exactas, Universidad Nacional del Centro de la Provincia de Buenos Aires, Pinto 399, B 7000 GHG Tandil (Argentina); Tovar, R; Castillo, J A, E-mail: lthomas@exa.unicen.edu.ar [Centro de Investigacion en EnergIa, Universidad Nacional Autonoma de Mexico Priv. Xochicalco s/n, Col. Centro, Temixco, Morelos (Mexico)
2011-05-01
The flow generated by a linear heat source inside a thermally insulated box with the upper boundary maintained at constant temperature is analyzed by means of a series of experiments. The attention is focused on the steady state during which the heat provided by the source is absorbed by the upper boundary giving place to a particular convective process in two well defined zones. One of them occupies most part of the box from the lower boundary and is characterized by a turbulent convective flow; the other is the thermal boundary layer developed below the cooled top contour where strong fluctuations are detected. Special interest is found in the analogies with the results obtained employing a configuration similar to that used to research the Rayleigh-Benard convection with lower and upper boundaries at constant temperatures but without internal sources. This study has particular relevance to understanding and predicting the use of the cooled-ceiling systems in buildings as a passive solution to enhance the comfort of its inhabitants during hot days with the consequential energy savings.
Wang, J. X.; Jia, P. Y.; Wang, Y. S.; Jiang, L.
2010-03-01
In this article, using Gibson-Ashby constitutive model, we suggest a new method for numerical investigation of forced convection heat transfer in porous foam metal, and try to consolidate the study for mechanical property and that for thermal characteristic. By available experimental data, we simulated to two cases, namely as the transfer in porous media for diameter is 0.6 mm and porosity is 0.402, and for diameter is 1.6 mm and porosity is 0.462. The result, from our constitutive model for single forced convection heat transfer, corresponds well with the experimental data. As for pressure drop prediction in porous is in good agreement with experiment, and the error is only 5% to 10%, but for transfer is less accurate, the error is about 20%, which is acceptable in practice. So it is done that constitutive model is used to simulate the transfer property.
Effect of Selected Factors on Drying Process of Tomato in Forced Convection Solar Energy Dryer
U.S. Muhammed; A.M.I. El-Okene; Isiaka, M
2012-01-01
The effect of air velocity, slice thickness and grazing materials in drying process of tomato in forced convection solar energy dryer was evaluated. The result is to serve as an input for solar energy development for drying of vegetable and fruit products in North West Ecological zone of Nigeria. In order to evaluate the effects of the above factors in drying operation, a split-split-plot experimental design was used. Differences among the treatments and their interactions were tested with or...
Cao, Y.; Faghri, A.
1991-01-01
The performance of a thermal energy storage module is simulated numerically. The change of phase of the phase-change material (PCM) and the transient forced convective heat transfer for the transfer fluid with low Prandtl numbers are solved simultaneously as a conjugate problem. A parametric study and a system optimization are conducted. The numerical results show that module geometry is crucial to the design of a space-based thermal energy storage system.
Directory of Open Access Journals (Sweden)
Prasenjit Dey
2016-01-01
Full Text Available An unsteady two-dimensional forced convection over a square cylinder with sharp and rounded corner edge is numerically analyzed for the low Reynolds number laminar flow regime. In this study, the analysis is carried out for Reynolds number (Re in the range of 80 to 180 with Prandtl number (Pr variation from 0.01 to 1000 for various corner radius (r=0.50, 0.51, 0.54, 0.59, 0.64 and 0.71. The lateral sides of the computational domain are kept constant to maintain the blockage as 5%. Heat transfer due to unsteady forced convection has been predicted by Artificial Neural network (ANN. The present ANN is trained by the input and output data which has been acquired from the numerical simulation, performed in finite volume based Computational Fluid Dynamics (CFD commercial software FLUENT. The heat transfer characteristics over the sharp and rounded corner square cylinder are evaluated by analyzing the local Nusselt number (Nulocal, average Nusselt number (Nuavg at various Reynolds number, Prandtl numbers and for various corner radii. It is found that the heat transfer rate of a circular cylinder can be enhanced by 12% when Re is varying and 14% when Prandtl number is varying by introducing a new cylinder geometry of corner radius r=0.51. It is found that the unsteady forced convection heat transfer over a cylinder can be predicted appropriately by ANN. It is also observed that the back propagation ANN can predict the heat transfer characteristics of forced convection very quickly compared to a standard CFD method.
Analysis of streamwise conduction in forced convection of microchannels using fin approach
Institute of Scientific and Technical Information of China (English)
Suhandran MUNIANDY; Yew Mun HUNG
2011-01-01
The effects induced by streamwise conduction on the thermal characteristics of forced convection for single-phase liquid flow in rectangular microchannel heat sinks under imposed constant wall temperature have been studied.By employing the fin approach in the first law of analysis,models with and without streamwise conduction term in the energy equation were developed for hydrodynamically and thermally fully-developed flow under local thermal non-equilibrium for the solid and fluid phases.These two models were solved to obtain closed form analytical solutions for the fluid and solid temperature distributions and the analysis emphasized details of the variations induced by the streamwise conduction on the fluid temperature distributions.The effects of the Peclet number,aspect ratio,and thermal conductivity ratio on the thermal characteristics of forced convection in microchannel heat sinks were analyzed and discussed.This study reveals the conditions under which the effect of streamwise conduction is significant and should not be neglected in the forced convective heat transfer analysis ofmicrochannel heat sinks.
Laboratory grey cast iron continuous casting line with electromagnetic forced convection support
Directory of Open Access Journals (Sweden)
J. Szajnar
2010-07-01
Full Text Available The article describes the construction of a 20 mm diameter grey cast iron ingots continuous casting laboratory line. This line is made ofthree main units: melting unit (induction furnace, casting unit and the pulling unit. In order to improve the homogeneity of themicrostructure of ingots (by applying forced convection of liquid metal during the crystallization process in this case a crystallizer system generating the forced movement of liquid metal based on a system of electrical power windings of the AC specific frequency. Thissolution allowed to obtain a homogeneous microstructure of the continuous casting of cast iron EN-GJL-200 species.
Beukema, K.J.
1980-01-01
Three different models of bulk-stored agricultural products with air flow through the bulk, predicting the temperature profiles or the velocity of natural convection, are developed. The temperature distribution in a cylindrical container with insulated walls and open top and bottom, filled with heat-generating model material is studied experimentally, used to calculate the velocity of natural convection and compared with model predictions. A two-dimensional two-phase model of temper...
The effect of natural and forced melt convection on dendritic solidification in Ga-In alloys
Shevchenko, N.; Roshchupkina, O.; Sokolova, O.; Eckert, S.
2015-05-01
The directional solidification of Ga-25 wt%In alloys within a Hele-Shaw cell was visualized by means of X-ray radioscopy. The experimental investigations are especially focused on the impact of melt convection on the dendritic growth. Natural convection occurs during a bottom up solidification because lighter solute is rejected at the solid-liquid interface leading to an unstable density stratification. Forced convection was produced by a rotating wheel with two parallel disks containing at their inner sides a set of permanent NdFeB magnets with alternating polarization. The direction of forced melt flow is almost horizontal at the solidification front whereas local flow velocities in the range between 0.1 and 1.0 mm/s were achieved by controlling the rotation speed of the magnetic wheel. Melt flow induces various effects on the grain morphology primarily caused by the convective transport of solute. Our observations show a facilitation of the growth of primary trunks or lateral branches, suppression of side branching, dendrite remelting and fragmentation. The manifestation of all phenomena depends on the dendrite orientation, local direction and intensity of the flow. The forced flow eliminates the solutal plumes and damps the local fluctuations of solute concentration. It provokes a preferential growth of the secondary arms at the upstream side of the primary dendrite arms, whereas the high solute concentration at the downstream side of the dendrites can inhibit the formation of secondary branches completely. Moreover, the flow changes the inclination angle of the dendrites and the angle between primary trunks and secondary arms.
HANARO cooling features: design and experience
Energy Technology Data Exchange (ETDEWEB)
Park, Cheol; Chae, Hee-Taek; Han, Gee-Yang; Jun, Byung-Jin; Ahn, Guk-Hoon [HANARO Operating Team, Korea Atomic Energy Research Institute, Taejon (Korea)
1999-08-01
In order to achieve the safe core cooling during normal operation and upset conditions, HANARO adopted an upward forced convection cooling system with dual containment arrangements instead of the forced downward flow system popularly used in the majority of forced convection cooling research reactors. This kind of upward flow system was selected by comparing the relative merits of upward and downward flow systems from various points of view such as safety, performance, maintenance. However, several operational matters which were not regarded as serious at design come out during operation. In this paper are presented the design and operational experiences on the unique cooling features of HANARO. (author)
Directory of Open Access Journals (Sweden)
O. D. Makinde
2014-01-01
Full Text Available Heat transfer characteristics of a Berman flow of water based nanofluids containing copper (Cu and alumina (Al2O3 as nanoparticles in a porous channel with Navier slip, viscous dissipation, and convective cooling are investigated. It is assumed that the exchange of heat with the ambient surrounding takes place at the channel walls following Newton’s law of cooling. The governing partial differential equations and boundary conditions are converted into a set of nonlinear ordinary differential equations using appropriate similarity transformations. These equations are solved analytically by regular perturbation methods with series improvement technique and numerically using an efficient Runge-Kutta Fehlberg integration technique coupled with shooting scheme. The effects of the governing parameters on the dimensionless velocity, temperature, skin friction, pressure drop, and Nusselt numbers are presented graphically and discussed quantitatively.
Solar drying of whole mint plant under natural and forced convection
Directory of Open Access Journals (Sweden)
Y.I. Sallam
2015-03-01
Full Text Available Two identical prototype solar dryers (direct and indirect having the same dimensions were used to dry whole mint. Both prototypes were operated under natural and forced convection modes. In the case of the later one the ambient air was entered the dryer with the velocity of 4.2 m s−1. The effect of flow mode and the type of solar dryers on the drying kinetics of whole mint were investigated. Ten empirical models were used to fit the drying curves; nine of them represented well the solar drying behavior of mint. The results indicated that drying of mint under different operating conditions occurred in the falling rate period, where no constant rate period of drying was observed. Also, the obtained data revealed that the drying rate of mint under forced convection was higher than that of mint under natural convection, especially during first hours of drying (first day. The values of the effective diffusivity coefficient for the mint drying ranged between 1.2 × 10−11 and 1.33 × 10−11 m2 s−1.
Solar drying of whole mint plant under natural and forced convection.
Sallam, Y I; Aly, M H; Nassar, A F; Mohamed, E A
2015-03-01
Two identical prototype solar dryers (direct and indirect) having the same dimensions were used to dry whole mint. Both prototypes were operated under natural and forced convection modes. In the case of the later one the ambient air was entered the dryer with the velocity of 4.2 m s(-1). The effect of flow mode and the type of solar dryers on the drying kinetics of whole mint were investigated. Ten empirical models were used to fit the drying curves; nine of them represented well the solar drying behavior of mint. The results indicated that drying of mint under different operating conditions occurred in the falling rate period, where no constant rate period of drying was observed. Also, the obtained data revealed that the drying rate of mint under forced convection was higher than that of mint under natural convection, especially during first hours of drying (first day). The values of the effective diffusivity coefficient for the mint drying ranged between 1.2 × 10(-11) and 1.33 × 10(-11) m(2) s(-1).
Energy Technology Data Exchange (ETDEWEB)
Farmer, M. T.; Kilsdonk, D. J.; Tzanos, C.P.; Lomperski, S.; Aeschlimann, R.W.; Pointer, D.; Nuclear Engineering Division
2005-09-01
As part of the Department of Energy (DOE) Generation IV roadmapping activity, the Very High Temperature gas cooled Reactor (VHTR) has been selected as the principal concept for hydrogen production and other process-heat applications such as district heating and potable water production. On this basis, the DOE has selected the VHTR for additional R&D with the ultimate goal of demonstrating emission-free electricity and hydrogen production with this advanced reactor concept. One of the key passive safety features of the VHTR is the potential for decay heat removal by natural circulation of air in a Reactor Cavity Cooling System (RCCS). The air-cooled RCCS concept is notably similar to the Reactor Vessel Auxiliary Cooling System (RVACS) that was developed for the General Electric PRISM sodium-cooled fast reactor. As part of the DOE R&D program that supported the development of this fast reactor concept, the Natural Convection Shutdown Heat Removal Test Facility (NSTF) was developed at ANL to provide proof-of-concept data for the RVACS under prototypic natural convection flow, temperature, and heat flux conditions. Due to the similarity between RVACS and the RCCS, current VHTR R&D plans call for the utilization of the NSTF to provide RCCS model development and validation data, in addition to supporting design validation and optimization activities. Both air-cooled and water-cooled RCCS designs are to be included. In support of this effort, ANL has been tasked with the development of an engineering plan for mechanical and instrumentation modifications to NSTF to ensure that sufficiently detailed temperature, heat flux, velocity and turbulence profiles are obtained to adequately qualify the codes under the expected range of air-cooled RCCS flow conditions. Next year, similar work will be carried out for the alternative option of a water-cooled RCCS design. Analysis activities carried out in support of this experiment planning task have shown that: (a) in the RCCS, strong
Energy Technology Data Exchange (ETDEWEB)
Silva, Joao Carlos Teles Ribeiro da; Mederos, Barbara Janet Teruel [Universidade Estadual de Campinas (FEAGRI/UNICAMP), SP (Brazil). Fac. de Engenharia Agricola
2008-07-01
The application of cooling technologies for the conservation of horticultural products is one of the stages the Cold Chain. In Brazil particularly, as a country with tropical climate with average high temperature almost all year, the application of these technologies is very important because the shelf-life of fresh horticultural products, with quality that the market demands, is directly related to temperature. In particular, the systems of forced air cooling operate according to the flow of air predetermined in the project according to the quantity of product to cool. When actual conditions differ from considerations of the project, as to the quantity of product, a situation very common in agricultural properties and packing houses, the fan will continue providing the nominal flow rate, causing alteration of the cost-benefit relation of process. This project aims at the development of a micro-processing equipment (output current of 4 to 20 mA) to control the rotational speed of the motor of the fan systems, air forced through an inverter of frequency. The objective is development of a Man-Machine Interface, based on an algorithm, which, through the introduction of mass product data and the automatic acquisition of data from temperature of the product and the camera, is calculated the cooling time. The rotation of the engine fan will be amended automatically, to maintain air flow with a proper cost-benefit, in connection with the reduction of cooling time, energy consumption, for the increasing the shelf life of products. (author)
Energy Technology Data Exchange (ETDEWEB)
Silva, Joao Carlos Teles Ribeiro da; Mederos, Barbara Janet Teruel [Universidade Estadual de Campinas (FEAGRI/UNICAMP), SP (Brazil). Fac. de Engenharia Agricola
2008-07-01
The application of cooling technologies for the conservation of horticultural products is one of the stages the Cold Chain. In Brazil particularly, as a country with tropical climate with average high temperature almost all year, the application of these technologies is very important because the shelf-life of fresh horticultural products, with quality that the market demands, is directly related to temperature. In particular, the systems of forced air cooling operate according to the flow of air predetermined in the project according to the quantity of product to cool. When actual conditions differ from considerations of the project, as to the quantity of product, a situation very common in agricultural properties and packing houses, the fan will continue providing the nominal flow rate, causing alteration of the cost-benefit relation of process. This project aims at the development of a micro-processing equipment (output current of 4 to 20 mA) to control the rotational speed of the motor of the fan systems, air forced through an inverter of frequency. The objective is development of a Man-Machine Interface, based on an algorithm, which, through the introduction of mass product data and the automatic acquisition of data from temperature of the product and the camera, is calculated the cooling time. The rotation of the engine fan will be amended automatically, to maintain air flow with a proper cost-benefit, in connection with the reduction of cooling time, energy consumption, for the increasing the shelf life of products. (author)
Zhang, Guang; Jiang, Shaohui; Yao, Wei; Liu, Changhong
2016-11-16
Owing to the outstanding properties of thermal conduction, lightweight, and chemical durability, carbon nanotubes (CNTs) have revealed promising applications in thermal management materials. Meanwhile, the increasingly popular portable electronics and the rapid development of space technology need lighter weight, smaller size, and more effective thermal management devices. Here, a novel kind of heat dissipation devices based on the superaligned CNT films and underlying microchannels is proposed, and the heat dissipation properties are measured at the natural condition. Distinctive from previous studies, by combining the advantages of microchannels and CNTs, such a novel heat dissipation device enables superior natural convection heat transfer properties. Our findings prove that the novel CNT-based devices could show an 86.6% larger total natural heat dissipation properties than bare copper plate. Further calculations of the radiation and natural convection heat transfer properties demonstrate that the excellent passive cooling properties of these CNT-based devices are primarily caused by the reinforcement of the natural convection heat transfer properties. Furthermore, the heat dissipation mechanisms are briefly discussed, and we propose that the very high heat transfer coefficients and the porous structures of superaligned CNT films play critical roles in reinforcing the natural convection. The novel CNT-based heat dissipation devices also have advantages of energy-saving, free-noise, and without additional accessories. So we believe that the CNT-based heat dissipation devices would replace the traditional metal-finned heat dissipation devices and have promising applications in electronic devices, such as photovoltaic devices, portable electronic devices, and electronic displays.
2D study of wind forces around multiple cooling towers using ...
African Journals Online (AJOL)
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2D study of wind forces around multiple cooling towers using computational ... Due to the complexity of wind turbulence near ground (Atmospheric Boundary. Layer), the .... In the past, before the computer revolution, the analysis used to be.
Beukema, K.J.
1980-01-01
Three different models of bulk-stored agricultural products with air flow through the bulk, predicting the temperature profiles or the velocity of natural convection, are developed. The temperature distribution in a cylindrical container with insulated walls and open top and bottom, filled
A theoretical study of the spheroidal droplet evaporation in forced convection
Li, Jie; Zhang, Jian
2014-11-01
In many applications, the shape of a droplet may be assumed to be an oblate spheroid. A theoretical study is conducted on the evaporation of an oblate spheroidal droplet under forced convection conditions. Closed-form analytical expressions of the mass evaporation rate for an oblate spheroid are derived, in the regime of controlled mass-transfer and heat-transfer, respectively. The variation of droplet size during the evaporation process is presented in the regime of shrinking dynamic model. Comparing with the droplets having the same surface area, an increase in the aspect ratio enhances the mass evaporation rate and prolongs the burnout time.
Sabin, C. M.; Poppendiek, H. F.
1971-01-01
A number of heat transfer and fluid flow mechanisms that control once-through, forced convection potassium boiling are studied analytically. The topics discussed are: (1) flow through tubes containing helical wire inserts, (2) motion of droplets entrained in vapor flow, (3) liquid phase distribution in boilers, (4) temperature distributions in boiler tube walls, (5) mechanisms of heat transfer regime change, and (6) heat transfer in boiler tubes. Whenever possible, comparisons of predicted and actual performances are made. The model work presented aids in the prediction of operating characteristics of actual boilers.
Numerical simulation of forced convection in a duct subjected to microwave heating
Energy Technology Data Exchange (ETDEWEB)
Zhu, J.; Kuznetsov, A.V. [North Carolina State University, Department of Mechanical and Aerospace Engineering, Campus Box 7910, Raleigh, NC (United States); Sandeep, K.P. [North Carolina State University, Department of Food Science, Raleigh, NC (United States)
2007-01-15
In this paper, forced convection in a rectangular duct subjected to microwave heating is investigated. Three types of non-Newtonian liquids flowing through the duct are considered, specifically, apple sauce, skim milk, and tomato sauce. A finite difference time domain method is used to solve Maxwell's equations simulating the electromagnetic field. The three-dimensional temperature field is determined by solving the coupled momentum, energy, and Maxwell's equations. Numerical results show that the heating pattern strongly depends on the dielectric properties of the fluid in the duct and the geometry of the microwave heating system. (orig.)
Critical Heat Flux in forced Convective Boiling with a Wall Jet
Institute of Scientific and Technical Information of China (English)
王迅
2004-01-01
The critical heat flux (CHF) in the forced convective boiling with a wall jet has been investigated.The experiments of CHF with a wall jet have been performed over a wide range of ρl/ρg=6.6-1 603 and ΔTsub=0-60 K.The mechanism on CHF is discussed and a CHF model based on heat balance in sublayer can provide a good clue for analyzing and deriving CHF.Finally,a generalized correlation is presented, which can predict CHF for saturated and subcooled conditions.
Institute of Scientific and Technical Information of China (English)
无
2008-01-01
Entropy generation for thermally developing forced convection in a porous medium bounded by two isothermal parallel plates is investigated analytically on the basis of the Darcy flow model where the viscous dissipation effects had also been taken into account.A parametric study showed that decreasing the group parameter and the Péclet number increases the entropy generation while for the Brinkman number the converse is true.Heatline visualization technique is applied with an emphasis on the Br ＜ 0 case where there is somewhere that heat transfer changes direction at some streamwise location to the wall instead of its original direction,i.e.,from the wall.
Kleinman, Leonid S.; Red, X. B., Jr.
1995-01-01
An algorithm has been developed for time-dependent forced convective diffusion-reaction having convection by a recirculating flow field within the drop that is hydrodynamically coupled at the interface with a convective external flow field that at infinity becomes a uniform free-streaming flow. The concentration field inside the droplet is likewise coupled with that outside by boundary conditions at the interface. A chemical reaction can take place either inside or outside the droplet, or reactions can take place in both phases. The algorithm has been implemented, and for comparison results are shown here for the case of no reaction in either phase and for the case of an external first order reaction, both for unsteady behavior. For pure interphase mass transfer, concentration isocontours, local and average Sherwood numbers, and average droplet concentrations have been obtained as a function of the physical properties and external flow field. For mass transfer enhanced by an external reaction, in addition to the above forms of results, we present the enhancement factor, with the results now also depending upon the (dimensionless) rate of reaction.
Irrigation Induced Surface Cooling in the Context of Modern and Increased Greenhouse Gas Forcing
Cook, Benjamin I.; Puma, Michael J.; Krakauer, Nir Y.
2010-01-01
There is evidence that expected warming trends from increased greenhouse gas (GHG) forcing have been locally masked by irrigation induced cooling, and it is uncertain how the magnitude of this irrigation masking effect will change in the future. Using an irrigation dataset integrated into a global general circulation model, we investigate the equilibrium magnitude of irrigation induced cooling under modern (Year 2000) and increased (A1B Scenario, Year 2050) GHG forcing, using modern irrigation rates in both scenarios. For the modern scenario, the cooling is largest over North America, India, the Middle East, and East Asia. Under increased GHG forcing, this cooling effect largely disappears over North America, remains relatively unchanged over India, and intensifies over parts of China and the Middle East. For North America, irrigation significantly increases precipitation under modern GHG forcing; this precipitation enhancement largely disappears under A1B forcing, reducing total latent heat fluxes and the overall irrigation cooling effect. Over India, irrigation rates are high enough to keep pace with increased evaporative demand from the increased GHG forcing and the magnitude of the cooling is maintained. Over China, GHG forcing reduces precipitation and shifts the region to a drier evaporative regime, leading to a relatively increased impact of additional water from irrigation on the surface energy balance. Irrigation enhances precipitation in the Middle East under increased GHG forcing, increasing total latent heat fluxes and enhancing the irrigation cooling effect. Ultimately, the extent to which irrigation will continue to compensate for the warming from increased GHG forcing will primarily depend on changes in the background evaporative regime, secondary irrigation effects (e.g. clouds, precipitation), and the ability of societies to maintain (or increase) current irrigation rates.
Human response to local convective and radiant cooling in a warm environment
DEFF Research Database (Denmark)
Melikov, Arsen Krikor; Krejcirikova, Barbora; Kaczmarczyk, Jan
2013-01-01
thermal comfort compared to the condition without cooling. The acceptability of the thermal environment was similar for all cooling devices. The acceptability of air movement and perceived air quality increased when local cooling methods were used. The best results were...... quality. The intensity of the reported sick building syndrome symptoms increased during the exposure time, with or without cooling devices in operation. Air movement had very little effect on sick building syndrome symptoms, but they increased when the pollution level was high. The lowest prevalence...... of symptoms was reported with personalized ventilation and with the radiant panel with attached fans, which also caused subjects to report less fatigue. Sick building syndrome symptoms increased most when the tabletop fan, generating movement of polluted room air, was in operation. The temperature...
A Forced Jet System for the Cooling of Cutting Tools.
Cutting tools , *Coolant pumps, *Machine tools, *Metals, Machine shop practice, High pressure, Force(Mechanics), Centrifugal pumps, Mist, Jet streams, Lubricants, Machining, Friction, Surface finishing, Safety
The onset of nonpenetrative convection in a suddenly cooled layer of fluid
Energy Technology Data Exchange (ETDEWEB)
Ihle, Christian F. [Program in Fluid Dynamics, Facultad de Ciencias Fisicas y Matematicas, Universidad de Chile, Blanco Encalada 2002 Of. 327, Santiago (Chile); Nino, Yarko [Departamento de Ingenieria Civil, Division de Recursos Hidricos y Medio Ambiente, Universidad de Chile, Av. Blanco Encalada 2002, Santiago (Chile)
2006-04-15
Conditions for the onset of nonpenetrative convection in a horizontal Boussinesq fluid layer subject to a step change in temperature are studied using propagation theory. A wide range of Prandtl numbers and two different kinematic boundary conditions are considered. It is shown that for high Rayleigh numbers, critical conditions for the onset of convective motion reproduce exactly those for the unsteady Rayleigh-Benard instability. Present results extend those of previous research and show a tendency of the rigid-rigid and free-rigid critical curves to converge for low Prandtl numbers. Comparison between present and previously reported results on critical conditions for the onset of instabilities and onset time using different methods yields good agreement on a middle to high Prandtl number range. A ratio of 10 between experimentally measured and theoretically predicted onset times is suggested for stress-free bounded systems. (author)
MHD simulations of near-surface convection in cool main-sequence stars
Beeck, Benjamin; Reiners, Ansgar
2014-01-01
The solar photospheric magnetic field is highly structured owing to its interaction with the convective flows. Its local structure has a strong influence on the profiles of spectral lines not only by virtue of the Zeeman effect, but also through the modification of the thermodynamical structure (e.g. line weakening in hot small-scale magnetic structures). Many stars harbor surface magnetic fields comparable to or larger than the Sun at solar maximum. Therefore, a strong influence of the field on the surface convection and on spectral line profiles can be expected. We carried out 3D local-box MHD simulations of unipolar magnetized regions (average fields of 20, 100, and 500G) with parameters corresponding to six main-sequence stars (spectral types F3V to M2V). The influence of the magnetic field on the convection and the local thermodynamical structure were analyzed in detail. For three spectral lines, we determined the impact of the magnetic field on the disc-integrated Stokes-I profiles. Line weakening has i...
Optical dipole-force cooling of anions in a Penning trap
Fesel, Julian; Gerber, Sebastian; Doser, Michael; Comparat, Daniel
2017-09-01
We discuss the possibility of using optical dipole forces for Sisyphus cooling of ions stored in a Penning trap by addressing the specific case of the molecular cooling candidate C2 -. Using a GPU accelerated code for Penning trap simulations, which we extended to include the molecule-light interaction, we show that this scheme can decrease the time required for cooling by an order of magnitude with respect to Doppler cooling. In our simulation we found that a reduction of the axial anion temperature from 10 K to 50 mK in around 10 s is possible. The temperature of the radial degrees of freedom was seen to thermalize to 150 mK . Based on the laser-cooled C2 -, a study on the sympathetic cooling of anions with masses 1-50 nucleon was performed, covering relevant candidates for investigations of chemical anion reactions at ultracold temperatures as well as for antimatter studies.
Effects of Cooling on Ankle Muscle Strength, Electromyography, and Gait Ground Reaction Forces
Directory of Open Access Journals (Sweden)
Amitava Halder
2014-01-01
Full Text Available The effects of cooling on neuromuscular function and performance during gait are not fully examined. The purpose of this study was to investigate the effects of local cooling for 20 min in cold water at 10°C in a climate chamber also at 10°C on maximal isometric force and electromyographic (EMG activity of the lower leg muscles. Gait ground reaction forces (GRFs were also assessed. Sixteen healthy university students participated in the within subject design experimental study. Isometric forces of the tibialis anterior (TA and the gastrocnemius medialis (GM were measured using a handheld dynamometer and the EMG was recorded using surface electrodes. Ground reaction forces during gait and the required coefficient of friction (RCOF were recorded using a force plate. There was a significantly reduced isometric maximum force in the TA muscle (P<0.001 after cooling. The mean EMG amplitude of GM muscle was increased after cooling (P<0.003, indicating that fatigue was induced. We found no significant changes in the gait GRFs and RCOF on dry and level surface. These findings may indicate that local moderate cooling 20 min of 10°C cold water, may influence maximal muscle performance without affecting activities at sub-maximal effort.
Nie, Ji; Sobel, Adam H
2016-01-01
Extratropical extreme precipitation events are usually associated with large-scale flow disturbances, strong ascent and large latent heat release. The causal relationships between these factors are often not obvious, however, and the roles of different physical processes in producing the extreme precipitation event can be difficult to disentangle. Here, we examine the large-scale forcings and convective heating feedback in the precipitation events which caused the 2010 Pakistan flood within the Column Quasi-Geostrophic framework. A cloud-revolving model (CRM) is forced with the large-scale forcings (other than large-scale vertical motion) computed from the quasi-geostrophic omega equation with input data from a reanalysis data set, and the large-scale vertical motion is diagnosed interactively with the simulated convection. Numerical results show that the positive feedback of convective heating to large-scale dynamics is essential in amplifying the precipitation intensity to the observed values. Orographic li...
Energy Technology Data Exchange (ETDEWEB)
Francisco Valentin; Narbeh Artoun; Masahiro Kawaji; Donald M. McEligot
2015-08-01
Fundamental high pressure/high temperature forced convection experiments have been conducted in support of the development of a Very High Temperature Reactor (VHTR) with a prismatic core. The experiments utilize a high temperature/high pressure gas flow test facility constructed for forced convection and natural circulation experiments. The test section has a single 16.8 mm ID flow channel in a 2.7 m long, 108 mm OD graphite column with four 2.3kW electric heater rods placed symmetrically around the flow channel. This experimental study presents the role of buoyancy forces in enhancing or reducing convection heat transfer for helium at high pressures up to 70 bar and high temperatures up to 873 degrees K. Wall temperatures have been compared among 10 cases covering the inlet Re numbers ranging from 500 to 3,000. Downward flows display higher and lower wall temperatures in the upstream and downstream regions, respectively, than the upward flow cases due to the influence of buoyancy forces. In the entrance region, convection heat transfer is reduced due to buoyancy leading to higher wall temperatures, while in the downstream region, buoyancyinduced mixing causes higher convection heat transfer and lower wall temperatures. However, their influences are reduced as the Reynolds number increases. This experimental study is of specific interest to VHTR design and validation of safety analysis codes.
Study and Application of Forced Convection in Road Bus Heating System
Directory of Open Access Journals (Sweden)
Scheila Sandi Biazus
2015-03-01
Full Text Available This work deals with a replacing the heating system of intercity buses, made by long heat exchangers distributed on the sides of the passenger saloon by small and compact ones that use forced convection to heat dissipation. At the beginning the calculation was made of the heat capacity of a heat exchanger with the parameters defined by the supplier, and then conducted an analysis of change in these parameters in order to improve its efficiency. The method for examining the heat exchanger is based on the correlations available in the convection flow inside and outside to determine the respective convective coefficients, and therefore the overall coefficient of heat transfer. Following, the heat exchanger in its original form, was tested on bench. Thus, from the thermal load of the bus model studied, the amount of exchangers required has been defined to satisfy the thermal comfort of passengers. Field tests of the heating systems with new heat exchangers were performed to obtain actual data of the proposed system. The original factory system showed to be efficient in meeting the needs thermal load and economically viable, such that the critical thermal load can be met with ten heaters installed along the body. Furthermore, this system offers the possibility of varying the parameters to best fit to the project, where the flow variation of the fans or the spacing between the fins of the heat exchanger decreases the amount of heaters required. At the same time, the system showed the need for further study to assess its distribution along the passenger saloon in order to meet a best uniformity temperature distribution.
Khilkevich, Andrei; Shemyakin, Alexander V
2012-01-01
In Fermilab's electron cooler, a 0.1A, 4.3MeV DC electron beam propagates through the 20 m cooling section, which is immersed in a weak longitudinal magnetic field. A proper adjustment of 200 dipole coils, installed in the cooling section for correction of the magnetic field imperfections, can create a helix-like trajectory with the wavelength of 1-10 m. The longitudinal cooling force is measured in the presence of such helixes at different wavelengths and amplitudes. The results are compared with a model calculating the cooling force as a sum of collisions with small impact parameters, where the helical nature of the coherent angle is ignored, and far collisions, where the effect of the coherent motion is neglected. A qualitative agreement is found.
Sympathetic laser-cooling of graphene with Casimir-Polder forces
Ribeiro, Sofia
2016-01-01
We propose a scheme to actively cool the fundamental flexural (out-of-plane) mode of a graphene sheet via vacuum forces. Our setup consists in a cold atom cloud placed close to a graphene sheet at distances of a few micrometers. The atoms couple to the graphene membrane via Casimir-Polder forces. By deriving a self-consistent set of equations governing the dynamics of the atomic gas and the flexural modes of the graphene, we show to be possible to cool graphene from room temperatures by actively (laser) cooling an atomic gas. By choosing the right set of experimental parameter we are able to cool a graphene sheet down to ~ 60 microkelvin.
Thermal environment in a simulated double office room with convective and radiant cooling systems
DEFF Research Database (Denmark)
Mustakallio, Panu; Bolashikov, Zhecho Dimitrov; Rezgals, Lauris
2017-01-01
The thermal environment in a double office room obtained with chilled beam (CB), chilled beam with radiant panel (CBR), chilled ceiling with ceiling installed mixing ventilation (CCMV) and overhead mixing total volume ventilation (MTVV) under summer (cooling) condition was compared. Design (peak......) and usual (average) heat load from solar radiation, office equipment, lighting and occupants was simulated, respectively at 62 W/m2 and 38 W/m2 under four different workstation layouts. Air temperature, globe (operative) temperature, radiant asymmetry, air velocity and turbulent intensity were measured...... CCMV especially in the design heat load cases. With CBR, the thermal environment was found to be between CB and CCMV. MTVV generated high draught level under the tested design heat load cases. All cooling systems generated similar thermal environment in the usual heat load cases. It would...
2014-09-01
the test section, a metal sleeve was placed around the entire ceramic transition as seen at the bottom left of Fig. 3.4. Despite the structural...Wade, W. R. Measurements of Total Hemispherical Emissivity of Several Stably Oxidized Metals and Some Refractory Oxide Coatings. Technical report...materials. Ceramics are capable of withstanding the high Taw condition but crack after repeated heating and cooling cycles. Thus, ceramics not viable in
Single layer solar drying behaviour of Citrus aurantium leaves under forced convection
Energy Technology Data Exchange (ETDEWEB)
Ait Mohamed, L.; Lahsasni, S. [Ecole Normale Superieure, Marrakech (Morocco). Laboratoire d' Energie Solaire et des Plantes Aromatiques et Medicinales; Unite de Chimie Agroalimentaire, Marrakech (Morocco). Faculte des Sciences Semlalia; Kouhila, M.; Jamali, A. [Ecole Normale Superieure, Marrakech (Morocco). Laboratoire d' Energie Solaire et des Plantes Aromatiques et Medicinales; Kechaou, N. [Ecole Nationale d' Ingenieurs de Sfax (Tunisia); Mahrouz, M. [Unite de Chimie Agroalimentaire, Marrakech (Morocco). Faculte des Sciences Semlalia
2005-06-01
Convective solar drying experiments in thin layers of Citrus aurantium leaves grown in Marrakech, morocco, were conducted. An indirect forced convection solar dryer consisting of a solar air collector, an auxiliary heater, a circulation fan and a drying cabinet is used for the experiments. The air temperature was varied from 50 to 60{sup o}C; the relative humidity from 41% to 53%; and the drying air flow rate from 0.0277 to 0.0833 m{sup 3}/s. Thirteen statistical models, which are semi-theoretical and/or empirical, were tested for fitting the experimental data. A nonlinear regression analysis using a statistical computer program was used to evaluate the constants of the models. The Midilli-Kucuk drying model was found to be the most suitable for describing the solar drying curves of Citrus aurantium leaves with a correlation coefficient (r) of 0.99998, chi-square ({chi}{sup 2}) of 4.664 x 10{sup -6} and MBE of 4.8381 x 10{sup -4}. (author)
Kleinman, Leonid S.; Reed, X. B., Jr.
1995-01-01
An algorithm has been developed for the forced convective diffusion-reaction problem for convection inside and outside a droplet by a recirculating flow field hydrodynamically coupled at the droplet interface with an external flow field that at infinity becomes a uniform streaming flow. The concentration field inside the droplet is likewise coupled with that outside by boundary conditions at the interface. A chemical reaction can take place either inside or outside the droplet or reactions can take place in both phases. The algorithm has been implemented and results are shown here for the case of no reaction and for the case of an external first order reaction, both for unsteady behavior. For pure interphase mass transfer, concentration isocontours, local and average Sherwood numbers, and average droplet concentrations have been obtained as a function of the physical properties and external flow field. For mass transfer enhanced by an external reaction, in addition to the above forms of results, we present the enhancement factor, with the results now also depending upon the (dimensionless) rate of reaction.
Transient-forced convection film boiling on an isothermal flat plate.
Nagendra, H. R.
1972-01-01
Development of a new approach for the solution of transient-forced convection film boiling on an isothermal flat plate using the boundary layer model. The similarity variables are used to convert the governing partial differential equations to ordinary ones. The results of numerical solutions of these ordinary equations indicate that the transient process can be classified as one-dimensional conduction, intermediate, and the steady-state regions. The time required for the one-dimensional conduction and the time necessary to attain a steady-state condition are obtained. The use of local similarity approximations for the intermediate regime facilitates prediction of complete boundary layer growth. Using the ratio of time at any instant to the steady-state time as abscissa, the curves representing the boundary layer growth can be merged into a single mean curve within 5%. Further, the analysis shows that the average rate of heat transfer during transient is 50 to 100% higher than those at steady state. The average rate of vapor convected away is 10 to 15% lower than at steady state while the average rate of accumulation to form the vapor layer is 1 to 14 times larger.
Kiani, Hossein; Sun, Da-Wen; Zhang, Zhihang
2012-11-01
were the main cause of heating effect. The results showed that closer distances to the transducer surface showed higher cooling rates. On the other hand, despite having a bigger distance from the transducer, when the sphere was located close to the gas-liquid interface the enhancement factor of heat transfer was higher. Ultrasound irradiation showed promising effect for the enhancement of convective heat transfer rate during immersion cooling. More investigations are required to demonstrate the behavior of ultrasound assisted heat transfer and resolve the proper way of the application of ultrasound to assist the cooling and/or freezing processes. Copyright © 2012 Elsevier B.V. All rights reserved.
Plasma acceleration and cooling by strong laser field due to the action of radiation reaction force.
Berezhiani, V I; Mahajan, S M; Yoshida, Z
2008-12-01
It is shown that for super intense laser pulses propagating in a hot plasma, the action of the radiation reaction force (appropriately incorporated into the equations of motion) causes strong bulk plasma motion with the kinetic energy raised even to relativistic values; the increase in bulk energy is accompanied by a corresponding cooling (intense cooling) of the plasma. The effects are demonstrated through explicit analytical calculations.
Forced Two-Phase Helium Cooling Scheme for the Mu2e Transport Solenoid
Energy Technology Data Exchange (ETDEWEB)
Tatkowski, G. [Fermilab; Cheban, S. [Fermilab; Dhanaraj, N. [Fermilab; Evbota, D. [Fermilab; Lopes, M. [Fermilab; Nicol, T. [Fermilab; Sanders, R. [Fermilab; Schmitt, R. [Fermilab; Voirin, E. [Fermilab
2015-01-01
The Mu2e Transport Solenoid (TS) is an S-shaped magnet formed by two separate but similar magnets, TS-u and TS-d. Each magnet is quarter-toroid shaped with a centerline radius of approximately 3 m utilizing a helium cooling loop consisting of 25 to 27 horizontal-axis rings connected in series. This cooling loop configuration has been deemed adequate for cooling via forced single phase liquid helium; however it presents major challenges to forced two-phase flow such as “garden hose” pressure drop, concerns of flow separation from tube walls, difficulty of calculation, etc. Even with these disadvantages, forced two-phase flow has certain inherent advantages which make it a more attractive option than forced single phase flow. It is for this reason that the use of forced two-phase flow was studied for the TS magnets. This paper will describe the analysis using helium-specific pressure drop correlations, conservative engineering approach, helium properties calculated and updated at over fifty points, and how the results compared with those in literature. Based on the findings, the use of forced-two phase helium is determined to be feasible for steady-state cooling of the TS solenoids
Okken, A; Blijham, C; Franz, W; Bohn, E
1982-07-01
To assess the effect of forced convection of heated air exchange in preterm infants in conventional incubators, we measured insensible water loss and total heat loss in preterm infants in a conventional forced convection incubator (air velocity 15 to 25 cm/second) and in a specially constructed still-air incubator (air velocity 0 to 2 cm/second) at equal operative temperature and humidity. Under the forced conditions, insensible water loss in the preterm infants increased by a mean 52% from 1.04 +/- 0.24 (mean +/- SD) to 1.58 +/- 0.51 ml/kg/hour (P less than 0.001). The ensuing increase in evaporative heat loss was partly reflected in the small but significant increase in total heat loss from 1.65 +/- 0.47 to 1.80 +/- 0.44 kcal/kg/hour (P less than 0.02). In the forced convection incubator, the increased evaporative heat loss in preterm infants was apparently partly compensated by a decreased nonevaporative heat loss. If reduction of insensible water loss is required, preterm infants should not be subjected to forced convection in incubators.
Experiments on forced convection form a horizontal heated plate in a packed bed of glass spheres
Energy Technology Data Exchange (ETDEWEB)
Renken, K.J. (Univ. of Wisconsin, Milwaukee (USA)); Poulikakos, D. (Univ. of Illinois, Chicago (USA))
1989-02-01
This paper presents an experimental investigation of boundary-layer forced convective heat transfer from a flat isothermal plate in a packed bed of spheres. Extensive experimental results are reported for the thermal boundary-layer thickness, the temperature field, and the local wall heat flux (represented by the local Nusselt number). Theoretical findings of previous investigations using the Darcy flow model as well as a general model for themomentum equation accouting for flow inertia and macroscopic shear wtih and without variable porosity are used to evaluate the theoretical models. Several trends are revealed regarding the conditions of validity of these flow models. Overall the general flow model including variable porosity appears to perform better, even through the need for serious improvements in modeling becomes apparent.
Experimental-theoretical analysis of laminar internal forced convection with nanofluids
Energy Technology Data Exchange (ETDEWEB)
Cerqueira, Ivana G.; Cotta, Renato M. [Lab. of Transmission and Technology of Heat-LTTC. Mechanical Eng. Dept. - POLI and COPPE/UFRJ, Rio de Janeiro, RJ (Brazil)], E-mail: cotta@mecanica.coppe.ufrj.br; Mota, Carlos Alberto A. [Conselho Nacional de Pesquisas - CNPq, Brasilia, DF (Brazil)], e-mail: carlosal@cnpq.br; Nunes, Jeziel S. [INPI, Rio de Janeiro, RJ (Brazil)], e-mail: jeziel@inpi.gov.br
2010-07-01
This work reports fundamental experimental-theoretical research related to heat transfer enhancement in laminar channel flow with nanofluids, which are essentially modifications of the base fluid with the dispersion of metal oxide nanoparticles. The theoretical work was performed by making use of mixed symbolic-numerical computation (Mathematica 7.0 platform) and a hybrid numerical-analytical methodology (Generalized Integral Transform Technique - GITT) in accurately handling the governing partial differential equations for the heat and fluid flow problem formulation with temperature dependency in all the thermophysical properties. Experimental work was also undertaken based on a thermohydraulic circuit built for this purpose, and sample results are presented to verify the proposed model. The aim is to illustrate detailed modeling and robust simulation attempting to reach an explanation of the controversial heat transfer enhancement observed in laminar forced convection with nanofluids. (author)
Wen, Mao-Yu; Yeh, Cheng-Hsiung
2016-12-01
This paper presents a numerical simulation of the heat transfer performance under forced convection for two different types of circular pin fin heat sinks with (Type A) and without (Type B) a hollow in the heated base. COMSOL Multiphysics, which is used for the thermal hydraulic analyses, has proven to be a powerful finite-element-based simulation tool for solving multiple physics-based systems of partial and ordinary differential equations. The standard κ- ɛ two-equations turbulence model is employed to describe the turbulent structure and behavior. The numerical results are validated with the experimental results, and are shown to be in good agreement. The effects of the Reynolds number, height of the fin, finning factor and the perforated base plate on the heat-transfer coefficient are investigated and evaluated. The present study strongly recommends the use of a small hollow ( (Dh /Db ) sink.
Institute of Scientific and Technical Information of China (English)
李志锦; 丑纪范
1994-01-01
According to the theory of the nonlinear dynamical systems with forcing and dissipation,theconceptual ideas are proposed that the intense convective system is an attractor of the meso-scale dynamicalsystem,in which the large-scale fields serve as the bifurcation parameters that evolve slowly.The intenseconvective systems occur explosively when the intensity of the large-scale system passes a bifurcation point.Thus,its prediction is just to predict the evolution of the attractor,independent of the mesoscale initial con-ditions.A primitive spectral limited-area model has been developed and two realistic cases,which were cap-tured by the Eastern China area mesoscale experiment,have been simulated by using this model.The resultshave proved the theoretical points.
Wen, Mao-Yu; Yeh, Cheng-Hsiung
2017-06-01
This paper presents a numerical simulation of the heat transfer performance under forced convection for two different types of circular pin fin heat sinks with (Type A) and without (Type B) a hollow in the heated base. COMSOL Multiphysics, which is used for the thermal hydraulic analyses, has proven to be a powerful finite-element-based simulation tool for solving multiple physics-based systems of partial and ordinary differential equations. The standard κ - \\varepsilon two-equations turbulence model is employed to describe the turbulent structure and behavior. The numerical results are validated with the experimental results, and are shown to be in good agreement. The effects of the Reynolds number, height of the fin, finning factor and the perforated base plate on the heat-transfer coefficient are investigated and evaluated. The present study strongly recommends the use of a small hollow ( (Dh /Db ) heat sink.
