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…
Forced heat convection in annular spaces
International Nuclear Information System (INIS)
This report deals with the experimental study of forced heat convection in annular spaces through which flow of air is passing when a uniform heat flux is dissipated across the inner wall. These observations took place chiefly in the region where thermal equilibrium are not yet established. Amongst other things it became apparent that, both in the region where thermal equilibrium conditions are on the way to establishment and where they are already established, the following relationship held good: the longitudinal temperature gradient, either on the wall or in the fluid stream, is proportional to the heat flux dissipated q, and inversely proportional to the average flow rate V: dT/dx = B (q/V). From this result the next step is to express the variations of the local convection coefficient ? (or of the Margoulis number M) in a relationship of the form: 1/M = ?(V) + F(x). If this relationship is compared with the classical empirical relationship ? = KVn (where n is close to 0.8), the relationship: 1/M = ?V1-n + F(x) is obtained (? is a constant for a given annular space); from this it was possible to coordinate the whole set of experimental results. (author)
Theory of heat transfer with forced convection film flows
Shang, Deyi
2010-01-01
Developing a new treatment of ""Free Convection Film Flows and Heat Transfer"" began in Shang's first monograph and is continued in this monograph. The current book displays the recent developments of laminar forced convection and forced film condensation. It is aimed at revealing the true features of heat and mass transfer with forced convection film flows to model the deposition of thin layers. The novel mathematical similarity theory model is developed to simulate temperature - and concentration - dependent physical processes. The following topics are covered in this book: Mathematical meth
NANOFLUID PROPERTIES FOR FORCED CONVECTION HEAT TRANSFER: AN OVERVIEW
Directory of Open Access Journals (Sweden)
W.H.Azmi
2013-06-01
Full Text Available Nano?uids offer a significant advantage over conventional heat transfer ?uids 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.
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). PMID:17748675
Mechanistic Multidimensional Modeling of Forced Convection Boiling Heat Transfer
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Raf M. Podowski
2008-12-01
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.
Energy Technology Data Exchange (ETDEWEB)
Ghadimi, Mohammad; Ghadamian, Hossein [Islamic Azad Univ., Tehran (Iran, Islamic Republic of). Dept. of Energy Engineering, Science and Research Branch; Hamidi, Aliasghar A. [Tehran Univ. (Iran, Islamic Republic of). Dept. of Chemical Engineering; Fazelpour, Farivar [Islamic Azad Univ. of South Tehran Branch, Tehran (Iran, Islamic Republic of). Dept. of Energy System Engineering; Behghadam, Mehdi [Islamic Azad Univ. of Roudehen Branch, Tehran (Iran, Islamic Republic of). Dept. of Mechanical Engineering
2012-11-01
The present paper describes a two-dimensional finite volume numerical simulation of flow and heat transfer in airflow windows by free and forced convection techniques. The governing equations are the fully elliptic, Reynolds-averaged Navier-Stokes equations. The simple algorithm is employed to correct the pressure term. The second-order upwind scheme is used to discretize the convection terms. The (k-{epsilon}/RNG) turbulence model is applied for the flow simulation. The mesh used is the body-fitted, multi-plane grid system. Results on the variations of velocity and temperature profiles with geometrical parameters, at different temperature and velocity, for heat transfer by free and forced convection techniques are presented. Comparisons of the present results on temperature distribution for forced convection and for free convection with the available experimental forced convection data indicate that the airflow-influenced forced convection methods are considerably enhanced. (orig.)