Institute of Scientific and Technical Information of China (English)
YANG Xiao; LIU Xuemei
2007-01-01
Based on the Darcy fluid model, by considering the effects of viscous dissipation due to the interaction between solid skeleton and pore fluid flow and thermal conduction in the direction of the fluid flow, the thermally developing forced convection of the local thermal equili- brium and the corresponding thermal stresses in a semi- infmite saturated porous plate channel are investigated in this paper. The expressions of temperature, local Nusselt number and corresponding thermal stresses are obtained by means of the Fourier series, and the distributions of the same are also shown. Furthermore, influences of the Péclet number (Pe) and Brinkman number (Br) on temperature, Nusselt number (Nu) and thermal stress are revealed numerically.
The effects of magnetic field on forced and free convection flow
Abdel-Khalek, M M
2003-01-01
The effects of magnetic field and permeability of the porous medium on unsteady forced and free convection flow past an infinite vertical porous plate have been studied when the temperature of the plate is oscillating with time about a constant nonzero mean value and in the presence of temperature dependent heat source. The governing equations for the hydromagnetic fluid flow and the heat transfer are solved subject to the relevant boundary conditions. The perturbation technique is used to obtain expressions for velocity field, skin friction and Nusselt number. Also, the effects of magnetic parameter, heat source parameter, suction parameter and permeability of porous medium on velocity field, skin friction and Nusselt number are discussed.
Forced convection of power-law fluids flow over a rotating nonisothermal body
Kim, H. W.; Essemyi, A. J.
1993-10-01
Presented is an analysis of steady laminar flow of power-law fluids past a rotating body with nonisothermal surfaces. A coordinate transformation combined with the Merk-type series expansion is employed to transform the governing momentum equations into a set of coupled ordinary differential equations. The equations are numerically integrated to obtain the axial and tangential velocity gradients for determining the friction coefficient. For forced convection, a generalized coordinate transformation is used to analyze the temperature field of the power-law flow. Solutions to the transformed energy equations are obtained in the form of universal functions. The heat transfer coefficients in terms of NuRe(sup 1/(n + 1)) are presented for a rotating sphere. The effects of power-law index, rotation sphere, Prandtl number, and the location of step discontinuity in surface temperature on the local Nusselt number are fully investigated and demonstrated.
Effect of Heated Perimeter on Forced Convection Heat Transfer of he i at a Supercritical Pressure
Doi, D.; Shiotsu, M.; Shirai, Y.; Hama, K.
2008-03-01
The forced convection heat transfer coefficients were measured on two pairs of test plates all 6.0 mm in width and located face to face on inner walls of a rectangular duct. Each pair having length of 20 mm and 80 mm, respectively, was connected in series electrically. The rectangular duct was 420 mm in length and 5 mm×6 mm in inner cross section. The experiments were performed for inlet temperatures from 2.2 to 6.5 K, flow velocities from 0.1 to 5.6 m/s, and at a supercritical pressure of 2.8 atm. Comparison of the obtained Nusselt numbers with the former results with a single test plate showed the clear effect of a heated perimeter. Non-dimensional heat transfer equation including the effect of heated perimeter is presented.
Combined effect of free and forced convection on MHD flow in a rotating porous channel
Directory of Open Access Journals (Sweden)
D. R. V. Prasada Rao
1982-01-01
Full Text Available This paper gives a steady linear theory of the combined effect of the free and forced convection in rotating hydromagnetic viscous fluid flows in a porous channel under the action of a uniform magnetic field. The flow is governed by the Grashof number G, the Hartmann number H, the Ekman number E, and the suction Reynolds number S. The solutions for the velocity field, temperature distribution, magnetic field, mass rate of flow and the shear stresses on the channel boundaries are obtained using a perturbation method with the small parameter S. The nature of the associated boundary layers is investigated for various values of the governing flow parameters. The velocity, the temperature, and the shear stresses are discussed numerically by drawing profiles with reference to the variations in the flow parameters.
Forced convection analysis for generalized Burgers nanofluid flow over a stretching sheet
Khan, Masood; Khan, Waqar Azeem
2015-10-01
This article reports the two-dimensional forced convective flow of a generalized Burgers fluid over a linearly stretched sheet under the impacts of nano-sized material particles. Utilizing appropriate similarity transformations the coupled nonlinear partial differential equations are converted into a set of coupled nonlinear ordinary differential equations. The analytic results are carried out through the homotopy analysis method (HAM) to investigate the impact of various pertinent parameters for the velocity, temperature and concentration fields. The obtained results are presented in tabular form as well as graphically and discussed in detail. The presented results show that the rate of heat transfer at the wall and rate of nanoparticle volume fraction diminish with each increment of the thermophoresis parameter. While incremented values of the Brownian motion parameter lead to a quite opposite effect on the rates of heat transfer and nanoparticle volume fraction at the wall.
CFD Analysis of nanofluid forced convection heat transport in laminar flow through a compact pipe
Yu, Kitae; Park, Cheol; Kim, Sedon; Song, Heegun; Jeong, Hyomin
2017-08-01
In the present paper, developing laminar forced convection flows were numerically investigated by using water-Al2O3 nano-fluid through a circular compact pipe which has 4.5mm diameter. Each model has a steady state and uniform heat flux (UHF) at the wall. The whole numerical experiments were processed under the Re = 1050 and the nano-fluid models were made by the Alumina volume fraction. A single-phase fluid models were defined through nano-fluid physical and thermal properties calculations, Two-phase model(mixture granular model) were processed in 100nm diameter. The results show that Nusselt number and heat transfer rate are improved as the Al2O3 volume fraction increased. All of the numerical flow simulations are processed by the FLUENT. The results show the increment of thermal transfer from the volume fraction concentration.
Directory of Open Access Journals (Sweden)
Stefano Guarino
2017-08-01
Full Text Available In this paper, the heat transfer performances of aluminum metal foams, placed on horizontal plane surface, was evaluated in forced convection conditions. Three different types of contacts between the sample and the heated base plate have been investigated: simple contact, brazed contact and grease paste contact. First, in order to perform the study, an ad hoc experimental set-up was built. Second, the value of thermal contact resistance was estimated. The results show that both the use of a conductive paste and the brazing contact, realized by means of a copper electro-deposition, allows a great reduction of the global thermal resistance, increasing de facto the global heat transfer coefficient of almost 80%, compared to the simple contact case. Finally, it was shown that, while the contribution of thermal resistance is negligible for the cases of brazed and grease paste contact, it is significantly high for the case of simple contact.
A theoretical study of the spheroidal droplet evaporation in forced convection
Energy Technology Data Exchange (ETDEWEB)
Li, Jie, E-mail: leejay1986@163.com; Zhang, Jian
2014-11-07
In many applications, the shape of a droplet may be assumed to be an oblate spheroid. A theoretical study is conducted on the evaporation of an oblate spheroidal droplet under forced convection conditions. Closed-form analytical expressions of the mass evaporation rate for an oblate spheroid are derived, in the regime of controlled mass-transfer and heat-transfer, respectively. The variation of droplet size during the evaporation process is presented in the regime of shrinking dynamic model. Comparing with the droplets having the same surface area, an increase in the aspect ratio enhances the mass evaporation rate and prolongs the burnout time. - Highlights: • Fully algebraic solutions for the spheroidal droplet evaporation rate is obtained. • We examine the effect of aspect ratio on the droplet evaporation. • We propose a calculation method of Nusselt number for spheroidal droplet.
Energy Technology Data Exchange (ETDEWEB)
Bucci, M., E-mail: matteo.bucci@cea.fr [Univ. of Pisa, DIMNP, Pisa (Italy); CEA Saclay, CEA/DEN/DANS/DM2S/SFME/LETR (France); Ambrosini, W.; Forgione, N. [Univ. of Pisa, DIMNP, Pisa (Italy); Lioce, D [Univ. of Pisa, DIMNP, Pisa (Italy); Westinghouse Electric Belguim, Nivelles (Belgium)
2011-07-01
This paper discusses the results obtained from recent experimental investigations devoted to the study of steam condensation in the presence of air and a light noncondensable gas. The experiments are intended to provide data for the validation of engineering models and CFD codes. The original experimental data herein discussed focus on forced convection turbulent boundary layer conditions and involve atmospheric pressure, different conditions for mixture velocity (from 1.5 to 3.5 m/s), mixture composition (form 0 to 75 per cent of the light species in the overall amount of noncondensable gases) and two nominal electrical power supply of the steam generator. The experimental data are qualified against correlations based on the heat and mass transfer analogy and to the predictions obtained by an in house condensation model implemented in a commercial CFD code. (author)
Laminar forced convection slip-flow in a micro-annulus between two concentric cylinders
Energy Technology Data Exchange (ETDEWEB)
Avci, Mete; Aydin, Orhan [Karadeniz Technical University, Department of Mechanical Engineering, 61080 Trabzon (Turkey)
2008-07-01
Forced convection heat transfer in hydrodynamically and thermally fully developed flows of viscous dissipating gases in annular microducts between two concentric micro cylinders is analyzed analytically. The viscous dissipation effect, the velocity slip and the temperature jump at the wall are taken into consideration. Two different cases of the thermal boundary conditions are considered: uniform heat flux at the outer wall and adiabatic inner wall (Case A) and uniform heat flux at the inner wall and adiabatic outer wall (Case B). Solutions for the velocity and temperature distributions and the Nusselt number are obtained for different values of the aspect ratio, the Knudsen number and the Brinkman number. The analytical results obtained are compared with those available in the literature and an excellent agreement is observed. (author)
The effects of outward forced convective flow on inward diffusion in human dentine in vitro.
Pashley, D H; Matthews, W G
1993-07-01
In vitro experiments were conducted to evaluate the influence of outward forced convective flow on the inward diffusion of radioactive iodide. When the smear layer was present, application of 15 cmH2O (1.47 kPa) outward-directed filtration pressure reduced the inward flux of iodide by about 10-20% depending upon the hydraulic conductance of each specimen. When the smear layer was removed by acid etching, the same 1.47 kPa pressure lowered the inward iodide flux by as much as 60%, depending on the hydraulic conductance. The results demonstrate the importance of the balance between inward diffusion and outward bulk-fluid movement on the rate of permeation of exogenous solutes.
Enhancing filling of interconnect deep trenches using forced convection magneto-electroplating
Said, R. A.
2006-01-01
Filling deep trenches and cavities is currently accomplished by copper electro-less plating technology utilizing super-conformal deposition methods. Unlike typical electrolyses processes, where an electric potential is applied between the anodes to activate the plating reaction, electro-less plating relies on chemical agents to activate deposition. To achieve super-conformal deposition, special electrolytic paths must be used. This poses a challenge to the fabrication of narrower trenches, and thus requires the development of other deposition schemes. This work proposes an alternative solution to the filling of deep trenches that avoids the difficulties outlined above, using a forced convection magneto-electroplating method. The technique operates as in typical electrolysis processes, however, with forcing the flow of the plating electrolyte, by hydro-dynamic means, in the presence of an externally applied magnetic field. This arrangement introduces a Lorentz type of force that enhances the transport of deposit species toward desired locations, such as deep regions in interconnect trenches. The proposed method is demonstrated by filling interconnect trenches with aspect ratio as high as 3:1. Quality of samples filled using the proposed magneto-electroplating method is compared with the quality of samples filled by typical electroplating method.
Performance of a Forced-Convection Greenhouse Dryer for Fish Drying
Directory of Open Access Journals (Sweden)
Martunis Martunis
2013-04-01
Full Text Available ABSTRACT. This research present experimental performance of a forced convection greenhouse dryer for drying of fish. The greenhouse dryer was installed at Aceh province, Indonesia. It has a concrete floor with the area of 6×4 m2. The roof of dryer is built in semi-cylindrical shape and covered with transparent polycarbonate sheets. Two axial flow fans powered by a 50-watt solar cell module was used to generate forced convection for ventilating the dryer.To investigate its performance, the dryer was used to dry two batches of fish. The Results showed that to dry 50 kg fish with initial moisture 68 % required 11 hours. Incontrast, to dry the same amount of fish using sun drying take a time about 2 days. The air temperature inside greenhouse dryer at noon in the clear day was 45-55°C. Kinerja Pengering Rumah Kaca Dengan Metode Konveksi Paksa Untuk Pengeringan Ikan ABSTRAK. Penelitian ini memperlihatkan hasil percobaan terhadap kinerja pengering rumah kaca dengan metode konveksi udara secara paksa pada pengeringan ikan. Penelitian pengering rumah kaca ini dilakukan di Propinsi Aceh, Indonesia. Pengering ini berlantaikan beton dengan luas sebesar 6x4 m2. .Atapnya dibuat berbentuk semi-selinder dan ditutup dengan lembaran plastik transparan berbahan polikarbonat. Dua buah kipas dengan aliran udara secara aksial dipasang dengan sumber daya berasal dari panel surya sebesar 50 Watt dan dipakai untuk menghasilkan konveksi udara paksa pada ventilasi pengering. Untuk menginvestigasi kinerja rumah kaca ini, pengering ini digunakan untuk mengeringkan dua tumpukan ikan. Hasilnya menunjukkan bahwa untuk mengeringkan sebanyak 50 kg ikan dengan kadar air awalnya sebesar 68% membutuhkan waktu selama 11 jam. Sebaliknya, dengan menggunakan sinar matahari secara langsung, untuk mengeringkan ikan dengan jumlah yang sama, maka waktu yang dibutuhkan lebih lama yaitu sekitar 2 hari. Suhu udara di dalam rumah pengering tepat pada siang hari yang cerah berkisar antara 45
Batubara, Fatimah; Misran, Erni; Dina, Sari Farah; Heppy
2017-06-01
Research on potato drying using the indirect solar dryer with flat-plate finned collector and forced convection has been done. The research was conducted at the outdoor field of Laboratory of Institute for Research and Standardization of Industry on June 14th-23rd, 2016 from 9:00 am to 4:00 pm. This research aims to obtain the drying kinetics model of potato (Solanumtuberosum L.) using an indirect solar dryer's (ISD) with flat plate-finned collector and forced convection. The result will be compared to the open sun drying (OSD) method. Weather conditions during the drying process took place as follows; surrounding air temperature was in the range 27 to 34.7 °C, relative humidity (RH) 29.5 to 61.0% and the intensity of solar radiation 105.6 to 863.1 Watt/m2. The dried potato thicknesses were 1.0 cm, 1.5 cm and 2.0 cm, with the average initial water content of 76.46%. The average temperature in the collector chamber ranged from 42.2 to 57.4 °C and the drying chamber was at 46.2 °C. The best drying result was obtained from a sample size of 1 cm thickness using the IDS method with an average drying rate of 0.018 kg H2O per kg dry-weight.hour and the water content was constant at 5.02% in 21 hours of drying time. The most suitable kinetics model is Page model, equation MR = exp (-0.049 t1,336) for 1.0 cm thickness, exp (-0.066 t1,222) for 1.5 cm thickness and exp (-0.049 t1,221) for 2.0 cm thickness. The quality of potato drying using ISD method is better than using OSD which can be seen from the color produced.
Comparison of radiant and convective cooling of office room: effect of workstation layout
DEFF Research Database (Denmark)
Bolashikov, Zhecho Dimitrov; Melikov, Arsen Krikor; Rezgals, Lauris;
2014-01-01
The impact of heat source location (room layout) on the thermal environment generated in a double office room with four cooling ventilation systems - overhead ventilation, chilled ceiling with overhead ventilation, active chilled beam and active chilled beam with radiant panels was measured...... and compared. The room was furnished with two workstations, two laptops and two thermal manikins resembling occupants. Two heat load levels, design (65 W/m2) and usual (39 W/m2), were generated by adding heat from warm panels simulating solar radiation. Two set-ups were studied: occupants sitting...
Enhanced heat transfer of forced convective fin flow with transverse ribs
Energy Technology Data Exchange (ETDEWEB)
Chang, Shyy Woei; Chiou, Shyr Fuu [Thermal Fluids Laboratory, National Kaohsiung Institute of Marine Technology, No. 142, Hai-Chuan Road, Nan-Tzu District, Post code 811, Kaohsiung, Taiwan (China); Su, Lo May [Department of Electrical Engineering, Tung Fang Institute of Technology, Taiwan (China); Yang, Tsun Lirng [Department of Electrical Engineering, Fortune Institute of Technology, Taiwan (China)
2004-02-01
This experimental study investigates the heat transfers in three side-opened and bottom-sealed rectangular channels with two opposite walls roughened by 90 staggered ribs, which simulate the enhanced cooling passages in the fin-type heat sinks of electronic chip-sets. The various degrees of interactive effects due to the surface ribs, side-profile leakage flows and stream-wise weakened coolant flow are functionally related with Reynolds number (Re) and channel length-to-gap ratio (L/B), which unravel the considerable impacts on local and spatially averaged heat transfers over the rib-roughened fin surfaces. A selection of detailed heat transfer measurements over the rib-roughened fin surfaces illustrates the manner by which the isolated and interactive influences of Re and L/B-ratio affect the local and spatially averaged heat transfers. Relative to the heat transfer results acquired from the smooth-walled test channels, the augmentations of spatially averaged heat transfers generated by the present surface ribs are in the range of 140-200% of the flat fin reference levels. In conformity with the experimentally revealed heat transfer physics, a regression-type analysis is performed to develop the correlation of spatially-averaged Nusselt number over rib-roughened fin surface, which permits the individual and interactive effect of Re and L/B on heat transfer to be evaluated. A criterion for selecting the optimal length-to-gap ratio of a fin channel, which provides the maximum convective heat flux from the rib-roughened fin surface, is formulated as an engineering tool to assist the design activity for the cooling device of electronic chip-sets. (authors)
Vanapalli, Srinivas; ter Brake, Hermanus J.M.
2013-01-01
Nanofluids are considered for improving the heat exchange in forced convective flow. In literature, the benefit of nanofluids compared to the corresponding base fluid is represented by several figures-of-merit in which the heat transfer benefit and the cost of pumping the fluid are considered. These
Vanapalli, S.; Brake, ter H.J.M.
2013-01-01
Nanofluids are considered for improving the heat exchange in forced convective flow. In literature, the benefit of nanofluids compared to the corresponding base fluid is represented by several figures-of-merit in which the heat transfer benefit and the cost of pumping the fluid are considered. These
Institute of Scientific and Technical Information of China (English)
WeizhongLi; WeichengWang; 等
1995-01-01
Physical and mathematical models are developed to describe the forced convection condensation heat transfer of saturated vapor flowing axially outside a horizontal tube.The numerical solution of the models indicates the effects of vapor velocity on the liquid film thickness.The result verifies the enhancement of condensation heat transfer caused by such flow.
Nie, Ji; Shaevitz, Daniel A.; Sobel, Adam H.
2016-09-01
Extratropical extreme precipitation events are usually associated with large-scale flow disturbances, strong ascent, and large latent heat release. The causal relationships between these factors are often not obvious, however, the roles of different physical processes in producing the extreme precipitation event can be difficult to disentangle. Here we examine the large-scale forcings and convective heating feedback in the precipitation events, which caused the 2010 Pakistan flood within the Column Quasi-Geostrophic framework. A cloud-revolving model (CRM) is forced with large-scale forcings (other than large-scale vertical motion) computed from the quasi-geostrophic omega equation using input data from a reanalysis data set, and the large-scale vertical motion is diagnosed interactively with the simulated convection. Numerical results show that the positive feedback of convective heating to large-scale dynamics is essential in amplifying the precipitation intensity to the observed values. Orographic lifting is the most important dynamic forcing in both events, while differential potential vorticity advection also contributes to the triggering of the first event. Horizontal moisture advection modulates the extreme events mainly by setting the environmental humidity, which modulates the amplitude of the convection's response to the dynamic forcings. When the CRM is replaced by either a single-column model (SCM) with parameterized convection or a dry model with a reduced effective static stability, the model results show substantial discrepancies compared with reanalysis data. The reasons for these discrepancies are examined, and the implications for global models and theoretical models are discussed.
Multimode laser cooling and ultra-high sensitivity force sensing with nanowires
Hosseini, Mahdi; Slatyer, Harri J; Buchler, Ben C; Lam, Ping Koy
2015-01-01
Photo-induced forces can be used to manipulate and cool the mechanical motion of oscillators. When the oscillator is used as a force sensor, such as in atomic force microscopy, active feedback is an enticing route to enhancing measurement performance. Here, we show broadband multimode cooling of $-23$ dB down to a temperature of $8 \\pm 1$~K in the stationary regime. Through the use of periodic quiescence feedback cooling, we show improved signal-to-noise ratios for the measurement of transient signals. We compare the performance of real feedback to numerical post-processing of data and show that both methods produce similar improvements to the signal-to-noise ratio of force measurements. We achieved a room temperature force measurement sensitivity of $< 2\\times10^{-16}$ N with integration time of less than $0.1$ ms. The high precision and fast force microscopy results presented will potentially benefit applications in biosensing, molecular metrology, subsurface imaging and accelerometry.
Influence of Forced Convection on the Solidification of Metal in Cast-Iron and Ceramic Ingot Molds
Kalashnikova, O. A.; Dremov, V. V.
2016-09-01
A nonstationary problem of solidification of an ingot in molds with walls of varying thermal conductivity has been solved by the variational method with account taken of forced convection in the liquid phase of the metal. A formula for the temperature distribution in the liquid phase and the dependence of the time of advance of the solidification front on its coordinates have been obtained. Numerical calculations of the time of solidification of the ingot in castiron and ceramic molds have been done at different convection rates.
Bednarz, Tomasz; Fornalik, Elzbieta; Tagawa, Toshio; Ozoe, Hiroyuki; Szmyd, Janusz S.
The magnetic convection of paramagnetic fluid is studied in a strong magnetic field. The fluid in a cubic enclosure is heated from one vertical wall and cooled from the opposite one. The fluid is the 80% mass aqueous solution of glycerol with 0.8 mol/kg concentration of gadolinium nitrate hexahydrate to make the working fluid paramagnetic. The small amount of liquid crystal slurry is added to the fluid in order to visualize the temperature profiles in a vertical cross-section. This system is placed directly below the solenoid of the superconducting magnet which is oriented vertically. The temperature of cold wall is constantly controlled by the water flowing from a thermostating bath. On the other hand, the hot wall is heated by a nichrome wire from a DC power supply. In the numerical computations, the configuration of the system is modeled to be as close as possible to the real system. The physical properties of the working fluid are used to compute dimensionless parameters in the numerical model and the computations are carried out for corresponding cases. Later, the numerical and experimental results are compared with each other.
Forced heat convection in a reciprocating duct fitted with 45 degree ribs
Energy Technology Data Exchange (ETDEWEB)
Shyy Woei, Chang [National Kaohsiung Institute of Marine Technology, Dept. of Marine Engineering, Taiwan (China)
2002-03-01
This experimental study investigated heat-transfer physics of forced convection in a reciprocating square duct fitted with 45 deg C crossed ribs on two opposite walls. The parametric conditions involved several Reynolds, pulsating and buoyancy numbers, respectively, in the ranges of 600-10 000, 0-10, and 0-0.14 with five different reciprocating frequencies tested, namely, 0.67, 1, 1.33, 1.67 and 2 Hz. The rib-induced flows in the static duct produced an augmentation of heat transfer in the range of 260-300% compared to the smooth-walled situation. The reciprocating heat-transfer data reconfirmed the appearance of large-scale wavy-like axial heat transfer distribution that differed significantly from the stationary results. The manner in which the pulsating force and convective inertia, with and without buoyancy interaction, interactively affected the local heat transfer along the rib-roughened surface was illustrated using a number of experimentally based observations and extrapolations. The buoyancy interaction in the reciprocating duct reduced heat transfer, which effect was enhanced by increasing the pulsating number, but appeared to be a weak function of Reynolds number. When the Reynolds and pulsating numbers were relatively low, a range of heat transfer impediments, that could lead the spatial-time averaged heat-transfer to levels about 71 % of non-reciprocating values, was observed. A further increase of pulsating number resulted in a subsequent heat-transfer recovery, which tendency could lead to heat-transfer improvement from the non-reciprocating level. An empirical correlation to evaluate the spatial-time averaged heat transfer over the reciprocating ribbed duct was developed to assist the design activity. The possibility to further enhance heat-transfer via the use of angled ribs in a reciprocating duct is confirmed, but it is important to ensure that the range of reciprocating flow parameters produced does not create heat-transfer impediment in order
Conceptual Design of Forced Convection Molten Salt Heat Transfer Testing Loop
Energy Technology Data Exchange (ETDEWEB)
Manohar S. Sohal; Piyush Sabharwall; Pattrick Calderoni; Alan K. Wertsching; S. Brandon Grover
2010-09-01
This report develops a proposal to design and construct a forced convection test loop. A detailed test plan will then be conducted to obtain data on heat transfer, thermodynamic, and corrosion characteristics of the molten salts and fluid-solid interaction. In particular, this report outlines an experimental research and development test plan. The most important initial requirement for heat transfer test of molten salt systems is the establishment of reference coolant materials to use in the experiments. An earlier report produced within the same project highlighted how thermophysical properties of the materials that directly impact the heat transfer behavior are strongly correlated to the composition and impurities concentration of the melt. It is therefore essential to establish laboratory techniques that can measure the melt composition, and to develop purification methods that would allow the production of large quantities of coolant with the desired purity. A companion report describes the options available to reach such objectives. In particular, that report outlines an experimental research and development test plan that would include following steps: •Molten Salts: The candidate molten salts for investigation will be selected. •Materials of Construction: Materials of construction for the test loop, heat exchangers, and fluid-solid corrosion tests in the test loop will also be selected. •Scaling Analysis: Scaling analysis to design the test loop will be performed. •Test Plan: A comprehensive test plan to include all the tests that are being planned in the short and long term time frame will be developed. •Design the Test Loop: The forced convection test loop will be designed including extensive mechanical design, instrument selection, data acquisition system, safety requirements, and related precautionary measures. •Fabricate the Test Loop. •Perform the Tests. •Uncertainty Analysis: As a part of the data collection, uncertainty analysis will
DEFF Research Database (Denmark)
Schellen, L.; Loomans, M.G.L.C.; de Wit, M.H.;
2012-01-01
, thermal comfort and productivity in response to thermal non-uniform environmental conditions. Twenty healthy subjects (10 males and 10 females, age 20–29years) were exposed to two different experimental conditions: a convective cooling situation (CC) and a radiant cooling situation (RC). During...... the experiments physiological responses, thermal comfort and productivity were measured. The results show that under both experimental conditions the actual mean thermal sensation votes significantly differ from the PMV-index; the subjects are feeling colder than predicted. Furthermore, the females are more...
Directory of Open Access Journals (Sweden)
C. Wang
2009-10-01
Full Text Available Previous works have suggested that the direct radiative forcing (DRF of black carbon (BC aerosols are able to force a significant change in tropical convective precipitation ranging from the Pacific and Indian Ocean to the Atlantic Ocean. In this in-depth analysis, the sensitivity of this modeled effect of BC on tropical convective precipitation to the emissions of BC from 5 major regions of the world has been examined. In a zonal mean base, the effect of BC on tropical convective precipitation is a result of a displacement of ITCZ toward the forcing (warming hemisphere. However, a substantial difference exists in this effect associated with BC over different continents. The BC effect on convective precipitation over the tropical Pacific Ocean is found to be most sensitive to the emissions from Central and North America due to a persistent presence of BC aerosols from these two regions in the lowermost troposphere over the Eastern Pacific. The BC effect over the tropical Indian and Atlantic Ocean is most sensitive to the emissions from South as well as East Asia and Africa, respectively. Interestingly, the summation of these individual effects associated with emissions from various regions mostly exceeds their actual combined effect as shown in the model run driven by the global BC emissions, so that they must offset each other in certain locations and a nonlinearity of this type of effect is thus defined. It is known that anthropogenic aerosols contain many scattering-dominant constituents that might exert an effect opposite to that of absorbing BC. The combined aerosol forcing is thus likely differing from the BC-only one. Nevertheless, this study along with others of its kind that isolates the DRF of BC from other forcings provides an insight of the potentially important climate response to anthropogenic forcings particularly related to the unique particulate solar absorption.
Directory of Open Access Journals (Sweden)
Wei Cai
2014-06-01
Full Text Available The convective drying kinetics of porous medium was investigated numerically. A mathematical model for forced convective drying was established to estimate the evolution of moisture content and temperature inside multilayered porous medium. The set of coupled partial differential equations with the specified boundary and initial conditions were solved numerically using a MATLAB code. An experimental setup of convective drying had been constructed and validated the theoretical model. The temperature and moisture content of the potato samples were dynamically measured and recorded during the drying process. Results indicate that thermal diffusion coefficient has significant positive impact on temperature distribution and mass diffusion coefficient might directly affect the moisture content distribution. Soret effect has a significant impact on heat flux and temperature distribution in the presence of large temperature gradient.
DEFF Research Database (Denmark)
Petersen, H H; Chen, X; Pietersen, A
1999-01-01
A variety of basic factors such as electrode tip pressure, flow around the electrode and electrode orientation influence lesion size during radiofrequency ablation, but importantly is dependent on the chosen mode of ablation. However, only little information is available for the frequently used t...... temperature-controlled mode. The purpose of the present experimental study was to evaluate the impact during temperature-controlled radiofrequency ablation of three basic factors regarding electrode-tissue contact and convective cooling on lesion size....
HVAC cable systems with forced water cooling for wind energy transmission
Energy Technology Data Exchange (ETDEWEB)
Brakelmann, Heinrich; Zhang, Dongping [Duisburg-Essen Univ., Duisburg (DE). Dept. Energy Transport and Storage (ETS)
2008-07-01
This paper presents a solution for an efficient wind energy transmission onshore: HVAC cable system with forced water cooling, which provides a substantial increase of the cable ampacity without any modification of the cable construction and design. This work shows the projecting and planning of such HVAC cable systems in combination with a cooling system, especially considering the faulty (n-1)-case. The efficiency utilizing the short-term load capacity of the cable systems transmitting wind energy is shown by computations provided by specialized and adapted FEM (Finite Element Method) software. (orig.)
Shornikov, A; Wolf, A
2014-01-01
We present design and commissioning results of a forced flow cooling system utilizing neon at 30 K. The cryogen is pumped through the system by a room-temperature compression stage. To decouple the cold zone from the compression stage a recuperating counterflow tube-in-tube heat exchanger is used. Commissioning demonstrated successful condensation of neon and transfer of up to 30 W cooling power to the load at 30 K using only 30 g of the cryogen circulating in the system at pressures below 170 kPa.
Short-range force detection using optically-cooled levitated microspheres
Geraci, Andrew A; Kitching, John
2010-01-01
We propose an experiment using optically trapped and cooled dielectric microspheres for the detection of short-range forces. The center-of-mass motion of a microsphere trapped in vacuum can experience extremely low dissipation and quality factors of $10^{12}$, leading to yoctonewton force sensitivity. Trapping the sphere in an optical field enables positioning at less than 1 $\\mu$m from a surface, a regime where exotic new forces may exist. We expect that the proposed system could advance the search for non-Newtonian gravity forces via an enhanced sensitivity of $10^5-10^7$ over current experiments at the 1 $\\mu$m length scale. Moreover, our system may be useful for characterizing other short-range physics such as Casimir forces.
Directory of Open Access Journals (Sweden)
azzedine ABDEDOU
2015-01-01
Full Text Available Two criteria are used and compared to investigate the local thermal equilibrium assumption in a forced convection through a porous channel. The first criterion is based on the maximum local temperature difference between the solid and fluid phases, while the second is based on the average of the local differences between the temperature of the solid phase and the fluid phase. For this purpose, the momentum and energy equations based on the Darcy-Brinkman-Forchheimer and the local thermal non equilibrium models are solved numerically using the finite volume method. The analysis focused on searching thermophysical parameters ranges which validate local thermal equilibrium hypothesis. Thus, by using the two criteria, the obtained results mainly revealed that this local thermal equilibrium assumption is verified for low thermal conductivity ratio and Reynolds number values and for high interstitial Biot number and porosity, while it is unfavorably affected by the high values of Prandtl number. However, it is also found that the parameters ranges corresponding to the local equilibrium validity depends on the selected local thermal non equilibrium criterion.
Energy Technology Data Exchange (ETDEWEB)
Jamradloedluk, Jindaporn; Wiriyaumpaiwong, Songchai [Mahasarakham Univ. Khamriang, Kantarawichai, Mahasarakham (Thailand)
2008-07-01
Solar energy, a form of sustainable energy, has a great potential for a wide variety of applications because it is abundant and accessible, especially for countries located in the tropical region. Drying process is one of the prominent techniques for utilization of solar energy. This research work proposes a forced convection solar drying of osmotically pretreated fruits viz. mango, guava, and pineapple. The fruit cubes with a dimension of 1cm x 1cm x 1cm were immersed in 35% w./w. sucrose solution prior to the drying process. Drying kinetics, color and hardness of the final products obtained from solar drying were investigated and compared with those obtained from open air-sun drying. Desorption isotherms of the osmosed fruits were also examined and five mathematical models were used to fit the desorption curves. Experimental results revealed that solar drying provided higher drying rate than natural sun drying. Color of glace fruit processed by solar drying was more intense, indicated by lower value of lightness and higher value of yellowness, than that processed by sun drying. Hardness of the products dehydrated by both drying methods, however, was not significantly different (p>0.05). Validation of the mathematical models developed showed that the GAB model was most effective for describing desorption isotherms of osmotically pretreated mango and pineapple whereas Peleg's model was most effective for describing desorption isotherms of osmotically pretreated guava. (orig.)
Munir, Asif; Shahzad, Azeem; Khan, Masood
2014-01-01
The major focus of this article is to analyze the forced convective heat transfer in a steady boundary layer flow of Sisko fluid over a nonlinear stretching sheet. Two cases are studied, namely (i) the sheet with variable temperature (PST case) and (ii) the sheet with variable heat flux (PHF case). The heat transfer aspects are investigated for both integer and non-integer values of the power-law index. The governing partial differential equations are reduced to a system of nonlinear ordinary differential equations using appropriate similarity variables and solved numerically. The numerical results are obtained by the shooting method using adaptive Runge Kutta method with Broyden's method in the domain[Formula: see text]. The numerical results for the temperature field are found to be strongly dependent upon the power-law index, stretching parameter, wall temperature parameter, material parameter of the Sisko fluid and Prandtl number. In addition, the local Nusselt number versus wall temperature parameter is also graphed and tabulated for different values of pertaining parameters. Further, numerical results are validated by comparison with exact solutions as well as previously published results in the literature.
Munir, Asif; Shahzad, Azeem; Khan, Masood
2014-01-01
The major focus of this article is to analyze the forced convective heat transfer in a steady boundary layer flow of Sisko fluid over a nonlinear stretching sheet. Two cases are studied, namely (i) the sheet with variable temperature (PST case) and (ii) the sheet with variable heat flux (PHF case). The heat transfer aspects are investigated for both integer and non-integer values of the power-law index. The governing partial differential equations are reduced to a system of nonlinear ordinary differential equations using appropriate similarity variables and solved numerically. The numerical results are obtained by the shooting method using adaptive Runge Kutta method with Broyden’s method in the domain. The numerical results for the temperature field are found to be strongly dependent upon the power-law index, stretching parameter, wall temperature parameter, material parameter of the Sisko fluid and Prandtl number. In addition, the local Nusselt number versus wall temperature parameter is also graphed and tabulated for different values of pertaining parameters. Further, numerical results are validated by comparison with exact solutions as well as previously published results in the literature. PMID:24949738
Institute of Scientific and Technical Information of China (English)
LU Wenqiang; BAI Fengwu
2004-01-01
In this paper, a new model to analyze laminar forced convective enhanced heat transfer in latent functionally thermal fluid is developed. The main characteristics of the model are: I) a new formula of the specific heat at constant pressure is used; ii) a real heat transfer process is considered; that is, heat transfer processes occur not only between working fluid and microcapsules, but also between the mixture and tube wall; iii) the new method, which combines the newly developed axisymmetrical dual reciprocity boundary element method (DRBEM) with finite difference method (FDM), is used to solve the control equations of this problem. The new model is validated by experimental data.Some new physical results on the variational characteristics of the specific heat at constant pressure with space and time during phase-change process, the time-marching history of the phase-change interfaces and so on are obtained. Several main physical factors that affect enhanced heat transfer in latent functionally thermal fluid are numerically analyzed.Some new understandings for the mechanism of enhanced heat transfer in the functionally fluid are obtained.
López, R.; Vaca, M.; Terres, H.; Lizardi, A.; Morales, J.; Flores, J.; Chávez, S.
2015-01-01
The sunflower is an annual plant native to the Americas. It possesses a large inflorescence (flowering head), and its name is derived from the flower's shape and image, which is often used to capture the sun. The plant has a rough, broad, hairy stem, coarsely toothed, with rough leaves, and circular flower heads. The sunflower seeds are appreciated for their oil, which has become a widespread cooking ingredient. Leaves of the sunflower can be used as cattle feed, while the stems contain a fiber that may be used in paper production. Recently this flower has been used in phytoremediation of soils, contaminated with heavy metals. Sunflower has been probed as an efficient phytoextractor of chromium, lead, aluminum, zinc, cadmium from soil. In this work we present the experimental results of the drying of the sunflower stem, cut in 100 mm longitudinal sections, with diameters in the range of 11-18 mm. The aim was to obtain a dry and easy-to-handle final product, since these plants were originally cultivated in order to extract heavy metals from a polluted soil. The dried stems could then be easily confined or sent to recycle premises to concentrate the metals. The drying process was done in forced convection within a hot air tunnel. The used temperature was 60 °C, the velocity of air was 3 m/s and the required times were 8 hours. The initial average wet mass was 28 g and the final value was 5 g, resulting in the aimed product.
Transient forced convection in the entrance region of a porous tube
Energy Technology Data Exchange (ETDEWEB)
Al-Nimr, M.A.; Aldoss, T.; Naji, M.I. (Jordan Univ. of Science and Technology, Irbid (Jordan))
1994-04-01
Study of the flow of energy-carrying fluids through porous media is a rapidly growing branch of fluid mechanics and heat transfer since its applications include thermal insulation, direct contact heat exchangers, nuclear waste repositories, and the petroleum and geothermal industries. The problem of transient forced convection in the entrance region of a cylindrical porous medium with developing thermal boundary layer is investigated. The hydrodynamic behavior of the flow is assumed to be steady and fully developed, and both Darcian and non-Darcian effects on the flow are considered. The numerical solution uses finite-difference discretization. Thermal transients are created by a step change in the temperature of the tube wall. The effect of different fluid and solid-matrix parameters on Nusselt number, thermal entrance length, total heat absorbed and mixing cup temperature are described. Numerical solutions are obtained for the velocity and temperature fields. It is found that increasing the inertia parameter increases the heat absorbed by the fluid. 15 refs., 11 figs.
Energy Technology Data Exchange (ETDEWEB)
Ledezma, G.A. [Duke Univ., Durham, NC (United States). Mechanical Engineering Dept.; Campo, A. [Idaho State Univ., Pocatello, ID (United States). Coll. of Engineering
1999-04-01
The utilization of internal longitudinal finned tubes has received unparallel attention in the heat transfer literature over the years as a result of its imminent application in high performance compact heat exchangers to enhance the heat transfer between laminar streams of viscous fluids and tube walls. Here, the central goal of this paper is to report a simple approximate way for the prediction of the two asymptotes for the local Nusselt number in laminar forced convection flows inside internal longitudinal finned tubes. The computational attributes of the Method Of Lines (MOL) are propitious for the determination of asymptotic temperature solutions and corresponding heat transfer rates (one for Z {r_arrow} 0 and the other for z {r_arrow} {infinity}). The two local Nusselt number sub-distributions, namely Nu{sub z{r_arrow}0} and Nu{sub z{r_arrow}{infinity}}, blend themselves into an approximate Nusselt number distribution that covers the entire z-domain in a natural way.
Effect of Selected Factors on Drying Process of Tomato in Forced Convection Solar Energy Dryer
Directory of Open Access Journals (Sweden)
U.S. Muhammed
2012-09-01
Full Text Available The effect of air velocity, slice thickness and grazing materials in drying process of tomato in forced convection solar energy dryer was evaluated. The result is to serve as an input for solar energy development for drying of vegetable and fruit products in North West Ecological zone of Nigeria. In order to evaluate the effects of the above factors in drying operation, a split-split-plot experimental design was used. Differences among the treatments and their interactions were tested with orthogonal contrast test to access their significance while further analysis were done to compare all possible pairs of treatment means using Duncan’s Multiple Range Test (DMRT. The results showed that there is no significant difference, at 95% probability level, between the means of the three glazing materials used. However, the variations in mean slice thickness and in mean air flow rate are highly significant at 99% probability level. The results further revealed that drying rate increasing with decrease in slice thickness and increase in air flow rate. Drying of 15, 20 and 25 mm slice thickness of tomato was achieved in time range of 21-24, 27-29 and 30-50 h, respectively.
Arquiza, J. M. R. Apollo; Morrow, Robert; Remiker, Ross; Hunter, Jean B.
2017-09-01
During long-term space missions, astronauts generate wet trash, including food containers with uneaten portions, moist hygiene wipes and wet paper towels. This waste produces two problems: the loss of water and the generation of odors and health hazards by microbial growth. These problems are solved by a closed-loop, forced-convection, heat-pump drying system which stops microbial activity by both pasteurization and desiccation, and recovers water in a gravity-independent porous media condensing heat exchanger. A transient, pseudo-homogeneous continuum model for the drying of wet ersatz trash was formulated for this system. The model is based on the conservation equations for energy and moisture applied to the air and solid phases and includes the unique trash characteristic of having both dry and wet solids. Experimentally determined heat and mass transfer coefficients, together with the moisture sorption equilibrium relationship for the wet material are used in the model. The resulting system of differential equations is solved by the finite-volume method as implemented by the commercial software COMSOL. Model simulations agreed well with experimental data under certain conditions. The validated model will be used in the optimization of the entire closed-loop system consisting of fan, air heater, dryer vessel, heat-pump condenser, and heat-recovery modules.
Skin-friction drag analysis from the forced convection modeling in simplified underwater swimming.
Polidori, G; Taïar, R; Fohanno, S; Mai, T H; Lodini, A
2006-01-01
This study deals with skin-friction drag analysis in underwater swimming. Although lower than profile drag, skin-friction drag remains significant and is the second and only other contribution to total drag in the case of underwater swimming. The question arises whether varying the thermal gradient between the underwater swimmer and the pool water may modify the surface shear stress distribution and the resulting skin-friction drag acting on a swimmer's body. As far as the authors are aware, such a question has not previously been addressed. Therefore, the purpose of this study was to quantify the effect of this thermal gradient by using the integral formalism applied to the forced convection theory. From a simplified model in a range of pool temperatures (20-30 degrees C) it was demonstrated that, whatever the swimming speeds, a 5.3% reduction in the skin-friction drag would occur with increasing average boundary-layer temperature provided that the flow remained laminar. However, as the majority of the flow is actually turbulent, a turbulent flow analysis leads to the major conclusion that friction drag is a function of underwater speed, leading to a possible 1.5% reduction for fast swimming speeds above 1m/s. Furthermore, simple correlations between the surface shear stress and resulting skin-friction drag are derived in terms of the boundary-layer temperature, which may be readily used in underwater swimming situations.
Directory of Open Access Journals (Sweden)
AR. Umayal Sundari
2013-06-01
Full Text Available A forced convection solar drier is designed with evacuated tube collector and a blower. The performance of the designed drier is evaluated by carrying drying experiments at Thanjavur District, Tamilnadu, India with amla. Solar drying of amla is carried at different air velocity flow rates – 4m/s, 4.25m/s and 4.5m/s and is compared with natural sun drying. The temperature of the dryingchamber ranges from 53ºC to 82ºC while the ambient temperature ranges from 29ºC to 32ºC. The efficiency of the designed drier varies from 38.61% to 43.7% where as the efficiency of sun drying varies from 12.5% to 14.15%. It is observed that the efficiency of the drier increases with increase in air velocity flow rates. Initial moisture content of amla ranges between 83.6% and 84.3% and the equilibrium moisture content ranges between 0.1% and 0.6%. Solar drying takes 5 to 7 hours to reach safe moisture content where as sun drying takes 13 to15 hours. Also the quality of solar dried amla is better in terms of colour, odour, flavour and appearance than the sun dried amla. The observed result of the present work shows that the proposed solar drier is good for drying amla in this region.
Nanofluid flow and forced convection heat transfer over a stretching surface considering heat source
Mohammadpour, M.; Valipour, P.; Shambooli, M.; Ayani, M.; Mirparizi, M.
2015-07-01
In this paper, magnetic field effects on the forced convection flow of a nanofluid over a stretching surface in the presence of heat generation/absorption are studied. The equations of continuity, momentum and energy are transformed into ordinary differential equations and solved numerically using the fourth-order Runge-Kutta integration scheme featuring the shooting technique. Different types of nanoparticles as copper (Cu), silver (Ag), alumina (Al2O3) and titania (TiO2) with water as their base fluid has been considered. The influence of significant parameters, such as magnetic parameter, volume fraction of the nanoparticles, heat generation/absorption parameter, velocity ratio parameter and temperature index parameter on the flow and heat transfer characteristics are discussed. The results show that the values of temperature profiles increase with increasing heat generation/absorption and volume fraction of the nanoparticles but they decrease with increasing velocity ratio parameter and temperature index parameter. Also, it can be found that selecting silver as nanoparticle leads to the highest heat transfer enhancement.
Ilday, Serim; Akguc, Gursoy B.; Tokel, Onur; Makey, Ghaith; Yavuz, Ozgun; Yavuz, Koray; Pavlov, Ihor; Ilday, F. Omer; Gulseren, Oguz
We report a new dynamical self-assembly mechanism, where judicious use of convective and strong Brownian forces enables effective patterning of colloidal nanoparticles that are almost two orders of magnitude smaller than the laser beam. Optical trapping or tweezing effects are not involved, but the laser is used to create steep thermal gradients through multi-photon absorption, and thereby guide the colloids through convective forces. Convective forces can be thought as a positive feedback mechanism that helps to form and reinforce pattern, while Brownian motion act as a competing negative feedback mechanism to limit the growth of the pattern, as well as to increase the possibilities of bifurcation into different patterns, analogous to the competition observed in reaction-diffusion systems. By steering stochastic processes through these forces, we are able to gain control over the emergent pattern such as to form-deform-reform of a pattern, to change its shape and transport it spatially within seconds. This enables us to dynamically initiate and control large patterns comprised of hundreds of colloids. Further, by not relying on any specific chemical, optical or magnetic interaction, this new method is, in principle, completely independent of the material type being assembled.
Energy Technology Data Exchange (ETDEWEB)
Nickell, T.W.
1988-01-01
This study numerically analyzes combined radiative and natural or forced convective heat transfer between vertical parallel plates with two-dimensional discrete heat sources. The numerical method was verified by comparing its results with other published experimental data and the agreement was excellent. It is shown that radiative heat transfer is a significant and useful mode of heat transfer in combination with both natural and forced convection in this situation and cannot be neglected. Radiative heat transfer accounted for 50-60% or more of the total heat transfer in some cases, and usually approximately 30-35% on the top of a discrete heat source. This fact can be used to advantage in the thermal design of electronic circuit boards.
Directory of Open Access Journals (Sweden)
M. MOHANRAJ
2009-09-01
Full Text Available An indirect forced convection solar drier integrated with different sensible heat storage maternal has been developed and tested its performance for drying chili under the metrological conditions of Pollachi, India. The system consists of a flat plate solar air heater with heat storage unit, a drying chamber and a centrifugal blower. Drying experiments have been performed at an air flow rate of 0.25 kg/s. Drying of chili in a forced convection solar drier reduces the moisture content from around 72.8% (wet basis to the final moisture content about 9.1% in 24 h. Average drier efficiency was estimated to be about 21%. The specific moisture extraction rate was estimated to be about 0.87 kg/kWh.
Cao, Y.; Faghri, A.; Juhasz, A.
1991-01-01
Latent heat energy storage systems with both annular and countercurrent flows are modeled numerically. The change of phase of the phase-change material (PCM) and the transient forced convective heat transfer for the transfer fluid are solved simultaneously as a conjugate problem. A parametric study and a system optimization are conducted. It is found that the energy storage system with the countercurrent flow is an efficient way to absorb heat energy in a short period for pulsed power load space applications.
Energy Technology Data Exchange (ETDEWEB)
Humphrey, J.A.C.
1982-01-01
A summary is provided of the first of three years of experimental and theoretical research on free-forced convection flows in cavity-type solar receivers. New experimental and theoretical results are presented and discussed. The implication of these findings, with respect to the future thrust of the research program, is clarified as well as is possible at the present time. Following various related conclusions a summary and tentative schedule of work projected for year two of research are presented.
Yang, Xuegeng; Mühlenhoff, Sascha; Nikrityuk, Petr A.; Eckert, Kerstin
2013-03-01
Magnetic fields are well-established in electrochemistry as an attractive tool to improve both the quality of the deposit as well as the deposition rate. The key mechanism is a mass transfer enhancement by Lorentz-force-driven convection. However, during electrolysis this convection interacts with buoyancy-driven convection, which arises from concentration differences, in a sometimes intriguing way. In the case of a Lorentz force opposing buoyancy, this is due to the growth of a bubble-like zone of less-concentrated cupric ion solution at the lower part of the vertical cathode when copper electrolysis is performed. If buoyancy is strong enough to compete with the Lorentz force, this zone rises along the cathode and causes surprisingly unsteady initial transient behaviour. We explore this initial transient under galvanostatic conditions by analyzing the development of the concentration and velocity boundary layers obtained by Mach-Zehnder interferometry and particle image velocimetry. Particular attention is also paid to higher current densities above the limiting current, obtained from potentiodynamic measurements, at which a chaotic advection takes place. The results are compared by scaling analysis.
Increase in rate of force development with skin cooling during isometric knee extension.
Shimose, Ryota; Ushigome, Nobuyuki; Tadano, Chigaya; Sugawara, Hitoshi; Yona, Masae; Matsunaga, Atsuhiko; Muro, Masuo
2014-12-01
Rate of force development (RFD) plays an important role when performing rapid and forceful movements. Cold-induced afferent input with transient skin cooling (SC) can modulate neural drive. However, the relationship between RFD and SC is unknown. The purpose of this study was to investigate whether SC increases RFD during isometric knee extension. Fifteen young healthy men (25 ± 8 yrs old) contracted their quadriceps muscle as fast and forcefully as possible with or without SC. Skin cooling was administered to the front of the thigh. Torque and electromyographic activity were measured simultaneously. Peak torque was not affected by SC. Skin cooling induced a significant increase in RFD at the phase 0-30 and 0-50 ms. The root mean square of the electromyography of vastus medialis, rectus femoris and vastus lateralis at the phases 0-30-50-100 ms increased significantly or tended to increase with SC. These results suggest that SC may increase neural drive and improve RFD in the very early phases of contraction.
Directory of Open Access Journals (Sweden)
Garg P.
2016-12-01
Full Text Available This paper studies the mathematical implications of the two dimensional viscous steady laminar combined free-forced convective flow of an incompressible fluid over a semi infinite fixed vertical porous plate embedded in a porous medium. It is assumed that the left surface of the plate is heated by convection from a hot fluid which is at a temperature higher than the temperature of the fluid on the right surface of the vertical plate. To achieve numerical consistency for the problem under consideration, the governing non linear partial differential equations are first transformed into a system of ordinary differential equations using a similarity variable and then solved numerically under conditions admitting similarity solutions. The effects of the physical parameters of both the incompressible fluid and the vertical plate on the dimensionless velocity and temperature profiles are studied and analysed and the results are depicted both graphically and in a tabular form. Finally, algebraic expressions and the numerical values are obtained for the local skin-friction coefficient and the local Nusselt number.
Chen, Bingyan; Fowler, Alex; Bhowmick, Sankha
2006-06-01
Trehalose is believed to offer desiccation protection to mammalian cells by forming stable glassy matrices. The goal of the current study was to explore the desiccation kinetics of thin films of trehalose-water solution under forced and natural convective conditions and to investigate the thermophysical state of mammalian cells at the bottom of the thin film. We developed a finite difference model based on the mass and energy conservation equations coupled to the water transport model from the cells. The boundary conditions were obtained from correlations or experimental measurements and the Gordon-Taylor equation was used to predict the glass transition temperature at every location. Results indicated that there are three distinct regimes for drying for both forced and natural convection, characterized by the slope of the moisture content plot as a function of time. Our results also indicate that the surface of the solution reached the glassy state in less than 10 min for the Reynolds (forced) numbers explored and approximately 30 min for some Rayleigh (natural convective) numbers; however, significant water was trapped at this instant. Larger drying force hastened quicker glass formation but trapped more water. The numerical model was capable of predicting the drying kinetics for the dilute region accurately, but deviated while predicting the other regimes. Based on these experimental validations of the model, the osmotic response of different cells located at the bottom of the solution with orders of magnitude difference in their membrane permeability (Lp) was predicted. The results suggested that extracellular glass formed around cells at the bottom of a trehalose-water solution by the propagation of glass into the solution; however it takes more than an order of magnitude time (approximately 7 min to >100 min for forced convective drying) to remove sufficient water to form glass around cells from the time when the first surface glass is formed. This is
Sub-Kelvin Cooling of a Macroscopic Oscillator and femto-Newton Force Measurement
Müller, F; Wang, L J
2007-01-01
Measuring very small forces, particularly those of a gravitational nature, has always been of great interest, as fundamental tests of our understanding of the physical laws. Ultra-long period mechanical oscillators, typically used in such measurements, will have kT/2 of thermal energy associated with each degree of freedom, owing to the equal-partition of energy. Moreover, additional seismic fluctuations in the low frequency band can raise this equivalent temperature significantly to 10^5 K. Recently, various methods using opto-mechanical forces have been reported to decrease this thermal energy for MHz, micro-cantilever oscillators, effectively cooling them. Here we show the direct, dynamical cooling of a gram-size, macroscopic oscillator to 300 mK in equivalent temperature - noise reduction by a factor of 10^6. By precisely measuring the torsional oscillator's position, we dynamically provide an external 'viscous' damping force. Such an added, dissipative force is essentially free of noise, resulting in rap...
Bogerd, Cornelis P; Brühwiler, Paul A; Heus, Ronald
2008-05-01
Both radiant and forced convective heat flow were measured for a prototype rowing headgear and white and black cotton caps. The measurements were performed on a thermal manikin headform at a wind speed of 4.0 m . s(-1) (s = 0.1) in a climate chamber at 22.0 degrees C (s = 0.05), with and without radiant heat flow from a heat lamp, coming from either directly above (90 degrees ) or from above at an angle of 55 degrees . The effects of hair were studied by repeating selected measurements with a wig. All headgear reduced the radiant heat gain compared with the nude headform: about 80% for the caps and 95% for the prototype rowing headgear (P convective heat loss was reduced more by the caps (36%) than by the prototype rowing headgear (9%) (P heat gain contributed maximally 13% to the net heat transfer, with or without headgear, showing that forced convective heat loss is the dominant heat transfer parameter under the chosen conditions. The results of the headgear - wig combinations were qualitatively similar, with lower absolute heat transfer.
Boiling of subcooled water in forced convection; Ebullition locale de l'eau en convection forcee
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Ricque, R.; Siboul, R. [Commissariat a l' Energie Atomique, Grenoble (France). Centre d' Etudes Nucleaires
1970-07-01
As a part of a research about water cooled high magnetic field coils, an experimental study of heat transfer and pressure drop is made with the following conditions: local boiling in tubes of small diameters (2 and 4 mm), high heat fluxes (about 1000 W/cm{sup 2}), high coolant velocities (up to 25 meters/s), low outlet absolute pressures (below a few atmospheres). Wall temperatures are determined with a good accuracy, because very thin tubes are used and heat losses are prevented. Two regimes of boiling are observed: the establishment regime and the established boiling regime and the inception of each regime is correlated. Important delays on boiling inception are also observed. The pressure drop is measured; provided the axial temperature distribution of the fluid and the axial distributions of the wall temperatures, in other words the axial distribution of the heat transfer coefficients under boiling and non boiling conditions, at the same heat flux or the same wall temperatures, are taken in account, then total pressure drop can be correlated, but probably under certain limits of void fraction only. Using the same parameters, it seems possible to correlate the experimental values on critical heat flux obtained previously, which show very important effect of length and hydraulic diameter of the test sections. (authors) [French] Dans le cadre d'une etude sur le refroidissement par l'eau des bobines electromagnetiques a champ intense, on etudie experimentalement l'echange thermique et la perte de pression avec ebullition locale a la paroi dans des tubes de petit diametre (2 et 4 mm), a flux thermique eleve (environ 1000 W/cm{sup 2}), pour des vitesses de circulation elevees (jusqu'a 25 m/s) et des pressions basses (quelques atmospheres). La paroi des tubes etant tres mince et les fuites thermiques etant annulees, les temperatures de paroi sont determinees de facon assez precise. On distingue deux phases dans l'ebullition locale; la phase d
Parvin, Salma; Ahmed, Sajid; Chowdhury, Raju
2017-06-01
The present work investigates numerically the convective and radiative heat transfer performance and entropy generation of forced convection through a direct absorption solar collector (DASC). Four different fluids; Cu-water nanofluid, Al2O3-waternanofluid, TiO2-water nanofluid and pure water are used as the working fluid. Entropy production has been taken into account in addition to the collector efficiency and heat transfer enhancement. Penalty finite element method with Galerkin's weighted residual technique is used to solve the governing non-linear partial differential equations. Numerical simulations are performed for the variation of solar irradiation (I). The outcomes are presented in the form of isotherms, average output temperature, the average Nusselt number, collector efficiency, average entropy generation and Bejan number. The results present that the rate of heat transfer and collector efficiency enhance significantly for raising the values of I upto a certain range.
Thermal and Lorentz force analysis of beryllium windows for a rectilinear muon cooling channel
Energy Technology Data Exchange (ETDEWEB)
Luo, T. [Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); Stratakis, D. [Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); Li, D. [Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); Virostek, S. [Brookhaven National Lab. (BNL), Upton, NY (United States); Palmer, R. B. [Brookhaven National Lab. (BNL), Upton, NY (United States); Bowring, D. [Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
2015-05-03
Reduction of the 6-dimensional phase-space of a muon beam by several orders of magnitude is a key requirement for a Muon Collider. Recently, a 12-stage rectilinear ionization cooling channel has been proposed to achieve that goal. The channel consists of a series of low frequency (325 MHz-650 MHz) normal conducting pillbox cavities, which are enclosed with thin beryllium windows (foils) to increase shunt impedance and give a higher field on-axis for a given amount of power. These windows are subject to ohmic heating from RF currents and Lorentz force from the EM field in the cavity, both of which will produce out of the plane displacements that can detune the cavity frequency. In this study, using the TEM3P code, we report on a detailed thermal and mechanical analysis for the actual Be windows used on a 325 MHz cavity in a vacuum ionization cooling rectilinear channel for a Muon Collider.
Thermal and Lorentz Force Analysis of Beryllium Windows for the Rectilinear Muon Cooling Channel
Energy Technology Data Exchange (ETDEWEB)
Luo, Tianhuan [LBL, Berkeley; Li, D. [LBL, Berkeley; Virostek, S. [LBL, Berkeley; Palmer, R. [Brookhaven; Stratakis, Diktys [Brookhaven; Bowring, D. [Fermilab
2015-06-01
Reduction of the 6-dimensional phase-space of a muon beam by several orders of magnitude is a key requirement for a Muon Collider. Recently, a 12-stage rectilinear ionization cooling channel has been proposed to achieve that goal. The channel consists of a series of low frequency (325 MHz-650 MHz) normal conducting pillbox cavities, which are enclosed with thin beryllium windows (foils) to increase shunt impedance and give a higher field on-axis for a given amount of power. These windows are subject to ohmic heating from RF currents and Lorentz force from the EM field in the cavity, both of which will produce out of the plane displacements that can detune the cavity frequency. In this study, using the TEM3P code, we report on a detailed thermal and mechanical analysis for the actual Be windows used on a 325 MHz cavity in a vacuum ionization cooling rectilinear channel for a Muon Collider.
Fast cooling techniques for gravitational wave antennas
Furtado, S R
2002-01-01
The resonant-mass technique for the detection of gravitational waves may involve, in the near future, the cooling of very large masses (about 100 tons) from room temperature (300 K) to extreme cryogenic temperatures (20 mK). To cool these detectors to cryogenic temperatures an exchange gas (helium) is used, and the heat is removed from the antenna to the cold reservoir by thermal conduction and natural convection. With the current technique, cooling times of about 1 month can be obtained for cylindrical bar antennas of 2.5 tons. Should this same technique be used to cool a 100 ton spherical antenna the cooling time would be about 10 months, making the operation of these antennas impracticable. In this paper, we study the above-mentioned cooling technique and others, such as thermal switching and forced convection from room temperature to liquid nitrogen temperature (77 K) using an aluminium truncated icosahedron of 19 kg weight and 25 cm diameter.
Simulation of Thermomagnetic Convection in a Cavity Using the Lattice Boltzmann Model
Directory of Open Access Journals (Sweden)
Mahshid Hadavand
2011-01-01
Full Text Available Thermomagnetic convection in a differentially heated square cavity with an infinitely long third dimension is numerically simulated using the single relaxation time lattice Boltzmann method (LBM. This problem is of considerable interest when dealing with cooling of microelectronic devices, in situations where natural convection does not meet the cooling requirements, and forced convection is not viable due to the difficulties associated with pumping a ferrofluid. Therefore, circulation is achieved by imposing a magnetic field, which is created and controlled by placing a dipole at the bottom of the enclosure. The magnitude of the magnetic force is controlled by changing the electrical current through the dipole. In this study, the effects of combined natural convection and magnetic convection, which is commonly known as “thermomagnetic convection,” are analysed in terms of the flow modes and heat transfer characteristics of a magnetic fluid.
DEFF Research Database (Denmark)
Taherian, Hessam; Yazdanshenas, Eshagh
2006-01-01
Due to scarcity of literature on forced-convection heat transfer in a solar collector with rhombic cross-section absorbing tubes, a series of experiments was arranged and conducted to determine heat transfer coefficient. In this study, a typical rhombic cross-section finned tube of flat......-plate collectors used as the test section. Two correlations were proposed for the Nusselt number as a function of the Reynolds number and the Prandtl number based on hydraulic diameter for various heat fluxes. The temperature distribution along the finned tube for the fluid and the wall were also illustrated....
Directory of Open Access Journals (Sweden)
M. R. Astaraki
2012-01-01
Full Text Available In the present study analytical solution for forced convection heat transfer in a circular duct with a special boundary condition has been presented, because the external wall temperature is a periodic function of axial direction. Local energy balance equation is written with reference to the fully developed regime. Also governing equations are two-dimensionally solved, and the effect of duct wall thickness has been considered. The temperature distribution of fluid and solid phases is assumed as a periodic function of axial direction and finally temperature distribution in the flow field, solid wall, and local Nusselt number, is obtained analytically.
The effect of Coriolis force on nonlinear convection in a porous medium
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D. H. Riahi
1994-01-01
Full Text Available Nonlinear convection in a porous medium and rotating about vertical axis is studied in this paper. An upper bound to the heat flux is calculated by the method initiated first by Howard [6] for the case of infinite Prandtl number.
Forced Convective Heat Transfer in a Plate Channel Filled with Solid Particles
Institute of Scientific and Technical Information of China (English)
Pei－XueJiang; Ze－PeiRen; 等
1996-01-01
A numerical study of fluid flow and convective heat transfer in a plate channel filled with solid(metallic)perticles is presented in this paper,The study uses the thermal equilibrium model and a newly developed numerical model which does not assume idealized local thermal equilibrium between the solid particles and the fluid.The numerical simulation results are compared with the experimental data in reference[2].The paper investigates the effects of the assumption of local thermal equilibrium versus non-thermal equilibrium,the thermal conductivity of the solid particles and the particle diameter on convective heat transfer.For the conditions studied.the convective heat transfer and the temperature filed assuming local thermal equilibrium are much different from that for the non-thermal equilibrium assumption when the difference between the solid and fluid thermal conductivities is large,The relative values of the thermal conductivities of the solid particles and the fluid also have a profound effect on the temperature distribution in the channel.The pressure drop decreases as the particle diameter increases and the convective heat transfer coefficient may decrease of increase as the particle diameter increasws depending on the values of ε，λs，λf,λd,αu，ρu.
R-HPDC Process with Forced Convection Mixing Device for Automotive Part of A380 Aluminum Alloy
Directory of Open Access Journals (Sweden)
Bing Zhou
2014-04-01
Full Text Available The continuing quest for cost-effective and complex shaped aluminum castings with fewer defects for applications in the automotive industries has aroused the interest in rheological high pressure die casting (R-HPDC. A new machine, forced convection mixing (FCM device, based on the mechanical stirring and convection mixing theory for the preparation of semisolid slurry in convenience and functionality was proposed to produce the automotive shock absorber part by R-HPDC process. The effect of barrel temperature and rotational speed of the device on the grain size and morphology of semi-solid slurry were extensively studied. In addition, flow behavior and temperature field of the melt in the FCM process was investigated combining computational fluid dynamics simulation. The results indicate that the microstructure and pore defects at different locations of R-HPDC casting have been greatly improved. The vigorous fluid convection in FCM process has changed the temperature field and composition distribution of conventional solidification. Appropriately increasing the rotational speed can lead to a uniform temperature filed sooner. The lower barrel temperature leads to a larger uniform degree of supercooling of the melt that benefits the promotion of nucleation rate. Both of them contribute to the decrease of the grain size and the roundness of grain morphology.
Experimental study on convective boiling heat transfer in narrow-gap annulus tubes
Institute of Scientific and Technical Information of China (English)
LI Bin; ZHAO Jian-Fu; ZHOU Fang-De; TANG Ze-Mei; HU Wen-Rui
2004-01-01
Since convective boiling or highly subcooled single-phase forced convection in micro-channels is an effective cooling mechanism with a wide range of applications, more experimental and theoretical studies are required to explain and verify the forced convection heat transfer phenomenon in narrow channels. In this experimental study, we model the convective boiling behavior of water with low latent heat substance Freon 113 (R-113), with the purpose of saving power consumption and visualizing experiments. Both heat transfer and pressure drop characteristics were measured in subcooled and saturated concentric narrow gap forced convection boiling. Data were obtained to qualitatively identify the effects of gap size, pressure, flow rate and wall superheat on boiling regimes and the transition between various regimes. Some significant differences from unconfined forced convection boiling were found,and also, the flow patterns in narrow vertical annulus tubes have been studied quantitatively.
Energy Technology Data Exchange (ETDEWEB)
Wittig, S.; Schiele, R.; Sieger, K.; Schulz, A.
1995-12-31
The objective of this project is the integration of aerodynamic and thermal design in the example of convection cooled turbine blades. For this purpose, in a first section of this project, a boundary-layer numerical method is to be further developed to permit the calculation of heat transfer and fluidic losses of convection cooled cascades. Using this improved calculation method, a prototype aerothermic design of a turbine stator is to be conducted in a second section of the project. Finally, the aerodynamic losses and the heat transfer behavior of the new cascade are to be experimentally investigated to make a design assessment. Due to its decisive importance to aerodynamics and heat transfer in gas-turbine blades, special interest is focused on the process of laminar-turbulent transition. (orig.) [Deutsch] Ziel des Vorhabens ist daher die Zusammenfuehrung der aerodynamischen und thermischen Auslegung am Beispiel konvektionsgekuehlter Turbinenschaufeln. Dazu soll in einem ersten Teil des Vorhabens ein Grenzschichtrechenverfahren weiterentwickelt werden, das die Berechnung des Waermeuebergangs und der stroemungsmechanischen Verluste konvektionsgekuehlter Schaufelgitter ermoeglicht. Mit Hilfe dieses verbesserten Berechnungsverfahrens soll in einem zweiten Teil des Vorhabens eine prototypische aerothermische Auslegung eines Turbinenleitrades durchgefuehrt werden. Schliesslich sollen die aerodynamischen Verluste und das Waermeuebergangsverhalten des neuen Schaufelgitters experimentell untersucht und somit die Auslegung bewertet werden. Aufgrund seiner entscheidenden Bedeutung fuer Aerodynamik und Waermeuebergang an Gasturbinenschaufeln gilt dem Vorgang des laminar-turbulenten Umschlags dabei besonderes Interesse. (orig.)
Energy Technology Data Exchange (ETDEWEB)
Lisowski, D. D. [Argonne National Lab. (ANL), Argonne, IL (United States); Farmer, M. T. [Argonne National Lab. (ANL), Argonne, IL (United States); Lomperski, S. [Argonne National Lab. (ANL), Argonne, IL (United States); Kilsdonk, D. J. [Argonne National Lab. (ANL), Argonne, IL (United States); Bremer, N. [Argonne National Lab. (ANL), Argonne, IL (United States); Aeschlimann, R. W. [Argonne National Lab. (ANL), Argonne, IL (United States)
2014-06-01
The Natural convection Shutdown heat removal Test Facility (NSTF) is a large scale thermal hydraulics test facility that has been built at Argonne National Laboratory (ANL). The facility was constructed in order to carry out highly instrumented experiments that can be used to validate the performance of passive safety systems for advanced reactor designs. The facility has principally been designed for testing of Reactor Cavity Cooling System (RCCS) concepts that rely on natural convection cooling for either air or water-based systems. Standing 25-m in height, the facility is able to supply up to 220 kW at 21 kW/m^{2} to accurately simulate the heat fluxes at the walls of a reactor pressure vessel. A suite of nearly 400 data acquisition channels, including a sophisticated fiber optic system for high density temperature measurements, guides test operations and provides data to support scaling analysis and modeling efforts. Measurements of system mass flow rate, air and surface temperatures, heat flux, humidity, and pressure differentials, among others; are part of this total generated data set. The following report provides an introduction to the top level-objectives of the program related to passively safe decay heat removal, a detailed description of the engineering specifications, design features, and dimensions of the test facility at Argonne. Specifications of the sensors and their placement on the test facility will be provided, along with a complete channel listing of the data acquisition system.
Design of Test Loops for Forced Convection Heat Transfer Studies at Supercritical State
Balouch, Masih N.
Worldwide research is being conducted to improve the efficiency of nuclear power plants by using supercritical water (SCW) as the working fluid. One such SCW reactor considered for future development is the CANDU-Supercritical Water Reactor (CANDU-SCWR). For safe and accurate design of the CANDU-SCWR, a detailed knowledge of forced-convection heat transfer in SCW is required. For this purpose, two supercritical fluid loops, i.e. a SCW loop and an R-134a loop are developed at Carleton University. The SCW loop is designed to operate at pressures as high as 28 MPa, temperatures up to 600 °C and mass fluxes of up to 3000 kg/m2s. The R-134a loop is designed to operate at pressures as high as 6 MPa, temperatures up to 140 °C and mass fluxes in the range of 500-6000 kg/m2s. The test loops designs allow for up to 300 kW of heating power to be imparted to the fluid. Both test loops are of the closed-loop design, where flow circulation is achieved by a centrifugal pump in the SCW loop and three parallel-connected gear pumps in the R-134a loop, respectively. The test loops are pressurized using a high-pressure nitrogen cylinder and accumulator assembly, which allows independent control of the pressure, while simultaneously dampening pump induced pressure fluctuations. Heat exchangers located upstream of the pumps control the fluid temperature in the test loops. Strategically located measuring instrumentation provides information on the flow rate, pressure and temperature in the test loops. The test loops have been designed to accommodate a variety of test-section geometries, ranging from a straight circular tube to a seven-rod bundle, achieving heat fluxes up to 2.5 MW/m2 depending on the test-section geometry. The design of both test loops allows for easy reconfiguration of the test-section orientation relative to the gravitational direction. All the test sections are of the directly-heated design, where electric current passing through the pressure retaining walls of the
Properties of forced convection experimental with silicon carbide based nano-fluids
Soanker, Abhinay
-fluids. The nano-fluid properties were tested at three different volume concentrations; 0.55%, 1% and 1.6%. Thermal conductivity was measured for the three-volume concentration as function of temperature. Thermal conductivity enhancement increased with the temperature and may be attributed to increased Brownian motion of colloidal particles at higher temperatures. Measured thermal conductivity values are compared with results obtained by theoretical model derived in this work. Effect of temperature and volume concentration on viscosity was also measured and reported. Viscosity increase and related consequences are important issues for the use of nano-fluids. Extensive measurements of heat transfer and pressure drop for forced convection in circular pipes with nano-fluids was also conducted. Parameters such as heat transfer coefficient, Nusselt number, pressure drop and a thermal hydraulic performance factor that takes into account the gains made by increase in thermal conductivity as well as penalties related to increase in pressure drop are evaluated for laminar and transition flow regimes. No significant improvement in heat transfer (Nusselt number) compared to its based fluid was observed. It is also observed that the values evaluated for the thermal-hydraulic performance factor (change in heat transfer/change in pressure drop) was under unity for many flow conditions indicating poor overall applicability of SiC based nano-fluids.
Sterl, Sebastian; Zhong, Jin-Qiang
2016-01-01
In this paper, we present results from an experimental study into turbulent Rayleigh-Benard convection forced externally by periodically modulated unidirectional rotation rates. We find that the azimuthal rotation velocity $\\dot{\\theta}$(t) and thermal amplitude $\\delta$(t) of the large-scale circulation (LSC) are modulated by the forcing, exhibiting a variety of dynamics including increasing phase delays and a resonant peak in the amplitude of $\\dot{\\theta}$(t). We also focus on the influence of modulated rotation rates on the frequency of occurrence $\\eta$ of stochastic cessation/reorientation events, and on the interplay between such events and the periodically modulated response of $\\dot{\\theta}$(t). Here we identify a mechanism by which $\\eta$ can be amplfied by the modulated response and these normally stochastic events can occur with high regularity. We provide a modeling framework that explains the observed amplitude and phase responses, and extend this approach to make predictions for the occurrence ...
RCCS Experiments and Validation for High Temperature Gas-Cooled Reactor
Energy Technology Data Exchange (ETDEWEB)
Chang Oh; Cliff Davis; Goon C. Park
2007-09-01
A reactor cavity cooling system (RCCS), an air-cooled helical coil RCCS unit immersed in the water pool, was proposed to overcome the disadvantages of the weak cooling ability of air-cooled RCCS and the complex structure of water-cooled RCCS for the high temperature gas-cooled reactor (HTGR). An experimental apparatus was constructed to investigate the various heat transfer phenomena in the water pool type RCCS, such as the natural convection of air inside the cavity, radiation in the cavity, the natural convection of water in the water pool and the forced convection of air in the cooling pipe. The RCCS experimental results were compared with published correlations. The CFX code was validated using data from the air-cooled portion of the RCCS. The RELAP5 code was validated using measured temperatures from the reactor vessel and cavity walls.
Ahmed, Mahmoud; Eslamian, Morteza
2015-01-01
Laminar natural convection in differentially heated (β = 0°, where β is the inclination angle), inclined (β = 30° and 60°), and bottom-heated (β = 90°) square enclosures filled with a nanofluid is investigated, using a two-phase lattice Boltzmann simulation approach. The effects of the inclination angle on Nu number and convection heat transfer coefficient are studied. The effects of thermophoresis and Brownian forces which create a relative drift or slip velocity between the particles and th...
Lee, Man
2012-02-22
A microchannel heat sink, integrated with pressure and temperature microsensors, is utilized to study single-phase liquid flow forced convection under a uniform heat flux boundary condition. Utilizing a waferbond-and-etch- back technology, the heat source, temperature and pressure sensors are encapsulated in a thin composite membrane capping the microchannels, thus allowing experimentally good control of the thermal boundary conditions. A three-dimensional physical model has been constructed to facilitate numerical simulations of the heat flux distribution. The results indicate that upstream the cold working fluid absorbs heat, while, within the current operating conditions, downstream the warmer working fluid releases heat. The Nusselt number is computed numerically and compared with experimental and analytical results. The wall Nusselt number in a microchannel can be estimated using classical analytical solutions only over a limited range of the Reynolds number, Re: both the top and bottom Nusselt numbers approach 4 for Re < 1, while the top and bottom Nusselt numbers approach 0 and 5.3, respectively, for Re > 100. The experimentally estimated Nusselt number for forced convection is highly sensitive to the location of the temperature measurements used in calculating the Nusselt number. © 2012 IOP Publishing Ltd.
Energy Technology Data Exchange (ETDEWEB)
Rosenfeld, Daniel [Hebrew Univ. of Jerusalem (Israel)
2015-12-23
Quantifying the aerosol/cloud-mediated radiative effect at a global scale requires simultaneous satellite retrievals of cloud condensation nuclei (CCN) concentrations and cloud base updraft velocities (Wb). Hitherto, the inability to do so has been a major cause of high uncertainty regarding anthropogenic aerosol/cloud-mediated radiative forcing. This can be addressed by the emerging capability of estimating CCN and Wb of boundary layer convective clouds from an operational polar orbiting weather satellite. Our methodology uses such clouds as an effective analog for CCN chambers. The cloud base supersaturation (S) is determined by Wb and the satellite-retrieved cloud base drop concentrations (Ndb), which is the same as CCN(S). Developing and validating this methodology was possible thanks to the ASR/ARM measurements of CCN and vertical updraft profiles. Validation against ground-based CCN instruments at the ARM sites in Oklahoma, Manaus, and onboard a ship in the northeast Pacific showed a retrieval accuracy of ±25% to ±30% for individual satellite overpasses. The methodology is presently limited to boundary layer not raining convective clouds of at least 1 km depth that are not obscured by upper layer clouds, including semitransparent cirrus. The limitation for small solar backscattering angles of <25º restricts the satellite coverage to ~25% of the world area in a single day. This methodology will likely allow overcoming the challenge of quantifying the aerosol indirect effect and facilitate a substantial reduction of the uncertainty in anthropogenic climate forcing.
Institute of Scientific and Technical Information of China (English)
无
1996-01-01
An experimental investigation is described for the characteristics of convective boiling flow instabilities in horizontally helically coiled tubes using a steam-water two-phase closed circulation test loop at pressure from 0.5 MPa to 3.5MPa.Three kinds of oscillation are reported.density waves;pressure drop excorsions;thermal fluctuations.We describe their dependence on main system parameters such as system pressure,mass flowrate,inlet subcooling,compressible volume and heat flux.Utilising the experimental data together with conservation constraints,a dimensionless correlation is proposed for the occurrence of density waves.
Directory of Open Access Journals (Sweden)
Grković Vojin R.
2013-01-01
Full Text Available The paper presents the mathematical model and the procedure for calculation of the resultant force acting on the air cooled gas turbine blade(s based on the aerodynamic theory and computation of the circulation around the blade profile. In the conducted analysis was examined the influence of the cooling air mass flow expressed through the cooling air flow parameter λc, as well as, the values of the inlet and outlet angles β1 and β2, on the magnitude of the tangential and axial forces. The procedure and analysis were exemplified by the calculation of the tangential and axial forces magnitudes. [Projekat Ministarstva nauke Republike Srbije: Development and building the demonstrative facility for combined heat and power with gasification
Directory of Open Access Journals (Sweden)
S K Mandal
2016-06-01
Full Text Available In an effort to increase processor speeds, 3D IC architecture is being aggressively pursued by researchers and chip manufacturers. This architecture allows extremely high level of integration with enhanced electrical performance and expanded functionality, and facilitates realization of VLSI and ULSI technologies. However, utilizing the third dimension to provide additional device layers poses thermal challenges due to the increased heat dissipation and complex electrical interconnects among different layers. The conflicting needs of the cooling system requiring larger flow passage dimensions to limit the pressure drop, and the IC architecture necessitating short interconnect distances to reduce signal latency warrant paradigm shifts in both of their design approach. Additional considerations include the effects due to temperature non-uniformity, localized hot spots, complex fluidic connections, and mechanical design. This paper reviews the advances in electronic chip cooling in the last decade and provides a vision for code signing integrated cooling systems. For various heat fluxes on each side of a chip acting as discrete heat source, the current single-phase cooling technology is projected to provide adequate cooling, albeit with high pressure drops. Effectively mitigating the high temperatures surrounding local hot spots remains a challenging issue. Various forms of tabulators above the chips, different geometric arrangements of the chips positioned top and bottom wall of the duct serves very well in the heat augmentation technique with better performance
Directory of Open Access Journals (Sweden)
Yu Ji
2017-03-01
Full Text Available The entropy generation analysis of fully turbulent convective heat transfer to nanofluids in a circular tube is investigated numerically using the Reynolds Averaged Navier–Stokes (RANS model. The nanofluids with particle concentration of 0%, 1%, 2%, 4% and 6% are treated as single phases of effective properties. The uniform heat flux is enforced at the tube wall. To confirm the validity of the numerical approach, the results have been compared with empirical correlations and analytical formula. The self-similarity profiles of local entropy generation are also studied, in which the peak values of entropy generation by direct dissipation, turbulent dissipation, mean temperature gradients and fluctuating temperature gradients for different Reynolds number as well as different particle concentration are observed. In addition, the effects of Reynolds number, volume fraction of nanoparticles and heat flux on total entropy generation and Bejan number are discussed. In the results, the intersection points of total entropy generation for water and four nanofluids are observed, when the entropy generation decrease before the intersection and increase after the intersection as the particle concentration increases. Finally, by definition of Ep, which combines the first law and second law of thermodynamics and attributed to evaluate the real performance of heat transfer processes, the optimal Reynolds number Reop corresponding to the best performance and the advisable Reynolds number Read providing the appropriate Reynolds number range for nanofluids in convective heat transfer can be determined.
Kahani, M.; Zeinali Heris, S.; Mousavi, S. M.
2014-05-01
Coiled tubes and nanofludics are two significant techniques to enhance the heat transfer ability of thermal equipments. The forced convective heat transfer and the pressure drop of nanofluid inside straight tube and helical coiled one with a constant wall heat flux were studied experimentally. Distilled water was used as a host fluid and Nanofluids of aqueous TiO2 nanoparticles (50 nm) suspensions were prepared in various volume concentrations of 0.25-2 %. The heat transfer coefficient of nanofluids is obtained for different nanoparticle concentrations as well as various Reynolds numbers. The experiments covered a range of Reynolds number of 500-4,500. The results show the considerable enhancement of heat transfer rate, which is due to the nanoparticles present in the fluid. Heat transfer coefficient increases by increasing the volume concentration of nanoparticles as well as Reynolds number. Moreover, due to the curvature of the tube when fluid flows inside helical coiled tube instead of straight one, both convective heat transfer coefficient and the pressure drop of fluid grow considerably. Also, the thermal performance factors for tested nanofluids are greater than unity and the maximum thermal performance factor of 3.72 is found with the use of 2.0 % volume concentration of nanofluid at Reynolds number of 1,750.
Qi, Mingfan; Kang, Yonglin; Zhu, Guoming; Li, Yangde; Li, Weirong
2017-01-01
A simple process so-called forced convection stirring casting (FCSC) was proposed to prepare large-diameter 7075 Al alloy ingots. The flow behavior, temperature, and composition fields of the melt in the FCSC process were simulated. The macromorphology, macrosegregation, microstructure, and mechanical properties of the ingots prepared by the FCSC were studied and compared with those prepared by normal casting (NC). The results showed that in the FCS device, the strong convection caused by the axial flow and circular flow rapidly promoted the uniformity of the temperature and composition fields of the melt. Microstructures of the FCSC ingots from the edge to the center were all nearly spherical grains, which were much finer and more uniform than that of the NC ingots. The rotation speed played an important role in the microstructure of the FCSC ingots, and the grains became finer and rounder as the speed increasing. The FCSC process effectively eliminated cracks, improved macrosegregation, and decreased the eutectic phase area fraction and the average grain boundary thickness of ingots. Mechanical properties of the ingots prepared by the FCSC are far better than that of the NC ingots.
Energy Technology Data Exchange (ETDEWEB)
Nanjundappa, C.E., E-mail: cenanju@hotmail.com [Department of Mathematics, Dr. Ambedkar Institute of Technology, Bangalore-560 056 (India); Shivakumara, I.S., E-mail: shivakumarais@gmail.com [Department of Mathematics, Bangalore University, Bangalore-560 001 (India); Prakash, H.N., E-mail: prakashahn83@gmail.com [Government Pre-University College, B H Road, Tumkur-572 102 (India)
2014-12-15
We investigate the influence of Coriolis force on the onset of thermomagnetic convection in ferrofluid saturating a porous layer in the presence of a uniform vertical magnetic field using both linear and weakly non-linear analyses. The modified Brinkman–Forchheimer-extended Darcy equation with Coriolis term has been used to describe the fluid flow. The linear theory based on normal mode method is considered to find the criteria for the onset of stationary thermomagnetic Convection and weakly non-linear analysis based on minimal representation of truncated Fourier series analysis containing only two terms has been used to find the Nusselt number Nu as functions of time. The range of thermal Rayleigh number R beyond which the bifurcation becomes subcritical increases with increasing Λ, Da{sup −1} and Ta. The global quantity of the heat transfer rate decreases by increasing the Taylor number Ta. The results obtained, during the above analyses, have been presented graphically and the effects of various parameters on heat and mass transfer have been discussed. Finally, we have drawn the steady streamlines for various parameters.
Antoine, R.; Geshi, N.; Kurita, K.; Aoki, Y.; Ichihara, M.; Staudacher, T.; Bachelery, P.
2012-04-01
Subsurface airflow in the unsaturated zone of the soil has been extensively investigated in a variety of disciplines such as mining, nuclear waste or agriculture science. In volcanology, the recent discovery of subsurface airflow close to the terminal cone of Piton de La Fournaise volcano (La Réunion Island, France) provides for the first time insights into the convective behavior of air within the unsaturated layer [1]. The characteristics of the aerothermal system, its occurrence in other volcanoes, its ability to transport heat during quiescent periods and the perturbation of this system before eruptions are the key questions we want to address following this discovery. In this study, we present observations of subsurface convective airflow within opened fractures located at the summit of Miyakejima and Piton de la Fournaise volcanoes from anemometric and temperature data. Two anemometers and thermocouples were placed at the surface and at the center of the fracture at two-meter depth during a diurnal cycle. Six thermocouples also measured the temperature at 1 meter-depth, on a profile set perpendicularly to the fracture. Finally, a thermal camera was used to make punctual measurements of the surface temperature of the fracture. At Miyakejima, two surveys were realized in winter 2010 and summer 2011. During the winter, mild air exit was detected from the fracture with a central vertical velocity of 20 to 50 cm/s. The temperature of the site was constant during the diurnal cycle (~ 22°C), leading to a maximum temperature contrast of 15°C between the fracture and the atmosphere just before sunrise. During summer, a different hydrodynamic behavior was observed: Air inflow was detected during the whole diurnal cycle with a mean velocity of 20 cm/s. The temperature of the fracture followed the temperature of the atmosphere at 2 meters-depth. In the case of Piton de la Fournaise volcano, the same convective behavior was observed at two different fractures during
Park, Hae-Kyun; Chung, Bum-Jin
2016-12-01
The turbulent forced convection heat transfer of rectangular fins in a duct was investigated by varying the tip clearance and Pr. Mass transfer experiments using a H2SO4-CuSO4 electroplating system were performed based on the analogy between heat and mass transfers. FLUENT 6.3 was used for calculations. Turbulent models were tested and the Reynolds Stress Model was chosen, which showed a 1.15 % discrepancy with the existing correlation for a simple tube flow when Pr = 2, but 13 % when Pr = 2014. For a more complex fin channel, the discrepancy increased up to 30 %. The optimal tip clearances, corresponding to maximum heat transfer rates, did not vary with Pr, which is explained using the temperature contours. The results were also compared with the laminar case where Pr influenced the optimal tip clearance.
Energy Technology Data Exchange (ETDEWEB)
Magyari, E. [ETH-Zuerich, Institute of Building Technology, Zuerich (Switzerland); Barletta, A. [Universita di Bologna, Dipartimento di Ingegneria Energetica, Nucleare e del Controllo Ambientale (DIENCA), Bologna (Italy)
2007-12-15
Laminar forced convection flow of a liquid in the fully developed region of a circular duct with isothermal wall is analyzed. The effects of viscous dissipation as well as of temperature dependent viscosity are taken into account. The coupled momentum and energy equations are solved analytically by means of a power series method. Then, reference is made to the Poiseuille model for the temperature change of viscosity. For a fixed value of the axial pressure gradient along the duct, dual solutions are found for the velocity and temperature fields. Although dual solutions correspond to the same value of the axial pressure gradient, they lead in general to different values of the average fluid velocity, of the average fluid temperature and of the wall heat flux. It is shown that, for a given fluid and for a fixed duct radius, the absolute value of the axial pressure gradient has an upper bound above which no steady laminar solution can exist. (orig.)
Institute of Scientific and Technical Information of China (English)
YANG Xiao; LIU Xue-mei
2006-01-01
Based on the two-energy equation model, taking into account viscous dissipation due to the interaction between solid skeleton and pore fluid flow, temperature expressions of the solid skeleton and pore fluid flow are obtained analytically for the thermally developing forced convection in a saturated porous medium parallel plate channel,with walls being at constant temperature. It is proved that the temperatures of the two phases for the local thermal nonequilibrium approach to the temperature derived from the one-energy equation model for the local thermal equilibrium when the heat exchange coefficient goes to infinite. The temperature profiles are shown in figures for different dimensionless parameters and the effects of the parameters on the local thermal nonequilibrium are revealed by parameter study.
Guedda, M.; Benlahsen, M.; Sriti, M.; Achemlal, D.
2017-09-01
In this paper, we revisit a mathematical model representing a two-dimensional forced convection boundary-layer flow over a horizontal impermeable plate with a variable heat flux and viscosity. It is assumed that the fluid viscosity varies as an inverse linear function of temperature, the free stream velocity varies as an inverse linear of x and the wall heat flux varies with x as x^{λ}; where λ > -1 and x measures the distance along the surface. Analytical local similarity solutions are presented which reveal that there are two competing effects: λ and θe; where θe is the variable viscosity parameter. It has been shown that for θe > 0 dual solutions exist and boundary separation occurs, while a unique local similarity solution exists for any θe < 0.
Tatsumoto, H.; Shirai, Y.; Shiotsu, M.; Naruo, Y.; Kobayashi, H.; Inatani, Y.
2014-12-01
Forced convection heat transfer from a PtCo wire with a length of 120 mm and a diameter of 1.2 mm that was inserted into a vertically-mounted pipe with a diameter of 8.0 mm to liquid hydrogen flowing upward was measured with a quasi-steady increase of a heat generation rate for wide ranges of flow rate under saturated conditions. The pressures were varied from 0.4 MPa to 1.1 MPa. The non-boiling heat transfer characteristic agrees with that predicted by Dittus-Boelter correlation. The critical heat fluxes are higher for higher flow rates and lower pressures. Effect of Weber number on the CHF was clarified and a CHF correlation that can describe the experimental data is derived based on our correlation for a pipe.
Directory of Open Access Journals (Sweden)
M.M. Rahman
2012-12-01
Full Text Available This paper presents the numerical study on two-dimensional forced convection heat transfer across three in-line flat tubes confined in a channel under incompressible, steady-state conditions. This system is solved in body-fitted coordinates (BFC using the finite volume method (FVM. The constant heat flux is imposed on the surface of the tubes as the thermal boundary conditions. The range of the longitudinal pitch-to-diameter ratio (SL/Ds of 2.0–4.0 is considered, the Reynolds number varies within the range 25–300, and the Prandtl number is taken as 0.7. The temperature contours, local Nusselt number distributions at the tube surface and mean Nusselt number were analyzed. The strength of the heat transfer between the surface of the tubes and the air flow increases with an increase in Reynolds number and pitch-to-diameter ratio.
Energy Technology Data Exchange (ETDEWEB)
Luna, N. [Direccion de Operacion Petrolera, Direccion General de Exploracion y Explotacion de Hidrocarburos, Secretaria de Energia, 03100 Mexico DF (Mexico); Mendez, F. [Facultad de Ingenieria, UNAM, 04510 Mexico DF (Mexico)
2005-07-01
The steady-state analysis of conjugated heat transfer process for the hydrodynamically developed forced convection flow on a heated flat plate embedded in a porous medium is studied. The governing equations for the fluid-saturated porous medium are solved analytically using the integral boundary layer approximation. This integral solution is coupled to the energy equation for the flat plate, where the longitudinal heat conduction effects are taken into account. The resulting equations are then reduced to an integro-differential equation which is solved by regular perturbation techniques and numerical methods. The analytical and numerical predictions for the temperature profile of the plate and appropriate local and average Nusselt numbers are plotted for finite values of the conduction parameter, {alpha}, which represents the presence of the longitudinal heat conduction effects. (authors)
Forced convection to laminar flow of liquid egg yolk in circular and annular ducts
Directory of Open Access Journals (Sweden)
M. Bernardi
2009-06-01
Full Text Available The steady-state heat transfer in laminar flow of liquid egg yolk - an important pseudoplastic fluid food - in circular and concentric annular ducts was experimentally investigated. The average convection heat transfer coefficients, determined by measuring temperatures before and after heating sections with constant temperatures at the tube wall, were used to obtain simple new empirical expressions to estimate the Nusselt numbers for fully established flows at the thermal entrance of the considered geometries. The comparisons with existing correlations for Newtonian and non-Newtonian fluids resulted in excellent agreement. The main contribution of this work is to supply practical and easily applicable correlations, which are, especially for the case of annulus, rather scarce and extensively required in the design of heat transfer operations dealing with similar shear-thinning products. In addition, the experimental results may support existing theoretical analyses.