Effect of Buoyancy on Forced Convection Heat Transfer in Vertical Channels - a Literature Survey
International Nuclear Information System (INIS)
This report contains a short resume of the available information from various sources on the effect of free convection flow on forced convection heat transfer in vertical channels. Both theoretical and experimental investigations are included. Nearly all of the theoretical investigations are concerned with laminar flow with or without internal heat generation. More consistent data are available for upward flow than for downward flow. Curves are presented to determine whether free convection or forced convection mode of heat transfer is predominant for a particular Reynolds number and Rayleigh number. At Reb > 105 free convection effects are negligible. Downward flow through a heated channel at low Reynolds number is unstable. Under similar conditions the overall heat transfer coefficient for downward flow tends to be higher than that for upward flow
Directory of Open Access Journals (Sweden)
S. I. ANWAR
2012-10-01
Full Text Available In this paper, convective heat transfer coefficient of Indian gooseberry (Emblica officinalis, in three different forms (shreds, slices and pieces, under forced convection mode has been determined. These forms were dried in laboratory drier. Values of constants C and n have been determined using experimental data and regression analysis for calculating values of convective heat transfer coefficient. It was found that the convective heat transfer coefficient varies with form of commodity being dried and decreases as the drying progresses. The value of convective heat transfer coefficient was highest for shredded form (30.39 W/m2oC followed by slices (25.88 W/m2oC and pieces (18.67 W/m2oC when compared at certain final moisture content. The data were also analyzed for per cent uncertainty.
Evaluating a tobacco-curing oven using a forced-convection heat exchanger USCO — MADR
Néstor Enrique Cerquera Peña; Yaneth Liliana Ruiz Osorio; Eduardo Pastrana Bonilla
2010-01-01
A traditional oven for curing tobacco leaves was redesigned (based on existing infrastructure); a forced-convection heat exchan- ger system was implemented in it which worked with coffee hulls as fuel. This oven (called a forced-convection tobacco leaf curing oven) was evaluated during the harvesting season. It was found that temperature and relative humidity inside the furnace could be controlled with this assembly during the three stages involved in curing tobacco leaves. The equipment ...
NUMERICAL ANALYSIS OF FORCED CONVECTIVE HEAT TRANSFER THROUGH HELICAL CHANNELS
Dr. K. E. REBY ROY.; Rohit N Shenoy; Bibin Prasad
2012-01-01
Helical ducts are used in a variety of applications including food processing, thermal processing plants and refrigeration. They are advantageous due to their high heat transfer coefficient and compactness compared to straight tubes. The curvature of the coil governs the centrifugal force resulting in development of secondaryflow i.e. the fluid stream in the outer side of the pipe moves faster than the fluid streams in the inner side of the pipe. In the present study, Computational Fluid Dyna...
NUMERICAL ANALYSIS OF FORCED CONVECTIVE HEAT TRANSFER THROUGH HELICAL CHANNELS
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Dr. K. E. Reby Roy
2012-07-01
Full Text Available Helical ducts are used in a variety of applications including food processing, thermal processing plants and refrigeration. They are advantageous due to their high heat transfer coefficient and compactness compared to straight tubes. The curvature of the coil governs the centrifugal force resulting in development of secondaryflow i.e. the fluid stream in the outer side of the pipe moves faster than the fluid streams in the inner side of the pipe. In the present study, Computational Fluid Dynamics (CFD simulations using Fluent 6.3.26 are carried out for helical rectangular ducts wound over a cylindrical passage. The cylindrical passage is oriented horizontallyand acts as a counterflow heat exchanger. The analysis is done by changing the flow rates of four different fluids like Ethylene Glycol, Kerosene, Nano Fluid and Water. The fluid flow and heat transfer characteristics of the fluids are studied and Nusselt Number correlations with Dean Number are developed.