Jakkareddy, Pradeep S.; Balaji, C.
2017-02-01
This paper reports the results of an experimental study to estimate the heat flux and convective heat transfer coefficient using liquid crystal thermography and Bayesian inference in a heat generating sphere, enclosed in a cubical Teflon block. The geometry considered for the experiments comprises a heater inserted in a hollow hemispherical aluminium ball, resulting in a volumetric heat generation source that is placed at the center of the Teflon block. Calibrated thermochromic liquid crystal sheets are used to capture the temperature distribution at the front face of the Teflon block. The forward model is the three dimensional conduction equation which is solved within the Teflon block to obtain steady state temperatures, using COMSOL. Match up experiments are carried out for various velocities by minimizing the residual between TLC and simulated temperatures for every assumed loss coefficient, to obtain a correlation of average Nusselt number against Reynolds number. This is used for prescribing the boundary condition for the solution to the forward model. A surrogate model obtained by artificial neural network built upon the data from COMSOL simulations is used to drive a Markov Chain Monte Carlo based Metropolis Hastings algorithm to generate the samples. Bayesian inference is adopted to solve the inverse problem for determination of heat flux and heat transfer coefficient from the measured temperature field. Point estimates of the posterior like the mean, maximum a posteriori and standard deviation of the retrieved heat flux and convective heat transfer coefficient are reported. Additionally the effect of number of samples on the performance of the estimation process has been investigated.
Jakkareddy, Pradeep S.; Balaji, C.
2016-05-01
This paper reports the results of an experimental study to estimate the heat flux and convective heat transfer coefficient using liquid crystal thermography and Bayesian inference in a heat generating sphere, enclosed in a cubical Teflon block. The geometry considered for the experiments comprises a heater inserted in a hollow hemispherical aluminium ball, resulting in a volumetric heat generation source that is placed at the center of the Teflon block. Calibrated thermochromic liquid crystal sheets are used to capture the temperature distribution at the front face of the Teflon block. The forward model is the three dimensional conduction equation which is solved within the Teflon block to obtain steady state temperatures, using COMSOL. Match up experiments are carried out for various velocities by minimizing the residual between TLC and simulated temperatures for every assumed loss coefficient, to obtain a correlation of average Nusselt number against Reynolds number. This is used for prescribing the boundary condition for the solution to the forward model. A surrogate model obtained by artificial neural network built upon the data from COMSOL simulations is used to drive a Markov Chain Monte Carlo based Metropolis Hastings algorithm to generate the samples. Bayesian inference is adopted to solve the inverse problem for determination of heat flux and heat transfer coefficient from the measured temperature field. Point estimates of the posterior like the mean, maximum a posteriori and standard deviation of the retrieved heat flux and convective heat transfer coefficient are reported. Additionally the effect of number of samples on the performance of the estimation process has been investigated.
Directory of Open Access Journals (Sweden)
Abimanyu Purusothaman
2016-01-01
Full Text Available This paper deals with the study of natural convection cooling of a discrete heater array in Cu-EG-water nanofluid filled rectangular enclosure. A 3 × 3 array of non-protruding heat sources is embedded on one of the vertical walls of the enclosure while the top horizontal and opposite vertical walls are assumed to be isothermally cold. The remaining portions in which the heaters are mounted and all other walls are insulated. The above setup is modeled into a system of partial differential equations which are solved numerically using finite volume method based on the Semi-Implicit Method for Pressure Linked Equation (SIMPLE algorithm and power law scheme. The wide range of parameters for computation are the aspect ratio of the enclosure, the mixture proportion of Ethylene glycol-water, the solid volume fraction of the nanoparticle along with two different thermal conductivity models. It is observed that the proper choice of the computation parameters and thermal conductivity models could be able to maximize the heat transfer rate from the heater array. Also, the results obtained in this study will provide new guidelines in the field of electronic equipment cooling.
Primary Issues of Mixed Convection Heat Transfer Phenomena
Energy Technology Data Exchange (ETDEWEB)
Chae, Myeong-Seon; Chung, Bum-Jin [Kyung Hee University, Yongin (Korea, Republic of)
2015-10-15
The computer code analyzing the system operating and transient behavior must distinguish flow conditions involved with convective heat transfer flow regimes. And the proper correlations must be supplied to those flow regimes. However the existing safety analysis codes are focused on the Light Water Reactor and they are skeptical to be applied to the GCRs (Gas Cooled Reactors). One of the technical issues raise by the development of the VHTR is the mixed convection, which occur when the driving forces of both forced and natural convection are of comparable magnitudes. It can be encountered as in channel of the stacked with fuel elements and a decay heat removal system and in VHTR. The mixed convection is not intermediate phenomena with natural convection and forced convection but independent complicated phenomena. Therefore, many researchers have been studied and some primary issues were propounded for phenomena mixed convection. This paper is to discuss some problems identified through reviewing the papers for mixed convection phenomena. And primary issues of mixed convection heat transfer were proposed respect to thermal hydraulic problems for VHTR. The VHTR thermal hydraulic study requires an indepth study of the mixed convection phenomena. In this study we reviewed the classical flow regime map of Metais and Eckert and derived further issues to be considered. The following issues were raised: (1) Buoyancy aided an opposed flows were not differentiated and plotted in a map. (2) Experimental results for UWT and UHF condition were also plotted in the same map without differentiation. (3) The buoyancy coefficient was not generalized for correlating with buoyancy coefficient. (4) The phenomenon analysis for laminarization and returbulization as buoyancy effects in turbulent mixed convection was not established. (5) The defining to transition in mixed convection regime was difficult.
Climatic forcing before, during, and after the 8.2 Kyr B.P. global cooling event
Indian Academy of Sciences (India)
Devendra Lal; William G Large; Stephan G Walker
2007-06-01
This paper attempts at full characterization of the unique global 8.2Kyr B.P. cooling event. Significant atmospheric cooling started during 9.5–8.5Kyr B.P. when the Sun was extremely quiet during three periods of ∼50–100 years. The flood of melt water in the N. Atlantic from glacial lakes during the demise of the Laurentide ice sheet, starting at ∼8.5Kyr B.P., adds to the atmospheric cooling. Climatic forcing events occurred at 8.5Kyr B.P., at 8.2Kyr B.P. and finally at 8.06Kyr B.P., leading to concurrent increases or decreases in the atmospheric 14C levels, completely consistent with the climatic forcing proposed here.
The effects of outward forced convective flow on inward diffusion of potassium across human dentin.
Pashley, David H; Agee, Kelli; Zhang, Yi; Smith, Anita; Tavss, Edward A; Gambogi, Robert J
2002-08-01
To measure the inward flux of potassium across human dentin disks, in vitro, in the absence and then in the presence of a simulated pulpal pressure (PP), before and after brushing with two desensitizing dentifrices. Dentin discs were made from extracted unerupted third molars and etched with 6% citric acid to remove the smear layers. The hydraulic conductance (Lp or outward fluid movement) of the discs was measured before and after brushing for 2 minutes with two desensitizing dentifrices, Colgate Sensitive Maximum Strength or Sensodyne Fresh Mint, both dentifrices containing 5% potassium nitrate. The potassium flux was measured at a pressure of 0 cm H2O, and a simulated pulpal pressure of 20 cm H2O. The results showed the Colgate-brushed specimens had lower Lp (P Sensodyne or with saline. The Colgate-brushed specimens had a corresponding lower K+ flux, at a PP = 0 cm H2O, than specimens brushed with saline, but were not significantly different than specimens brushed with Sensodyne dentifrice. However, when the pressure was increased from 0 to 20 cm H2O, to simulate outward dentin fluid pressure, there were significant decreases in K+ flux across all three treatment groups, but the K+ flux of the Colgate-brushed specimens were affected the least, resulting in the Colgate-brushed specimens having significantly (P Sensodyne dentifrice-brushed groups. The magnitude of reduction in K+ flux in going from 0 cm H2O to 20 cm H2O was demonstrated to be linearly related to the magnitude of the Lp of the brushed specimens. Hence, a small Lp corresponded to a small reduction in K+ flux, and a correspondingly large K+ flux at 20 cm H2O. An explanation of this phenomenon is that a decrease in the Lp of dentin, produced by brushing, corresponds to a decreased outward dentin fluid flow. This lowers the outward K+ convective flow and also lowers the inward K+ diffusion. However, since the decrease of the outward K+ convective flow is greater than the decrease of the inward K
A zonally symmetric model for the monsoon-Hadley circulation with stochastic convective forcing
De La Chevrotière, Michèle; Khouider, Boualem
2016-09-01
Idealized models of reduced complexity are important tools to understand key processes underlying a complex system. In climate science in particular, they are important for helping the community improve our ability to predict the effect of climate change on the earth system. Climate models are large computer codes based on the discretization of the fluid dynamics equations on grids of horizontal resolution in the order of 100 km, whereas unresolved processes are handled by subgrid models. For instance, simple models are routinely used to help understand the interactions between small-scale processes due to atmospheric moist convection and large-scale circulation patterns. Here, a zonally symmetric model for the monsoon circulation is presented and solved numerically. The model is based on the Galerkin projection of the primitive equations of atmospheric synoptic dynamics onto the first modes of vertical structure to represent free tropospheric circulation and is coupled to a bulk atmospheric boundary layer (ABL) model. The model carries bulk equations for water vapor in both the free troposphere and the ABL, while the processes of convection and precipitation are represented through a stochastic model for clouds. The model equations are coupled through advective nonlinearities, and the resulting system is not conservative and not necessarily hyperbolic. This makes the design of a numerical method for the solution of this system particularly difficult. Here, we develop a numerical scheme based on the operator time-splitting strategy, which decomposes the system into three pieces: a conservative part and two purely advective parts, each of which is solved iteratively using an appropriate method. The conservative system is solved via a central scheme, which does not require hyperbolicity since it avoids the Riemann problem by design. One of the advective parts is a hyperbolic diagonal matrix, which is easily handled by classical methods for hyperbolic equations, while
A zonally symmetric model for the monsoon-Hadley circulation with stochastic convective forcing
De La Chevrotière, Michèle; Khouider, Boualem
2017-02-01
Idealized models of reduced complexity are important tools to understand key processes underlying a complex system. In climate science in particular, they are important for helping the community improve our ability to predict the effect of climate change on the earth system. Climate models are large computer codes based on the discretization of the fluid dynamics equations on grids of horizontal resolution in the order of 100 km, whereas unresolved processes are handled by subgrid models. For instance, simple models are routinely used to help understand the interactions between small-scale processes due to atmospheric moist convection and large-scale circulation patterns. Here, a zonally symmetric model for the monsoon circulation is presented and solved numerically. The model is based on the Galerkin projection of the primitive equations of atmospheric synoptic dynamics onto the first modes of vertical structure to represent free tropospheric circulation and is coupled to a bulk atmospheric boundary layer (ABL) model. The model carries bulk equations for water vapor in both the free troposphere and the ABL, while the processes of convection and precipitation are represented through a stochastic model for clouds. The model equations are coupled through advective nonlinearities, and the resulting system is not conservative and not necessarily hyperbolic. This makes the design of a numerical method for the solution of this system particularly difficult. Here, we develop a numerical scheme based on the operator time-splitting strategy, which decomposes the system into three pieces: a conservative part and two purely advective parts, each of which is solved iteratively using an appropriate method. The conservative system is solved via a central scheme, which does not require hyperbolicity since it avoids the Riemann problem by design. One of the advective parts is a hyperbolic diagonal matrix, which is easily handled by classical methods for hyperbolic equations, while
Heat Transfer and Cooling Techniques at Low Temperature
Baudouy, B
2014-07-17
The first part of this chapter gives an introduction to heat transfer and cooling techniques at low temperature. We review the fundamental laws of heat transfer (conduction, convection and radiation) and give useful data specific to cryogenic conditions (thermal contact resistance, total emissivity of materials and heat transfer correlation in forced or boiling flow for example) used in the design of cooling systems. In the second part, we review the main cooling techniques at low temperature, with or without cryogen, from the simplest ones (bath cooling) to the ones involving the use of cryocoolers without forgetting the cooling flow techniques.
Energy Technology Data Exchange (ETDEWEB)
Jin, L.F.; Tou, K.W. [Nanyang Technological Univ., Singapore (Singapore). School of Mechanical and Production Engineering; Tso, C.P. [Multimedia Univ., Melaka (Malaysia). Faculty of Engineering and Technology
2005-09-01
Two-dimensional unsteady numerical studies are made on an air-filled enclosure rotated about its horizontal axis with an array of three rows of heat sources on one of the walls, revealing three physically realizable phenomena, namely, uni-periodic oscillation, multi-periodic oscillation and chaotic oscillation. The evolutionary process of flow field and natural convection characteristics from stationary to rotating situation is studied. Rotation results in an imbalance between clockwise and counter-clockwise circulations, increases heat transfer in the worst scenario, reduces the oscillation of Nusselt number, and improves or reduces mean performance in each cycle. The optimal distribution of heaters in rotating fluid is close to the results in the stationary situation if they have same dominated circulation direction. (author)
Numerical simulation of laminar forced convection of water-CuO nanofluid inside a triangular duct
Aghanajafi, Amir; Toghraie, Davood; Mehmandoust, Babak
2017-01-01
In this article, distilled water and CuO particles with volume fraction of 1%, 2% and 4% are numerically studied. The steady state flow regime is considered laminar with Reynolds number of 100, and nano-particles diameters are assumed 20 nm and 80 nm. The hydraulic diameter and the length of equilateral triangular channel are 8 mm and 1000 mm, respectively. The problem is solved for two different boundary conditions; firstly, constant heat flux for all sides as a validation approach; and secondly, constant heat flux for two sides and constant temperature for one side (hot plate). Convective heat transfer coefficient, Nusselt number, pressure loss through the channel, velocity distribution in cross section and temperature distribution on walls are investigated in detail. The fluid flow is supposed to be one-phase flow. It can be observed that nano-fluid leads to a remarkable enhancement on heat transfer coefficient. Furthermore, CuO particles increase pressure loss through the channel and velocity distribution in fully developed cross section of channel, as well. The computations reveal that the size of nano-particles has no significant influence on heat transfer properties. Besides, the study shows a good agreement between provided outcomes and experimental data available in the literature.
Energy Technology Data Exchange (ETDEWEB)
Aydin, Orhan; Avci, Mete [Karadeniz Technical University, Trabzon (Turkey). Department of Mechanical Engineering
2006-08-15
In this study, analytical solutions are obtained to predict laminar heat-convection in a Couette-Poiseuille flow between two plane parallel plates with a simultaneous pressure gradient and an axial movement of the upper plate. A Newtonian fluid with constant properties is considered with an emphasis on the viscous-dissipation effect. Both hydrodynamically and thermally fully-developed flow cases are investigated. The axial heat-conduction in the fluid is neglected. Two different orientations of the thermal boundary-conditions are considered: the constant heat-flux at the upper plate with an adiabatic lower plate (Case A) and the constant heat-flux at the lower plate with an adiabatic upper plate (Case B). For different values of the relative velocity of the upper plate, the effect of the modified Brinkman number on the temperature distribution and the Nusselt number are discussed. Comparison of the present analytical results for a special case with those available in the literature indicates an excellent agreement. (author)
Lorentz force effect on mixed convection micropolar flow in a vertical conduit
Abdel-wahed, Mohamed S.
2017-05-01
The present work provides a simulation of control and filtration process of hydromagnetic blood flow with Hall current under the effect of heat source or sink through a vertical conduit (pipe). This work meets other engineering applications, such as nuclear reactors cooled during emergency shutdown, geophysical transport in electrically conducting and heat exchangers at low velocity conditions. The problem is modeled by a system of partial differential equations taking the effect of viscous dissipation, and these equations are simplified and solved analytically as a series solution using the Differential Transformation Method (DTM). The velocities and temperature profiles of the flow are plotted and discussed. Moreover, the conduit wall shear stress and heat flux are deduced and explained.
Institute of Scientific and Technical Information of China (English)
WU Chunqiang; ZHOU Tianjun; SUN De-Zheng; BAO Qing
2011-01-01
Characteristics of the total clear-sky greenhouse effect (GA) and cloud radiative forcings (CRFs), along with the radiative-related water vapor and cloud properties simulated by the Spectral Atmospheric Model developed by LASG/IAP (SAMIL) are evaluated. Impacts of the convection scheme on the simulation of CRFs are discussed by using two AMIP (Atmospheric Model Inter-comparison Project) type simulations employing different convection schemes: the new Zhang-McFarlane (NZH) and Tiedtke (TDK) convection schemes. It shows that both the climatological GA and its response to El Nifio warming are simulated well, both in terms of spatial pattern and magnitude. The impact of the convection scheme on GA is not significant. The climatological longwav e CRF (LWCRF) and its response to El Nino warming are simulated well, but with a prominently weaker magnitude. The simulation of the climatology (response) of LWCRF in the NZH (TDK) run is slightly more realistic than in the TDK (NZH) simulation, indicating significant impacts of the convection scheme. The shortwave CRF (SWCRF) shows large biases in both spatial pattern and magnitude, and the results from the TDK run are better than those from the NZH run. A spuriously excessive negative climatological SWCRF over the southeastern Pacific and an insufficient response of SWCRF to El Nifio warming over the tropical Pacific are seen in the NZH run. These two biases are alleviated in the TDK run, since it produces vigorous convection, which is related to the low threshold for convection to take place. Also, impacts of the convection scheme on the cloud profile are discussed.
Institute of Scientific and Technical Information of China (English)
Yang Wan-Min; Zhou Lian; Feng Yong; Zhang Ping-Xiang; R. Nicolsky
2004-01-01
The levitation forces under different field cooling states were measured at 77K by changing the field cooling distance 0Zfc between a YBCO bulk and a permanent magnet. It is found that the relationship between the absolute maximum attractive force (Fmaf) and the corresponding gap distance (Zmaf) to Fmaf can be well described by exponential laws as a function of Zfc, which allow us to predict these values according to Zfc. It is also found that the distance between the Z0fa (gap distance corresponding to the zero force) and Zmaf in the ascending process is a constant value, which is closely related to the constant reduction factor of the axial component of flux density along the axial line of the magnet if Zmaf - Z0fa is a constant value. These results are very interesting for fundamental research and helpful in practical designing and applications.
Convective forcing of mercury and ozone in the Arctic boundary layer induced by leads in sea ice.
Moore, Christopher W; Obrist, Daniel; Steffen, Alexandra; Staebler, Ralf M; Douglas, Thomas A; Richter, Andreas; Nghiem, Son V
2014-02-06
The ongoing regime shift of Arctic sea ice from perennial to seasonal ice is associated with more dynamic patterns of opening and closing sea-ice leads (large transient channels of open water in the ice), which may affect atmospheric and biogeochemical cycles in the Arctic. Mercury and ozone are rapidly removed from the atmospheric boundary layer during depletion events in the Arctic, caused by destruction of ozone along with oxidation of gaseous elemental mercury (Hg(0)) to oxidized mercury (Hg(II)) in the atmosphere and its subsequent deposition to snow and ice. Ozone depletion events can change the oxidative capacity of the air by affecting atmospheric hydroxyl radical chemistry, whereas atmospheric mercury depletion events can increase the deposition of mercury to the Arctic, some of which can enter ecosystems during snowmelt. Here we present near-surface measurements of atmospheric mercury and ozone from two Arctic field campaigns near Barrow, Alaska. We find that coastal depletion events are directly linked to sea-ice dynamics. A consolidated ice cover facilitates the depletion of Hg(0) and ozone, but these immediately recover to near-background concentrations in the upwind presence of open sea-ice leads. We attribute the rapid recoveries of Hg(0) and ozone to lead-initiated shallow convection in the stable Arctic boundary layer, which mixes Hg(0) and ozone from undepleted air masses aloft. This convective forcing provides additional Hg(0) to the surface layer at a time of active depletion chemistry, where it is subject to renewed oxidation. Future work will need to establish the degree to which large-scale changes in sea-ice dynamics across the Arctic alter ozone chemistry and mercury deposition in fragile Arctic ecosystems.
Patel, Kevin H; Evenhuis, Christopher J; Cherney, Leonid T; Krylov, Sergey N
2012-03-01
Temperature increase due to resistive electrical heating is an inherent limitation of capillary electrophoresis (CE). Active cooling systems are used to decrease the temperature of the capillary, but their capacity is limited; and in addition, they leave "hot spots" at the detection interface and at the capillary ends. Until recently, the matter was complicated by the lack of a fast and generic method for temperature determination in efficiently and inefficiently cooled regions of the capillary. Our group recently introduced such a method, termed "Universal Method for determining Electrolyte Temperatures" (UMET). UMET is a probe-less approach that requires only measuring current versus voltage for different voltages and processing the data using an iterative algorithm. Here, we apply UMET to develop a Simplified Universal Method of Temperature Determination (SUMET) for a CE instrument with a forced-air cooling system using an Agilent 7100 CE instrument (Agilent Technologies, Saint Laurent, Quebec, Canada) as an example. We collected a wide set of empirical voltage-current data for a variety of buffers and capillary diameters. We further constructed empirical equations for temperature calculation in efficiently and inefficiently cooled parts of the capillary that require only the data from a single 1-min voltage-current measurement. The equations are specific for the Agilent 7100 CE instrument (Agilent Technologies) but can be applied to all kinds of capillaries and buffers. Similar SUMET approaches can be developed for other CE instruments with forced-air cooling using our approach.
Brazaitis, Marius; Skurvydas, Albertas; Vadopalas, Kazys; Daniusevičiūtė, Laura; Senikienė, Zibuoklė
2011-01-01
The aim of this study was to investigate the effect of heating and cooling on time course of voluntary and electrically induced muscle force variation. Ten volunteers performed 50 maximal voluntary and electrically induced contractions of the knee extensors at an angle of 120 degrees under the control conditions and after passive lower body heating and cooling in the control, heating, and cooling experiments. Peak torque, torque variation, and half-relaxation time were assessed during the exercise. Passive lower body heating increased muscle and core temperatures, while cooling lowered muscle temperature, but did not affect core temperature. We observed significantly lower muscle fatigue during voluntary contraction compared with electrically induced contractions. Body heating (opposite to cooling) increased involuntarily induced muscle force, but caused greater electrically induced muscle fatigue. In the middle of the exercise, the coefficient of correlation for electrically induced muscle torque decreased significantly as compared with the beginning of the exercise, while during maximal voluntary contractions, this relation for torque remained significant until the end of the exercise. It was shown that time course of voluntary contraction was more stable than in electrically induced contractions.
Directory of Open Access Journals (Sweden)
H. Pietersen
2014-07-01
Full Text Available We study the disturbances of CBL dynamics due to large-scale atmospheric contributions for a representative day observed during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST campaign. We first reproduce the observed boundary-layer dynamics by combining the Dutch Atmospheric Large-Eddy Simulation (DALES model with a mixed-layer theory based model. We find that by only taking surface and entrainment fluxes into account, the boundary-layer height is overestimated by 70%. If we constrain our numerical experiments with the BLLAST comprehensive data set, we are able to quantify the contributions of advection of heat and moisture, and subsidence. We find that subsidence has a clear diurnal pattern. Supported by the presence of a nearby mountain range, this pattern suggests that not only synoptic scales exert their influence on the boundary layer, but also mesoscale circulations. Finally, we study whether the vertical and temporal evolution of turbulent variables are influenced by these large-scale forcings. Our model results show good correspondence of the vertical structure of turbulent variables with observations. Our findings further indicate that when large-scale advection and subsidence are applied, the values for turbulent kinetic are lower than without these large-scale forcings. We conclude that the prototypical CBL can still be used as a valid representation of the boundary-layer dynamics near regions characterized by complex topography and small-scale surface heterogeneity, provided that surface- and large-scale forcings are well characterized.
Forced Convection Boiling and Critical Heat Flux of Ethanol in Electrically Heated Tube Tests
Meyer, Michael L.; Linne, Diane L.; Rousar, Donald C.
1998-01-01
Electrically heated tube tests were conducted to characterize the critical heat flux (transition from nucleate to film boiling) of subcritical ethanol flowing at conditions relevant to the design of a regeneratively cooled rocket engine thrust chamber. The coolant was SDA-3C alcohol (95% ethyl alcohol, 5% isopropyl alcohol by weight), and tests were conducted over the following ranges of conditions: pressure from 144 to 703 psia, flow velocities from 9.7 to 77 ft/s, coolant subcooling from 33 to 362 F, and critical heat fluxes up to 8.7 BTU/in(exp 2)/sec. For the data taken near 200 psia, critical heat flux was correlated as a function of the product of velocity and fluid subcooling to within +/- 20%. For data taken at higher pressures, an additional pressure term is needed to correlate the critical heat flux. It was also shown that at the higher test pressures and/or flow rates, exceeding the critical heat flux did not result in wall burnout. This result may significantly increase the engine heat flux design envelope for higher pressure conditions.
Energy Technology Data Exchange (ETDEWEB)
Heselhaus, A.
1997-05-01
In this work a hybrid program system consisting of a 3D finite-volume Navier-Stokes flow solver and a 3D finite-element heat conduction solver has been developed. It enables the coupled calculation of structure temperatures in diabatic solid/fluid configurations. The grids of both the finite element and the finite volume computational domain may be completely independent. The coupled program fully resolves the thermal interaction between heat transfer and the resulting material temperatures. The developed coupling algorithm is numerically stable, conservative and works without the need to define ambient temperatures in the flowfield. This allows for the simulation of any solid/fluid configuration. When simulating combined blade/endwall cooling or filmcooling, only a coupled procedure is capable to completely account for the interaction between all relevant thermal parameters. It is found that the coupled calculation of convective cooling in a realistic guide vane leads locally to 45 K higher and 107 K lower blade temperatures than the uncoupled calculation. This shows that accounting for the thermal interaction between the flow and the structure offers both potential to save cooling air and a lower margin of safety when designing cooling systems close to the thermal limits of the blade material. (orig.) [Deutsch] Im Rahmen der vorliegenden Arbeit wurde ein Verfahren zur Berechnung der Temperaturverteilung in diabat umstroemten Koerpern entwickelt, bei dem ein 3D-Finite Volumen Navier-Stokes Stroemungsloeser und ein 3D-Finite Elemente Waermeleitungsloeser zu einem hybriden Programmsystem gekoppelt werden. Dabei besteht die Moeglichkeit, voellig unabhaengige Rechennetze fuer Stroemung und Struktur zu verwenden. Mit dem gekoppelten Verfahren kann die Wechselwirkung zwischen resultierenden Materialtemperaturen und dem davon rueck-beeinflussten Waermeuebergang beruecksichtigt werden. Weiterhin ist der hier entwickelte, stabile und konservative Kopplungsalgorithmus nicht
Forced convection and transport effects during hyperbaric laser chemical vapor deposition
Energy Technology Data Exchange (ETDEWEB)
Maxwell, James L [Los Alamos National Laboratory; Chavez, Craig A [Los Alamos National Laboratory; Espinoza, Miguel [Los Alamos National Laboratory; Black, Marcie [Los Alamos National Laboratory; Maskaly, Karlene [Los Alamos National Laboratory; Boman, Mats [UPPSALA UNIV
2009-01-01
This work explores mass transport processes during HP-LCYD, including the transverse forced-flow of precursor gases through a nozzle to enhance fiber growth rates. The use of laser trapping and suspension of nano-scale particles in the precursor flow is also described, providing insights into the nature of the gas flow, including jetting from the fiber tip and thermodiffusion processes near the reaction zone. The effects of differing molecular-weight buffer gases is also explored in conjunction with the Soret effect, and it is found that nucleation at the deposit surface (and homogeneous nucleation in the gas phase) can be enhanced/ retarded, depending on the buffer gas molecular weight. To demonstrate that extensive microstructures can be grown simultaneously, three-dimensional fiber arrays are also grown in-parallel using diffractive optics--without delatory effects from neighboring reaction sites.
Cooling Technology for Electronic Computers
Nakayama, Wataru
The rapid growth of data processing speed in computers has been sustained by the advances in cooling technology. This article first presents a review of the published data of heat loads in recent Japanese large-scale computers. The survey indicates that, since around 1980, the high-level integration of microelectronic circuits has brought about almost four fold increase in the power dissipation from logic chips. The integration also has invited the evolutions of multichip modules and new schemes of electronic interconnections. Forced convection air-cooling and liquid cooling coupled with thermal connectors are discussed with reference to the designs employed in actual computers. More advanced cooling schemes are also discussed. Finally, the importance of thermal environmental control of computer rooms is emphasized.
Directory of Open Access Journals (Sweden)
Nemat Dalir
2014-12-01
Full Text Available Entropy generation for the steady two-dimensional laminar forced convection flow and heat transfer of an incompressible Jeffrey non-Newtonian fluid over a linearly stretching, impermeable and isothermal sheet is numerically investigated. The governing differential equations of continuity, momentum and energy are transformed using suitable similarity transformations to two nonlinear coupled ordinary differential equations (ODEs. Then the ODEs are solved by applying the numerical implicit Keller’s box method. The effects of various parameters of the flow and heat transfer including Deborah number, ratio of relaxation to retardation times, Prandtl number, Eckert number, Reynolds number and Brinkman number on dimensionless velocity, temperature and entropy generation number profiles are analyzed. The results reveal that the entropy generation number increases with the increase of Deborah number while the increase of ratio of relaxation to retardation times causes the entropy generation number to reduce. A comparative study of the numerical results with the results from an exact solution for the dimensionless velocity gradient at the sheet surface is also performed. The comparison shows excellent agreement within 0.05% error.
Energy Technology Data Exchange (ETDEWEB)
Wenji, Song [Guangzhou Institute of Energy Conversion, CAS, No. 2 Nengyuan Road, Tianhe District, Guangzhou 510640 (China); Key Laboratory of Renewable Energy and Gas Hydrate, CAS, No. 2 Nengyuan Road, Tianhe District, Guangzhou 510640 (China); Graduate School of Chinese Academy of Sciences, Beijing 100039 (China); Rui, Xiao; Chong, Huang; Shihui, He; Kaijun, Dong; Ziping, Feng [Guangzhou Institute of Energy Conversion, CAS, No. 2 Nengyuan Road, Tianhe District, Guangzhou 510640 (China); Key Laboratory of Renewable Energy and Gas Hydrate, CAS, No. 2 Nengyuan Road, Tianhe District, Guangzhou 510640 (China)
2009-11-15
Tetra-n-butyl-ammonium bromide (TBAB) clathrate hydrate slurry (CHS) is one kind of secondary refrigerants, which is promising to be applied into air-conditioning or latent-heat transportation systems as a thermal storage or cold carrying medium for energy saving. It is a solid-liquid two phase mixture which is easy to produce and has high latent heat and good fluidity. In this paper, the heat transfer characteristics of TBAB slurry were investigated in a horizontal stainless steel tube under different solid mass fractions and flow velocities with constant heat flux. One velocity region of weakened heat transfer was found. Moreover, TBAB CHS was treated as a kind of Bingham fluids, and the influences of the solid particles, flow velocity and types of flow on the forced convective heat transfer coefficients of TBAB CHS were investigated. At last, criterial correlations of Nusselt number for laminar and turbulent flows in the form of power function were summarized, and the error with experimental results was within {+-}20%. (author)
Institute of Scientific and Technical Information of China (English)
李明春; 赵中亮; 静宇; 刘家涛; 吴玉胜
2013-01-01
To study the influence of the Soret and Dufour effects on the reactive characteristics of a porous packed bed with endothermic reactions and forced convection, a two-dimensional mathematical model considering the cross-diffusion effects was developed in accordance with the thermodynamics of irreversible processes and the lo-cal thermal non-equilibrium model. The simulation results were validated by comparing with experimental data. The influence of the Soret and Dufour effects on the heat transfer, mass transfer and endothermic chemical reaction in the non-thermal equilibrium packed bed is discussed. It was found that when the Peclet number reaches 1865, the maximum relative error of the concentration of gas product induced by the Soret effect is 34.7% and that of the solid fractional conversion caused by the Dufour effect is 10.8%at reaction time 160 s and initial temperature 1473 K. The differences induced by the Soret and Dufour effects are demonstrated numerically to increase gradually with the initial temperature of feeding gas and the Peclet number.
Indian Academy of Sciences (India)
ASHOK K BARIK; PRASANTA K SATAPATHY; SUDHANSU S SAHOO
2016-07-01
In this paper, the forced convective heat transfer enhancement with nanofluids in a 90° pipe bend has been presented. Numerical investigation is carried out for the turbulent flow through the pipe employing finite volume method. The governing differential equations are discretized using hexahedral cells, and theresulting algebraic equations are solved using Commercial solver Fluent 6.3. In order to close the time averaged Navier–Stokes equations, the two-equation k–e turbulence model with a standard wall function have been used.The duct Reynolds number is varied in the range of 2,500–6,000. It is observed that the heat transfer is enhanced significantly by varying the volume fraction of the nanofluid. It is also found that the heat transfer is increased with Reynolds number. A strong secondary flow is observed due to the presence of the wall. Turbulent kinetic energy near outer wall is found to be higher than the inner wall of the bend. A comparative assessment for the heat transfer enhancement with different types of nanofluids is also presented. The computed results of areaweighted average Nusselt numbers are validated with some of the existing literature
Directory of Open Access Journals (Sweden)
Amnart Boonloi
2014-09-01
Full Text Available Thermal performance analysis for laminar forced convection in an isothermal wall square channel with 30° V-baffle is presented numerically. The parameters of the V-baffle, blockage ratio (b/H, BR, pitch ratio (P/H, PR, flow direction (V-Downstream and V-Upstream, and arrangement (in-line and staggered, are studied and compared with the previous works, 20° and 45° V-baffle. The Reynolds number based on the hydraulic diameter of the channel (Dh, Re = 100–2000, is used in range study. The results show that the flow configurations of 30° V-baffle are found similar as 20° and 45° V-baffle. The fully developed periodic flow and heat transfer are created around 7th-8th module, while the periodic flow and heat transfer profiles are found at 2nd module in all cases. Except for the periodic concept, the 30° V-baffle can help to reduce the pressure loss around 2.3 times in comparison with the 45° V-baffle at the maximum f/f0 value (BR = 0.3, PR = 1, V-Downstream. The optimum thermal enhancement factor for the 30° V-baffle is found around 4.25 at BR = 0.15, PR = 1, and Re = 2000 for V-Downstream case with in-line arrangement.
Directory of Open Access Journals (Sweden)
Moh'd A. Al-Nimr
2004-06-01
Full Text Available Magnetic field effect on local entropy generation due to steady two-dimensional laminar forced convection flow past a horizontal plate was numerically investigated. This study was focused on the entropy generation characteristics and its dependency on various dimensionless parameters. The effect of various dimensionless parameters, such as Hartmann number (Ha, Eckert number (Ec, Prandtl number (Pr, Joule heating parameter (R and the free stream temperature parameter (ÃŽÂ¸Ã¢ÂˆÂž on the entropy generation characteristics is analyzed. The dimensionless governing equations in Cartesian coordinate were solved by an implicit finite difference technique. The solutions were carried out for Ha2=0.5-3, Ec=0.01-0.05, Pr=1-5 and ÃŽÂ¸Ã¢ÂˆÂž=1.1-2.5. It was found that, the entropy generation increased with increasing Ha, Ec and R. While, increasing the free stream temperature parameter, and Prandtl number tend to decrease the local entropy generation.
Directory of Open Access Journals (Sweden)
Hsien-Hung Ting
2015-01-01
Full Text Available This numerical study is aimed at investigating the forced convection heat transfer and flow characteristics of water-based Al2O3 nanofluids inside a horizontal circular tube in the laminar flow regime under the constant wall temperature boundary condition. Five volume concentrations of nanoparticle, 0.1, 0.5, 1, 1.5, and 2 vol.%, are used and diameter of nanoparticle is 40 nm. Characteristics of heat transfer coefficient, Nusselt number, and pressure drop are reported. The results show that heat transfer coefficient of nanofluids increases with increasing Reynolds number or particle volume concentration. The heat transfer coefficient of the water-based nanofluid with 2 vol.% Al2O3 nanoparticles is enhanced by 32% compared with that of pure water. Increasing particle volume concentration causes an increase in pressure drop. At 2 vol.% of particle concentration, the pressure drop reaches a maximum that is nearly 5.7 times compared with that of pure water. It is important to note that the numerical results are in good agreement with published experimental data.
Wstawska, Iwona; Ślimak, Krzysztof
2016-12-01
Titanium alloys are one of the materials extensively used in the aerospace industry due to its excellent properties of high specific strength and corrosion resistance. On the other hand, they also present problems wherein titanium alloys are extremely difficult materials to machine. In addition, the cost associated with titanium machining is also high due to lower cutting velocities and shorter tool life. The main objective of this work is a comparison of different cooling techniques during cryogenic machining of titanium alloys. The analysis revealed that applied cooling technique has a significant influence on cutting force and surface roughness (Ra parameter) values. Furthermore, in all cases observed a positive influence of cryogenic machining on selected aspects after turning and milling of titanium alloys. This work can be also the starting point to the further research, related to the analysis of cutting forces and surface roughness during cryogenic machining of titanium alloys.
Marcum, Jeremy W.; Olson, Sandra L.; Ferkul, Paul V.
2016-01-01
The axisymmetric rod geometry in upward axial stagnation flow provides a simple way to measure normal gravity blowoff limits to compare with microgravity Burning and Suppression of Solids - II (BASS-II) results recently obtained aboard the International Space Station. This testing utilized the same BASS-II concurrent rod geometry, but with the addition of normal gravity buoyant flow. Cast polymethylmethacrylate (PMMA) rods of diameters ranging from 0.635 cm to 3.81 cm were burned at oxygen concentrations ranging from 14 to 18% by volume. The forced flow velocity where blowoff occurred was determined for each rod size and oxygen concentration. These blowoff limits compare favorably with the BASS-II results when the buoyant stretch is included and the flow is corrected by considering the blockage factor of the fuel. From these results, the normal gravity blowoff boundary for this axisymmetric rod geometry is determined to be linear, with oxygen concentration directly proportional to flow speed. We describe a new normal gravity 'upward flame spread test' method which extrapolates the linear blowoff boundary to the zero stretch limit in order to resolve microgravity flammability limits-something current methods cannot do. This new test method can improve spacecraft fire safety for future exploration missions by providing a tractable way to obtain good estimates of material flammability in low gravity.
Drying of prickly pear cactus cladodes (Opuntia ficus indica) in a forced convection tunnel
Energy Technology Data Exchange (ETDEWEB)
Lopez, R.; de Ita, A.; Vaca, M. [Universidad Autonoma Metropolitana-Azcapotzalco, DePt. de Energia, Area de Termofluidos, Av. San Pablo 180, Col. Reynosa Tamaulipas, Del. Azcapotzalco, C.P. 02200, Mexico D.F. (Mexico)
2009-09-15
In this work we evaluated the kinetics of drying of Opuntia's cladodes observing two conditions: complete cladode with the protective cuticle of the intact product and with reduced cuticle (partially removed), using a drying tunnel with forced flow. The temperature of the air was set at 35, 45, and 60 C with velocities of 1.5 and 3.0 m/s. The conditions of the environment were controlled and maintained at 22 C and 30% of relative humidity. The results show that the drying time was considerably reduced when approximately 30% of the cuticle that protects the product was removed. Additionally, the temperature had greater influence than the velocity of the air. The numerical model that best describes the behavior of the drying process is the double logarithmic one, with the imposed restrictions of r close to the unit, the lowest possible {chi}{sup 2} and the RSEM tending to zero. The characteristic drying function of the product resulted in a third-grade exponential curve, where r and SD were the corresponding selection criteria. (author)
Bernstein, P.; Colson, L.; Dupont, L.; Noudem, J.
2017-06-01
We report levitation force cycles resulting from measurements carried out on a YBCO and a MgB2 disk cooled down in the field of a permanent magnet. In both cases the amplitude of the levitation force tends toward maximum values as the temperature decreases. Otherwise, the cycles are almost closed at low temperature and strongly hysteretic in the high temperature range. The hysteresis of the force cycles is attributed to the distribution of the currents induced in the sample by the field of the magnet. The saturation of the levitation forces at low temperature is related to that of the magnetic moment of the disks. We show that this type of measurement allows for the determination of the critical current density of superconductors in a restricted domain of temperatures.
Gokoglu, S. A.; Rosner, D. E.
1984-01-01
A simple engineering correlation scheme is developed to predict the variable property effects on dilute species laminar forced convection mass transfer applicable to all vapor molecules or Brownian diffusing small particle, covering the surface to mainstream temperature ratio of 0.25 T sub W/T sub e 4. The accuracy of the correlation is checked against rigorous numerical forced convection laminar boundary layer calculations of flat plate and stagnation point flows of air containing trace species of Na, NaCl, NaOH, Na2SO4, K, KCl, KOH, or K2SO4 vapor species or their clusters. For the cases reported here the correlation had an average absolute error of only 1 percent (maximum 13 percent) as compared to an average absolute error of 18 percent (maximum 54 percent) one would have made by using the constant-property results.
Theoretical and Computational Study of Forced-Convection Heat Transfer at Supercritical Pressures
Zhong, Jianguo
In the simulation of turbulent fluid flow and heat transfer at supercritical pressures, substantial difficulties have been encountered in the modeling of turbulence and bounda-ry layer. This is due to significant fluid property variations with respect to the local temperature and pressure, especially in the near-wall region of a heated wall, where large temperature differences occur. The classical turbulence models available in literature were typically developed for constant-property fluids, where an empirical wall function in the high-Re k-epsilon model, and a damping function in the low-Re k-epsilon model were derived based on the constant-property data to solve the boundary layer. As it can be found in the existing literature, large differences have been observed between the experimental and numerical simulation results of the heat transfer coefficient predictions in the en-hanced and deteriorated heat transfer situations for supercritical fluids. In this thesis, a novel near-wall treatment method is proposed to treat large property variations in the thermal and velocity sub-layers. In the near-wall region, the supercritical fluids can be considered thermal-conductive and viscous forces dominated. The thick-ness of the viscous sub-layer (VSL) and the conduction sub-layer (CSL) can be related to the wall shear stress and local Prandtl number information by using computational CFD models, such as that implemented in the NPHASE-CMFD code. The fluids' bulk and wall temperature information has been obtained from the literature review of experi-mental measurements. The wall temperature and heat transfer coefficient calculated from the k-epsilon model with the proposed wall treatment method have been found to be in good agreement with experimental data for both heat transfer enhancement and deterioration cases for two most widely used fluids: CO2 and water. The proposed model has been applied in the reactor-scale thermal-hydraulic analysis of different flow path
Ito, Takehiro; Nishikawa, Kaneyasu; Shigechi, Tooru
1981-01-01
Forced convection film boiling heat transfer from a horizontal cylinder to saturated liquid cross-flowing upward is analyzed based on the two-phase boundary-layer theory. Numerical solution of the conservation equations is determined by means of the integral method of boundary-layer for water, ethanol and hexane under the atmospheric pressure. The velocity profile, separation point of the boundary-layer, thickness of the boundary-layer, distribution of the heat transfer coefficients and avera...
Shigechi, Tooru; Ito, Takehiro; Nishikawa, Kaneyasu
1983-01-01
Forced convection film boiling heat transfer from a horizontal cylinder to a subcooled liquid cross-flowing upward is analysed based on the two-phase boundary-layer theory. Numerical solution of the conservation equations is determined for subcooled water, ethanol and hexane under the atmospheric pressure by the method similar to that of the first report for saturated liquid. The velocity profile, the separation point in the vapor film, the thickness of the boundary-layer and the average Nuss...
Qasim, S. M.; Sahar, A. F. A.; Firas, A. A.
2015-11-01
A numerical study has been carried out to investigate the heat transfer by laminar forced convection of nanofluid taking Titania (TiO2) and Alumina (Al2O3) as nanoparticles and the water as based fluid in a three dimensional plain and U-longitudinal finned tube heat exchanger. A Solid WORKS PREMIUM 2012 is used to draw the geometries of plain tube heat exchanger or U-longitudinal copper finned tube heat exchanger. Four U-longitudinal copper fins have 100 cm long, 3.8cm height and 1mm thickness are attached to a straight copper tube of 100 cm length, 2.2 cm inner diameter and 2.39 cm outer diameter. The governing equations which used as continuity, momentum and energy equations under assumptions are utilized to predict the flow field, temperature distribution, and heat transfer of the heat exchanger. The finite volume approach is used to obtain all the computational results using commercial ANSYS Fluent copy package 14.0 with assist of solid works and Gambit software program. The effect of various parameters on the performance of heat exchanger are investigated numerically such as Reynolds' number (ranging from 270 to 1900), volume consternation of nanoparticles (0.2%, 0.4%, 0.6%, 0.8%), type of nanoparticles, and mass flow rate of nanofluid in the hot region of heat exchanger. For 0.8% consternation of nanoparticles, heat transfer has significant enhancement in both nanofluids. It can be found about 7.3% for TiO2 and about 7.5% for Al2O3 compared with the water only as a working fluid.
Directory of Open Access Journals (Sweden)
Ghofrane Sekrani
2016-11-01
Full Text Available In the present paper, laminar forced convection nanofluid flows in a uniformly heated horizontal tube were revisited by direct numerical simulations. Single and two-phase models were employed with constant and temperature-dependent properties. Comparisons with experimental data showed that the mixture model performs better than the single-phase model in the all cases studied. Temperature-dependent fluid properties also resulted in a better prediction of the thermal field. Particular attention was paid to the grid arrangement. The two-phase model was used then confidently to investigate the influence of the nanoparticle size on the heat and fluid flow with a particular emphasis on the sedimentation process. Four nanoparticle diameters were considered: 10, 42, 100 and 200 nm for both copper-water and alumina/water nanofluids. For the largest diameter d n p = 200 nm, the Cu nanoparticles were more sedimented by around 80%, while the A l 2 O 3 nanoparticles sedimented only by 2 . 5 %. Besides, it was found that increasing the Reynolds number improved the heat transfer rate, while it decreased the friction factor allowing the nanoparticles to stay more dispersed in the base fluid. The effect of nanoparticle type on the heat transfer coefficient was also investigated for six different water-based nanofluids. Results showed that the Cu-water nanofluid achieved the highest heat transfer coefficient, followed by C, A l 2 O 3 , C u O , T i O 2 , and S i O 2 , respectively. All results were presented and discussed for four different values of the concentration in nanoparticles, namely φ = 0 , 0 . 6 % , 1 % and 1 . 6 % . Empirical correlations for the friction coefficient and the average Nusselt number were also provided summarizing all the presented results.
Heller, Reinhard; Hull, John R.
High-temperature superconductors (HTS's), consisting of Bi-2223 HTS tapes sheathed with Ag alloys are proposed for a 20-kA current lead for the planned stellarator WENDELSTEIN 7-X. Forced-flow He cooling is used, and 4-K He cooling of the whole lead as well as 60-K He cooling of the copper part of the lead, is discussed. Power consumption and behavior in case of loss of He flow are given.
Barrett, Joe H., III; Roeder, William P.
2010-01-01
The expected peak wind speed for the day is an important element in the daily morning forecast for ground and space launch operations at Kennedy Space Center (KSC) and Cape Canaveral Air Force Station (CCAFS). The 45th Weather Squadron (45 WS) must issue forecast advisories for KSC/CCAFS when they expect peak gusts for >= 25, >= 35, and >= 50 kt thresholds at any level from the surface to 300 ft. In Phase I of this task, the 45 WS tasked the Applied Meteorology Unit (AMU) to develop a cool-season (October - April) tool to help forecast the non-convective peak wind from the surface to 300 ft at KSC/CCAFS. During the warm season, these wind speeds are rarely exceeded except during convective winds or under the influence of tropical cyclones, for which other techniques are already in use. The tool used single and multiple linear regression equations to predict the peak wind from the morning sounding. The forecaster manually entered several observed sounding parameters into a Microsoft Excel graphical user interface (GUI), and then the tool displayed the forecast peak wind speed, average wind speed at the time of the peak wind, the timing of the peak wind and the probability the peak wind will meet or exceed 35, 50 and 60 kt. The 45 WS customers later dropped the requirement for >= 60 kt wind warnings. During Phase II of this task, the AMU expanded the period of record (POR) by six years to increase the number of observations used to create the forecast equations. A large number of possible predictors were evaluated from archived soundings, including inversion depth and strength, low-level wind shear, mixing height, temperature lapse rate and winds from the surface to 3000 ft. Each day in the POR was stratified in a number of ways, such as by low-level wind direction, synoptic weather pattern, precipitation and Bulk Richardson number. The most accurate Phase II equations were then selected for an independent verification. The Phase I and II forecast methods were
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S.K Ghosh
2013-01-01
Full Text Available A theoretical analysis is presented for transient, fully-developed magnetohydrodynamic free and forced convection flow of a viscous, incompressible, Newtonian fluid in a rotating horizontal parallel-plate channel subjected to a uniform strength, static, oblique magnetic field acting at an angle to the positive direction of the axis of rotation. A constant pressure gradient is imposed along the longitudinal axis of the channel. Magnetic Reynolds number is sufficiently small to negate the effects of magnetic induction. The channel plates are electrically non-conducting. The conservation equations are formulated in an (x,y,z coordinate system and normalized using appropriate transformations. The resulting non-dimensional coupled ordinary differential equations for primary and secondary velocity components and transformed boundary conditions are found to be reciprocal of the Ekman number ( 2 K =1/Ek, non-dimensional pressure gradient parameter (Px, Hartmann number ( 2 M , Grashof number (Gr, magnetic field inclination ( and oscillation frequency (. Complex variables are employed to solve the two-point boundary value problem. A steady state resonance of the velocity field is identified for 4 4 4 1/ 2 16 2 1 K M Sin . Furthermore the solutions indicate that the condition 1/2 1 4 4 4 cos 16 2 T K M Sin signifies an oscillatory turbulent dynamo mechanism. A critical Grashof number (Grcx is also evaluated for which primary flow reversal arises at the upper channel plate. A similar criterion for Grashof number (Grcy is established for the onset of secondary flow reversal at the upper plate. A detailed assessment of the influence of the control parameters on primary and secondary velocity evolution in the channel is also conducted. The model finds applications in MHD (Magneto Hydro Dynamic energy generators and also magnetic materials processing systems.
Ahmed, Mahmoud; Eslamian, Morteza
2015-07-01
Laminar natural convection in differentially heated ( β = 0°, where β is the inclination angle), inclined ( β = 30° and 60°), and bottom-heated ( β = 90°) square enclosures filled with a nanofluid is investigated, using a two-phase lattice Boltzmann simulation approach. The effects of the inclination angle on Nu number and convection heat transfer coefficient are studied. The effects of thermophoresis and Brownian forces which create a relative drift or slip velocity between the particles and the base fluid are included in the simulation. The effect of thermophoresis is considered using an accurate and quantitative formula proposed by the authors. Some of the existing results on natural convection are erroneous due to using wrong thermophoresis models or simply ignoring the effect. Here we show that thermophoresis has a considerable effect on heat transfer augmentation in laminar natural convection. Our non-homogenous modeling approach shows that heat transfer in nanofluids is a function of the inclination angle and Ra number. It also reveals some details of flow behavior which cannot be captured by single-phase models. The minimum heat transfer rate is associated with β = 90° (bottom-heated) and the maximum heat transfer rate occurs in an inclination angle which varies with the Ra number.
Ahmed, Mahmoud; Eslamian, Morteza
2015-12-01
Laminar natural convection in differentially heated (β = 0°, where β is the inclination angle), inclined (β = 30° and 60°), and bottom-heated (β = 90°) square enclosures filled with a nanofluid is investigated, using a two-phase lattice Boltzmann simulation approach. The effects of the inclination angle on Nu number and convection heat transfer coefficient are studied. The effects of thermophoresis and Brownian forces which create a relative drift or slip velocity between the particles and the base fluid are included in the simulation. The effect of thermophoresis is considered using an accurate and quantitative formula proposed by the authors. Some of the existing results on natural convection are erroneous due to using wrong thermophoresis models or simply ignoring the effect. Here we show that thermophoresis has a considerable effect on heat transfer augmentation in laminar natural convection. Our non-homogenous modeling approach shows that heat transfer in nanofluids is a function of the inclination angle and Ra number. It also reveals some details of flow behavior which cannot be captured by single-phase models. The minimum heat transfer rate is associated with β = 90° (bottom-heated) and the maximum heat transfer rate occurs in an inclination angle which varies with the Ra number.
Thermal behaviour of cooling ceilings; Thermisches Verhalten von Kuehldecken
Energy Technology Data Exchange (ETDEWEB)
Beck, C.
2002-07-01
The thermal behaviour of cooling ceilings is so far described by characteristic equations in the form of q=C x {delta}{theta}{sup n}. In special cases these equations are extended by terms which take into account the radiation exchange between heat sink and space user (operational ambient temperature). This is insufficient. Therefore a model is presented, which describes more exactly the individual processes (radiation, convection) at the cooled surface. As a first step the ceiling area is divided into three ranges in which different phenomena are affecting the heat transfer considerably: 1. Range under the influence of free convection in the space. 2. Range with forced convection caused by a plume along the window. 3. Range with forced convection caused by air flow through supply air outlets. For each of these ranges the fundamentals of heat transfer are applied using the substantial variables. The results of these models are then compared in a second step with results from laboratory tests. This model is developed first for steady-state conditions. It is extended by a second model to describe the dynamic processes (storage). Finally, both steady-state and unsteady conditions are investigated systematically. Results are presented for the cooling rate effected by the temperature of cooling ceiling, ambient conditions, supply air flow rate, supply air temperature and type of the air outlet. The results computed indicate that the maximum total cooling rate is always gained by a combination of a cooling ceiling and a supply air with minimum permissible temperature. In these cases the cooling rate of the ceiling is lower than that without ventilation of the space. However, the additional cooling rate of the ventilation compensates this effect in all cases investigated. This is an important finding, since cooling ceilings usually must be combined with an HVAC system for reasons of dehumidifying. Dried, cooled supply air can thus be supplied to the space without
Energy Technology Data Exchange (ETDEWEB)
Bataille, Arnaud; Genthon, Pierre; Rabinowicz, Michel [Laboratoire de Dynamique Terrestre et Planetaire, UMR 5562, Observatoire Midi-Pyrenees, 14 Avenue Edouard Belin, 31400 Toulouse (France); Fritz, Bertrand [Centre de Geochimie de Surface, UMR 7517, Ecole de l' Observatoire des Sciences de la Terre, 1 rue Blessig, F-67084 Strasbourg (France)
2006-10-15
The aim of the hydraulic stimulations in the Soultz-sous-Forets, France, Enhanced Geothermal System (EGS) project was to create, in crystalline rocks, a fractured reservoir 750 m high, 750 m long and 35 m thick interconnecting the injection and production wells. Increasing the permeability in a zone with a high geothermal gradient will trigger free convection, which will interact with the forced flow driven by pumping. A systematic numerical study of the coupling between forced and free convective flows has been performed by considering a large range of injection rates and Rayleigh numbers. The simulations showed that if there is weak or no free convection in an EGS reservoir, economic exploitation of the system will rapidly end because of a decrease in produced fluid temperature. The maximum injection rate preventing such a temperature drop increases with the Rayleigh number and the height of the stimulated domain. The model establishes constraints on the conditions for achieving optimal heat extraction at the Soultz-sous-Forets EGS site. It was also shown that, although mineral precipitation may partially close or heal some open fissures, it does not lead to a major decrease of the hydraulic conductivity in the stimulated reservoir. (author)
Bune, Andris V.; Sen, Subhayu; Mukherjee, Sundeep; Catalina, Adrian; Stefanescu, Doru M.
2000-04-01
Numerical modeling was undertaken to analyze the influence of both radial and axial thermal gradients on convection patterns and velocities during solidification of pure Al and an Al-4 wt% Cu alloy. The objective of the numerical task was to predict the influence of convective velocity on an insoluble particle near a solid/liquid (s/l) interface. These predictions were then be used to define the minimum gravity level ( g) required to investigate the fundamental physics of interactions between a particle and a s/l interface. This is an ongoing NASA funded flight experiment entitled "particle engulfment and pushing by solidifying interfaces (PEP)". Steady-state calculations were performed for different gravity levels and orientations with respect to the gravity vector. The furnace configuration used in this analysis is the quench module insert (QMI-1) proposed for the Material Science Research Facility (MSRF) on board the International Space Station (ISS). The general model of binary alloy solidification was based on the finite element code FIDAP. At a low g level of 10 -4g 0 ( g 0=9.8 m/s 2) maximum melt convection was obtained for an orientation of 90°. Calculations showed that even for this worst case orientation the dominant forces acting on the particle are the fundamental drag and interfacial forces.
Evidence of convective heat transfer enhancement induced by spinodal decomposition.
Poesio, P; Lezzi, A M; Beretta, G P
2007-06-01
Spinodal decomposition can be driven by either diffusion or self-induced convection; the importance of convection relative to diffusion depends on the Péclet number, defined as the ratio between convective and diffusive mass fluxes. Diffusion is the dominating mechanism of phase segregation when the Péclet number is small - i.e., when viscosity and diffusivity are large - or when the domain characteristic size is small. For low-viscosity mixtures, convection is the dominating process and the segregation is very rapid as it takes a few seconds compared to the hours needed in the case of pure diffusion. In such cases, strong convective motion of the phase segregating domains is generated even in small-size systems and is almost independent of the temperature difference as long as it is below the transition value. We study experimentally the enhancement of heat transfer in a 1-mm -thick cell. A water-acetonitrile-toulene mixture is quenched into a two-phase region so as to induce convection-driven spinodal decomposition. The heat transfer rate is measured and compared to that obtained in the absence of convective motion. A substantial reduction in the cooling time obtains in the case of spinodal decomposition. The heat transfer enhancement induced by this self-induced, disordered but effectively convective effect may be exploited in the cooling or heating of small-scale systems whereby forced convection cannot be achieved because of the small sizes involved. A scaling analysis of the data based on the diffuse interface H model for a symmetric mixture near the equilibrium point yields very encouraging agreement and insights.
Boiling incipience and convective boiling of neon and nitrogen
Papell, S. S.; Hendricks, R. C.
1977-01-01
Forced convection and subcooled boiling heat transfer data for liquid nitrogen and liquid neon were obtained in support of a design study for a 30 tesla cryomagnet cooled by forced convection of liquid neon. This design precludes nucleate boiling in the flow channels as they are too small to handle vapor flow. Consequently, it was necessary to determine boiling incipience under the operating conditions of the magnet system. The cryogen data obtained over a range of system pressures, fluid flow rates, and applied heat fluxes were used to develop correlations for predicting boiling incipience and convective boiling heat transfer coefficients in uniformly heated flow channels. The accuracy of the correlating equations was then evaluated. A technique was also developed to calculate the position of boiling incipience in a uniformly heated flow channel. Comparisons made with the experimental data showed a prediction accuracy of plus or minus 15 percent
Resfriamento de banana-prata com ar forçado Forced-air cooling of banana
Directory of Open Access Journals (Sweden)
BÁRBARA TERUEL
2002-04-01
Full Text Available Este trabalho apresenta as curvas de resfriamento de banana-prata (Musa balbisiana Colla e os valores do tempo de meio e sete oitavos de resfriamento, partindo do cálculo da Taxa Adimensional de Temperatura. Os frutos foram resfriados num sistema com ar forçado a 7ºC, umidade relativa de 87,6±3,8%, e velocidade do ar entre 1 e 0,2 m/s. Aplicou-se um delineamento experimental inteiramente casualizado, usando um esquema fatorial 2x2 (dois fluxos de ar (fatores e duas embalagens (níveis, para um nível de significância de 10%. Os fluxos de ar foram 1.933 a 1.160 m³/h, e as embalagens se diferenciaram pela porcentagem de área de abertura disponível para a ventilação (40 e 3,2%. Foi constatada uma diferença significativa no tempo de resfriamento, tanto quando aplicadas as duas taxas de ar como quando usadas as duas embalagens. O menor tempo de resfriamento foi atingido no tratamento que combinou a maior taxa de ar (1.933 m³/h com a embalagem de maior área de aberturas (40%. O maior tempo de resfriamento foi atingido no tratamento que combinou a menor taxa de ar (1160 m³/h com a embalagem de 3,2% de área efetiva de abertura. Os resultados obtidos demonstram que o tempo de resfriamento depende, em grande medida, da taxa de ar e do tipo de embalagem usada. O tempo de resfriamento variou em média entre 117 a 555 min, dependendo do tratamento aplicado. Não se constatou diferença significativa nas perdas de massa entre os diferentes tratamentos.This work presents the cooling curves for bananas Prata, (Musa balbisiana Colla, and determinates half-cooling and seven-eight cooling times and the cooling rate. Bananas were kept in a cold room with a forced-air system at 7ºC and RH = 87.6±3.8%. The experiment was conducted in a 2x2 factorial design, to test the effects of two flow rates (factors of air passing through the product, and two types of boxes (levels. The statistical analysis was performed at p<0.10. The air flow rates were 1933
Enhancing the efectiveness of film cooling
Institute of Scientific and Technical Information of China (English)
Tom I-P.Shih; Sangkwon Na
2007-01-01
Advanced gas turbine stages are designed to operate at increasingly higher inlet temperatures to increase thermal efficiency and specific power output.To maintain durability and reasonable life,film cooling is needed in addition to internal cooling,especially for the first stage.Film cooling lowers material temperature by forced convection inside film-cooling holes and by forming a layer of coolant about component surfaces to insulate them from the hot gases.Unfortunately,each cooling jet forms a pair of counter-rotating vortices that entrains hot gas and causes the film-cooling jet to lift off from the surface that it is intended to protect.This paper gives an overview of efforts to enhance the effectiveness of film-cooling.This paper also describes two new design concepts.One design concept seeks to minimize the entrainment of hot gases underneath of film-cooling jets by using flow-aligned blockers.The other design concept shifts the interaction between the approaching hot gas and the cooling jet to occur further above the surface by using an upstream ramp.For both design concepts,computational fluid dynamics results are presented to examine their usefulness in enhancing film-cooling effectiveness.
Institute of Scientific and Technical Information of China (English)
ZHOU Yushu; Xiaofan LI
2011-01-01
Both water vapor and heat processes play key roles in producing surface rainfall.While the water vapor effects of sea surface temperature and cloud radiative and microphysical processes on surface rainfall have been investigated in previous studies,the thermal effects on rainfall are analyzed in this study using a series of two-dimensional equilibrium cloud-resolving model experiments forced by zonally-uniform,constant,large-scale zonal wind and zero large-scale vertical velocity.The analysis of thermally-related surface rainfall budget reveals that the model domain mean surface rain rate is primarily associated with the mean infrared cooling rate.Convective rainfall and transport of hydrometeor concentration from convective regions to raining stratiform regions corresponds to the heat divergence over convective regions,whereas stratiform rainfall corresponds to the transport of hydrometeor concentration from convective regions and heat divergence over raining stratiform regions.The heat divergence over convective regions is mainly balanced by the heat convergence over rainfall-free regions,which is,in turn,offset by the radiative cooling over rainfall-free regions.The sensitivity experiments of rainfall to the effects of sea surface temperature and cloud radiative and microphysical processes show that the sea surface temperature and cloud processes affect convective rainfall through the changes in infrared cooling rate over rainfall-free regions and transport rate of heat from convective regions to rainfall-free regions.
Verdoold, J.
2010-01-01
This dissertation focuses on turbulent thermal convection, which occurs in a wide range of (geo)physical situations, like in the atmosphere, the oceans, the interior of stars or planets, and engineering applications, like metal casting or crystal growth processes. In this work, a special type of th
Directory of Open Access Journals (Sweden)
Amin Kashani
2013-04-01
Full Text Available Laminar forced convection of a nanofluid consisting of water and Al2O3 in a horizontal annulus has been studied numerically. Two-phase mixture model has been used to investigate thermal behaviors of the nanofluid over constant temperature thermal boundary condition and with different volume concentration of nanoparticles. Comparisons with previously published experimental and analytical works on flow behavior in horizontal annulus show good agreements between the results as volume fraction is zero. In general convective heat transfer coefficient increases with nanoparticle concentration. ABSTRAK: Kertaskerja ini mengkaji secara numerik olakan paksa bendalir lamina yang menganduangi air dan Al2O3 didalam anulus mendatar. Model campuran dua fasa digunakan bagi mengkaji tingkah laku haba bendalir nano pada keadaan suhu malar dengan kepekatan nanopartikel berbeza. Perbandingan dengan karya eksperimen dan analitikal yang telah diterbitkan menunjukkan bahawa kelakuan aliran didalm anulus mendatar adalah baik apabila pecahan isipadu adalah sifar. Pada amnya, pekali pemindahan haba olakan meningkat dengan kepekatan nanopartikel. KEYWORDS: nanofluid; volume concentration; heat transfer enhancement; laminar flow convection; annulus
Convective Regimes in Crystallizing Basaltic Magma Chambers
Gilbert, A. J.; Neufeld, J. A.; Holness, M. B.
2015-12-01
Cooling through the chamber walls drives crystallisation in crustal magma chambers, resulting in a cumulate pile on the floor and mushy regions at the walls and roof. The liquid in many magma chambers, either the bulk magma or the interstitial liquid in the mushy regions, may convect, driven either thermally, due to cooling, or compositionally, due to fractional crystallization. We have constructed a regime diagram of the possible convective modes in a system containing a basal mushy layer. These modes depend on the large-scale buoyancy forcing characterised by a global Rayleigh number and the proportion of the chamber height constituting the basal mushy region. We have tested this regime diagram using an analogue experimental system composed of a fluid layer overlying a pile of almost neutrally buoyant inert particles. Convection in this system is driven thermally, simulating magma convection above and within a porous cumulate pile. We observe a range of possible convective regimes, enabling us to produce a regime diagram. In addition to modes characterised by convection of the bulk and interstitial fluid, we also observe a series of regimes where the crystal pile is mobilised by fluid motions. These regimes feature saltation and scouring of the crystal pile by convection in the bulk fluid at moderate Rayleigh numbers, and large crystal-rich fountains at high Rayleigh numbers. For even larger Rayleigh numbers the entire crystal pile is mobilised in what we call the snowglobe regime. The observed mobilisation regimes may be applicable to basaltic magma chambers. Plagioclase in basal cumulates crystallised from a dense magma may be a result of crystal mobilisation from a plagioclase-rich roof mush. Compositional convection within such a mush could result in disaggregation, enabling the buoyant plagioclase to be entrained in relatively dense descending liquid plumes and brought to the floor. The phenocryst load in porphyritic lavas is often interpreted as a
Stability of cable-in-conduit, force-cooled conductors: elementary theory
Energy Technology Data Exchange (ETDEWEB)
Dresner, L.
1979-01-01
The recovery of cable-in-conduit conductors cooled by supercritical helium has been studied. The initial perturbation is a sudden, uniform deposition of heat. The problem has been solved, and answers have been obtained in the form of simple closed formulas under the simplifying assumption of constant thermophysical properties. Stability-optimized conductors have been identified and simple formulas for their material composition are given. A rule of design of cable-in-conduit conductors based on the existence of stability-optimized conductors is suggested. Some ''zero-flow'' experiments of Miller et al. have been analyzed, and it has been determined that for these experiments the effective heat transfer coefficient during the period of recovery is approximately 1000 Wm/sup -2/K/sup -1/.
Khurana, Deepak; Choudhary, Rajesh; Subudhi, Sudhakar
2017-01-01
Nanofluid is the colloidal suspension of nanosized solid particles like metals or metal oxides in some conventional fluids like water and ethylene glycol. Due to its unique characteristics of enhanced heat transfer compared to conventional fluid, it has attracted the attention of research community. The forced convection heat transfer of nanofluid is investigated by numerous researchers. This paper critically reviews the papers published on experimental studies of forced convection heat transfer and pressure drop of Al2O3, TiO2 and CuO based nanofluids dispersed in water, ethylene glycol and water-ethylene glycol mixture. Most of the researchers have shown a little rise in pressure drop with the use of nanofluids in plain tube. Literature has reported that the pumping power is appreciably high, only at very high particle concentration i.e. more than 5 %. As nanofluids are able to enhance the heat transfer at low particle concentrations so most of the researchers have used less than 3 % volume concentration in their studies. Almost no disagreement is observed on pressure drop results of different researchers. But there is not a common agreement in magnitude and mechanism of heat transfer enhancement. Few studies have shown an anomalous enhancement in heat transfer even at low particle concentration. On the contrary, some researchers have shown little heat transfer enhancement at the same particle concentration. A large variation (2-3 times) in Nusselt number was observed for few studies under similar conditions.
Cornut, R; Poirier, S; Mauzeroll, Janine
2012-04-17
In scanning electrochemical microscopy (SECM), an approach curve performed in feedback mode involves the downward displacement of a microelectrode toward a substrate while applying a bias to detect dissolved electroactive species at a diffusion-limited rate. The resulting measured current is said to be at steady state. In order to reduce the required measurement time, the approach velocity can be increased. In this paper, we investigate experimentally and theoretically the combination of diffusion and convection processes related to a moving microdisk electrode during feedback approaches. Transient modeling and numerical simulations with moving boundaries are performed, and the results are compared to the experimental approach curves obtained in aqueous solution. The geometry and misalignment of the microelectrode influence the experimental approach curves recorded at high approach velocities. The effects are discussed through the decomposition of the current into transient diffusional, radial convectional, and axial convectional contributions. Finally a ready-to-use expression is provided to rapidly evaluate the maximal approach velocity for steady state measurements as a function of the microelectrode geometry and the physical properties of the media. This expression holds for the more restrictive case of negative feedback as well as other modes, such as SECM approach curves performed at substrates displaying first order kinetics.
Improvement of thermal comfort by cooling clothing in warm climate
DEFF Research Database (Denmark)
Sakoi, Tomonori; Melikov, Arsen Krikor; Kolencíková, Sona;
2014-01-01
comfort compared to convective cooling alone. The supply of a small amount of water allowed the cooling clothing to provide a continuous cooling effect, whereas the effect of convective cooling alone decreased as sweat dried. However, the controllability of the cooling clothing needs to be improved....
Energy Technology Data Exchange (ETDEWEB)
Perroud, P.; Rebiere, J.; Strittmatter, R. [Commissariat a l' Energie Atomique, Grenoble (France). Centre d' Etudes Nucleaires
1969-07-01
Heat transfer coefficients and pressure drop of gaseous ammonia in forced convection are experimentally determined. The fluid flows (mass flow rate 0.6 to 2.4 g/s) in a long tungsten tube (d{sub i} = 2.8 mm, d{sub e} = 5.1 mm, L = 700 mm) electrically heated. The temperature of the wall reaches 3000 deg. K and the fluid 2500 deg. K; maximum heat flux 530 w/cm{sup 2}. Ammonia is completely dissociated and the power necessary for dissociation reaches 30 per cent of the total power exchanged. Inlet pressure varies between 6 and 16 bars and the maximum pressure drop in the tube reaches 15 bars. Two regimes of dissociation have been shown: catalytic and homogeneous and the variation of dissociation along the length of the tube is studied. The measured heat transfer coefficients may be about 10 times these calculated by the means of classical formulae. A correlation of experimental results using enthalpy as a driving force for heat transmission is presented. Pressure drops may be calculated by the means of a classical friction factor. (authors) [French] On determine experimentalement les coefficients d'echange thermique et les pertes de charge de l'ammoniac gazeux en convection forcee. Le fluide circule avec un debit en masse compris entre 0.6 et 2.4 g/s (G = 10 a 40 g/cm{sup 2}.s) dans un tube long en tungstene (d{sub i} = 2.8 mm, d{sub e} = 5.1 mm, L = 700 mm), chauffe electriquement. La temperature de paroi atteint 3000 deg. K, celle du fluide 2500 deg. K et le flux de chaleur maximal est de 530 W/cm{sup 2}. L'ammoniac se dissocie completement, la puissance correspondant a la dissociation atteint 30 pour cent de la puissance totale echangee. La pression d'entree varie entre 6 et 16 bars et la chute de pression maximale dans le canal est de 15 bars. On distingue deux regimes de dissociation, catalytique et homogene, et on etudie la variation du taux de dissociation en fonction de la longueur du tube. Les coefficients d'echange thermique mesures
Sivakumar, A.; Alagumurthi, N.; Senthilvelan, T.
2016-07-01
The microchannels are device used to remove high heat fluxes from smaller area. In this experimental research work the heat transfer performance of nanofluids of Al2O3/water and CuO/water were compared. The important character of such fluids is the enhanced thermal conductivity, in comparison with base fluid without considerable alteration in physical and chemical properties. The effect of forced convective heat transfer coefficient was calculated using serpentine shaped microchannel heat exchanger. Furthermore we calculated the forced convective heat transfer coefficient of the nanofluids using theoretical correlations in order to compare the results with the experimental data. The heat transfer coefficient for different particle concentration and temperature were analysed using forced convection heat transfer using nanofluids. The findings indicate considerable enhancement in convective heat transfer coefficient of the nanofluids as compared to the basefluid. The results also shows that CuO/water nanofluid has increased heat transfer coefficient compared with Al2O3/water and base fluids. Moreover the experimental results indicate there is increased forced convective heat transfer coefficient with the increase in nano particle concentration.
Rosner, D. E.; Gokoglu, S. A.; Israel, R.
1982-01-01
A multiparameter correlation approach to the study of particle deposition rates in engineering applications is discussed with reference to two specific examples, one dealing with thermophoretically augmented small particle convective diffusion and the other involving larger particle inertial impaction. The validity of the correlations proposed here is demonstrated through rigorous computations including all relevant phenomena and interactions. Such representations are shown to minimize apparent differences between various geometric, flow, and physicochemical parameters, allowing many apparently different physicochemical situations to be described in a unified way.
de Jong, M.F.; de Steur, L.
2016-01-01
Deep convection is presumed to be vital for the North Atlantic Meridional Overturning Circulation,even though observational evidence for the link remains inconclusive. Modeling studies have suggested thatconvection will weaken as a result of enhanced freshwater input. The emergence of anomalously lo
Measurement of the convective heat-transfer coefficient
Conti, Rosaria; Fiordilino, Emilio
2014-01-01
We propose an experiment for investigating how objects cool down toward the thermal equilibrium with its surrounding through convection. We describe the time dependence of the temperature difference of the cooling object and the environment with an exponential decay function. By measuring the thermal constant tau, we determine the convective heat-transfer coefficient, which is a characteristic constant of the convection system.
Cooling Augmentation with Microchanneled Structures
Institute of Scientific and Technical Information of China (English)
X.F.Peng; B.X.Wang
1993-01-01
Experiments were conducted to investigate the heat transfer characteristics and cooling performance of subcooled liquid,water,flowing through rectangular cross-section microchanneled structures machined on a stainless steel plate.Heat transfer or flow mode stransition was observed when the heating rate or wall temperature was increased.This transition was found to be suggestively induced by the variation in liquid thermophysical properties due to the significant rise of liquid temperature in the microstructures.The influence of such parameters as liquid velocity,subcooling,property variation,and microchannel geometric configuration on the heat transfer behavior,cooling performance and the heat transfer and liquid flow mode transition were also investigated.The experiments indicated that both single-phase forced convection and flow boiling characteristics were quite different from those in normal-sized tubes and the heat transfer was obviously intensified.
Mixed convection around calandria tubes in a ¼ scale CANDU-6 moderator circulation tank
Energy Technology Data Exchange (ETDEWEB)
Atkins, M.D.; Rossouw, D.J.; Boer, M. [Nuclear Science Division, School of Mechanical and Aeronautical Engineering, University of the Witwatersrand, Johannesburg (South Africa); Kim, T., E-mail: tong.kim@wits.ac.za [Nuclear Science Division, School of Mechanical and Aeronautical Engineering, University of the Witwatersrand, Johannesburg (South Africa); Rhee, B.W.; Kim, H.T. [Severe Accident and PHWR Safety Research Division, Korea Atomic Energy Research Institute, Daejeon (Korea, Republic of)
2017-05-15
Highlights: • A secondary jet is formed at a stagnation region and is directed towards the center of the MCT. • The secondary jet undergoes the significant dissipation and mixing due to calandria tubes (CTs). • Its cooling effectiveness is reduced on the CTs in the bottom of the MCT. • With forced convection dominance, peak heat transfer is on the upper CT surface. • With natural convection dominance, peak heat transfer is on the lower CT surface. - Abstract: This study experimentally characterizes mixed convection around calandria tubes (CTs) in a ¼ scale CANDU-6 moderator circulation tank (MCT) that uses air as the working fluid. In a full scale CANDU-6 reactor that undergoes a postulated dual failure with a loss-of-coolant accident without the emergency core cooling system available, mixed convection heat transfer occurs around the CTs. The cooling effectiveness of the moderator is diminished as an emergency heat sink if overheating eventually leads to film boiling. To prevent the onset of film boiling, local sub-cooling margins of the moderator needs to be maintained or else the critical heat flux should be increased. Circulating the moderator which interacts with the overheated CTs increases the heat transfer into the moderator which may suppress film boiling. The present experimental results demonstrate that the cooling effectiveness of the circulating moderator, in particular the secondary jet, is attenuated substantially as it is convected away from the inner wall towards the center of the MCT. The momentum of the secondary jet is diffused through the CTs. At a low jet Reynolds number, the secondary jet becomes ineffective so that some overheated CTs positioned in the other half of the MCT are cooled only by natural convection.
Wolf, Bart J.; Johnson, D. R.
1995-01-01
A kinetic energy (KE) analysis of the forcing of a mesoscale upper-tropospheric jet streak by organized diabatic processes within the simulated convective system (SCS) that was discussed in Part 1 is presented in this study. The relative contributions of the ageostrophic components of motion to the generation of KE of the convectively generated jet streak are compared, along with the KE generation by the rotational (nondivergent) and irrotational (divergent) mass transport. The sensitivity of the numerical simulations of SCS development to resolution is also briefly examined. Analysis within isentropic coordinates provides for an explicit determination of the influence of the diabatic processes on the generation of KE. The upper-level production of specific KE is due predominatly to the inertial advective ageostrophic component (IAD), and as such represents the primary process through which the KE of the convectively generated jet streak is realized. A secondary contribution by the inertial diabatic (IDI) term is observed. Partitioning the KE generation into its rotational and irrotational components reveals that the latter, which is directly linked to the diabatic heating within the SCS through isentropic continuity requirements, is the ultimate source of KE generation as the global area integral of generation by the rotational component vanishes. Comparison with an identical dry simulation reveals that the net generation of KE must be attributed to latent heating. Both the IAD and IDI ageostrophic components play important roles in this regard. Examination of results from simulations conducted at several resolutions supports the previous findings in that the effects of diabatic processes and ageostrophic motion on KE generation remain consistent. Resolution does impact the location and timing of SCS development, a result that has important implications in forecasting the onset of convection that develops from evolution of the large-scale flow and moisture
Umer, Asim; Naveed, Shahid; Ramzan, Naveed
2016-10-01
Nanofluids, having 1-100 nm size particles in any base fluid are promising fluid for heat transfer intensification due to their enhanced thermal conductivity as compared with the base fluid. The forced convection of nanofluids is the major practical application in heat transfer equipments. In this study, heat transfer enhancements at constant wall heat flux under laminar flow conditions were investigated. Nanofluids of different volume fractions (1, 2 and 4 %) of copper (I) oxide nanoparticles in deionized water were prepared using two step technique under mechanical mixing and ultrasonication. The results were investigated by increasing the Reynolds number of the nanofluids at constant heat flux. The trends of Nusselt number variation with dimensionless length (X/D) and Reynolds numbers were studied. It was observed that heat transfer coefficient increases with increases particles volume concentration and Reynolds number. The maximum enhancement in heat transfer coefficient of 61 % was observed with 4 % particle volume concentration at Reynolds number (Re ~ 605).
2016-08-03
the structural inertial force and ∗ can be thought of as the ratio between the time scale related to the reed’s natural oscillation in vacuum and...tip. Such contact clearly affects the evolution of the flow along the surface and may produce some suction force that leads to progressive...Heat Transfer Enhancement in High-Power Heat Sinks using Active Reed Technology,” THERMINIC, Barcelona, Spain, October, 2010. 29. Huang, W., Shin
Actively convected liquid metal divertor
Shimada, Michiya; Hirooka, Yoshi
2014-12-01
The use of actively convected liquid metals with j × B force is proposed to facilitate heat handling by the divertor, a challenging issue associated with magnetic fusion experiments such as ITER. This issue will be aggravated even more for DEMO and power reactors because the divertor heat load will be significantly higher and yet the use of copper would not be allowed as the heat sink material. Instead, reduced activation ferritic/martensitic steel alloys with heat conductivities substantially lower than that of copper, will be used as the structural materials. The present proposal is to fill the lower part of the vacuum vessel with liquid metals with relatively low melting points and low chemical activities including Ga and Sn. The divertor modules, equipped with electrodes and cooling tubes, are immersed in the liquid metal. The electrode, placed in the middle of the liquid metal, can be biased positively or negatively with respect to the module. The j × B force due to the current between the electrode and the module provides a rotating motion for the liquid metal around the electrodes. The rise in liquid temperature at the separatrix hit point can be maintained at acceptable levels from the operation point of view. As the rotation speed increases, the current in the liquid metal is expected to decrease due to the v × B electromotive force. This rotating motion in the poloidal plane will reduce the divertor heat load significantly. Another important benefit of the convected liquid metal divertor is the fast recovery from unmitigated disruptions. Also, the liquid metal divertor concept eliminates the erosion problem.
Observing Convective Aggregation
Holloway, Christopher E.; Wing, Allison A.; Bony, Sandrine; Muller, Caroline; Masunaga, Hirohiko; L'Ecuyer, Tristan S.; Turner, David D.; Zuidema, Paquita
2017-06-01
Convective self-aggregation, the spontaneous organization of initially scattered convection into isolated convective clusters despite spatially homogeneous boundary conditions and forcing, was first recognized and studied in idealized numerical simulations. While there is a rich history of observational work on convective clustering and organization, there have been only a few studies that have analyzed observations to look specifically for processes related to self-aggregation in models. Here we review observational work in both of these categories and motivate the need for more of this work. We acknowledge that self-aggregation may appear to be far-removed from observed convective organization in terms of time scales, initial conditions, initiation processes, and mean state extremes, but we argue that these differences vary greatly across the diverse range of model simulations in the literature and that these comparisons are already offering important insights into real tropical phenomena. Some preliminary new findings are presented, including results showing that a self-aggregation simulation with square geometry has too broad distribution of humidity and is too dry in the driest regions when compared with radiosonde records from Nauru, while an elongated channel simulation has realistic representations of atmospheric humidity and its variability. We discuss recent work increasing our understanding of how organized convection and climate change may interact, and how model discrepancies related to this question are prompting interest in observational comparisons. We also propose possible future directions for observational work related to convective aggregation, including novel satellite approaches and a ground-based observational network.
Turbulent mixed convection in asymmetrically heated vertical channel
Directory of Open Access Journals (Sweden)
Mokni Ameni
2012-01-01
Full Text Available In this paper an investigation of mixed convection from vertical heated channel is undertaken. The aim is to explore the heat transfer obtained by adding a forced flow, issued from a flat nozzle located in the entry section of a channel, to the up-going fluid along its walls. Forced and free convection are combined studied in order to increase the cooling requirements. The study deals with both symmetrically and asymmetrically heated channel. The Reynolds number based on the nozzle width and the jet velocity is assumed to be 3 103 and 2.104; whereas, the Rayleigh number based on the channel length and the wall temperature difference varies from 2.57 1010 to 5.15 1012. The heating asymmetry effect on the flow development including the mean velocity and temperature the local Nusselt number, the mass flow rate and heat transfer are examined.
Energy Technology Data Exchange (ETDEWEB)
Matzen, Gehard W. [Univ. of California, Berkeley, CA (United States)
1997-01-01
Three-dimensional creeping flow around single, axisymmetric protrusions is studied numerically using the boundary-integral technique. Emphasis is placed upon cylindrical protrusions on plane walls for various height-to-radius (h-to-a) aspect ratios, but cones and sections of spheres protruding from plane walls are also briefly examined. The presented items include shear-stress distributions, shear-stress contours, extents of the fluid-flow disturbance, total forces and torques on the cylinders, streamlines, and skin-friction lines. Also included is a discussion of flow topology around axisymmetric geometries. No flow reversal is observed for cylindrical protrusions with aspect ratios greater than 2.4 to 2.6. At higher aspect ratios, the fluid tends to be swept around cylindrical protrusions with little vertical motion. At lower aspect ratios, the strength of the recirculation increases, and the recirculation region becomes wider in the transverse direction and narrower in the flow direction. Also, the recirculation pattern begins to resemble the closed streamline patterns in two-dimensional flow over square ridges. However, unlike two-dimensional flow, closed streamline patterns are not observed. For arbitrary axisymmetric geometries, the extent of the fluid-flow disturbance can be estimated with the total force that is exerted on the protrusion. When the same force is exerted on protrusions with different aspect ratios, the protrusion with the higher aspect ratio tends to have a greater disturbance in the flow direction and a smaller disturbance in the transverse direction. The total force exerted on cylindrical protrusions with rounded corners is only slightly lower than the total force exerted on cylindrical protrusions with sharp corners.
Mhaisekar, Ashutosh; Kazmierczak, Michael J; Banerjee, Rupak
2005-05-01
The differential momentum and thermal energy equations for fluid flow and convective heat-transfer around the sample biocrystal, with coupled internal heat conduction, are solved using advanced computational fluid dynamics techniques. Average \\bar{h} as well as local h(theta) values of the convective heat-transfer coefficients are obtained from the fundamental equations. The results of these numerical solutions show the three-dimensional fluid flow field around the sample in conjunction with the detailed internal temperature distribution inside the crystal. The external temperature rise and maximum internal temperature increase are reported for various cases. The effect of the important system parameters, such as gas velocity and properties, crystal size and thermal conductivity and incident beam conditions (intensity and beam size), are all illustrated with comparative examples. For the reference case, an external temperature rise of 7 K and internal temperature increase of 0.5 K are calculated for a 200 microm-diameter cryocooled spherical biocrystal subjected to a 13 keV X-ray beam of 4 x 10(14) photons s(-1) mm(-2) flux density striking half the sample. For all the cases investigated, numerical analysis shows that the controlling thermal resistance is the rate of convective heat-transfer and not internal conduction. Thermal diffusion results in efficient thermal spreading of the deposited energy and this results in almost uniform internal crystal temperatures (DeltaT(internal) approximately 0.5 K), in spite of the non-uniform h(theta) with no more than 1.3 K internal temperature difference for the worst case of localized and focused beam heating. Rather, the major temperature variation occurs between the outer surface of the crystal/loop system and the gas stream, T(s) - T(gas), which itself is only about DeltaT(external) approximately 5-10 K, and depends on the thermal loading imposed by the X-ray beam, the rate of convection and the size of the loop
Energy Technology Data Exchange (ETDEWEB)
Sridharan, Kumar; Anderson, Mark; Allen, Todd; Corradini, Michael
2012-01-30
on Cr-carbide on the graphite surface. Ni-electroplating dramatically reduced corrosion of alloys, although some diffusion of Fe and Cr were observed occur through the Ni plating. A pyrolytic carbon and SiC (PyC/SiC) CVD coating was also investigated and found to be effective in mitigating corrosion. The KCl-MgCl2 molten salt was less corrosive than FLiNaK fluoride salts for corrosion tests performed at 850oC. Cr dissolution in the molten chloride salt was still observed and consequently Ni-201 and Hastelloy N exhibited the least depth of attack. Grain-boundary engineering (GBE) of Incoloy 800H improved the corrosion resistance (as measured by weight loss and maximum depth of attack) by nearly 50% as compared to the as-received Incoloy 800H sample. Because Cr dissolution is an important mechanism of corrosion, molten salt electrochemistry experiments were initiated. These experiments were performed using anodic stripping voltammetry (ASV). Using this technique, the reduction potential of Cr was determined against a Pt quasi-reference electrode as well as against a Ni(II)-Ni reference electrode in molten FLiNaK at 650 oC. The integrated current increased linearly with Cr-content in the salt, providing for a direct assessment of the Cr concentration in a given salt of unknown Cr concentration. To study heat transfer mechanisms in these molten salts over the forced and mixed convection regimes, a forced convective loop was constructed to measure heat transfer coefficients, friction factors and corrosion rates in different diameter tubes in a vertical up flow configuration in the laminar flow regime. Equipment and instrumentation for the forced convective loop was designed, constructed, and tested. These include a high temperature centrifugal pump, mass flow meter, and differential pressure sensing capabilities to an uncertainty of < 2 Pa. The heat transfer coefficient for the KCl-MgCl2 salt was measured in two different diameter channels (0.083 and 0.370Ã). In the 0
Optimization of convective heat transfer coefficient in water jet cooled mirror%射流式水冷镜对流换热系数的优化
Institute of Scientific and Technical Information of China (English)
李斌; 焦路光; 刘亮; 李兰; 周琼; 袁圣付; 刘文广
2011-01-01
介绍了射流水冷技术并将其应用到高能激光器反射镜的冷却中,设计了射流式水冷镜.采用计算流体力学的方法,利用通用有限元软件ANSYS的流体分析模块FLOTRAN对单孔内射流进行了数值模拟,详细讨论了各孔参数对换热性能的影响.实现了孔参数的优化.结果表明:在水冷镜中采用射流技术可以获得很好的对流换热效果,换热系数可达100 KW/(m·K)以上,且具有结构简单、可控性强等特点.%The jet cooling technology was introduced and used in the cooling of high power laser mirror, then the water jet cooled mirror was designed.Computational fluid dynamics method was adopted to simulate the jet in a hole by the fluid analysis module FLOTRAN of the finite element analysis software ANSYS, then the influence on the capability of heat transfer of the parameters of the hole was discussed and optimization of the parameters was achieved.The results show that great capability of heat transfer can be achieved by using the jet cooling technology, with simple structure and good controllability, and the heat transfer coefficient can be higher than 100 kW/(m2 · K).
Convection in stellar envelopes a changing paradigm
Spruit, H C
1996-01-01
Progress in the theory of stellar convection over the past decade is reviewed. The similarities and differences between convection in stellar envelopes and laboratory convection at high Rayleigh numbers are discussed. Direct numerical simulation of the solar surface layers, with no other input than atomic physics, the equations of hydrodynamics and radiative transfer is now capable of reproducing the observed heat flux, convection velocities, granulation patterns and line profiles with remarkably accuracy. These results show that convection in stellar envelopes is an essentially non-local process, being driven by cooling at the surface. This differs distinctly from the traditional view of stellar convection in terms of local concepts such as cascades of eddies in a mean superadiabatic gradient. The consequences this has for our physical picture of processes in the convective envelope are illustrated with the problems of sunspot heat flux blocking, the eruption of magnetic flux from the base of the convection ...
Extended Subadiabatic Layer in Simulations of Overshooting Convection
Käpylä, Petri J.; Rheinhardt, Matthias; Brandenburg, Axel; Arlt, Rainer; Käpylä, Maarit J.; Lagg, Andreas; Olspert, Nigul; Warnecke, Jörn
2017-08-01
We present numerical simulations of hydrodynamic overshooting convection in local Cartesian domains. We find that a substantial fraction of the lower part of the convection zone (CZ) is stably stratified according to the Schwarzschild criterion while the enthalpy flux is outward directed. This occurs when the heat conduction profile at the bottom of the CZ is smoothly varying, based either on a Kramers-like opacity prescription as a function of temperature and density or a static profile of a similar shape. We show that the subadiabatic layer arises due to nonlocal energy transport by buoyantly driven downflows in the upper parts of the CZ. Analysis of the force balance of the upflows and downflows confirms that convection is driven by cooling at the surface. We find that the commonly used prescription for the convective enthalpy flux being proportional to the negative entropy gradient does not hold in the stably stratified layers where the flux is positive. We demonstrate the existence of a non-gradient contribution to the enthalpy flux, which is estimated to be important throughout the convective layer. A quantitative analysis of downflows indicates a transition from a tree-like structure where smaller downdrafts merge into larger ones in the upper parts to a structure in the deeper parts where a height-independent number of strong downdrafts persist. This change of flow topology occurs when a substantial subadiabatic layer is present in the lower part of the CZ.