Analysis of Rectangular Microchannel under Forced convection heat transfer condition
Directory of Open Access Journals (Sweden)
Dr. B.S.Gawali,
2011-03-01
Full Text Available Micro-convection is a strategic area in transport phenomena, since it is the basis for a wide range of miniaturized high-performance pplications. Surface area is one of the important concepts for high flux heat transfer in Microchannel performance. Microchannel with hydraulic diameters 440?m, 476?m, 500?m and 550?m are analyzed for optimize microchannel hydraulic diameter. The microchannel having height of 400?m, 450?m, 500?m, 600?m with width of 500?m is analyzed numerically. Spacing between microchannel is also varied in range of 250?m, 300?m, 350?mand 400?m are considered for the analysis. Cu material microchannel having length of 30mm which carries 20 microchannels on top surface of the cu piece is to be considered. Flow rate also varied from 5lpm to 30 lpm for optimization with water as a medium. From numerical study it is observed that as hydraulic diameter increases from 444?m to 545 ?m the flow rate pressure drop also increases with decreases in diameter. Also heat in put to icrochannel as increases from 5 watt to 80 watt temperature drop is high at flow rate of 17lpm to 20 lpm. From analysis it is observed that as hydraulic diameter of microchannel is major concept of microchannel heat transfer which is dependent on flow rate of waterin microchannel. The microchannels with hydraulic diameter of 440?m to 600?m will follow temperature drop up-to 6 degree Kelvin with heat input of 5 watt to 80watt with flow rate of 5lpm to 25lpm.
Boiling heat transfer with forced convection of LiBr-water solution in horizontal tube
International Nuclear Information System (INIS)
This paper examines the phenomena of boiling heat transfer and calculates the local heat transfer coefficient of a one-through boiler types regenerator for an absorption refrigerating machine. Boiling heat transfer with forced convection of Br-water solution in a horizontal tube is experimentally investigated. The results are summarized as follows. The flow patterns in the tube change from single phase flow to bubble flow, to stratified flow, to slug flow and finally, to annular flow. Local heat flux paries in proportion to the temperature difference between the tube wall and the solution. In the section where solution and vapor are supposed to flow as annular flow, the boiling heat transfer coefficient can be explained by the Dengler-Addoms expression. An the forced convection boiling heat transfer coefficients are higher than the pool boiling heat transfer coefficient
Prediction of forced convection heat transfer to Lead-Bismuth-Eutectic
Thiele, Roman
2013-01-01
The goal of this work is to investigate the capabilities of two different commercial codes, OpenFOAM and ANSYS CFX, to predict forced convection heat transfer in low Prandtl number fluids and investigate the sensitivity of these predictions to the type of code and to several input parameters.The goal of the work is accomplished by predicting forced convection heat transfer in two different experimental setups with the codes OpenFOAM and ANSYS CFX using three different turbulence models and va...
Convective Heat Transfer Coefficients of the Human Body under Forced Convection from Ceiling
DEFF Research Database (Denmark)
Kurazumi, Yoshihito; Rezgals, Lauris
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 temperature distribution of an average person. The measurements were performed in a room with controlled thermal environment. Air temperature was set at 26ºC for cooling and at 20ºC for heating. The radiant temperature asymmetry in horizontal and vertical direction was close to zero, i.e. mean radiant temperature was equal to the air temperature. The air velocity of the isothermal downward flow from the ceiling at height of 1.5 m above the floor (above the top of the head) was set in a range between still air and 0.73 m/s. Based on the analyses of the results relationships for determination 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<0.3 m/s, due to the natural convection were found. The results may be useful during design of air distribution in rooms, e.g. low impulse ventilation, diffuse ventilation, etc.
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.
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 temperature distribution of an average person. The measurements were performed in a room with controlled thermal environment. Air temperature was set at 26ºC for cooling and at 20ºC for heating. The radiant ...