Interaction of Mesoscale Convection and Frontogenesis
Institute of Scientific and Technical Information of China (English)
彭加毅; 方娟; 伍荣生
2004-01-01
On the basis of the MM5 simulation data of the severe storm that occurred over the southeastern part of Hubei province on 21 July 1998, the interaction of mesoscale convection and frontogenesis is dealt with using the thermodynamical equation and frontogenetical function. The results show that the outbreak of the severe storm is closely related to the local frontogenesis. In fact, the interaction between the shearing instability of the low-level jet (LLJ) and the topographic forcing generates an gravity-inertia wave as well as local frontogenesis (the first front), which consequently induce the onset of the severe storm. From then on, owing to the horizontal and vertical advection of the potential temperature, the new frontogenesis (the second front) is formed to the northeast side of the severe storm, which initiates the second rain belt.Meanwhile, a two-front structure emerges over the southeastern part of Hubei province. Accompanied with the further intensification of the convection, the rain droplets evaporation cooling strengthens the first front and weakens the second front, resulting in single front structure over the southeastern part of Hubei province in the period of the strong convection.
Convective heat transfer in engine coolers influenced by electromagnetic fields
Karcher, C.; Kühndel, J.
2017-08-01
In engine coolers of off-highway vehicles, convective heat transfer at the coolant side limits both efficiency and performance density of the apparatus. Here, due to restrictions in construction and design, backwater areas and stagnation regions cannot be avoided. Those unwanted changes in flow characteristics are mainly triggered by flow deflections and sudden cross-sectional expansions. In application, mixtures of water and glysantine are used as appropriate coolants. Such coolants typically show an electrical conductivity of a few S/m. Coolant flow and convective heat transfer can then be controlled using Lorentz forces. These body forces are generated within the conducting fluid by the interactions of an electrical current density and a localized magnetic field, both of which are externally superimposed. In future application, this could be achieved by inserting electrodes in the cooler wall and a corresponding arrangement of permanent magnets. In this paper we perform numerical simulations of such magnetohydrodynamic flow in three model geometries that frequently appear in engine cooling applications: Carnot-Borda diffusor, 90° bend, and 180° bend. The simulations are carried out using the software package ANSYS Fluent. The present study demonstrates that, depending on the electromagnetic interaction parameter and the specific geometric arrangement of electrodes and magnetic field, Lorentz forces are suitable to break up eddy waters and separation zones and thus significantly increase convective heat transfer in these areas. Furthermore, the results show that hydraulic pressure losses can be reduced due to the pumping action of the Lorentz forces.
Potential of enhancing a natural convection loop with a thermomagnetically pumped ferrofluid
Aursand, Eskil; Lervåg, Karl Yngve; Lund, Halvor
2016-01-01
The feasibility of using a thermomagnetically pumped ferrofluid to enhance the performance of a natural convection cooling loop is investigated. First, a simplified analytical estimate for the thermomagnetic pumping action is derived, and then design rules for optimal solenoid and ferrofluid are presented. The design rules are used to set up a medium-scale (1 m, 10-1000 W) case study, which is modeled using a previously published and validated model (Aursand et al. [1]). The results show that the thermomagnetic driving force is significant compared to the natural convection driving force, and may in some cases greatly surpass it. The results also indicate that cooling performance can be increased by factors up to 4 and 2 in the single-phase and two- phase regimes, respectively, even when taking into the account the added heat from the solenoid. The performance increases can alternatively be used to obtain a reduction in heat-sink size by up to 75 %.
Directory of Open Access Journals (Sweden)
M. E. Nicholls
2015-03-01
Full Text Available Recent cloud-resolving numerical modeling results suggest that radiative forcing causes accelerated rates of tropical cyclogenesis and early intensification. Furthermore, observational studies of tropical cyclones have found that oscillations of the cloud canopy areal extent often occur that are clearly related to the solar diurnal cycle. A theory is put forward to explain these findings. The primary mechanism that seems responsible can be considered a refinement of the mechanism proposed by Gray and Jacobson (1977 to explain diurnal variations of oceanic tropical deep cumulus convection. It is hypothesized that differential radiative cooling or heating between a relatively cloud-free environment and a developing tropical disturbance generates circulations that can have very significant influences on convective activity in the core of the system. It is further suggested that there are benefits to understanding this mechanism by viewing it in terms of the lateral propagation of thermally driven gravity wave circulations, also known as buoyancy bores. Numerical model experiments indicate that mean environmental radiative cooling outside the cloud system is playing an important role in causing a significant horizontal differential radiative forcing and accelerating the rate of tropical cyclogenesis. As an expansive stratiform cloud layer forms aloft within a developing system the mean low level radiative cooling is reduced while at mid levels small warming occurs. During the daytime there is not a very large differential radiative forcing between the environment and the cloud system, but at nighttime when there is strong radiative clear sky cooling of the environment it becomes significant. Thermally driven circulations develop, characterized by relatively weak subsidence in the environment but much stronger upward motion in the cloud system. This upward motion leads to a cooling tendency and increased relative humidity. The increased relative
DEFF Research Database (Denmark)
Grunert, Klaus G; Brock, Steen; Brunsø, Karen
2016-01-01
such a product requires an interdisciplinary effort where researchers with backgrounds in psychology, anthropology, media science, philosophy, sensory science and food science join forces. We present the COOL SNACKS project, where such a blend of competences was used first to obtain thorough insight into young...
Near-wall measurements of the bubble- and Lorentz-force-driven convection at gas-evolving electrodes
Baczyzmalski, Dominik; Weier, Tom; Kähler, Christian J.; Cierpka, Christian
2015-08-01
Chemical energy storage systems, e.g., in the form of hydrogen or methanol, have a great potential for the establishment of volatile renewable energy sources due to the large energy density. The efficiency of hydrogen production through water electrolysis is, however, limited by gas bubbles evolving at the electrode's surface and can be enhanced by an accelerated bubble detachment. In order to characterize the complex multi-phase flow near the electrode, simultaneous measurements of the fluid velocities and the size and trajectories of hydrogen bubbles were performed in a water electrolyzer. The liquid phase velocity was measured by PIV/PTV, while shadowgraphy was used to determine the bubble trajectories. Special measurement and evaluation techniques had to be applied as the measurement uncertainty is strongly affected by the high void fraction close to the wall. In particular, the application of an advanced PTV scheme allowed for more precise fluid velocity measurements closer to electrode. Based on these data, stability characteristics of the near-wall flow were evaluated and compared to that of a wall jet. PTV was used as well to investigate the effect of Lorentz forces on the near-wall fluid velocities. The results show a significantly increased wall parallel liquid phase velocity with increasing Lorentz forces. It is presumed that this enhances the detachment of hydrogen bubbles from the electrode surface and, consequently, decreases the fractional bubble coverage and improves the efficiency. In addition, the effect of large rising bubbles with path oscillations on the near-wall flow was investigated. These bubbles can have a strong impact on the mass transfer near the electrode and thus affect the performance of the process.
Energy Technology Data Exchange (ETDEWEB)
Zhang, Dongping
2009-10-26
Lateral forced cooling can significantly increase the temporary overload capacity of a cable system, but the design of such systems requires a time-dependent 3D analysis of the nonlinear thermal behavior as the cooling water along the cable is heated up, resulting in position-dependent and time-dependent heat uptake. For this, a new calculation method was developed on the basis of an available 3D FEM software. The new method enables 3D simulation of force-cooled cables in consideration of the potential partial dryout of soil and of thermal stabilizations. The new method was first applied to a 110 kV wind power transmission cable for different configurations and grid conditions. It was found that with lateral forced cooling, the 110 kV will have a temporal 50 percent overload capacity. Further, the thermal characteristics and limiting capacity of a force-cooled 380 kV cable system were investigated. According to the results so far, laterally cooled cable systems open up new operating options, with advantages in terms of availability, economic efficiency, and flexibility. (orig.) [German] Eine laterale Zwangskuehlung kann die temporaere Ueberlastbarkeit einer Kabelanlage signifikant erhoehen. Der Entwurf solcher zwangsgekuehlter Kabelanlagen erfordert jedoch eine zeitabhaengige, dreidimensionale Analyse des nichtlinearen thermischen Verhaltens, da sich das Kuehlwasser entlang der Trasse erwaermt und sich so eine orts- und zeitabhaengige Waermeaufnahme ergibt. Zu diesem Zweck wurde auf der Basis eines vorhandenen zweidimensionalen FEM-Programms ein neues Berechnungsverfahren entwickelt, das die dreidimensionale Simulation zwangsgekuehlter Kabelanlagen unter Beruecksichtigung einer moeglicherweise auftretenden partiellen Bodenaustrocknung und von thermischen Stabilisierungen erlaubt. Mit Hilfe dieses Berechnungsverfahrens wurde zuerst eine 110-kV-Kabelanlage zur Windenergieuebertragung bei unterschiedlichen Anordnungen und unterschiedlichen Netzsituationen untersucht
Energy Technology Data Exchange (ETDEWEB)
Karimipour, Arash; Taghipour, Abdolmajid [Department of Mechanical Engineering, Najafabad Branch, Islamic Azad University, Najafabad (Iran, Islamic Republic of); Malvandi, Amir, E-mail: amirmalvandi@aut.ac.ir [Department of Mechanical Engineering, Neyshabur Branch, Islamic Azad University, Neyshabur (Iran, Islamic Republic of)
2016-12-01
This paper aims to investigate magnetic field and slip effects on developing laminar forced convection of nanofluids in the microchannels. A novel mixture of water and FMWNT carbon nanotubes is used as the working fluid. To do this, fluid flow and heat transfer through a microchannel is simulated by a computer code in FORTRAN language. The mixture of FMWNT carbon nanotubes suspended in water is considered as the nanofluid. Slip velocity is supposed as the hydrodynamic boundary condition while the microchannel's lower wall is insulated and the top wall is under the effect of a constant heat flux. Moreover, the flow field is subjected to a magnetic field with a constant strength. The results are presented as the velocity, temperature and Nusselt number profiles. It is observed that nanofluid composed of water and carbon nanotubes (FMWNT) can work well to increase the heat transfer rate along the microchannel walls. Furthermore, it is indicated that imposing the magnetic field is very effective at the thermally developing region. In contrast, the magnetic field effect at fully developed region is insignificant, especially at low values of Reynolds number. - Highlights: • Simulation of water/FMWNT carbon nanotubes flow in a microchannel. • The effects of magnetic field strength on nanofluid's slip velocity. • The effects of Ha, Re, ϕ and slip coefficient on averaged Nusselt number. • Magnetic field effect at developing flow region is significant.
Julia, J. E.; Hernández, L.; Martínez-Cuenca, R.; Hibiki, T.; Mondragón, R.; Segarra, C.; Jarque, J. C.
2012-11-01
Forced convective heat transfer coefficient and pressure drop of SiO2- and Al2O3-water nanofluids were characterized. The experimental facility was composed of thermal-hydraulic loop with a tank with an immersed heater, a centrifugal pump, a bypass with a globe valve, an electromagnetic flow-meter, a 18 kW in-line pre-heater, a test section with band heaters, a differential pressure transducer and a heat exchanger. The test section consists of a 1000 mm long aluminium pipe with an inner diameter of 31.2 mm. Eighteen band heaters were placed all along the test section in order to provide a uniform heat flux. Heat transfer coefficient was calculated measuring fluid temperature using immersed thermocouples (Pt100) placed at both ends of the test section and surface thermocouples in 10 axial locations along the test section (Pt1000). The measurements have been performed for different nanoparticles (Al2O3 and SiO2 with primary size of 11 nm and 12 nm, respectively), volume concentrations (1% v., 5% v.), and flow rates (3 103Re<105). Maximum heat transfer coefficient enhancement (300%) and pressure drop penalty (1000%) is obtained with 5% v. SiO2 nanofluid. Existing correlations can predict, at least in a first approximation, the heat transfer coefficient and pressure drop of nanofluids if thermal conductivity, viscosity and specific heat were properly modelled.
Simard, C; Dube, C
2016-01-01
We perform a mean-field analysis of the EULAG-MHD millenium simulation of global magnetohydrodynamical convection presented in Passos et al. 2014. The turbulent electromotive force operating in the simulation is assumed to be linearly related to the cyclic axisymmetric mean magnetic field and its first spatial derivatives. At every grid point in the simulation's meridional plane, this assumed relationship involves 27 independent tensorial coefficients. Expanding on Racine et al. 2011, we extract these coefficients from the simulation data through a least-squares minimization procedure based on singular value decomposition. The reconstructed alpha-tensor shows good agreement with that obtained by Racine et al. 2011, who did not include derivatives of the mean-field in their fit, as well as with the alpha-tensor extracted by Augustson et al. 2015 from a distinct ASH MHD simulation. The isotropic part of the turbulent magnetic diffusivity tensor beta is positive definite and reaches values of 5.0x10^7 m2s-1 in t...
Energy Technology Data Exchange (ETDEWEB)
Park, Hi Yong; Choi, Moon Suk [Hanyang Univ., Seoul (Korea, Republic of); Park, Kyoung Woo [LGIS Co., Ltd., Incheon (Korea, Republic of)
1998-04-01
The flow and heat transfer characteristics of combined forced convection and radiation in the entrance region of internally finned tubes are investigated numerically in this paper. The uniform flow is considered for an inlet flow condition. A three dimensional parabolic problem is solved by a marching-type procedure envolving a series of two dimensional elliptic problems in the cross-stream plane. The SIMPLER-algorithm and Raithby's pressure-velocity coupling method are employed to analyze the flow and heat transfer characteristics. For the calculation of radiative heat transfer, the P{sub 1}-approximation and the Weighted Sum of Gray Gases Method (WSGGM) are used. The effects of fin height, number of fins, optical thickness, reference temperature, and Planck number on the flow and heat transfer characteristics are examined. It was found that the effect of fin-height on the heat transfer characteristic is more dominant than that of number of fins. The present results show that the optimal non-dimensional fin height and number of fins are 0.4 and 16, respectively.
Karimipour, Arash; Taghipour, Abdolmajid; Malvandi, Amir
2016-12-01
This paper aims to investigate magnetic field and slip effects on developing laminar forced convection of nanofluids in the microchannels. A novel mixture of water and FMWNT carbon nanotubes is used as the working fluid. To do this, fluid flow and heat transfer through a microchannel is simulated by a computer code in FORTRAN language. The mixture of FMWNT carbon nanotubes suspended in water is considered as the nanofluid. Slip velocity is supposed as the hydrodynamic boundary condition while the microchannel's lower wall is insulated and the top wall is under the effect of a constant heat flux. Moreover, the flow field is subjected to a magnetic field with a constant strength. The results are presented as the velocity, temperature and Nusselt number profiles. It is observed that nanofluid composed of water and carbon nanotubes (FMWNT) can work well to increase the heat transfer rate along the microchannel walls. Furthermore, it is indicated that imposing the magnetic field is very effective at the thermally developing region. In contrast, the magnetic field effect at fully developed region is insignificant, especially at low values of Reynolds number.
Atmospheric forcing of cool subsurface water events in Bahía Culebra, Gulf of Papagayo, Costa Rica
Directory of Open Access Journals (Sweden)
Eric J. Alfaro
2012-04-01
Full Text Available Bahía Culebra, at Gulf of Papagayo on the north Pacific coast of Costa Rica, is an area of seasonal upwelling where more intense cooling events may occur during some boreal winter weeks mainly. To study these extreme cool events, records of nine sea subsurface temperature stations from 1998 to 2010 were analyzed. Five events associated with extremely cool temperatures in this region were identified from these records and taken as study cases. Sea temperatures decreased about 8-9ºC during these events and occurred while cold fronts were present in the Caribbean, with strong trade wind conditions over Central America. These strong wind conditions may have favored the offshore displacement of the sea surface water. The axis of Bahía Culebra runs northeastsouthwest, a condition that favors and triggers cool water events, mainly because the displaced water is replaced by water from deeper levels.
Directory of Open Access Journals (Sweden)
Mahmoodi Mostafa
2014-01-01
Full Text Available The problem of free convection fluid flow and heat transfer in a square cavity with a flush mounted heat source on its bottom wall and two heat sinks on its vertical side walls has been investigated numerically. Via changing the location of the heat sinks, six different arrangements have been generated. The cavity was filled with Cu-water nanofluid. The governing equations were discretized using the finite volume method and SIMPLER algorithm. Using the developed code, a parametric study was undertaken, and effects of Rayleigh number, arrangements of the heat sinks and volume fraction of the nanoparticles on fluid flow and heat transfer inside the cavity were investigated. Also for the middle-middle heat sinks arrangement, capability of five different water based nanofluids on enhancement of the rate of heat transfer was examined and compared. From the obtained results it was found that the average Nusselt number, for all six different arrangements of the heat sinks, was an increasing function of the Rayleigh number and the volume fraction of the nanoparticles. Also it was found that at high Rayleigh numbers, maximum and minimum average Nusselt number occurred for middle-middle and top-bottom arrangement, respectively. Moreover it was found that for the middle-middle arrangement, at high Rayleigh numbers, maximum and minimum rate of heat transfer was obtained by Cu-water and TiO2-water nanofluids respectively.
Energy Technology Data Exchange (ETDEWEB)
Veloso, Maria Auxiliadora Fortini
2004-07-01
The STHIRP-1 computer program, which fundamentals are described in this work, uses the principles of the subchannels analysis and has the capacity to simulate, under steady state and transient conditions, the thermal and hydraulic phenomena which occur inside the core of a water-refrigerated research reactor under a natural convection regime. The models and empirical correlations necessary to describe the flow phenomena which can not be described by theoretical relations were selected according to the characteristics of the reactor operation. Although the primary objective is the calculation of research reactors, the formulation used to describe the fluid flow and the thermal conduction in the heater elements is sufficiently generalized to extend the use of the program for applications in power reactors and other thermal systems with the same features represented by the program formulations. To demonstrate the analytical capacity of STHIRP-l, there were made comparisons between the results calculated and measured in the research reactor TRIGA IPR-R1 of CDTN/CNEN. The comparisons indicate that the program reproduces the experimental data with good precision. Nevertheless, in the future there must be used more consistent experimental data to corroborate the validation of the program. (author)
高温散状料强制冷却问题研究%Study on Forced Cooling Problem of High Temperature Balk Cargo
Institute of Scientific and Technical Information of China (English)
陈文仲; 廖佰凤; 史学敏; 刘慧
2012-01-01
连续式石墨化炉生产的散状料温度可达2 600℃左右,由于高温下实验条件的限制,为研究高温散状料的强制冷却情况,运用Fluent软件对优化后的冷却器进行数值模拟,得到了高温物料冷却过程的温度分布.结果表明,在冷却器入口处温度较高,为保护冷却器,需添加耐高温材料;冷却器出口处物料温度已降到324 K,远低于指标要求,冷却效果显著;两级冷却器的出水温度均满足要求.模拟结果与测试结果比较吻合,该模拟方法适用.对优化后的冷却器效果进行了检验,并为进一步优化提供了参考依据.%The temperature of balk cargo graphite produced by continuous graphitizing furnace was approximately 2 600 ℃. Experiments were limited by the high temperature. In order to study the forced cooling situation of high temperature balk cargo, the numerical simulation was conducted on the optimized cooling unit by using the software Fluent. The temperature distributions of the cooling process for graphite were obtained. The results showed that the inlet temperature of the cooling unit is very high. To protect the cooling unit, heat-resisting materials are required. The outlet temperature falls to 324 K, which is much lower than the requirement. The cooling effect is remarkable. Both of the outlet temperatures of the water in two stage cooling units meet the requirements. There are in good agreement between the simulation results and the measurements. According to the simulation results, the cooling effect is tested and the further optimization for the optimized cooling unit is guided.
Energy Technology Data Exchange (ETDEWEB)
Hennig, W.; Parks, D.; Weinstein, R.; Sawh, R.-P.; Ren, Y. [Beam Particles Dynamics Laboratories and TCSUH, University of Houston, Houston, TX 77204-5506 (United States)
2000-10-01
Both high levitation pressures (up to 22 N cm{sup -2}) and high restoring pressures (up to 11 N cm{sup -2}) are obtained for a superconducting trapped field magnet of 1.5 T levitating above the centre of a ring of zero-field cooled high-temperature superconductors. The ratio of levitation force to restoring force can be varied between 2.9 and 0.3 by changing the quality of the superconductors. This significantly improves the stability of levitation compared to commonly used single sample configurations. (author)
Simard, Corinne; Charbonneau, Paul; Dubé, Caroline
2016-10-01
We perform a mean-field analysis of the EULAG-MHD millenium simulation of global magnetohydrodynamical convection presented in Passos and Charbonneau (2014). The turbulent electromotive force (emf) operating in the simulation is assumed to be linearly related to the cyclic axisymmetric mean magnetic field and its first spatial derivatives. At every grid point in the simulation's meridional plane, this assumed relationship involves 27 independent tensorial coefficients. Expanding on Racine et al. (2011), we extract these coefficients from the simulation data through a least-squares minimization procedure based on singular value decomposition. The reconstructed α -tensor shows good agreement with that obtained by Racine et al. (2011), who did not include derivatives of the mean-field in their fit, as well as with the α -tensor extracted by Augustson et al. (2015) from a distinct ASH MHD simulation. The isotropic part of the turbulent magnetic diffusivity tensor β is positive definite and reaches values of 5.0 ×107 m2 s-1 in the middle of the convecting fluid layers. The spatial variations of both αϕϕ and βϕϕ component are well reproduced by expressions obtained under the Second Order Correlation Approximation, with a good matching of amplitude requiring a turbulent correlation time about five times smaller than the estimated turnover time of the small-scale turbulent flow. By segmenting the simulation data into epochs of magnetic cycle minima and maxima, we also measure α - and β -quenching. We find the magnetic quenching of the α -effect to be driven primarily by a reduction of the small-scale flow's kinetic helicity, with variations of the current helicity playing a lesser role in most locations in the simulation domain. Our measurements of turbulent diffusivity quenching are restricted to the βϕϕ component, but indicate a weaker quenching, by a factor of ≃ 1.36, than of the α -effect, which in our simulation drops by a factor of three between
Directory of Open Access Journals (Sweden)
V. Echarri Iribarren
2016-12-01
Full Text Available Porcelain stoneware is a widely used building material. In recent years, its range of uses has expanded to encompass a new spectrum of innovative and inventive applications in architecture. In this research, we analysed the patented Thermal Ceramic Panel. This consists of a thin porcelain stoneware panel that incorporates a capillary system of polypropylene tubes measuring 3.5 mm in diameter embedded in a conductive ceramic interface. The system works with hot or cold water, producing healthy heating and cooling by means of radiant surfaces. Following an initial prototype test in which panels were placed on the walls of an office, we conducted simulations at the University of Alicante Museum using wall, ceiling and baffle panels, having previously monitored the state of the building. Thermal behaviour parameters were analysed and compared with those of other standard finishing materials, obtaining results for thermal comfort and energy savings in comparison with all-air systems.
Mechanisms for convection triggering by cold pools
Torri, Giuseppe; Tian, Yang
2015-01-01
Cold pools are fundamental ingredients of deep convection. They contribute to organizing the sub-cloud layer and are considered key elements in triggering convective cells. It was long known that this could happen mechanically, through lifting by the cold pools' fronts. More recently, it has been suggested that convection could also be triggered thermodynamically, by accumulation of moisture around the edges of cold pools. A method based on Lagrangian tracking is here proposed to disentangle the signatures of both forcings and quantify their importance in a given environment. Results from a simulation of radiative-convective equilibrium over the ocean show that parcels reach their level of free convection through a combination of both forcings, each being dominant at different stages of the ascent. Mechanical forcing is an important player in lifting parcels from the surface, whereas thermodynamic forcing reduces the inhibition encountered by parcels before they reach their level of free convection.
Energy Technology Data Exchange (ETDEWEB)
Tripathi, D.; Dey, T.K., E-mail: tapasdey@hijli.iitkgp.ernet.in
2014-12-15
Highlights: • Levitation force between PM and starch/PS/MWCNT added MgB{sub 2} are studied in FC mode. • MgB{sub 2} added with 1 wt.% PS gives best result. • Levitation forces do not display hysteresis during ascending and descending mode. • Exponential variation in Max. Levitation (F{sub MLF}) and attractive forces (F{sub MAF}). • The gap between PM and SC for F{sub MAF} and F{sub 0AF} varies linearly with FC height. - Abstract: A series of MgB{sub 2} pellets with and without addition of carbon from different sources (viz. starch, polystyrene and carbon nanotubes) have been synthesized by solid state reaction under argon atmosphere. XRD analysis indicates a decrease in lattice parameters of MgB{sub 2} with addition of starch, polystyrene (PS) and MWCNT and confirms substitution of carbon in boron sites. The presence of nanosized carbon inclusions between the grain boundaries in the present set of samples is evident in TEM photographs. Resistivity data confirms a decrease in superconducting transition temperature (T{sub c0}) for MgB{sub 2} doped with starch/PS/MWCNT. The effect of different field cooling heights (H{sub IFC}) at 20 K on maximum levitation force (F{sub MLF}) and maximum attractive force (F{sub MAF}) of pure MgB{sub 2} and MgB{sub 2} doped with starch/PS/MWCNT have been investigated. Except for MWCNT, doping of starch and PS in MgB{sub 2} is found to improve F{sub MLF} and F{sub MAF} and the best result is obtained for MgB{sub 2} doped with 1 wt.% PS. Levitation force measured as a function of decreasing initial field cooling height indicates exponential dependence of both maximum levitation force (F{sub MLF}) and maximum attractive force (F{sub MAF}). However, the gap distance between PM and the sample (H{sub 0AF} and H{sub MAF}) corresponding to maximum attractive force (F{sub MAF}) and zero attractive force (F{sub 0AF}) varies linearly and their difference remains constant. This constancy in (H{sub MAF} − H{sub 0AF}) is understood in
Thermal convection driven by acoustic field under microgravity
Tanabe, Mitsuaki; 田辺 光昭
2007-01-01
Natural convection is suppressed in space environment due to the weightlessness. Only centrifugal force is utilized currently to drive gas-phase thermal convection in space. This paper presents an alternative way to drive thermal convection. From the investigation of combustion oscillation in rocket motors, a new thermal convection had been found in stationary acoustic fields. Analyzing the phenomena, acoustic radiation force is found to be the candidate driving force. With a simplified syste...
NUMERICAL STUDY ON MIXED CONVECTIVE FLOW IN A SOLAR COLLECTOR
Institute of Scientific and Technical Information of China (English)
无
2002-01-01
In a solar energy heat collector forced convection and free convection will occur concurrently. In this paper, the mixed convective flow was investigated. The dimensionless equation was derived and the results was verified by experiments. The numerical solution shows that error is less than 5% if the effect of free convection is ignored.
Graybill, George
2007-01-01
Forces are at work all around us. Discover what a force is, and different kinds of forces that work on contact and at a distance. We use simple language and vocabulary to make this invisible world easy for students to ""see"" and understand. Examine how forces ""add up"" to create the total force on an object, and reinforce concepts and extend learning with sample problems.
Anomalously Weak Solar Convection
Hanasoge, Shravan M.; Duvall, Thomas L.; Sreenivasan, Katepalli R.
2012-01-01
Convection in the solar interior is thought to comprise structures on a spectrum of scales. This conclusion emerges from phenomenological studies and numerical simulations, though neither covers the proper range of dynamical parameters of solar convection. Here, we analyze observations of the wavefield in the solar photosphere using techniques of time-distance helioseismology to image flows in the solar interior. We downsample and synthesize 900 billion wavefield observations to produce 3 billion cross-correlations, which we average and fit, measuring 5 million wave travel times. Using these travel times, we deduce the underlying flow systems and study their statistics to bound convective velocity magnitudes in the solar interior, as a function of depth and spherical- harmonic degree l..Within the wavenumber band l convective velocities are 20-100 times weaker than current theoretical estimates. This constraint suggests the prevalence of a different paradigm of turbulence from that predicted by existing models, prompting the question: what mechanism transports the heat flux of a solar luminosity outwards? Advection is dominated by Coriolis forces for wavenumbers l convection may be quasi-geostrophic. The fact that isorotation contours in the Sun are not coaligned with the axis of rotation suggests the presence of a latitudinal entropy gradient.
Energy Technology Data Exchange (ETDEWEB)
Sridharan, Kumar; Anderson, Mark; Allen, Todd; Corradini, Michael
2012-01-30
on Cr-carbide on the graphite surface. Ni-electroplating dramatically reduced corrosion of alloys, although some diffusion of Fe and Cr were observed occur through the Ni plating. A pyrolytic carbon and SiC (PyC/SiC) CVD coating was also investigated and found to be effective in mitigating corrosion. The KCl-MgCl2 molten salt was less corrosive than FLiNaK fluoride salts for corrosion tests performed at 850oC. Cr dissolution in the molten chloride salt was still observed and consequently Ni-201 and Hastelloy N exhibited the least depth of attack. Grain-boundary engineering (GBE) of Incoloy 800H improved the corrosion resistance (as measured by weight loss and maximum depth of attack) by nearly 50% as compared to the as-received Incoloy 800H sample. Because Cr dissolution is an important mechanism of corrosion, molten salt electrochemistry experiments were initiated. These experiments were performed using anodic stripping voltammetry (ASV). Using this technique, the reduction potential of Cr was determined against a Pt quasi-reference electrode as well as against a Ni(II)-Ni reference electrode in molten FLiNaK at 650 oC. The integrated current increased linearly with Cr-content in the salt, providing for a direct assessment of the Cr concentration in a given salt of unknown Cr concentration. To study heat transfer mechanisms in these molten salts over the forced and mixed convection regimes, a forced convective loop was constructed to measure heat transfer coefficients, friction factors and corrosion rates in different diameter tubes in a vertical up flow configuration in the laminar flow regime. Equipment and instrumentation for the forced convective loop was designed, constructed, and tested. These include a high temperature centrifugal pump, mass flow meter, and differential pressure sensing capabilities to an uncertainty of < 2 Pa. The heat transfer coefficient for the KCl-MgCl2 salt was measured in two different diameter channels (0.083 and 0.370Ã). In the 0
Directory of Open Access Journals (Sweden)
Thamir K. Jassem
2013-05-01
Full Text Available An experimental forced laminar study was presented in this research for an air flowing through a circular channel for different angles ( ,30o,45o,60o, the channel was heated at constant heat flux , the channel also was packed with steel and glass spheres respectively . The tests were done for three values of Peclets number (2111.71,3945.42,4575.47 with changing the heat flux for each case and five times for each number.The results showed that the dimensionless temperature distribution will decrease with increasing the dimensionless channel length for all cases with changing Peclet number, heat flux and inclination angles, and its lowest value will be for glass spheres at highest flux, while at lower flux for , and the decreasing in dimensionless temperature was closed for both types of packed at other inclination angles.The study declared that the local Nusselt number decreases with increasing the dimensionless length of the channel for both packeds and for different applied heat flux, also through this study it was declared that the average Nusselt increases as Peclet number increases for both packed. Its value for the glass spheres is greater than the steel spheres with percentage (98.3% at small Peclet, and percentage (97.2% at large Peclet number for the horizontal tube, and (98.3% at small Peclet number and (97.8% at large Peclet number at .Through this study its was found that average Nusselt number increases along the channel as the heat flux increases, because the bulk temperature will increase as the flow proceeds toward the end of the channel , so the heat transfer coefficient will increase. It was declared from this study that in the case of the steel packed the heat transfer will occur mainly by conduction, while in the case of glass packed the heat transfer will occur mainly by laminar forced convection, where the lowest Nusselt number (Nu=3.8 was found when the pipe is horizontal and lowest heat flux and lowest Peclet number.
An Approximate Model of Microchannel Cooling
Institute of Scientific and Technical Information of China (English)
ShipingYu; MingdaoXin
1994-01-01
Forced convective heat transfer in micro-rectangular channels can be described by a group of two-dimensional differential equations.These equations take the conduction in microchannel wall along the direction of flow of coolants into account,which are more generalized than those which neglect the conduction.For the same reason,they are suitable particularly for gases-cooled microchannels.With only numerical solution to the equations till today,an approximate analytic solution is derived here,From this solution,a rather simple formula can be introduced further,by which the differences between considering the conduction and neglecting it are easily found.In addition,the reasonableness of the classical fin method is also discussed.An experimental example of air-cooled microchannels is illustrated.
a Cabinet Level Thermal Test Vehicle to Evaluate Hybrid Double-Sided Cooling Schemes
Nie, Qihong; Joshi, Yogendra
Packaging of power semiconductor devices presents some of the greatest thermal design challenges due to the resulting high heat fluxes. Advanced cooling techniques are desired to help meet these demands for current and future devices. A hybrid double-sided approach combining micro-channel liquid cooling, thermoelectric cooling, and forced air convection is investigated via a test vehicle for the thermal management of electronic cabinets. A reduction of 74% in the chip junction temperature rise was achieved by using double-sided cooling, compared to single-sided air convection. Further reduction can be achieved by utilizing thermoelectric cooling (TEC). Additional reductions of 22.4% and 6.5% were achieved by utilizing TEC in single-sided air cooling and double-sided cooling, respectively. The effect of water flow rates through the air-to-liquid heat exchanger and the microchannel heat sink on the chip junction temperature rise was insignificant, compared to the effect of TEC, and cooling configuration.
Rotating thermal convection at very large Rayleigh numbers
Weiss, Stephan; van Gils, Dennis; Ahlers, Guenter; Bodenschatz, Eberhard
2016-11-01
The large scale thermal convection systems in geo- and astrophysics are usually influenced by Coriolis forces caused by the rotation of their celestial bodies. To better understand the influence of rotation on the convective flow field and the heat transport at these conditions, we study Rayleigh-Bénard convection, using pressurized sulfur hexaflouride (SF6) at up to 19 bars in a cylinder of diameter D=1.12 m and a height of L=2.24 m. The gas is heated from below and cooled from above and the convection cell sits on a rotating table inside a large pressure vessel (the "Uboot of Göttingen"). With this setup Rayleigh numbers of up to Ra =1015 can be reached, while Ekman numbers as low as Ek =10-8 are possible. The Prandtl number in these experiment is kept constant at Pr = 0 . 8 . We report on heat flux measurements (expressed by the Nusselt number Nu) as well as measurements from more than 150 temperature probes inside the flow. We thank the Deutsche Forschungsgemeinschaft (DFG) for financial support through SFB963: "Astrophysical Flow Instabilities and Turbulence". The work of GA was supported in part by the US National Science Foundation through Grant DMR11-58514.
大功率IGBT散热器水冷热阻计算%Forced water cooled thermal-resistance calculation for high-power IGBT radiator
Institute of Scientific and Technical Information of China (English)
罗冰洋; 黄丽婷; 莫易敏; 袁慕
2013-01-01
为了优化水冷散热器散热能力,保障其可靠工作,引用了传热学中的基本原理与公式,以散热器外形的机械尺寸、水的强制对流换热系数和水的导热系数作为参数及变量推导了散热器水冷热阻的计算公式.同时为了满足实际应用,开发了一种专用水冷散热器热阻计算和曲线绘制软件,可以显示热阻随参数变化而变化的各种曲线,也可以直接计算显示热阻值.为散热器的设计中参数的优化选择提供直观方便的参考.%In order to enhance the heat-transfer capability of the water-cooled radiator and ensure its reliability, the basic principle and formula of the heat transfer theory was employed. Taking the radiator dimension, water conductivity and water con-vective heat-transfer coefficient as parameters, the calculation formula of the water cooled thermal resistance of radiator was de-rived. In order to the practical applications of the radiator, a software to draw the curve and calculate the thermal resistance was developed. It can not only display various kinds of curves that thermal resistance changes with the parameters, but also can di-rectly show the calculation result of thermal resistance. With these functions, the parameter optimizing design method which is of good directive significance can be widely used in engineering projects.
Convection in complex shaped vessel; Convection dans des enceintes de forme complexe
Energy Technology Data Exchange (ETDEWEB)
NONE
2000-07-01
The 8 november 2000, the SFT (Societe Francaise de Thermique) organized a technical day on the convection in complex shaped vessels. Nine papers have been presented in the domains of the heat transfers, the natural convection, the fluid distribution, the thermosyphon effect, the steam flow in a sterilization cycle and the transformers cooling. Eight papers are analyzed in ETDE and one paper dealing with the natural convection in spent fuels depository is analyzed in INIS. (A.L.B.)
Workshop 4 Converter cooling & recuperation
Iles, Peter; Hindman, Don
1995-01-01
Cooling the PV converter increases the overall TPV system efficiency, and more than offsets the losses incurred in providing cooling systems. Convective air flow methods may be sufficient, and several standard water cooling systems, including thermo-syphon radiators, capillary pumps or microchannel plates, are available. Recuperation is used to increase system efficiency, rather than to increase the emitter temperature. Recuperators operating at comparable high temperatures, such as in high temperature turbines have worked effectively.
Tropical convection and climate sensitivity
Williams, Ian Nobuo
Surface temperature has become a popular measure of climate change, but it does not provide the most critical test of climate models. This thesis presents new methods to evaluate climate models based on processes determining the climate sensitivity to radiative forcing from atmospheric greenhouse gases. Cloud radiative feedbacks depend on temperature and relative humidity profiles in addition to surface temperature, through the dependence of cloud type on boundary layer buoyancy. Buoyancy provides a reference to which the onset of deep convection is invariant, and gives a compact description of sea surface temperature changes and cloud feedbacks suitable for diagnostics and as a basis for simplified climate models. This thesis also addresses uncertainties in climate sensitivity involving terrestrial ecosystem responses to global warming. Different diagnostics support different conclusions about atmospheric transport model errors that could imply either stronger or weaker northern terrestrial carbon sinks. Equilibrium boundary layer concepts were previously used in idealized tropical climate models, and are extended here to develop a diagnostic of boundary layer trace gas transport and mixing. Hypotheses linking surface temperature to climate and precipitation sensitivity were tested in this thesis using comprehensive and idealized climate model simulations, and observational datasets. The results do not support the thermostat hypothesis that predicts deep cloud cover will increase with radiative forcing and limit sea surface temperatures to the maximum present-day warm pool temperature. Warm pool temperatures increased along with or even faster than the tropical average over the past several decades, while diagnosed deep cloud cover has not significantly increased, in agreement with global warming simulations. Precipitation sensitivity also depends on more than surface temperature alone, including thermodynamic profiles and air-sea temperature differences. The
Atmospheric forcing of cool subsurface water events in Bahía Culebra, Gulf of Papagayo, Costa Rica
Directory of Open Access Journals (Sweden)
Eric J. Alfaro
2012-04-01
Full Text Available Bahía Culebra, at Gulf of Papagayo on the north Pacific coast of Costa Rica, is an area of seasonal upwelling where more intense cooling events may occur during some boreal winter weeks mainly. To study these extreme cool events, records of nine sea subsurface temperature stations from 1998 to 2010 were analyzed. Five events associated with extremely cool temperatures in this region were identified from these records and taken as study cases. Sea temperatures decreased about 8-9ºC during these events and occurred while cold fronts were present in the Caribbean, with strong trade wind conditions over Central America. These strong wind conditions may have favored the offshore displacement of the sea surface water. The axis of Bahía Culebra runs northeastsouthwest, a condition that favors and triggers cool water events, mainly because the displaced water is replaced by water from deeper levels.Bahía Culebra, localizada en el Golfo de Papagayo, al norte de la costa del Pacífico de Costa Rica, es un área de afloramiento estacional, en donde pueden ocurrir eventos de enfriamiento más intensos, principalmente durante algunas semanas del invierno boreal. Para estudiar estos eventos de enfriamiento extremo, se analizaron datos de nueve estaciones con registros de la temperatura subsuperficial del mar, desde 1998 hasta el 2010. A partir de estos registros, se identificaron cinco casos de estudio asociados a enfriamientos del mar en la región. Los descensos de la temperatura de estos eventos fueron de aproximadamente 8-9ºC y se asociaron al paso de empujes fríos en la región del Caribe y una fuerte magnitud del viento alisio sobre América Central. Este reforzamiento del flujo alisio favorece el desplazamiento del agua superficial hacia afuera de Bahía Culebra, cuyo eje principal se ubica en la dirección noreste-suroeste. Lo anterior favorece y provoca un enfriamiento de la temperatura del mar en la bahía, ya que el agua desplazada es
Gokoglu, Suleyman A.; Rosner, Daniel E.
1986-01-01
A formulation previously developed to predict and correlate the thermophoretically-augmented submicron particle mass transfer rate to cold surfaces is found to account for the thermophoretically reduced particle mass transfer rate to overheated surfaces such that thermophoresis brings about a 10-decade reduction below the convective mass transfer rate expected by pure Brownian diffusion and convection alone. Thermophoretic blowing is shown to produce effects on particle concentration boundary-layer (BL) structure and wall mass transfer rates similar to those produced by real blowing through a porous wall. The applicability of the correlations to developing BL-situations is demonstrated by a numerical example relevant to wet-steam technology.
Directory of Open Access Journals (Sweden)
Bárbara TERUEL
2001-08-01
positions along the bed and conditioned in package with 40% of effective opening area. The fruits were pre-cooled in a forced-air system (1.933m³/h, at 1°C and 7°C, RH= 88,4 ± 2,0% and air velocity around 1m/s. A two-dimensional mathematical model was applied, assuming prolate spheroid coordinate, to characterize the transient heat transfer process inside the fruits considering convective profile conditions at the surface of the body. The generated equations were numerically solved by finite-volume method. The convective heat transfer coefficients were obtained using the least square method by comparison between experimental and numerical data. The comparative analysis of the theoretical and experimental curves presented a satisfactory agreement, with error between 5% and 7%. It was noted that the convective heat transfer coefficient changes with the fruits position in the bed and cooling time present a variation of approximately 38% among the different points. The spatial distributions of temperature inside the fruits, for three times, demonstrate the existence of a temperature differential between the center and the surface of 30%, approximately. The model predicts that the cooling rate is in the extremes of the banana. The Bi and Fo numbers obtained represent very well the heat transfer process in the fruits under different relationships of volume/area.
Energy Technology Data Exchange (ETDEWEB)
Brown, D. P.
1976-04-15
A report is given of the refrigeration system proposed for ISABELLE. The system features a single refrigerator of about 25 kW capacity. The refrigerant helium is supplied to the 960 ISABELLE Magnets at a pressure of 15 atm and a temperature of 2.8^{0}K. The return to the refrigerator is at 14.3 atm and a 6.2^{0}K. As many as 60 magnets will be cooled in series. The steady-state design temperature for the warmest magnet is 4.3^{0}K. This temperature will rise to 4.5^{0}K during the acceleration cycle.
A Thermal Design of Forced Air Cooling Solid - state Power Amplifier%某固态功放设备的强迫风冷散热设计
Institute of Scientific and Technical Information of China (English)
冷献春
2012-01-01
通过对某发射机用固态功放的热设计,阐述了在设备强迫风冷热设计中的设计思路,介绍了结合式鳍片散热器在设备热设计中的应用,以及独立式风道的结构形式,并通过热测试验证了设计的合理性.%This paper provides an idea of forced air cooling devices' design through the thermal design for a solid - state power amplifier. The application of separate air duct and bonded fin heat sinks will be brought forth. Thermal test validates the rationality of design.
Simulations and scaling of horizontal convection
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Mehmet ILIcak
2012-05-01
Full Text Available In this paper we describe the results of various numerical simulations of sideways or horizontal convection. Specifically, a two-dimensional Boussinesq fluid is both heated and cooled from its upper surface, but the walls and the bottom of the tank are insulating and have no flux of heat through them. We perform experiments with a range of Rayleigh numbers up to 1011, obtained by systematically reducing the diffusivity. We also explore the effects of a nonlinear equation of state and of a mechanical force imposed on the top surface at a fixed Rayleigh number. We find that, when there is no mechanical forcing, both the energy dissipation and the strength of the circulation itself monotonically fall with decreasing diffusivity. At Rayleigh numbers greater than 1010 the flow is unsteady; however, the eddying flow is still much weaker than the steady flow at smaller Rayleigh numbers. At high Rayleigh numbers, the stratification and the mean circulation are increasingly confined to a thin layer at the upper surface, with the layer thickness decreasing according to Ra−1/5. There is no evidence that the flow ever enters a regime that is independent of Rayleigh number. Using a nonlinear equation of state makes little difference to the flow phenomenology at a moderate Rayleigh number. The addition of an imposed stress at the upper surface makes a significant difference in the flow. A strong, energy-dissipating circulation can be maintained even at Ra = 109, and the stratification extends more deeply into the fluid than in the unstressed case. Overall, our results are consistent with the notion that in the absence of mechanical forcing a fluid that is heated and cooled from above cannot maintain a deep stratification or a strong sustained flow at high Rayleigh numbers, even if the interior flow is unsteady.
Jinks, Steven L; Dominguez, Carmen L; Antognini, Joseph F
2005-03-01
Individuals with spinal cord injury may undergo multiple surgical procedures; however, it is not clear how spinal cord injury affects anesthetic requirements and movement force under anesthesia during both acute and chronic stages of the injury. The authors determined the isoflurane minimum alveolar concentration (MAC) necessary to block movement in response to supramaximal noxious stimulation, as well as tail-flick and hind paw withdrawal latencies, before and up to 28 days after thoracic spinal transection. Tail-flick and hind paw withdrawal latencies were measured in the awake state to test for the presence of spinal shock or hyperreflexia. The authors measured limb forces elicited by noxious mechanical stimulation of a paw or the tail at 28 days after transection. Limb force experiments were also conducted in other animals that received a reversible spinal conduction block by cooling the spinal cord at the level of the eighth thoracic vertebra. A large decrease in MAC (to Awake tail-flick and hind paw withdrawal latencies were facilitated or unchanged, whereas reflex latencies under isoflurane were depressed or absent. However, at 80-90% of MAC, noxious stimulation of the hind paw elicited ipsilateral limb withdrawals in all animals. Hind limb forces were reduced (by >/= 90%) in both chronic and acute cold-block spinal animals. The immobilizing potency of isoflurane increases substantially after spinal transection, despite the absence of a baseline motor depression, or "spinal shock." Therefore, isoflurane MAC is determined by a spinal depressant action, possibly counteracted by a supraspinal facilitatory action. The partial recovery in MAC at later time points suggests that neuronal plasticity after spinal cord injury influences anesthetic requirements.
Tripathi, D.; Dey, T. K.
2016-04-01
The measurement of superconducting levitation force between permanent magnet and polycrystalline samples of pure and MgB2 added with starch, polystyrene (PS) and multiwall carbon nanotube (MWCNT) have been performed under zero field cooling (ZFC) condition at 20 K in both descending and ascending modes. For this, the bulk pellets were synthesized by conventional solid state sintering technique. The XRD data indicate well developed MgB2 phase. However, a decrease in lattice parameter 'a = b' have been observed for doped MgB2 samples. Superconducting transition temperature of MgB2 also decreases with starch/PS/MWCNT addition. Unlike MWCNT, the addition of starch/polystyrene is found to enhance the levitation force of MgB2 superconductor. The levitation force between PM and investigated pellets in ZFC condition is explained well in terms of the updated version of modified frozen image model and the magnetic moment originated due to vertical motion of the superconductors have been estimated. It may be noted that except for MWCNT, addition of starch/PS in MgB2 improves the magnetic moment generated by vertical movement of pure MgB2. However, this improvement is more pronounced for 1 wt.% of PS added MgB2, which indicates more flux trapping and hence better levitation properties in 1 wt.% of PS added MgB2. The vertical stiffness estimated for pure and starch/PS/MWCNT doped MgB2 samples indicate that the levitation force are more sensitive in the region close to the PM.
Schneider, Steven J.
2015-01-01
Heat transfer correlations of data on flat plates are used to explore the parameters in the Coolit program used for calculating the quantity of cooling air for controlling turbine blade temperature. Correlations for both convection and film cooling are explored for their relevance to predicting blade temperature as a function of a total cooling flow which is split between external film and internal convection flows. Similar trends to those in Coolit are predicted as a function of the percent of the total cooling flow that is in the film. The exceptions are that no film or 100 percent convection is predicted to not be able to control blade temperature, while leaving less than 25 percent of the cooling flow in the convection path results in nearing a limit on convection cooling as predicted by a thermal effectiveness parameter not presently used in Coolit.
Barrett, Joe H., III; Roeder, William P.
2010-01-01
Peak wind speed is important element in 24-Hour and Weekly Planning Forecasts issued by 45th Weather Squadron (45 WS). Forecasts issued for planning operations at KSC/CCAFS. 45 WS wind advisories issued for wind gusts greater than or equal to 25 kt. 35 kt and 50 kt from surface to 300 ft. AMU developed cool-season (Oct - Apr) tool to help 45 WS forecast: daily peak wind speed, 5-minute average speed at time of peak wind, and probability peak speed greater than or equal to 25 kt, 35 kt, 50 kt. AMU tool also forecasts daily average wind speed from 30 ft to 60 ft. Phase I and II tools delivered as a Microsoft Excel graphical user interface (GUI). Phase II tool also delivered as Meteorological Interactive Data Display System (MIDDS) GUI. Phase I and II forecast methods were compared to climatology, 45 WS wind advisories and North American Mesoscale model (MesoNAM) forecasts in a verification data set.
Effects of a New Cooling Technology on Physical Performance in U.S Air Force Military Personnel
2015-03-25
1.9; and % body fat 10 ± .03) completed this study. Subjects performed a 70-minute weighted treadmill walking test and 10-minute, 50-pound sandbag...kg; VO2max: 57.96 ± 1.96 mL/kg/min; body fat : 10 ± 0.03%). Each provided written informed consent in accordance with the Air Force Research...composition was assessed using a three-site skinfold measurement of the chest, abdomen , and thigh [20]. Aerobic fitness was assessed using a single
Directory of Open Access Journals (Sweden)
Soraia Vilela Borges
2008-12-01
Full Text Available A abóbora (Cucurbita moschata, L. é uma importante fonte de provitamina A, de baixo custo, e sob a forma desidratada oferece diferentes opções de utilização e consumo. Secagens por convecção natural e forçada foram comparadas quanto ao grau de secagem atingido e encolhimento, em função da temperatura, velocidade de ar e dimensões do produto. Os resultados obtidos mostraram que o uso de fatias com volume de 6,25 cm³ em secador por convecção forçada a 50 °C e à velocidade de 5,5 x 10-4 ms-1 resultaram em produtos de menor encolhimento, sendo recomendadas estas condições.Besides its low price, Pumpkin (Cucurbita moschata, L. is an important source of provitamin-A, and when dehydrated it offers different options of utilizations and consumption. Natural and forced convection drying were compared according to the drying degree shrinkage as a function of temperature, and air velocity and product dimensions. The obtained results showed that slices with the volume of 6.25 cm³ in forced convection oven at 50 °C and at the velocity of 5.5 x 10-4 ms-1 resulted in a lower shrinkage products, so these conditions were recommended.
Institute of Scientific and Technical Information of China (English)
宋文芳
2013-01-01
The cooling system of linear induction motor for metro vehicle is researched and developed. It describes the cooling method of forced air linear motor, cooling structure and work principle to ifll the domestic blank.%研发了一种牵引轨道交通车辆用直线电机的冷却系统，阐述强迫风冷直线电机的冷却方式、冷却结构和工作原理，填补了国内空白。
Olson, Sandra
2011-01-01
To better evaluate the buoyant contributions to the convective cooling (or heating) inherent in normal-gravity material flammability test methods, we derive a convective heat transfer correlation that can be used to account for the forced convective stretch effects on the net radiant heat flux for both ignition delay time and burning rate. The Equivalent Low Stretch Apparatus (ELSA) uses an inverted cone heater to minimize buoyant effects while at the same time providing a forced stagnation flow on the sample, which ignites and burns as a ceiling fire. Ignition delay and burning rate data is correlated with incident heat flux and convective heat transfer and compared to results from other test methods and fuel geometries using similarity to determine the equivalent stretch rates and thus convective cooling (or heating) rates for those geometries. With this correlation methodology, buoyant effects inherent in normal gravity material flammability test methods can be estimated, to better apply the test results to low stretch environments relevant to spacecraft material selection.
Microtextured Surfaces for Turbine Blade Impingement Cooling
Fryer, Jack
2014-01-01
Gas turbine engine technology is constantly challenged to operate at higher combustor outlet temperatures. In a modern gas turbine engine, these temperatures can exceed the blade and disk material limits by 600 F or more, necessitating both internal and film cooling schemes in addition to the use of thermal barrier coatings. Internal convective cooling is inadequate in many blade locations, and both internal and film cooling approaches can lead to significant performance penalties in the engine. Micro Cooling Concepts, Inc., has developed a turbine blade cooling concept that provides enhanced internal impingement cooling effectiveness via the use of microstructured impingement surfaces. These surfaces significantly increase the cooling capability of the impinging flow, as compared to a conventional untextured surface. This approach can be combined with microchannel cooling and external film cooling to tailor the cooling capability per the external heating profile. The cooling system then can be optimized to minimize impact on engine performance.
Topology Optimisation for Coupled Convection Problems
DEFF Research Database (Denmark)
Alexandersen, Joe
This thesis deals with topology optimisation for coupled convection problems. The aim is to extend and apply topology optimisation to steady-state conjugate heat transfer problems, where the heat conduction equation governs the heat transfer in a solid and is coupled to thermal transport...... in a surrounding uid, governed by a convection-diffusion equation, where the convective velocity field is found from solving the isothermal incompressible steady-state Navier-Stokes equations. Topology optimisation is also applied to steady-state natural convection problems. The modelling is done using stabilised...... finite elements, the formulation and implementation of which was done partly during a special course as prepatory work for this thesis. The formulation is extended with a Brinkman friction term in order to facilitate the topology optimisation of fluid flow and convective cooling problems. The derived...
Economizer Based Data Center Liquid Cooling with Advanced Metal Interfaces
Energy Technology Data Exchange (ETDEWEB)
Timothy Chainer
2012-11-30
A new chiller-less data center liquid cooling system utilizing the outside air environment has been shown to achieve up to 90% reduction in cooling energy compared to traditional chiller based data center cooling systems. The system removes heat from Volume servers inside a Sealed Rack and transports the heat using a liquid loop to an Outdoor Heat Exchanger which rejects the heat to the outdoor ambient environment. The servers in the rack are cooled using a hybrid cooling system by removing the majority of the heat generated by the processors and memory by direct thermal conduction using coldplates and the heat generated by the remaining components using forced air convection to an air- to- liquid heat exchanger inside the Sealed Rack. The anticipated benefits of such energy-centric configurations are significant energy savings at the data center level. When compared to a traditional 10 MW data center, which typically uses 25% of its total data center energy consumption for cooling this technology could potentially enable a cost savings of up to $800,000-$2,200,000/year (assuming electricity costs of 4 to 11 cents per kilowatt-hour) through the reduction in electrical energy usage.
Numerical study on electromagnetics and thermal cooling of a switched reluctance motor
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Jer-Huan Jang
2015-09-01
Full Text Available In this study, the coupled thermomagnetic system of a switched reluctance motor for cooling performance improvement has been conducted numerically. The switched reluctance motor (SRM with output power of 3 kW and air cooling is under investigation. Aluminum cooling fins are fitted on the cooling casing of the motor in order to increase the convection surfaces. The commercial softwares used in this paper are JMAG-Designer and CFD-RC for electromagnetic field and thermal field analyzes, respectively. The electro-magnetic analysis for the switched reluctance motor is obtained in advanced in order to obtain the energy losses. They can then be treated as the heat sources for heat transfer analysis. Transient solutions for the temperatures of the components in SRM are calculated during operation. The transient temperature of SRM is analyzed for both with and without cooling casing. The influence of forced convection by the internal fan of the SRM is also under investigation. Results show the numerical model is in good agreement to the real practice. The model can be used for the SRM cooling design.
Coaxial heat pipe for cooling of a laser’s reflector
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Gershuni A. N.
2014-06-01
Full Text Available The paper presents the development and research results for a coaxial heat pipe designed for cooling of a reflector of a solid-state laser. A coaxial cylindrical heat pipe, designed to cool the laser reflector, provides that the temperature of the heat-removing surface does not exceed 120°C at any orientation in the gravitational field, if the heat is removed by forced convection of air with the temperature of 60°C in a pulsed mode of heat flow supply of 300 W. Thermal resistance of the developed heat pipe is 0,03 K/W, the specific thermal resistance — 1,1•10–3 m2•K/W. The developed cooling system based on the evaporation-condensation principle, allows ensuring temperature uniformity of the cooling surface at low thermal resistance.
WRF nested large-eddy simulations of deep convection during SEAC4RS
Heath, Nicholas Kyle
downdrafts are forced by evaporative cooling due to mixing near cloud edge and by vertical perturbation pressure gradient forces acting to restore mass continuity around neighboring updrafts. We conclude that the WRF nested-LES approach provides an effective method for studying deep convection for our real-world case. The method can be used to provide insight into physical processes that are important to understanding observations. The WRF nested-LES approach could be adapted for other case studies in which high-resolution observations are available for validation.
National Convective Weather Diagnostic
National Oceanic and Atmospheric Administration, Department of Commerce — Current convective hazards identified by the National Convective Weather Detection algorithm. The National Convective Weather Diagnostic (NCWD) is an automatically...
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Atul Bhardwaj
2013-12-01
Full Text Available In this paper we report the design and development of an electro-optically Q-switched diode pumped Nd:YAG laser with intracavity optical parametric oscillator, generating ~ 5 ns laser pulses of ~8 mJ energy at eye safe wavelength of 1534 nm. A Z-shaped laser resonator has been designed with porro prism end reflector in Q-switch arm containing RTP Q-Switch and a suitably oriented waveplate. The gain arm consists of a Ø3 x 72 mm Nd: YAG laser rod, pumped from one side by 3 x 5 bar laser diode array stack emitting total optical peak power of 740 W at 804 nm at 38 °C. Thermoelectric coolers (TECs have been employed to maintain the optimum temperature of laser diode arrays and the combined heat load from the pump chamber and TECs is distributed over the system base plate with embedded heat pipes. Such cooling mechanism has eliminated the requirement of fins and fans in the laser system. Eye safe radiation is out-coupled through intra cavity KTA OPO (5 x 5 x 20 mm3 placed in the gain arm. Laser was operated at 20 Hz for several duty cycles of 10 min on and 10 min off and output energy remained stable within ±0.5 mJ without any forced air/liquid cooling.Defence Science Journal, 2013, 63(6, pp.599-605, DOI:http://dx.doi.org/10.14429/dsj.63.5759
Transient Mixed Convection Validation for NGNP
Energy Technology Data Exchange (ETDEWEB)
Smith, Barton [Utah State Univ., Logan, UT (United States); Schultz, Richard [Idaho National Lab. (INL), Idaho Falls, ID (United States)
2015-10-19
The results of this project are best described by the papers and dissertations that resulted from the work. They are included in their entirety in this document. They are: (1) Jeff Harris PhD dissertation (focused mainly on forced convection); (2) Blake Lance PhD dissertation (focused mainly on mixed and transient convection). This dissertation is in multi-paper format and includes the article currently submitted and one to be submitted shortly; and, (3) JFE paper on CFD Validation Benchmark for Forced Convection.
Feasibility assessment of vacuum cooling followed by immersion vacuum cooling on water-cooked pork.
Dong, Xiaoguang; Chen, Hui; Liu, Yi; Dai, Ruitong; Li, Xingmin
2012-01-01
Vacuum cooling followed by immersion vacuum cooling was designed to cool water-cooked pork (1.5±0.05 kg) compared with air blast cooling (4±0.5°C, 2 m/s), vacuum cooling (10 mbar) and immersion vacuum cooling. This combined cooling method was: vacuum cooling to an intermediate temperature of 25°C and then immersion vacuum cooling with water of 10°C to the final temperature of 10°C. It was found that the cooling loss of this combined cooling method was significantly lower (Pvacuum cooling. This combined cooling was faster (Pvacuum cooling in terms of cooling rate. Moreover, the pork cooled by combined cooling method had significant differences (P<0.05) in water content, color and shear force. Copyright © 2011 Elsevier Ltd. All rights reserved.
Marangoni Convection and Deviations from Maxwells' Evaporation Model
Segre, P. N.; Snell, E. H.; Adamek, D. H.
2003-01-01
We investigate the convective dynamics of evaporating pools of volatile liquids using an ultra-sensitive thermal imaging camera. During evaporation, there are significant convective flows inside the liquid due to Marangoni forces. We find that Marangoni convection during evaporation can dramatically affect the evaporation rates of volatile liquids. A simple heat balance model connects the convective velocities and temperature gradients to the evaporation rates.
Energy Technology Data Exchange (ETDEWEB)
Noghrehabadi, Aminreza; Pourrajab, Rashid [Shahid Chamran University of Ahvaz, Ahvaz (Iran, Islamic Republic of)
2016-02-15
The effect of nanofluids on heat transfer inside circular tubes under uniform constant heat flux boundary condition was investigated. The working nanofluid was a suspension of γ-Al{sub 2}O{sub 3} nanoparticles of average diameter 20 nm. The heat transfer coefficients were calculated experimentally in the range of 1057 < Re < 2070 with different particle volume concentrations of 0.1%, 0.3% and 0.9%. Increasing the particle volume fraction led to enhancement of the convective heat transfer coefficient. The results show that the average heat transfer coefficient increased 16.8% at 0.9% volume concentration and Reynolds number of 2070 compared with distilled water. In addition, the enhancement of the convective heat transfer was particularly significant in the entrance region and decreased with axial distance. Finally, an empirical correlation for Nusselt number has been proposed for the present range of nanofluids. The mean deviation between the predicted Nusselt number and experimental values for the new correlation is 3.57%.
Transition to geostrophic convection: the role of the boundary conditions
Kunnen, R.P.J.; Ostilla-Monico, Rodolfo; Poel, van der E.P.; Verzicco, R.; Lohse, D.
2016-01-01
Rotating Rayleigh–Bénard convection, the flow in a rotating fluid layer heated from below and cooled from above, is used to analyse the transition to the geostrophic regime of thermal convection. In the geostrophic regime, which is of direct relevance to most geo- and astrophysical flows, the system
Chaotic asymmetric convection in the Bridgman-Stockbarger technique
Potts, H.; Wilcox, W. R.
1986-01-01
Convection was observed in naphthalene in a vertical Bridgman-Stockbarger arrangement. Differing from the assumptions of the theorists, the flow was neither steady nor axi-symmetric because of the heating and cooling conditions employed. It is suggested that such irregular convection may be common and cause compositional striations and azimuthal composition variations.
Institute of Scientific and Technical Information of China (English)
朱建军; 王建立; 李震; 张兴
2011-01-01
测量了水平微细圆管内蒸馏水和不同质量浓度的水基多壁碳纳米管纳米流体在低雷诺数下的强制对流换热特性。实验结果表明,与蒸馏水相比,纳米流体的对流换热系数显著提高,且随质量浓度和管内雷诺数的增大而增大;并且研究了流体管内流动阻力特性,得到的泊肃叶数f·Re值随着雷诺数的变化不明显,但纳米流体的f·Re值要明显小于纯水。%Measurements of forced convective heat transfer characteristics for distilled water and aqueous suspensions of multi-walled carbon nanotubes（CNT nanofluids）with different mass concentration are performed in a horizontal stainless circular tube.The experimental results show that the convective heat transfer coefficient of CNT nanofluids is significantly enhanced compared with distilled water,and increases with increasing mass concentration and Reynolds number;the flow resistance characteristics are also investigated,and the obtained Poiseuille number f？Re changes slightly with Re,meanwhile,f·Re of nanofluids is significantly smaller than that predicted by Hagen-Poiseuille Law.
Thermo-electro-hydrodynamic convection under microgravity: a review
Mutabazi, Innocent; Yoshikawa, Harunori N.; Tadie Fogaing, Mireille; Travnikov, Vadim; Crumeyrolle, Olivier; Futterer, Birgit; Egbers, Christoph
2016-12-01
Recent studies on thermo-electro-hydrodynamic (TEHD) convection are reviewed with focus on investigations motivated by the analogy with natural convection. TEHD convection originates in the action of the dielectrophoretic force generated by an alternating electric voltage applied to a dielectric fluid with a temperature gradient. This electrohydrodynamic force is analogous to Archimedean thermal buoyancy and can be regarded as a thermal buoyancy force in electric effective gravity. The review is concerned with TEHD convection in plane, cylindrical, and spherical capacitors under microgravity conditions, where the electric gravity can induce convection without any complexities arising from geometry or the buoyancy force due to the Earth’s gravity. We will highlight the convection in spherical geometry, comparing developed theories and numerical simulations with the GEOFLOW experiments performed on board the International Space Station (ISS).
Directory of Open Access Journals (Sweden)
João C. T. R. da Silva
2011-01-01
Full Text Available This work is a study of the implementation of a classical controller using a tuning method referred to as IMC (Internal Model Control and aimed at the reduction of electrical energy consumption by the appropriate relation between energy consumption and the cooling time with forced air. The supervisory system installed was able to manipulate the variable of frequency of the signal power of the exhaust fan engine (forced air module, to accelerate or decelerate the loss of heat from the product to be cooled by airflow variation that passes through the mass of the produce. The results demonstrated a reduction in energy consumption from 64% and an increase of only 8% in the cooling time to the system using PI/IMC (Proportional - Integral with IMC tuning method compared with the system in its operating nominal condition. This PI/IMC control may be implemented directly in a frequency converter, without the need to purchase a computer or PLC (programmable logic controller to run the dedicated application, increasing its economical viability.Este trabalho consiste no estudo da implementação de um controlador clássico utilizando o método de sintonia denominado por Controle de Modelo Interno, visando à redução do consumo de energia elétrica que decorra na adequada relação entre este consumo e o tempo de resfriamento do processo de resfriamento com ar forçado de produtos hortícolas. Para isto, o sistema supervisório instalado manipulou a variável de frequência do sinal de alimentação do motor de indução trifásico do exaustor (módulo de ar forçado, para acelerar ou desacelerar a perda de calor do produto a ser resfriado por intermédio da variação da vazão de ar que perpassava a massa deste produto. Obteve-se como resultado uma redução no consumo de energia elétrica de 64% e um acréscimo de apenas 8% no tempo de resfriamento para o sistema utilizando um controle proporcional e integral associado ao método de sintonia promovido
Radiative cooling for thermophotovoltaic systems
Zhou, Zhiguang; Sun, Xingshu; Bermel, Peter
2016-09-01
Radiative cooling has recently garnered a great deal of attention for its potential as an alternative method for photovoltaic thermal management. Here, we will consider the limits of radiative cooling for thermal management of electronics broadly, as well as a specific application to thermal power generation. We show that radiative cooling power can increase rapidly with temperature, and is particularly beneficial in systems lacking standard convective cooling. This finding indicates that systems previously operating at elevated temperatures (e.g., 80°C) can be passively cooled close to ambient under appropriate conditions with a reasonable cooling area. To examine these general principles for a previously unexplored application, we consider the problem of thermophotovoltaic (TPV) conversion of heat to electricity via thermal radiation illuminating a photovoltaic diode. Since TPV systems generally operate in vacuum, convective cooling is sharply limited, but radiative cooling can be implemented with proper choice of materials and structures. In this work, realistic simulations of system performance are performed using the rigorous coupled wave analysis (RCWA) techniques to capture thermal emitter radiation, PV diode absorption, and radiative cooling. We subsequently optimize the structural geometry within realistic design constraints to find the best configurations to minimize operating temperature. It is found that low-iron soda-lime glass can potentially cool the PV diode by a substantial amount, even to below ambient temperatures. The cooling effect can be further improved by adding 2D-periodic photonic crystal structures. We find that the improvement of efficiency can be as much as an 18% relative increase, relative to the non-radiatively cooled baseline, as well as a potentially significant improvement in PV diode lifetime.
Directory of Open Access Journals (Sweden)
Winifred Nduku Mutuku-Njane
2013-01-01
Full Text Available We examine the effect of magnetic field on boundary layer flow of an incompressible electrically conducting water-based nanofluids past a convectively heated vertical porous plate with Navier slip boundary condition. A suitable similarity transformation is employed to reduce the governing partial differential equations into nonlinear ordinary differential equations, which are solved numerically by employing fourth-order Runge-Kutta with a shooting technique. Three different water-based nanofluids containing copper (Cu, aluminium oxide (Al2O3, and titanium dioxide (TiO2 are taken into consideration. Graphical results are presented and discussed quantitatively with respect to the influence of pertinent parameters, such as solid volume fraction of nanoparticles (φ, magnetic field parameter (Ha, buoyancy effect (Gr, Eckert number (Ec, suction/injection parameter (fw, Biot number (Bi, and slip parameter (β, on the dimensionless velocity, temperature, skin friction coefficient, and heat transfer rate.
Alternative Room Cooling System
Directory of Open Access Journals (Sweden)
Md. Fazle Rabbi
2015-06-01
Full Text Available The rapidly growing population results in an increasing demand for much more residential and commercial buildings, which leads to vertical growth of the buildings and needs proper ventilation of those buildings. Natural air ventilation system is not sufficient for conventional building structures. Hence fans and air-conditioners are must to meet the requirement of proper ventilation as well as space conditioning. Globally building sector consumes largest energy in heating, cooling, ventilation and space conditioning. This load can be minimized by the application of solar chimney and modification in building structure for heating, cooling, ventilation and space conditioning. Passive solar cooling is a subject of interest to provide cooling by using the sun, a powerful energy source. This is done for ensuring human comfort in hot climates. ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers defines Comfort as ‘that state of mind which expresses satisfaction with the thermal environment.’ The present paper describes the development of a solar passive cooling system, which can provide thermal cooling throughout the summer season in hot and humid climates. The constructed passive system works on natural convection mode of air. Such system reduces the inside temperature of up to 5°C from the atmospheric temperature. Temperature can further be reduced by the judicious use of night ventilation.
Deciphering Core Collapse Supernovae Is Convection the Key?; 1, prompt convection
Mezzacappa, A; Bruenn, S W; Blondin, J M; Guidry, M W; Strayer, M R; Umar, A S
1996-01-01
We couple two-dimensional hydrodynamics to detailed one-dimensional multigroup flux-limited diffusion neutrino transport to investigate prompt convection in core collapse supernovae. Our initial conditions, time-dependent boundary conditions, and neutrino distributions for computing neutrino heating, cooling, and deleptonization rates are obtained from one-dimensional simulations that implement multigroup flux-limited diffusion neutrino transport and one-dimensional hydrodynamics. The development and evolution of prompt convection and its ramifications for the shock dynamics are investigated for both 15 and 25 solar mass models, representative of the two classes of stars with compact and extended iron cores, respectively. In the absence of neutrino transport, prompt convection develops and dissipates on a time scale $\\sim$15 ms for both models. Prompt convection seeds convection behind the shock, which causes distortions in the shock's sphericity, but on the average, the shock radius is not boosted significan...
Convective heat transfer measurement involving flow past stationary circular disks
Energy Technology Data Exchange (ETDEWEB)
Wedekind, G.L. (Oakland Univ., Rochester, MI (United States))
1989-11-01
Considerable empirical data exist in the literature for forced convection heat transfer involving external flow over a variety of geometries, and for various ranges of Reynolds number. This author is not aware of any published empirical data for forced convection heat transfer involving flow past a simple stationary circular disk, whose axis is perpendicular to the flow. Such is the purpose of this paper.
Meshkov, I.; Sidorin, A.
2004-10-01
The brief review of the most significant and interesting achievements in electron cooling method, which took place during last two years, is presented. The description of the electron cooling facilities-storage rings and traps being in operation or under development-is given. The applications of the electron cooling method are considered. The following modern fields of the method development are discussed: crystalline beam formation, expansion into middle and high energy electron cooling (the Fermilab Recycler Electron Cooler, the BNL cooler-recuperator, cooling with circulating electron beam, the GSI project), electron cooling in traps, antihydrogen generation, electron cooling of positrons (the LEPTA project).
Energy Technology Data Exchange (ETDEWEB)
Blaskiewicz, M.
2011-01-01
Stochastic Cooling was invented by Simon van der Meer and was demonstrated at the CERN ISR and ICE (Initial Cooling Experiment). Operational systems were developed at Fermilab and CERN. A complete theory of cooling of unbunched beams was developed, and was applied at CERN and Fermilab. Several new and existing rings employ coasting beam cooling. Bunched beam cooling was demonstrated in ICE and has been observed in several rings designed for coasting beam cooling. High energy bunched beams have proven more difficult. Signal suppression was achieved in the Tevatron, though operational cooling was not pursued at Fermilab. Longitudinal cooling was achieved in the RHIC collider. More recently a vertical cooling system in RHIC cooled both transverse dimensions via betatron coupling.
Topology optimisation for natural convection problems
Alexandersen, Joe; Andreasen, Casper Schousboe; Sigmund, Ole
2014-01-01
This paper demonstrates the application of the density-based topology optimisation approach for the design of heat sinks and micropumps based on natural convection effects. The problems are modelled under the assumptions of steady-state laminar flow using the incompressible Navier-Stokes equations coupled to the convection-diffusion equation through the Boussinesq approximation. In order to facilitate topology optimisation, the Brinkman approach is taken to penalise velocities inside the solid domain and the effective thermal conductivity is interpolated in order to accommodate differences in thermal conductivity of the solid and fluid phases. The governing equations are discretised using stabilised finite elements and topology optimisation is performed for two different problems using discrete adjoint sensitivity analysis. The study shows that topology optimisation is a viable approach for designing heat sink geometries cooled by natural convection and micropumps powered by natural convection.
Convection of Moist Saturated Air: Analytical Study
Directory of Open Access Journals (Sweden)
Robert Zakinyan
2016-01-01
Full Text Available In the present work, the steady-state stationary thermal convection of moist saturated air in a lower atmosphere has been studied theoretically. Thermal convection was considered without accounting for the Coriolis force, and with only the vertical temperature gradient. The analytical solution of geophysical fluid dynamics equations, which generalizes the formulation of the moist convection problem, is obtained in the two-dimensional case. The stream function is derived in the Boussinesq approximation with velocity divergence taken as zero. It has been shown that the stream function is asymmetrical in vertical direction contrary to the dry and moist unsaturated air convection. It has been demonstrated that the convection in moist atmosphere strongly depends on the vapor mass fraction gradient.
Topology Optimisation for Coupled Convection Problems
DEFF Research Database (Denmark)
Alexandersen, Joe; Andreasen, Casper Schousboe; Aage, Niels
conduction governs in the solid parts of the design domain and couples to convection-dominated heat transfer to a surrounding fluid. Both loosely coupled and tightly coupled problems are considered. The loosely coupled problems are convection-diffusion problems, based on an advective velocity field from......The work focuses on applying topology optimisation to forced and natural convection problems in fluid dynamics and conjugate (fluid-structure) heat transfer. To the authors' knowledge, topology optimisation has not yet been applied to natural convection flow problems in the published literature...... and the current work is thus seen as contributing new results to the field. In the literature, most works on the topology optimisation of weakly coupled convection-diffusion problems focus on the temperature distribution of the fluid, but a selection of notable exceptions also focusing on the temperature...
Peltier Junction heats and cools car seat
Energy Technology Data Exchange (ETDEWEB)
Gottschalk, M.A.
1994-10-10
Electrically heated seats may soon become heated and cooled seats. The design called the CCS module exploits the heat-pump capability of a class of semiconductor thermoelectric devices (TEDs) known as Peltier Junction. Every CCS module contain two TEDs. Heating and cooling occurs through convection and conduction. The heart of the system is the thermoelectric heat pump. This is originally conceived as the sole heating/cooling options for a prototype electric vehicle.
Organised convection embedded in a large-scale flow
Naumann, Ann Kristin; Stevens, Bjorn; Hohenegger, Cathy
2017-04-01
In idealised simulations of radiative convective equilibrium, convection aggregates spontaneously from randomly distributed convective cells into organized mesoscale convection despite homogeneous boundary conditions. Although these simulations apply very idealised setups, the process of self-aggregation is thought to be relevant for the development of tropical convective systems. One feature that idealised simulations usually neglect is the occurrence of a large-scale background flow. In the tropics, organised convection is embedded in a large-scale circulation system, which advects convection in along-wind direction and alters near surface convergence in the convective areas. A large-scale flow also modifies the surface fluxes, which are expected to be enhanced upwind of the convective area if a large-scale flow is applied. Convective clusters that are embedded in a large-scale flow therefore experience an asymmetric component of the surface fluxes, which influences the development and the pathway of a convective cluster. In this study, we use numerical simulations with explicit convection and add a large-scale flow to the established setup of radiative convective equilibrium. We then analyse how aggregated convection evolves when being exposed to wind forcing. The simulations suggest that convective line structures are more prevalent if a large-scale flow is present and that convective clusters move considerably slower than advection by the large-scale flow would suggest. We also study the asymmetric component of convective aggregation due to enhanced surface fluxes, and discuss the pathway and speed of convective clusters as a function of the large-scale wind speed.
Boundary layer control of rotating convection systems.
King, Eric M; Stellmach, Stephan; Noir, Jerome; Hansen, Ulrich; Aurnou, Jonathan M
2009-01-15
Turbulent rotating convection controls many observed features of stars and planets, such as magnetic fields, atmospheric jets and emitted heat flux patterns. It has long been argued that the influence of rotation on turbulent convection dynamics is governed by the ratio of the relevant global-scale forces: the Coriolis force and the buoyancy force. Here, however, we present results from laboratory and numerical experiments which exhibit transitions between rotationally dominated and non-rotating behaviour that are not determined by this global force balance. Instead, the transition is controlled by the relative thicknesses of the thermal (non-rotating) and Ekman (rotating) boundary layers. We formulate a predictive description of the transition between the two regimes on the basis of the competition between these two boundary layers. This transition scaling theory unifies the disparate results of an extensive array of previous experiments, and is broadly applicable to natural convection systems.
Natural convection in superposed fluid-porous layers
Bagchi, Aniruddha
2013-01-01
Natural Convection in Composite Fluid-Porous Domains provides a timely overview of the current state of understanding on the phenomenon of convection in composite fluid-porous layers. Natural convection in horizontal fluid-porous layers has received renewed attention because of engineering problems such as post-accident cooling of nuclear reactors, contaminant transport in groundwater, and convection in fibrous insulation systems. Because applications of the problem span many scientific domains, the book serves as a valuable resource for a wide audience.
Institute of Scientific and Technical Information of China (English)
邓自刚; 王家素; 郑珺; 刘伟; 林群煦; 马光同; 王为; 王素玉; 张娅
2009-01-01
文章通过对15块高温超导块材与永磁轨道相互作用的悬浮力测试,比较了零场冷和场冷两种冷却方式下块材的最大悬浮力关系.实验结果显示零场冷时悬浮力大的块材在场冷时悬浮力不一定就大,反之亦然,两者并无直接的对应关系.在实际的场冷应用中,推荐以场冷下的悬浮力数据为参考.%The paper compares the relationship of maximum levitation force of bulk high temperature superconductor in zero-field-cooling (ZFC) and field-cooling (FC) cases by the levitation measurement of 15 bulks interacting with permanent magnet guideway. The experimental results show that the maximum forces in the two cooling cases are not corresponding to each other. The bulk with large levitation force in ZFC case will not always obtain a large one in the FC case, and vice ver-sa. So, the levitation force data in FC case is recommended to the practical FC applications.
Mixed convection flow of nanofluid in a square enclosure with an intruded rectangular fin
Energy Technology Data Exchange (ETDEWEB)
Cong, Ran; Zhou, Xuanyu; De Souza Machado, Bruno; Das, Prodip K., E-mail: prodip.das@ncl.ac.uk [School of Mechanical and Systems Engineering Newcastle University Newcastle upon Tyne, NE1 7RU United Kingdom (United Kingdom)
2016-07-12
Mixed convection flow in enclosures has been a subject of interest for many years due to their ever increasing applications in solar collectors, electronic cooling, lubrication technologies, food processing, and nuclear reactors. In comparison, little effort has been given to the problem of mixed convection in enclosures filled with nanofluids, while the addition of nanoparticles in a fluid base to alter specific material properties is considered a feasible solution for many heat transfer problems. Mixed convection of nanofluids is a challenging problem as the addition of nanoparticles changes the fluid’s thermo-physical properties as well as due to the complex interactions among inertia, viscous, and buoyancy forces. In this study, a two-dimensional steady-state numerical model has been developed to investigate mixed convection flow of nanofluids in a square enclosure with an intruded rectangular fin and to optimize the fin geometry for maximizing the heat transfer using the Constructal design. The model has been developed using ANSYS-FLUENT for various fin geometries. Flow fields, temperature fields, and heat transfer rates are examined for different values of Rayleigh and Reynolds numbers for several geometries of the fin with the aim of maximizing the heat transfer from the fin to the surrounding flow. Outcome of this study provides important insight into the heat transfer behavior of nanofluids, which will help in developing novel geometries with enhanced and controlled heat transfer for solar collectors and electronic devices.
Mixed convection flow of nanofluid in a square enclosure with an intruded rectangular fin
Cong, Ran; Zhou, Xuanyu; De Souza Machado, Bruno; Das, Prodip K.
2016-07-01
Mixed convection flow in enclosures has been a subject of interest for many years due to their ever increasing applications in solar collectors, electronic cooling, lubrication technologies, food processing, and nuclear reactors. In comparison, little effort has been given to the problem of mixed convection in enclosures filled with nanofluids, while the addition of nanoparticles in a fluid base to alter specific material properties is considered a feasible solution for many heat transfer problems. Mixed convection of nanofluids is a challenging problem as the addition of nanoparticles changes the fluid's thermo-physical properties as well as due to the complex interactions among inertia, viscous, and buoyancy forces. In this study, a two-dimensional steady-state numerical model has been developed to investigate mixed convection flow of nanofluids in a square enclosure with an intruded rectangular fin and to optimize the fin geometry for maximizing the heat transfer using the Constructal design. The model has been developed using ANSYS-FLUENT for various fin geometries. Flow fields, temperature fields, and heat transfer rates are examined for different values of Rayleigh and Reynolds numbers for several geometries of the fin with the aim of maximizing the heat transfer from the fin to the surrounding flow. Outcome of this study provides important insight into the heat transfer behavior of nanofluids, which will help in developing novel geometries with enhanced and controlled heat transfer for solar collectors and electronic devices.
Experimental Studies of NGNP Reactor Cavity Cooling System With Water
Energy Technology Data Exchange (ETDEWEB)
Corradini, Michael; Anderson, Mark; Hassan, Yassin; Tokuhiro, Akira
2013-01-16
This project will investigate the flow behavior that can occur in the reactor cavity cooling system (RCCS) with water coolant under the passive cooling-mode of operation. The team will conduct separate-effects tests and develop associated scaling analyses, and provide system-level phenomenological and computational models that describe key flow phenomena during RCCS operation, from forced to natural circulation, single-phase flow and two-phase flow and flashing. The project consists of the following tasks: Task 1. Conduct separate-effects, single-phase flow experiments and develop scaling analyses for comparison to system-level computational modeling for the RCCS standpipe design. A transition from forced to natural convection cooling occurs in the standpipe under accident conditions. These tests will measure global flow behavior and local flow velocities, as well as develop instrumentation for use in larger scale tests, thereby providing proper flow distribution among standpipes for decay heat removal. Task 2. Conduct separate-effects experiments for the RCCS standpipe design as two-phase flashing occurs and flow develops. As natural circulation cooling continues without an ultimate heat sink, water within the system will heat to temperatures approaching saturation , at which point two-phase flashing and flow will begin. The focus is to develop a phenomenological model from these tests that will describe the flashing and flow stability phenomena. In addition, one could determine the efficiency of phase separation in the RCCS storage tank as the two-phase flashing phenomena ensues and the storage tank vents the steam produced. Task 3. Develop a system-level computational model that will describe the overall RCCS behavior as it transitions from forced flow to natural circulation and eventual two-phase flow in the passive cooling-mode of operation. This modeling can then be used to test the phenomenological models developed as a function of scale.
Destabilization of free convection by weak rotation
Gelfgat, Alexander
2011-01-01
This study offers an explanation of a recently observed effect of destabilization of free convective flows by weak rotation. After studying several models where flows are driven by a simultaneous action of convection and rotation, it is concluded that the destabilization is observed in the cases where centrifugal force acts against main convective circulation. At relatively low Prandtl numbers this counter action can split the main vortex into two counter rotating vortices, where the interaction leads to instability. At larger Prandtl numbers, the counter action of the centrifugal force steepens an unstable thermal stratification, which triggers Rayleigh-B\\'enard instability mechanism. Both cases can be enhanced by advection of azimuthal velocity disturbances towards the axis, where they grow and excite perturbations of the radial velocity. The effect was studied considering a combined convective/rotating flow in a cylinder with a rotating lid and a parabolic temperature profile at the sidewall. Next, explana...
Measurement of the Convective Heat-Transfer Coefficient
Conti, Rosaria; Gallitto, Aurelio Agliolo; Fiordilino, Emilio
2014-01-01
We propose an experiment for investigating how objects cool down toward the thermal equilibrium with their surroundings. We describe the time dependence of the temperature difference of the cooling objects and the environment with an exponential decay function. By measuring the thermal constant t, we determine the convective heat-transfer…
Experimental investigation of horizontal convection
Muñoz Córdoba, Lucía
2015-01-01
Fluid circulation driven by buoyancy forces due to a thermal gradient on a horizontal boundary, known as horizontal convection, is experimentally studied. For that purpose, a methacrylate box with inner dimensions 300x150x150 mm3 (LxWxH) whose bottom is composed by a heat exchanger and a printed circuit board is lled with water. The heat exchanger provides a uniform temperature boundary condition while the printed circuit board provides a boundary condition of uniform heat ...
Comparison of immersed liquid and air cooling of NASA's Airborne Information Management System
Hoadley, A. W.; Porter, A. J.
1992-01-01
The Airborne Information Management System (AIMS) is currently under development at NASA Dryden Flight Research Facility. The AIMS is designed as a modular system utilizing surface mounted integrated circuits in a high-density configuration. To maintain the temperature of the integrated circuits within manufacturer's specifications, the modules are to be filled with Fluorinert FC-72. Unlike ground based liquid cooled computers, the extreme range of the ambient pressures experienced by the AIMS requires the FC-72 be contained in a closed system. This forces the latent heat absorbed during the boiling to be released during the condensation that must take within the closed module system. Natural convection and/or pumping carries the heat to the outer surface of the AIMS module where the heat transfers to the ambient air. This paper will present an evaluation of the relative effectiveness of immersed liquid cooling and air cooling of the Airborne Information Management System.
Multicloud convective parametrizations with crude vertical structure
Energy Technology Data Exchange (ETDEWEB)
Khouider, Boualem [University of Victoria, Mathematics and Statistics, PO BOX 3045 STN CSC, Victoria, BC (Canada); Majda, Andrew J. [New York University, Department of Mathematics and Center for Atmosphere/Ocean Sciences, Courant Institute, New York, NY (United States)
2006-11-15
Recent observational analysis reveals the central role of three multi-cloud types, congestus, stratiform, and deep convective cumulus clouds, in the dynamics of large scale convectively coupled Kelvin waves, westward propagating two-day waves, and the Madden-Julian oscillation. The authors have recently developed a systematic model convective parametrization highlighting the dynamic role of the three cloud types through two baroclinic modes of vertical structure: a deep convective heating mode and a second mode with low level heating and cooling corresponding respectively to congestus and stratiform clouds. The model includes a systematic moisture equation where the lower troposphere moisture increases through detrainment of shallow cumulus clouds, evaporation of stratiform rain, and moisture convergence and decreases through deep convective precipitation and a nonlinear switch which favors either deep or congestus convection depending on whether the troposphere is moist or dry. Here several new facets of these multi-cloud models are discussed including all the relevant time scales in the models and the links with simpler parametrizations involving only a single baroclinic mode in various limiting regimes. One of the new phenomena in the multi-cloud models is the existence of suitable unstable radiative convective equilibria (RCE) involving a larger fraction of congestus clouds and a smaller fraction of deep convective clouds. Novel aspects of the linear and nonlinear stability of such unstable RCE's are studied here. They include new modes of linear instability including mesoscale second baroclinic moist gravity waves, slow moving mesoscale modes resembling squall lines, and large scale standing modes. The nonlinear instability of unstable RCE's to homogeneous perturbations is studied with three different types of nonlinear dynamics occurring which involve adjustment to a steady deep convective RCE, periodic oscillation, and even heteroclinic chaos in
Mixed convection flow and heat transfer in a vertical wavy channel ...
African Journals Online (AJOL)
user
Keywords: convective flow; wavy channel; porous medium; traveling thermal waves. ... the problems of forced convection in composite fluids and porous layers. ... Processes involving heat and mass transfer are often encountered in the ...
Aerosol-induced changes of convective cloud anvils produce strong climate warming
Directory of Open Access Journals (Sweden)
I. Koren
2010-01-01
Full Text Available The effect of aerosol on clouds poses one of the largest uncertainties in estimating the anthropogenic contribution to climate change. In contrast, even small human-induced perturbations in cloud coverage, lifetime, height or optical properties can change the instantaneous radiative energy flux by hundreds of watts per unit area, and this forcing can be either warming or cooling. Clouds and aerosols form a complex coupled system that, unlike greenhouse gases, have relatively short lifetime (hours to days and inhomogeneous distribution. This and the inherent complexity of cloud microphysics and dynamics, and the strong coupling with meteorology explain why the estimation of the overall effect of aerosol on climate is so challenging.
Here we focus on the effect of aerosol on cloud top properties of deep convective clouds over the tropical Atlantic. The tops of these vertically developed clouds consist of mostly ice and can reach high levels of the atmosphere, overshooting the lower stratosphere and reaching altitudes greater than 16 km. We show a link between aerosol, clouds and the free atmosphere wind profile that can change the magnitude and sign of the overall climate radiative forcing.
This study demonstrates the deep link between cloud shape and aerosol loading and that the overall aerosol effect in regions of deep convective clouds might be warming. Moreover we show how averaging the cloud height and optical properties over large regions may lead to a false cooling estimation.
National Convective Weather Forecast
National Oceanic and Atmospheric Administration, Department of Commerce — The NCWF is an automatically generated depiction of: (1) current convection and (2) extrapolated signficant current convection. It is a supplement to, but does NOT...
Mixed Convection in the VHTR in the Event of a LOFA
Energy Technology Data Exchange (ETDEWEB)
Richard W. Johnson
2012-05-01
The US Department of Energy, Office of Nuclear Energy (DOE-NE) is supporting the development of a very high temperature reactor (VHTR) concept as the primary focus of it next generation nuclear power plant (NGNP) program. The VHTR is cooled by forcing helium downwards through the core into the lower plenum and out the hot duct. In the event that the coolant circulators are lost, the driving pressure drop across the core will reduce to zero and there will be the opportunity for natural circulation to occur. During the time that the circulators are powering down, the heat transfer in the core from the graphite blocks to the helium coolant will transform from turbulent forced convection to mixed convection, where buoyancy effects become important, to free or natural convection, where buoyancy is dominant. Analysis of the nature of the forced, mixed and free convection is best done using computational fluid dynamic (CFD) software that can provide fine details of the flow and heat transfer. However, CFD analysis involves approximations in the results because of the finite nature of the spatial and temporal discretizations required, the inexact nature of the turbulence models that are used and the finite precision of the computers employed. Therefore, it is necessary to validate the CFD computations. Validation is accomplished by comparing results from specific CFD computations to experimental data that have been taken specifically for the purpose of validation and that are related to the physical phenomena in question. The present report examines the flow and heat transfer parameters (dimensionless numbers) that characterize the flow and reports ranges for their values based on specific CFD studies performed for the VHTR.
A new method to optimize natural convection heat sinks
Lampio, K.; Karvinen, R.
2017-08-01
The performance of a heat sink cooled by natural convection is strongly affected by its geometry, because buoyancy creates flow. Our model utilizes analytical results of forced flow and convection, and only conduction in a solid, i.e., the base plate and fins, is solved numerically. Sufficient accuracy for calculating maximum temperatures in practical applications is proved by comparing the results of our model with some simple analytical and computational fluid dynamics (CFD) solutions. An essential advantage of our model is that it cuts down on calculation CPU time by many orders of magnitude compared with CFD. The shorter calculation time makes our model well suited for multi-objective optimization, which is the best choice for improving heat sink geometry, because many geometrical parameters with opposite effects influence the thermal behavior. In multi-objective optimization, optimal locations of components and optimal dimensions of the fin array can be found by simultaneously minimizing the heat sink maximum temperature, size, and mass. This paper presents the principles of the particle swarm optimization (PSO) algorithm and applies it as a basis for optimizing existing heat sinks.
Baskys, Algirdas; Hopkins, Simon C.; Bader, Jakob; Glowacki, Bartek A.