Forced convection heat exchange inside porous sintered metals
International Nuclear Information System (INIS)
Methods and results of investigating heat exchange in the process of liquid flow inside porous sintered metals have been analyzed. It has been shown that experimental data available include extremely conflicting correlations between heat transfer coefficient and Reynolds number, porosity, and relative wall thickness. Scattering of the data can attain one order of magnitude. The volume coefficient of heat transfer inside pores determined in papers does not correspond to its real value in the initial equations of the inner problem of porous cooling. Calculating and experimental method of determining the heat transfer coefficient has been developed and realized on the unit of radiation heating. More accurate experimental data on intraporous heat exchange have been obtained. It has been established that relative wall thickness does not affect the intensity of heat transfer inside pores
International Nuclear Information System (INIS)
In a high-level waste (HLW) repository, heat is generated by the radioactive decay of the waste. This can affect the safety of the repository because the surrounding environment can be changed by the heat transfer through the rock. Thus, it is important to determine the heat transfer coefficient of the atmosphere in the underground repository. In this study, the heat transfer coefficient was estimated by measuring the indoor environmental factors in the Korea Atomic Energy Research Institute Underground Research Tunnel (KURT) under forced convection. For the experiment, a heater of 5 kw capacity, 2 meters long, was inserted through the tunnel wall in the heating section of KURT in order to heat up the inside of the rock to 90 .deg. C, and fresh air was provided by an air supply fan connected to the outside of the tunnel. The results showed that the average air velocity in the heating section after the provision of the air from outside of the tunnel was 0.81 m/s with the Reynolds number of 310,000 ? 340,000. The seasonal heat transfer coefficient in the heating section under forced convection was 7.68 W/m2 K in the summer and 7.24 W/mm2 K in the winter
Experimental and theoretical study on forced convection film boiling heat transfer
International Nuclear Information System (INIS)
Theoretical solutions of forced convection film boiling heat transfer from horizontal cylinders in saturated liquids were obtained based on a two-phase laminar boundary layer film boiling model. It was clarified that author's experimental data for the cylinders with the nondimensional diameters, D, of around 1.3 in water and in Freon-113 agreed with the values of theoretical numerical solutions based on the two-phase laminar boundary layer model with the smooth vapor-liquid interface except those for low flow velocities. A forced convection film boiling heat transfer correlation including the radiation contribution from the cylinders with various diameters in saturated and subcooled liquids was developed based on the two-phase laminar boundary layer film boiling model and the experimental data for water and Freon-113 at wide ranges of flow velocities, surface superheats, system pressures and cylinder diameters. (author)
Heat transfer tests under forced convection conditions with high wettable heater surface
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Mitsutake, Toru; Morooka, Shin-ichi; Miura, Shigeru; Akiba, Miyuki; Sato, Hisaki; Shirakawa, Ken-etsu; Oosato, Tetsuo; Yamamoto, Seiji [Toshiba Co., Kanagawa (Japan)
2002-07-01
Under forced convection and atmospheric pressure conditions, heat transfer tests were performed using the annulus channel of a heater rod with highly wettable surface. Improvement of boiling heat transfer requires that the cooling liquid can contact the heating surface, or a high-wettability heating surface, even if a vapor bubble layer is generated on the surface. >From this point of view, high-wettable heating surface was studied. As oxide semiconductor-coated materials are highly-wettable, we made a TiO{sub 2} coated heater rod. TiO{sub 2} coated surface has a high-wettability, in terms of contact angle and Leidenfrost temperature. The boiling curve was measured with and without TiO coated surface. The results showed difference between with and without TiO{sub 2} coating. TiO{sub 2} coating rod showed lower boiling onset heat flux, wider nucleate boiling region and higher critical heat flux than without coating. In summary, high wettablity heater surface produced higher boiling heat transfer characteristics under forced convection conditions. (author)
An Experimental Study of Heat Transfer During Forced Air Convection
Xiao, Bowang; Wang, Gang; Wang, Qigui; Maniruzzaman, Mohammed; Sisson, Richard D.; Rong, Yiming
2011-10-01
Cast aluminum alloys are usually subject to solution treatment, quenching, and aging hardening for improved mechanical properties. Cooling rate during quenching plays an important role in residual stress, distortion, and mechanical property distributions in the resultant cast aluminum components. As the cooling rates of work pieces heavily depend on the interfacial heat transfer coefficient (HTC) between work pieces and quenchants, it is important to understand how HTC varies with different quenching conditions so that optimal quenching process can be achieved. In this study, a quenching system and an experimental procedure of obtaining HTC are presented. A series of experiments have been conducted to study the variations of HTC with respect to air temperature, air humidity, air velocity, and part orientation.