2016-10-01
As superconducting materials find their way into applications, there is increasing need to verify their performance at operating conditions. Testing of critical current with respect to temperature and magnetic field is of particular importance. However, testing facilities covering a range of temperatures and magnetic fields can be costly, especially when considering the cooling power required in the cryogenic system in the temperature range below 65 K (inaccessible for LN2). Critical currents in excess of 500 A are common for commercial samples, making the testing of such samples difficult in setups cooled via a cryocooler, moreover it often does not represent the actual cooling conditions that the sample will experience in service. This work reports the design and operation of a low-cost critical current testing facility, capable of testing samples in a temperature range of 10-65 K, with magnetic field up to 1.6 T and measuring critical currents up to 900 A with variable cooling power.
Institute of Scientific and Technical Information of China (English)
赵宪锋; 周又和
2011-01-01
After the penetration depth of superconducting currents and the internal magnetic field interior of the HTS were calculated, the influence of critical current density (Jc) on the interaction forces between a high - Tc superconductor (HTS) and a permanent magnet (PM) was investigated under field cooling process based on Kim critical state model. It was found that the maximum levitation forces including the attractive force and the repulsive force increased as an exponential function with the increasing of the Jc. They became to saturate at high Jc. The hysteresis energy loss of vertical force - displacement curve under field cooling condition was larger than the one under zero -field cooling condition. There was a force ratio k, which was important on estimating the mechanics capability of superconducting levitation system with large Jc. All these relations could be explained with the penetration history of superconducting currents under field cooling condition.%基于Kim临界态模型,通过考虑超导块材内部屏蔽电流的穿透历史过程,讨论了场冷条件下临界电流密度对高温超导悬浮系统磁悬浮排斥力和吸引力的影响.结果显示:最大超导磁悬浮排斥力和吸引力均随临界电流密度的增加呈指数关系增加,并趋于饱和;场冷条件下的磁悬浮力回滞能量损耗远高于零场冷情况;存在一个磁悬浮力比率κ,κ值对于评价大电流超导磁悬浮系统的机械性能具有重要作用.磁悬浮力随临界电流密度的这些变化特征可以用超导块材内部屏蔽电流的穿透情况进行合理地解释,为高温超导悬浮系统的工业应用提供了重要的理论依据.
Mixed convection of nanofluids in a lid-driven rough cavity
Guo, Zhimeng; Wang, Jinyu; Mozumder, Aloke K.; Das, Prodip K.
2017-06-01
Mixed convection heat transfer and fluid flow of air, water or oil in enclosures have been studied extensively using experimental and numerical means for many years due to their ever-increasing applications in many engineering fields. In comparison, little effort has been given to the problem of mixed convection of nanofluids in spite of several applications in solar collectors, electronic cooling, lubrication technologies, food processing, and nuclear reactors. Mixed convection of nanofluids is a challenging problem due to the complex interactions among inertia, viscous, and buoyancy forces. In this study, mixed convection of nanofluids in a lid-driven square cavity with sinusoidal roughness elements at the bottom is studied numerically using the Navier-Stokes equations with the Boussinesq approximation. The numerical model is developed using commercial finite volume software ANSYS-FLUENT for Al2O3-water and CuO-water nanofluids inside a square cavity with various roughness elements. The effects of number and amplitude of roughness elements on the heat transfer and fluid flow are analysed for various volume concentrations of Al2O3 and CuO nanoparticles. The flow fields, temperature fields, and heat transfer rates are examined for different values of Rayleigh and Reynolds numbers. The outcome of this study provides some important insight into the heat transfer behaviour of Al2O3-water and CuO-water nanofluids inside a lid-driven rough cavity. This knowledge can be further used in developing novel geometries with enhanced and controlled heat transfer for solar collectors, electronic cooling, and food processing industries.
Directory of Open Access Journals (Sweden)
S. P. Anjali Devi
2014-01-01
Full Text Available A study has been carried out on MHD boundary layer forced convection flow along a shrinking surface with variable heat flux in the presence of heat source. The flow is generated due to linear shrinking of the sheet and is influenced by uniform transverse magnetic field. The basic boundary layer momentum and heat transfer equations, which are nonlinear partial differential equations, are converted into nonlinear ordinary differential equations by means of similarity transformation. Numerical solution of the resulting boundary value problem is obtained using Nachtsheim Swigert shooting iteration scheme for the satisfaction of asymptotic boundary conditions along with the Fourth Order Runge Kutta method. The effects of suction parameter, magnetic parameter, Prandtl number, heat source parameter, stretching/shrinking parameter and heat flux parameter on velocity and temperature are shown in several plots. The results are in good agreement with the earlier published works under some limiting cases. Skin friction coefficient and wall temperature are also explored for typical values of the parameter involved in the study.
Masalov, Vladimir M.; Vasilyeva, Natalia A.; Manomenova, Vera L.; Zhokhov, Andrei A.; Rudneva, Elena B.; Voloshin, Alexey E.; Emelchenko, Gennadi A.
2017-10-01
The technique and the scheme of the system for growing single crystals, including complex mixed composition, under stationary conditions of supercooling and forced convection of aqueous solution were described. Solubility in water of various compositions of K2CoxNi1-x(SO4)2·6H2O (KCNSH) and the dependence of Co content in the KCNSH crystal of Co concentration in the saline part of aqueous solutions of KCNSH have been measured in the temperature range of 30-70 °C. It was found that the growth sectors {0 0 1} and {1 1 0} differ in Ni and Co contents. The Ni/Co ratio is dependent on the value of solution supersaturation. The optical transmission spectra of crystals grown showed high transmittance in the UV region of the spectrum and the almost complete absorption of light in the visible spectrum. It is concluded that the crystals grown can be used as efficient UV filters.
Institute of Scientific and Technical Information of China (English)
全贞花; 陈永昌; 马重芳
2008-01-01
An experimental study was conducted to investigate the fouling process of calcium carbonate on the heat transfer surface,during forced convective heat transfer.The dynamic monitoring apparatus of fouling resistance was set up for the present experiments.The fouling behaviors were examined under different factors including fluid velocity,hardness,alkalinity,solution temperature,and wall temperature.Asymptotic fouling curves varying with time were obtained.The fouling rate and asymptotic fouling resistance increased and the induction periods were shortened with the fluid velocity decreasing,hardness and alkalinity increasing,and solution temperature and heat transfer surlace temperature lncreasmg.The components of touling that formed on the heat transfer surface included crystallization fouling and particulate fouling.The thermal performance parameter Of fouling,ρfλf,varied from 380 to 2600 kg·W·(m4·K)-1,increasing with growing velocity and decreasing solution ternperature,hardness or alkalinity.Furthermore,the thermal conductivity of fouling,λf,varied from 1.7 to 2.2 W·(m·K)-1.
Laser induced ponderomotive convection in water
Shneider, M N
2015-01-01
A new mechanism for inducing convection during IR laser interaction with water or any absorbing polar liquid is described theoretically. The numerical simulations performed using the developed model show that the ponderomotive force produces water flow in the direction of the laser beam propagation. In the later stage of interaction, when water temperature rises, the Archimedes force becomes first comparable and then dominant producing convection directed against the vector of gravitational acceleration (upward). The theoretical estimates and the numerical simulations predict fluid dynamics that is similar to the observed in the previous experiments.
Barker, P F
2010-01-01
Doppler cooling the center-of-mass motion of an optically levitated microsphere via the velocity dependent scattering force from narrow whispering gallery mode (WGM) resonances is described. Light that is red detuned from the WGM resonance can be used to damp the center-of-mass motion in a process analogous to the Doppler cooling of atoms. Leakage of photons out of the microsphere when the incident field is near resonant with the narrow WGM resonance acts to damp the motion of the sphere. The scattering force is not limited by saturation, but can be controlled by the incident power. Cooling times on the order of seconds are calculated for a 20 micron diameter silica microsphere trapped within optical tweezers, with a Doppler temperature limit in the microKelvin regime.
Electron Cooling Experiments in CSR
Xiaodong, Yang
2011-01-01
The six species heavy ion beam was accumulated with the help of electron cooling in the main ring of Cooler Storage Ring of Heavy Ion Research Facility in Lanzhou(HIRFL-CSR), the ion beam accumulation dependence on the parameters of cooler was investigated experimentally. The 400MeV/u 12C6+ and 200MeV/u 129Xe54+ was stored and cooled in the experimental ring CSRe, the cooling force was measured in different condition.
Electron cooling experiments in CSR
Institute of Scientific and Technical Information of China (English)
PARKHOMCHUK; Vasily; REVA; Vladimir
2011-01-01
The six species heavy ion beam was accumulated with the help of electron cooling in the main ring of Cooler Storage Ring of Heavy Ion Research Facility in Lanzhou (HIRFL-CSR). The ion beam accumulation dependence on the parameters of cooler was investigated experimentally. The 400 MeV/u 12C6+ and 200 MeV/u 129Xe54+ were stored and cooled in the experimental ring CSRe, and the cooling force was measured in different conditions.
Marcum, Jeremy W.; Ferkul, Paul V.; Olson, Sandra L.
2017-01-01
Normal gravity flame blowoff limits in an axisymmetric pmma rod geometry in upward axial stagnation flow are compared with microgravity Burning and Suppression of Solids II (BASS-II) results recently obtained aboard the International Space Station. This testing utilized the same BASS-II concurrent rod geometry, but with the addition of normal gravity buoyant flow. Cast polymethylmethacrylate (pmma) rods of diameters ranging from 0.635 cm to 3.81 cm were burned at oxygen concentrations ranging from 14 to 18 by volume. The forced flow velocity where blowoff occurred was determined for each rod size and oxygen concentration. These blowoff limits compare favorably with the BASS-II results when the buoyant stretch is included and the flow is corrected by considering the blockage factor of the fuel. From these results, the normal gravity blowoff boundary for this axisymmetric rod geometry is determined to be linear, with oxygen concentration directly proportional to flow speed. We describe a new normal gravity upward flame spread test method which extrapolates the linear blowoff boundary to the zero stretch limit to resolve microgravity flammability limits, something current methods cannot do. This new test method can improve spacecraft fire safety for future exploration missions by providing a tractable way to obtain good estimates of material flammability in low gravity.
Basics of lava-lamp convection
Gyüre, Balázs; Jánosi, Imre M.
2009-10-01
Laboratory experiments are reported in an immiscible two-fluid system, where thermal convection is initiated by heating at the bottom and cooling at the top. The lava-lamp regime is characterized by a robust periodic exchange process where warm blobs rise from the bottom, attach to the top surface for a while, then cold blobs sink down again. Immiscibility allows to reach real steady (dynamical equilibrium) states which can be sustained for several days. Two modes of lava-lamp convection could be identified by recording and evaluating temperature time series at the bottom and at the top of the container: a “slow” mode is determined by an effective heat transport speed at a given temperature gradient, while a second mode of constant periodicity is viscosity limited. Contrasting of laboratory and geophysical observations yields the conclusion that the frequently suggested lava-lamp analogy fails for the accepted models of mantle convection.
On the convective overstability in protoplanetary discs
Latter, Henrik
2015-01-01
This paper explores the driving of low-level hydrodynamical activity in protoplanetary-disc dead zones. A small adverse radial entropy gradient, ordinarily stabilised by rotation, excites oscillatory convection (`convective overstability') when thermal diffusion, or cooling, is neither too strong nor too weak. I revisit the linear theory of the instability, discuss its prevalence in protoplanetary discs, and show that unstable modes are exact nonlinear solutions in the local Boussinesq limit. Overstable modes cannot grow indefinitely, however, as they are subject to a secondary parametric instability that limits their amplitudes to relatively low levels. If parasites set the saturation level of the ensuing turbulence then the convective overstability is probably too weak to drive significant angular momentum transport or to generate vortices. But I also discuss an alternative, and far more vigorous, saturation route that generates radial `layers' or `zonal flows' (witnessed also in semiconvection). Numerical ...
Convection pump and method of operation
Energy Technology Data Exchange (ETDEWEB)
Steinhour, Leif Alexi
2017-07-11
This disclosure provides systems, methods, and apparatus related to a convection pump. In one aspect, an apparatus includes a chamber, the chamber having an inlet at a first end of the chamber and an outlet at a second end of the chamber. The chamber further has a first surface and a second surface, the first surface being opposite to the second surface. A baffle having a substantially helical shape is disposed inside the chamber. A heating device is configured to heat the first surface of the chamber. A cooling device is configured to cool the second surface of the chamber.
A continuous and prognostic convection scheme based on buoyancy, PCMT
Guérémy, Jean-François; Piriou, Jean-Marcel
2016-04-01
A new and consistent convection scheme (PCMT: Prognostic Condensates Microphysics and Transport), providing a continuous and prognostic treatment of this atmospheric process, is described. The main concept ensuring the consistency of the whole system is the buoyancy, key element of any vertical motion. The buoyancy constitutes the forcing term of the convective vertical velocity, which is then used to define the triggering condition, the mass flux, and the rates of entrainment-detrainment. The buoyancy is also used in its vertically integrated form (CAPE) to determine the closure condition. The continuous treatment of convection, from dry thermals to deep precipitating convection, is achieved with the help of a continuous formulation of the entrainment-detrainment rates (depending on the convective vertical velocity) and of the CAPE relaxation time (depending on the convective over-turning time). The convective tendencies are directly expressed in terms of condensation and transport. Finally, the convective vertical velocity and condensates are fully prognostic, the latter being treated using the same microphysics scheme as for the resolved condensates but considering the convective environment. A Single Column Model (SCM) validation of this scheme is shown, allowing detailed comparisons with observed and explicitly simulated data. Four cases covering the convective spectrum are considered: over ocean, sensitivity to environmental moisture (S. Derbyshire) non precipitating shallow convection to deep precipitating convection, trade wind shallow convection (BOMEX) and strato-cumulus (FIRE), together with an entire continental diurnal cycle of convection (ARM). The emphasis is put on the characteristics of the scheme which enable a continuous treatment of convection. Then, a 3D LAM validation is presented considering an AMMA case with both observations and a CRM simulation using the same initial and lateral conditions as for the parameterized one. Finally, global
Kelly, Patrick; Mapes, Brian; Hu, I.-Kuan; Song, Siwon; Kuang, Zhiming
2017-06-01
This paper describes a new intermediate global atmosphere model in which synoptic and planetary dynamics including the advection of water vapor are explicit in 10 layers, the time-mean flow is centered near a realistic state through the use of carefully calibrated time-independent 3-D forcings, and temporal anomalies of convective tendencies of heat and moisture in each column are represented as a linear matrix acting on the anomalous temperature and moisture profiles. Currently, this matrix is Kuang's [] linear response function (LRF) of a cyclic convection-permitting model (CCPM) in equilibrium with specified atmospheric cooling (i.e., without radiation or WISHE interactions, so it conserves column moist static energy exactly). The goal of this effort is to cleanly test the role of convection's free-tropospheric moisture sensitivity in tropical waves, without incurring large changes of mean climate that confuse the interpretation of experiments with entrainment parameters in the convection schemes of full-physics GCMs. When the sensitivity to free-tropospheric moisture is multiplied by a factor ranging from 0 to 2, the model's variability ranges from: (1) moderately strong convectively coupled Kelvin waves with speeds near 20 m s-1; to (0) similar but much weaker waves; to (2) similar but stronger and slightly faster waves as the water vapor field plays an increasingly important role. Longitudinal structure in the model's time-mean tropical flow is not fully realistic, and does change significantly with matrix-coupled variability, but further work on editing the anomaly physics matrix and calibrating the mean state could improve this class of models.
Numerical Simulation of Multiplicity and Stability of Mixed Convection in Rotating Curved Ducts
Directory of Open Access Journals (Sweden)
Wang Liqiu
2005-01-01
Full Text Available A numerical study is made on the fully developed bifurcation structure and stability of the mixed convection in rotating curved ducts of square cross-section with the emphasis on the effect of buoyancy force. The rotation can be positive or negative. The fluid can be heated or cooled. The study reveals the rich solution and flow structures and complicated stability features. One symmetric and two symmetric/asymmetric solution branches are found with seventy five limit points and fourteen bifurcation points. The flows on these branches can be symmetric, asymmetric, 2-cell, and up to 14-cell structures. Dynamic responses of the multiple solutions to finite random disturbances are examined by the direct transient computation. It is found that possible physically realizable fully developed flows evolve, as the variation of buoyancy force, from a stable steady multicell state at a large buoyancy force of cooling to the coexistence of three stable steady multicell states, a temporal periodic oscillation state, the coexistence of periodic oscillation and chaotic oscillation, a chaotic temporal oscillation, a subharmonic-bifurcation-driven asymmetric oscillating state, and a stable steady 2-cell state at large buoyancy force of heating.
Natural convective heat transfer from square cylinder
Novomestský, Marcel; Smatanová, Helena; Kapjor, Andrej
2016-06-01
This article is concerned with natural convective heat transfer from square cylinder mounted on a plane adiabatic base, the cylinders having an exposed cylinder surface according to different horizontal angle. The cylinder receives heat from a radiating heater which results in a buoyant flow. There are many industrial applications, including refrigeration, ventilation and the cooling of electrical components, for which the present study may be applicable
Experimental study of laminar mixed convection in a rod bundle with mixing vane spacer grids
Energy Technology Data Exchange (ETDEWEB)
Mohanta, Lokanath, E-mail: lxm971@psu.edu [Department of Mechanical and Nuclear Engineering, Pennsylvania State University, University Park, PA 16802 (United States); Cheung, Fan-Bill [Department of Mechanical and Nuclear Engineering, Pennsylvania State University, University Park, PA 16802 (United States); Bajorek, Stephen M.; Tien, Kirk; Hoxie, Chris L. [Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001 (United States)
2017-02-15
Highlights: • Investigated the heat transfer during mixed laminar convection in a rod bundle with linearly varying heat flux. • The Nusselt number increases downstream of the inlet with increasing Richardson number. • Developed an enhancement factor to account for the effects of mixed convection over the forced laminar heat transfer. - Abstract: Heat transfer by mixed convection in a rod bundle occurs when convection is affected by both the buoyancy and inertial forces. Mixed convection can be assumed when the Richardson number (Ri = Gr/Re{sup 2}) is on the order of unity, indicating that both forced and natural convection are important contributors to heat transfer. In the present study, data obtained from the Rod Bundle Heat Transfer (RBHT) facility was used to determine the heat transfer coefficient in the mixed convection regime, which was found to be significantly larger than those expected assuming purely forced convection based on the inlet flow rate. The inlet Reynolds (Re) number for the tests ranged from 500 to 1300, while the Grashof (Gr) number varied from 1.5 × 10{sup 5} to 3.8 × 10{sup 6} yielding 0.25 < Ri < 4.3. Using results from RBHT test along with the correlation from the FLECHT-SEASET test program for laminar forced convection, a new correlation is proposed for mixed convection in a rod bundle. The new correlation accounts for the enhancement of heat transfer relative to laminar forced convection.
DEFF Research Database (Denmark)
Toft, Anne Elisabeth
2016-01-01
Danish Cool. Keld Helmer-Petersen, Photography and the Photobook Handout exhibition text in English and Chinese by Anne Elisabeth Toft, Curator The exhibition Danish Cool. Keld Helmer-Petersen, Photography and the Photobook presents the ground-breaking work of late Danish photographer Keld Helmer...
Zhu, Ping; Wang, Yuting; Chen, Shuyi S.; Curcic, Milan; Gao, Cen
2016-01-01
Roll vortices in the atmospheric boundary layer (ABL) are important to oil operation and oil spill transport. This study investigates the impact of storm-induced sea surface temperature (SST) cooling on the roll vortices generated by the convective and dynamic instability in the ABL of Hurricane Isaac (2012) and the roll induced transport using hindcasting large eddy simulations (LESs) configured from the multiply nested Weather Research & Forecasting model. Two experiments are performed: one forced by the Unified Wave INterface - Coupled Model and the other with the SST replaced by the NCEP FNL analysis that does not include the storm-induced SST cooling. The simulations show that the roll vortices are the prevalent eddy circulations in the ABL of Isaac. The storm-induced SST cooling causes the ABL stability falls in a range that satisfies the empirical criterion of roll generation by dynamic instability, whereas the ABL stability without considering the storm-induced SST cooling meets the criterion of roll generation by convective instability. The ABL roll is skewed and the increase of convective instability enhances the skewness. Large convective instability leads to large vertical transport of heat and moisture; whereas the dominant dynamic instability results in large turbulent kinetic energy but relatively weak heat and moisture transport. This study suggests that failure to consider roll vortices or incorrect initiation of dynamic and convective instability of rolls in simulations may substantially affect the transport of momentum, energy, and pollutants in the ABL and the dispersion/advection of oil spill fume at the ocean surface.
Surface tension-driven convection patterns in two liquid layers
Juel, A; McCormick, W D; Swift, J B; Swinney, H L; Juel, Anne; Burgess, John M.; Swinney, Harry L.
1999-01-01
Two superposed liquid layers display a variety of convective phenomena that are inaccessible in the traditional system where the upper layer is a gas. We consider several pairs of immiscible liquids. Once the liquids have been selected, the applied temperature difference and the depths of the layers are the only independent control parameters. Using a perfluorinated hydrocarbon and silicone oil system, we have made the first experimental observation of convection with the top plate hotter than the lower plate. Since the system is stably stratified, this convective flow is solely due to thermocapillary forces. We also have found oscillatory convection at onset in an acetonitrile and n-hexane system heated from below.
Quantum thermodynamic cooling cycle
Palao, J P; Gordon, J M; Palao, Jose P.; Kosloff, Ronnie; Gordon, Jeffrey M.
2001-01-01
The quantum-mechanical and thermodynamic properties of a 3-level molecular cooling cycle are derived. An inadequacy of earlier models is rectified in accounting for the spontaneous emission and absorption associated with the coupling to the coherent driving field via an environmental reservoir. This additional coupling need not be dissipative, and can provide a thermal driving force - the quantum analog of classical absorption chillers. The dependence of the maximum attainable cooling rate on temperature, at ultra-low temperatures, is determined and shown to respect the recently-established fundamental bound based on the second and third laws of thermodynamics.
Active Cooling Of A Mobile Phone Handset
Grimes, Ronan; WALSH, EDMOND; Walsh, Pat
2010-01-01
Abstract Power dissipation levels in mobile phones continue to increase due to gaming, higher power applications, and increased functionality associated with the internet. The current cooling methodologies of natural convection and radiation limit the power dissipation within a mobile phone to between 1-2 W depending on size. As power dissipation levels increase, products such as mobile phones will require active cooling to ensure that the devices operate within an acceptable tempe...
3D thermal analysis of a permanent magnet motor with cooling fans
Institute of Scientific and Technical Information of China (English)
Zheng TAN; Xue-guan SONG; Bing JI; Zheng LIU; Ji-en MA; Wen-ping CAO
2015-01-01
Overheating of permanent magnet (PM) machines has become a major technical challenge as it gives rise to magnet demagnetization, degradation of insulation materials, and loss of motor efficiency. This paper proposes a state-of-the-art cooling system for an axial flux permanent magnet (AFPM) machine with the focus on its structural optimization. A computational fluid dynamics (CFD) simulation with thermal consideration has been shown to be an efficient approach in the literature and is thus employed in this work. Meanwhile, a simplified numerical approach to the AFPM machine with complex configuration in 3D consisting of conduction, forced convection, and conjugate heat transfer is taken as a case study. Different simplification meth-ods (including configuration and working conditions) and two optimized fans for forced convection cooling are designed and installed on the AFPM machine and compared to a natural convection cooling system. The results show that the proposed ap-proach is effective for analyzing the thermal performance of a complex AFPM machine and strikes a balance between reasona-ble simplification, accuracy, and computational resource.%目的：提出一种适合永磁电机的冷却系统设计方案，降低电机本体温度。 创新点：提出一种适合永磁电机热分析的CFD仿真模型。 方法：采用计算流体动力学方法对包含冷却风扇的永磁电机进行空间三维热力学分析和优化设计。 结论：本文提出并优化后的冷却风扇可有效降低永磁电机的最高和平均温度。
Modeling condensation with a noncondensable gas for mixed convection flow
Liao, Yehong
2007-05-01
This research theoretically developed a novel mixed convection model for condensation with a noncondensable gas. The model developed herein is comprised of three components: a convection regime map; a mixed convection correlation; and a generalized diffusion layer model. These components were developed in a way to be consistent with the three-level methodology in MELCOR. The overall mixed convection model was implemented into MELCOR and satisfactorily validated with data covering a wide variety of test conditions. In the development of the convection regime map, two analyses with approximations of the local similarity method were performed to solve the multi-component two-phase boundary layer equations. The first analysis studied effects of the bulk velocity on a basic natural convection condensation process and setup conditions to distinguish natural convection from mixed convection. It was found that the superimposed velocity increases condensation heat transfer by sweeping away the noncondensable gas accumulated at the condensation boundary. The second analysis studied effects of the buoyancy force on a basic forced convection condensation process and setup conditions to distinguish forced convection from mixed convection. It was found that the superimposed buoyancy force increases condensation heat transfer by thinning the liquid film thickness and creating a steeper noncondensable gas concentration profile near the condensation interface. In the development of the mixed convection correlation accounting for suction effects, numerical data were obtained from boundary layer analysis for the three convection regimes and used to fit a curve for the Nusselt number of the mixed convection regime as a function of the Nusselt numbers of the natural and forced convection regimes. In the development of the generalized diffusion layer model, the driving potential for mass transfer was expressed as the temperature difference between the bulk and the liquid-gas interface
Analogy between thermal convective and magnetohydrodynamic instabilities
Energy Technology Data Exchange (ETDEWEB)
Valdmanis, Ya.Ya.; Kukainis, O.A.
1977-01-01
An examination is made of the analogy between thermo-convective instability and instability produced by various electromagnetic forces both in steady and alternating thermal and electromagnetic fields. An example is given for calculating an assumed bubble instability which could occur in an alternating magnetic field. 17 references.
Energy Technology Data Exchange (ETDEWEB)
Favre, E.
1997-09-26
coupled buoyancy and thermo-capillary convection lead to a convective motion of the interface liquid/gas which drastically changes the heat and mass transfer across the liquid layer. Two experiments were considered, depending on the fluid: oil or mercury. The liquid is set in a cooled cylindrical vessel, and heated by a heat flux across the center of the free surface. The basic flow, in the case of oil, is a torus. When the heat parameter increases, a stationary flow appears as petals or rays when the aspect ratio. The lateral confinement selects the azimuthal wavelength. In the case of petals-like flow, a sub-critical Hopf bifurcation is underlined. The turbulence is found to be `weak`, even for the largest values of the Marangoni number (Ma = 1.3 10{sup 5}). In the case of mercury, the thermo-capillary effect is reduced to zero to impurities at the surface which have special trajectories we describe and compare to a simpler experiment. Only the buoyancy forces induce a unstationary, weakly turbulent flow as soon as the heating power exceeds 4W (Ra = 4.5 10{sup 3}, calculated with h = 1 mm). The past part concerns the analysis of the effect on the flow of the boundary conditions, the geometry, the Prandtl number and the buoyancy force with the help of the literature. Results concerning heat transfer, in particular the exponent of the law Nusselt number vs. heating power, were compared with available data. (author) 115 refs.
Energy Technology Data Exchange (ETDEWEB)
Ohk, Seung Min; Chung, Bum Jin [Kyunghee University, Yongin (Korea, Republic of)
2016-05-15
The Passive Cooling System (PCS) driven by natural forces drew research attention since Fukushima nuclear power plant accident. This study investigated the natural convection heat transfer inside of vertical pipe with emphasis on the phenomena regarding the boundary layer interaction. Numerical calculations were carried out using FLUENT 6.3. Experiments were performed for the parts of the cases to explore the accuracy of calculation. Based on the analogy, heat transfer experiment is replaced by mass transfer experiment using sulfuric acid copper sulfate (CuSO{sub 4}. H{sub 2}SO{sub 4}) electroplating system. The natural convection heat transfer inside a vertical pipe is studied experimentally and numerically. Experiments were carried out using sulfuric acid-copper sulfate (H{sub 2}SO{sub 4}-CuSO{sub 4}) based on the analogy concept between heat and mass transfer system. Numerical analysis was carried out using FLUENT 6.3. It is concluded that the boundary layer interaction along the flow passage influences the heat transfer, which is affected by the length, diameter, and Prandtl number. For the large diameter and high Prandtl number cases, where the thermal boundary layers do not interfered along the pipe, the heat transfer agreed with vertical flat plate for laminar and turbulent natural convection correlation within 8%. When the flow becomes steady state, the forced convective flow appears in the bottom of the vertical pipe and natural convection flow appears near the exit. It is different behavior from the flow on the parallel vertical flat plates. Nevertheless, the heat transfer was not different greatly compared with those of vertical plate.
Fan, Jiwen; Leung, L Ruby; Rosenfeld, Daniel; Chen, Qian; Li, Zhanqing; Zhang, Jinqiang; Yan, Hongru
2013-11-26
Deep convective clouds (DCCs) play a crucial role in the general circulation, energy, and hydrological cycle of our climate system. Aerosol particles can influence DCCs by altering cloud properties, precipitation regimes, and radiation balance. Previous studies reported both invigoration and suppression of DCCs by aerosols, but few were concerned with the whole life cycle of DCC. By conducting multiple monthlong cloud-resolving simulations with spectral-bin cloud microphysics that capture the observed macrophysical and microphysical properties of summer convective clouds and precipitation in the tropics and midlatitudes, this study provides a comprehensive view of how aerosols affect cloud cover, cloud top height, and radiative forcing. We found that although the widely accepted theory of DCC invigoration due to aerosol's thermodynamic effect (additional latent heat release from freezing of greater amount of cloud water) may work during the growing stage, it is microphysical effect influenced by aerosols that drives the dramatic increase in cloud cover, cloud top height, and cloud thickness at the mature and dissipation stages by inducing larger amounts of smaller but longer-lasting ice particles in the stratiform/anvils of DCCs, even when thermodynamic invigoration of convection is absent. The thermodynamic invigoration effect contributes up to ~27% of total increase in cloud cover. The overall aerosol indirect effect is an atmospheric radiative warming (3-5 W m(-2)) and a surface cooling (-5 to -8 W m(-2)). The modeling findings are confirmed by the analyses of ample measurements made at three sites of distinctly different environments.
Eskew, Matthew W.; Harrison, Jason; Simoyi, Reuben H.
2016-11-01
Oxidation reactions of thiourea by chlorite in a Hele-Shaw cell are excitable, autocatalytic, exothermic, and generate a lateral instability upon being triggered by the autocatalyst. Reagent concentrations used to develop convective instabilities delivered a temperature jump at the wave front of 2.1 K. The reaction zone was 2 mm and due to normal cooling after the wave front, this generated a spike rather than the standard well-studied front propagation. The reaction front has solutal and thermal contributions to density changes that act in opposite directions due to the existence of a positive isothermal density change in the reaction. The competition between these effects generates thermal plumes. The fascinating feature of this system is the coexistence of plumes and fingering in the same solution which alternate in frequency as the front propagates, generating hot and cold spots within the Hele-Shaw cell, and subsequently spatiotemporal inhomogeneities. The small ΔT at the wave front generated thermocapillary convection which competed effectively with thermogravitational forces at low Eötvös Numbers. A simplified reaction-diffusion-convection model was derived for the system. Plume formation is heavily dependent on boundary effects from the cell dimensions. This work was supported by Grant No. CHE-1056366 from the NSF and a Research Professor Grant from the University of KwaZulu-Natal.
A hybrid convection scheme for use in non-hydrostatic numerical weather prediction models
Directory of Open Access Journals (Sweden)
Volker Kuell
2008-12-01
Full Text Available The correct representation of convection in numerical weather prediction (NWP models is essential for quantitative precipitation forecasts. Due to its small horizontal scale convection usually has to be parameterized, e.g. by mass flux convection schemes. Classical schemes originally developed for use in coarse grid NWP models assume zero net convective mass flux, because the whole circulation of a convective cell is confined to the local grid column and all convective mass fluxes cancel out. However, in contemporary NWP models with grid sizes of a few kilometers this assumption becomes questionable, because here convection is partially resolved on the grid. To overcome this conceptual problem we propose a hybrid mass flux convection scheme (HYMACS in which only the convective updrafts and downdrafts are parameterized. The generation of the larger scale environmental subsidence, which may cover several grid columns, is transferred to the grid scale equations. This means that the convection scheme now has to generate a net convective mass flux exerting a direct dynamical forcing to the grid scale model via pressure gradient forces. The hybrid convection scheme implemented into the COSMO model of Deutscher Wetterdienst (DWD is tested in an idealized simulation of a sea breeze circulation initiating convection in a realistic manner. The results are compared with analogous simulations with the classical Tiedtke and Kain-Fritsch convection schemes.
Convection in Condensible-rich Atmospheres
Ding, F.; Pierrehumbert, R. T.
2016-05-01
Condensible substances are nearly ubiquitous in planetary atmospheres. For the most familiar case—water vapor in Earth’s present climate—the condensible gas is dilute, in the sense that its concentration is everywhere small relative to the noncondensible background gases. A wide variety of important planetary climate problems involve nondilute condensible substances. These include planets near or undergoing a water vapor runaway and planets near the outer edge of the conventional habitable zone, for which CO2 is the condensible. Standard representations of convection in climate models rely on several approximations appropriate only to the dilute limit, while nondilute convection differs in fundamental ways from dilute convection. In this paper, a simple parameterization of convection valid in the nondilute as well as dilute limits is derived and used to discuss the basic character of nondilute convection. The energy conservation properties of the scheme are discussed in detail and are verified in radiative-convective simulations. As a further illustration of the behavior of the scheme, results for a runaway greenhouse atmosphere for both steady instellation and seasonally varying instellation corresponding to a highly eccentric orbit are presented. The latter case illustrates that the high thermal inertia associated with latent heat in nondilute atmospheres can damp out the effects of even extreme seasonal forcing.
Toward a unified theory of atmospheric convective instability
Shirer, H. N.
1982-01-01
A nonlinear three-dimensional truncated spectral model of shallow and moist Boussinesq convection indicates that parallel instability and thermal forcing are linked, in view of the fact that only one convective mode exists in which either or both mechanisms are operating to generate convection in the planetary boundary layer. It is also established that the wind field causes two-dimensional roll convection formation, an alignment of the convection with the wind in a preferred manner, and a propagation speed that is related to the wind component perpendicular to the roll axis. Latent heating is responsible for the decrease of the critical value of the environmental lapse rate in accordance with the slice method stability criterion. When only the upper part of the upward branch is moist and all of the downward branch is dry, latent heating also causes a finite-amplitude convective solution for Rayleigh number values lower than the critical value of linear analysis.
Meridional Circulation in Solar and Stellar Convection Zones
Featherstone, Nicholas A
2015-01-01
We present a series of 3-D nonlinear simulations of solar-like convection, carried out using the Anelastic Spherical Harmonic (ASH) code, that are designed to isolate those processes that drive and shape meridional circulations within stellar convection zones. These simulations have been constructed so as to span the transition between solar-like differential rotation (fast equator/slow poles) and ``anti-solar' differential rotation (slow equator/fast poles). Solar-like states of differential rotation, arising when convection is rotationally constrained, are characterized by a very different convective Reynolds stress than anti-solar regimes, wherein convection only weakly senses the Coriolis force. We find that the angular momentum transport by convective Reynolds stress plays a central role in establishing the meridional flow profiles in these simulations. We find that the transition from single-celled to multi-celled meridional circulation profiles in strong and weak regimes of rotational constraint is lin...
Scaling regimes in spherical shell rotating convection
Gastine, T; Aubert, J
2016-01-01
Rayleigh-B\\'enard convection in rotating spherical shells can be considered as a simplified analogue of many astrophysical and geophysical fluid flows. Here, we use three-dimensional direct numerical simulations to study this physical process. We construct a dataset of more than 200 numerical models that cover a broad parameter range with Ekman numbers spanning $3\\times 10^{-7} \\leq E \\leq 10^{-1}$, Rayleigh numbers within the range $10^3 < Ra < 2\\times 10^{10}$ and a Prandtl number unity. We investigate the scaling behaviours of both local (length scales, boundary layers) and global (Nusselt and Reynolds numbers) properties across various physical regimes from onset of rotating convection to weakly-rotating convection. Close to critical, the convective flow is dominated by a triple force balance between viscosity, Coriolis force and buoyancy. For larger supercriticalities, a subset of our numerical data approaches the asymptotic diffusivity-free scaling of rotating convection $Nu\\sim Ra^{3/2}E^{2}$ in ...
Thermo-Elastic Analysis of Internally Cooled Structures Using a Higher Order Theory
Arnold, Steven M.; Bednarcyk, Brett A.; Aboudi, Jacob
2001-01-01
This paper presents the results of a study on the thermomechanical behavior of internally cooled silicon nitride structures. Silicon nitride is under consideration for elevated temperature aerospace engine applications. and techniques for lowering the operating temperature of structures composed of this material are under development. Lowering the operating temperature provides a large payoff in terms of fatigue life and may be accomplished through the use of thermal barrier coatings (TBC's) and the novel concept of included cooling channels. Herein, an in-depth study is performed on the behavior of a flame-impinged silicon nitride plate with a TBC and internal channels cooled by forced air. The analysis is performed using the higher order theory for functionally graded materials (HOTFGM), which has been developed through NASA Glenn Research Center funding over the past several years. HOTFGM was chosen over the traditional finite element approach as a prelude to an examination of functionally graded silicon nitride structures for which HOTFGM is ideally suited. To accommodate the analysis requirement% of the internally cooled plate problem, two crucial enhancements were made to the two-dimensional Cartesian-based version of HOTFGM. namely, incorporation of internal boundary capabilities and incorporation of convective boundary conditions. Results indicate the viability and large benefits of cooling the plate via forced air through cooling channels. Furthermore, cooling can positively impact the stress and displacement fields present in the plate, yielding an additional payoff in terms of fatigue life. Finally, a spin-off capability resulted from inclusion of internal boundaries within HOTFGM; the ability to simulate the thermo-elastic response of structures with curved surfaces. This new capability is demonstrated, and through comparison with an analytical solution, shown to be viable and accurate.
Forced convective heat transfer in curved diffusers
Rojas, J.; Whitelaw, J. H.; Yianneskis, M.
1987-01-01
Measurements of the velocity characteristics of the flows in two curved diffusers of rectangular cross section with C and S-shaped centerlines are presented and related to measurements of wall heat transfer coefficients along the heated flat walls of the ducts. The velocity results were obtained by laser-Doppler anemometry in a water tunnel and the heat transfer results by liquid crystal thermography in a wind tunnel. The thermographic technique allowed the rapid and inexpensive measurement of wall heat transfer coefficients along flat walls of arbitrary boundary shapes with an accuracy of about 5 percent. The results show that an increase in secondary flow velocities near the heated wall causes an increase in the local wall heat transfer coefficient, and quantify the variation for maximum secondary-flow velocities in a range from 1.5 to 17 percent of the bulk flow velocity.
Heat transfer in two-component internal mist cooling
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Novak, V.; Sadowski, D.L.; Schoonover, K.G.; Abdel-Khalik, S.I. [G.W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, 801 Ferst Drive, Atlanta, GA 30332-0405 (United States); Ghiaasiaan, S.M. [G.W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, 801 Ferst Drive, Atlanta, GA 30332-0405 (United States)], E-mail: mghiaasiaan@gatech.edu
2008-09-15
An experimental investigation was conducted to examine steady, internal, nozzle-generated, gas/liquid mist cooling in vertical channels. The ideal primary cooling mechanism in this situation is surface evaporation of an ultra-thin, subcooled liquid film that forms on the heated surface. The aim was to quantify the effects of various operating and design parameters on the cooling effectiveness. Parameters tested included the liquid atomization nozzle design, inlet flow condition (liquid mass fraction; carrier gas velocity, temperature and humidity; liquid temperature; liquid droplet size distribution; and gas/liquid combination), channel characteristics (cross-section geometry, length and surface wettability), and flow direction. Interest in this research has been motivated by the need for a highly efficient cooling mechanism in high-power lasers for inertial fusion reactor applications. A fully instrumented experimental test facility that included three cylindrical and two rectangular electrically heated test sections with different cross-sections and unheated entry lengths was used. The channel hydraulic diameters covered the range 16-26.7 mm, and the heated length-to-hydraulic-diameter ratio varied in the range from 23.3 to 51. Water was used as the mist liquid, with air or helium as the carrier gas. Three types of mist generating nozzles with significantly different spray characteristics were used. Local heat transfer coefficients, defined based on the temperature difference between the heated surface and the bulk gas, were obtained along the channels for a wide range of operating conditions. The data indicate that mist cooling can increase the heat transfer coefficient by more than an order of magnitude compared to forced convection using only the carrier gas.
Stochastic Convection Parameterizations
Teixeira, Joao; Reynolds, Carolyn; Suselj, Kay; Matheou, Georgios
2012-01-01
computational fluid dynamics, radiation, clouds, turbulence, convection, gravity waves, surface interaction, radiation interaction, cloud and aerosol microphysics, complexity (vegetation, biogeochemistry, radiation versus turbulence/convection stochastic approach, non-linearities, Monte Carlo, high resolutions, large-Eddy Simulations, cloud structure, plumes, saturation in tropics, forecasting, parameterizations, stochastic, radiation-clod interaction, hurricane forecasts
Houdek, G
2010-01-01
In this short review on stellar convection dynamics I address the following, currently very topical, issues: (1) the surface effects of the Reynolds stresses and nonadiabaticity on solar-like pulsation frequencies, and (2) oscillation mode lifetimes of stochastically excited oscillations in red giants computed with different time-dependent convection formulations.
Convective dust clouds in a complex plasma
Mitic, S; Ivlev, A V; Hoefner, H; Thoma, M H; Zhdanov, S; Morfill, G E
2008-01-01
The plasma is generated in a low frequency glow discharge within an elongated glass tube oriented vertically. The dust particles added to the plasma are confined above the heater and form counter-rotating clouds close to the tube centre. The shape of the clouds and the velocity field of the conveying dust particles are determined. The forces acting on the particles are calculated. It is shown that convection of the dust is affected by the convective gas motion which is triggered, in turn, by thermal creep of the gas along the inhomogeneously heated walls of the tube.
CPU Cooling of Desktop PC by Closed-end Oscillating Heat-pipe (CEOHP
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S. Rittidech
2005-01-01
Full Text Available The CEOHP cooling module consisted of two main parts, i.e., the aluminum housing and the CEOHP. The house casing was designed to be suitable for CEOHP. The housing to drilling for insert the CEOHP. The CEOHP design employed copper tubes: Two sets of capillary tubes with an inner diameter of 0.002 m, an evaporator length of 0.05 and a condenser length of 0.16 m and each of which has six meandering turns. The evaporator section was embraced in the aluminum housing and attached to the thermal pad of Pentium 4 CPU, model SL 6 PB, 2.26 GHZ. While the condenser section was embraced in the cooling fin housing and cooled by forced convection. R134a was used as the working fluid with filling ratio of 50%. In the experiment, the CPU chip with a power of 58 W was 70Â°C. Fan speed of 2000 and 4000 rpm. It was found that, if fan speed increases the cooling performance increases. The CEOHP cooling module had better thermal performance than conventional heat sink.
Simultaneous effects of water spray and crosswind on performance of natural draft dry cooling tower
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Ahmadikia Hossein
2013-01-01
Full Text Available To investigate the effect of water spray and crosswind on the effectiveness of the natural draft dry cooling tower (NDDCT, a three-dimensional model has been developed. Efficiency of NDDCT is improved by water spray system at the cooling tower entrance for high ambient temperature condition with and without crosswind. The natural and forced heat convection flow inside and around the NDDCT is simulated numerically by solving the full Navier-Stokes equations in both air and water droplet phases. Comparison of the numerical results with one-dimensional analytical model and the experimental data illustrates a well-predicted heat transfer rate in the cooling tower. Applying water spray system on the cooling tower radiators enhances the cooling tower efficiency at both no wind and windy conditions. For all values of water spraying rate, NDDCTs operate most effectively at the crosswind velocity of 3m/s and as the wind speed continues to rise to more than 3 m/s up to 12 m/s, the tower efficiency will decrease by approximately 18%, based on no-wind condition. The heat transfer rate of radiator at wind velocity 10 m/s is 11.5% lower than that of the no wind condition. This value is 7.5% for water spray rate of 50kg/s.
Health assessment of cooling fan bearings using wavelet-based filtering.
Miao, Qiang; Tang, Chao; Liang, Wei; Pecht, Michael
2012-12-24
As commonly used forced convection air cooling devices in electronics, cooling fans are crucial for guaranteeing the reliability of electronic systems. In a cooling fan assembly, fan bearing failure is a major failure mode that causes excessive vibration, noise, reduction in rotation speed, locked rotor, failure to start, and other problems; therefore, it is necessary to conduct research on the health assessment of cooling fan bearings. This paper presents a vibration-based fan bearing health evaluation method using comblet filtering and exponentially weighted moving average. A new health condition indicator (HCI) for fan bearing degradation assessment is proposed. In order to collect the vibration data for validation of the proposed method, a cooling fan accelerated life test was conducted to simulate the lubricant starvation of fan bearings. A comparison between the proposed method and methods in previous studies (i.e., root mean square, kurtosis, and fault growth parameter) was carried out to assess the performance of the HCI. The analysis results suggest that the HCI can identify incipient fan bearing failures and describe the bearing degradation process. Overall, the work presented in this paper provides a promising method for fan bearing health evaluation and prognosis.
Health Assessment of Cooling Fan Bearings Using Wavelet-Based Filtering
Directory of Open Access Journals (Sweden)
Qiang Miao
2012-12-01
Full Text Available As commonly used forced convection air cooling devices in electronics, cooling fans are crucial for guaranteeing the reliability of electronic systems. In a cooling fan assembly, fan bearing failure is a major failure mode that causes excessive vibration, noise, reduction in rotation speed, locked rotor, failure to start, and other problems; therefore, it is necessary to conduct research on the health assessment of cooling fan bearings. This paper presents a vibration-based fan bearing health evaluation method using comblet filtering and exponentially weighted moving average. A new health condition indicator (HCI for fan bearing degradation assessment is proposed. In order to collect the vibration data for validation of the proposed method, a cooling fan accelerated life test was conducted to simulate the lubricant starvation of fan bearings. A comparison between the proposed method and methods in previous studies (i.e., root mean square, kurtosis, and fault growth parameter was carried out to assess the performance of the HCI. The analysis results suggest that the HCI can identify incipient fan bearing failures and describe the bearing degradation process. Overall, the work presented in this paper provides a promising method for fan bearing health evaluation and prognosis.