Burnout in boiling heat transfer. Part III. High-quality forced-convection systems
International Nuclear Information System (INIS)
This is the final part of a review of burnout during boiling heat transfer. The status of burnout in high-quality forced-convection systems is reviewed, and recent developments are summarized in detail. A general guide to the considerable literature is given. Parametric effects and correlations for water in circular and noncircular ducts are presented. Other topics discussed include transients, steam-generator applications, correlations for other fluids, fouling, and augmentation
The effect of fouling on thermodynamic performance of forced convective heat transfer through a duct
International Nuclear Information System (INIS)
Based on the first and second thermodynamic laws, a new systematic approach to study in detail the effect of fouling on the thermodynamic performance of forced convective heat transfer through a duct with constant wall temperature and constant wall heat flux for thermally and hydrodynamically fully developed turbulent flow is investigated. When considering fouling exists inside the duct, the local and mean exergy variation coefficient, exergy variation flux, dimensionless exergy variation number and the equation of exergy variation rate of working fluids, etc. have been put forward and their generalized expressions derived. A criterion evaluating the effect of fouling on the exergy variation of working fluids of the forced convective heat transfer process, which is defined as the exergy variation degradation rate, has been put forward. By reference to a duct, the numerical results of the exergy variation of working fluids are obtained (considering fouling or not), the effects of Reynolds number, the thickness of the fouling layer, dimensionless inlet temperature difference and wall heat flux on the exergy variation of working fluids are discussed. The results show that the exergy variation degradation rate increases with the increase of Reynolds number and decreases with the increase of dimensionless inlet temperature and wall heat flux. The exergy variation caused by the heat conduction of fouling plays an important role in the total exergy variation of working fluids
Wall heat transfer coefficient for condensation and boiling in forced convection of sodium
International Nuclear Information System (INIS)
The wall heat transfer coefficient for sodium boiling and condensation in forced convective flow is theoretically derived by using the momentum-heat transfer analogy and the logarithmic law for velocity distribution in the liquid film. Only one constant in the logarithmic form is empirically determined. The results are insensitive to some of the approximations used, namely in the evaluation of E and of /tau/ /SUB i/ //tau/ /SUB w/. The results from the suggested correlation are in good agreement with the Zeigarnick and Litvinov data over the range of parameters that were specified in their experiments, while the Chen and NATOF correlations predict lower heat transfer coefficients. The suggested correlation predicts well the data in the high-heat transfer coefficient region. It can be argued that the data in the low heat transfer coefficient region are affected by unstable flow conditions and the uncertainties in the saturation-temperature measurements
Forced Convection Heat Transfer Experiments of the Finned Plate in a Duct
Energy Technology Data Exchange (ETDEWEB)
Chae, Myeongseon; Moon, Jeyoung; Chung, Bumjin [Kyung Hee Univ., Yongin (Korea, Republic of)
2014-05-15
The studies have been focused on the optimization of fin geometries to maximize the heat transfer rate. The forced convection heat transfer rates were affected largely by the fin spacing, fin height, and tip clearance. As the fin spacing decreases and fin height increases, heat transfers from the fins to the ambient are enhanced as they are directly proportional to the surface area. For a large tip clearance, the fluid tends to escape from the inner fin region to the outer wall region resulting in the decrease of the overall heat removal capability. Thus, the parametric influences of these variables are to be investigated to develop a generalized heat transfer correlation for the geometry. This study is a preliminary experimental study for plate-fin geometries such as fin spacing, fin height and duct width. Mass transfer experiments were carried out based on the analogy concept, using a copper sulfate electroplating system. The work has the relevance with the Reactor Cavity Cooling System performance enhancement study in the VHTR. Forced convection heat transfer experiments were performed for the vertical plate-fins in a duct. Based on the analogy between heat and mass transfer systems, mass transfer rates were measured using the cupric acid copper sulfate electroplating system. The fin spacings were varied from 0.002m to 0.007m, fin heights 0.01m and 0.015m, Re{sub Dh} from 10 to 6,500, and duct widths from 0.010m to 0.02m. The test results showed that the heat transfer rates enhanced with the increase of fin height and the decrease of fin spacing as they enlarge the heat transfer area. And the heat transfer rates were impaired with the increase of the duct width as the bypass flows increased to tip clearance region.
Analysis of Forced Convection Heat Transfer for Axial Annular Flow of Giesekus Viscoelastic Fluid
Energy Technology Data Exchange (ETDEWEB)
Mohseni, Mehdi Moayed; Rashidi, Fariborz; Movagar, Mohammad Reza Khorsand [Amirkabir University of Technology, Tehran (Iran, Islamic Republic of)
2015-02-15
Analytical solutions for the forced convection heat transfer of viscoelastic fluids obeying the Giesekus model are obtained in a concentric annulus under laminar flow for both thermal and hydrodynamic fully developed conditions. Boundary conditions are assumed to be (a) constant fluxes at the walls and (b) constant temperature at the walls. Temperature profiles and Nusselt numbers are derived from dimensionless energy equation. Subsequently, effects of elasticity, mobility parameter and viscous dissipation are discussed. Results show that by increasing elasticity, Nusselt number increases. However, this trend is reversed for constant wall temperature when viscous dissipation is weak. By increasing viscous dissipation, the Nusselt number decreases for the constant flux and increases for the constant wall temperature. For the wall cooling case, when the viscous dissipation exceeds a critical value, the generated heat overcomes the heat which is removed at the walls, and fluid heats up longitudinally.
International Nuclear Information System (INIS)
In the first part, free-convection and nucleate pool boiling heat transfer (up to burn-out heat flux) between a platinum wire of 0.15 mm in diameter in neon, deuterium and hydrogen has been studied at atmospheric pressure. These measurements were continued in liquid neon up to 23 bars (Pc ? 26.8 b). Film boiling heat transfer coefficients have been measured in pool boiling liquid neon at atmospheric pressure with three heating wires (diameters 0.2, 0.5, 2 mm). All the results have been compared with existing correlations. The second part is devoted to measurements of the critical heat flux limiting heat transfer with small temperature differences between the wall and the liquid neon flowing inside a tube (diameters 3 x 3.5 mm) heated by joule effect on 30 cm of length. Influences of flow stability, nature of electrical current, pressure, mass flow rate and subcooling are shown. In conclusion, the similarity of the heat transfer characteristics in pool boiling as well as in forced convection of liquid neon and hydrogen is emphasized. (author)
Performance of a forced convection solar drier integrated with gravel as heat storage material
Energy Technology Data Exchange (ETDEWEB)
Mohanraj, M. [Dr Mahalingam College of Engineering and Technology, Pollachi (India). Dept. of Mechanical Engineering; Chandrasekar, P. [Swinburne Univ. of Technology, Sarawak (Malaysia). School of Engineering Sciences
2009-07-01
Sun drying is the most common method used in India to dry agricultural products such as grains, fruits and vegetables. The rate of drying depends on solar radiation, ambient temperature, wind velocity, relative humidity, initial moisture content, type of crops, crop absorptivity and mass product per unit exposed area. However, this method of spreading the crop in a thin layer on the ground has several disadvantages. This paper reported on a study that focused on developing a forced convection solar drier integrated with heat storage materials for drying various agricultural crops. The indirect forced convection solar drier, integrated with gravel as a sensible heat material, was used to dry pineapple slices under conditions similar to those found in Pollachi, India. The performance of the system was discussed along with the drying characteristics, drying rate, and specific moisture extraction rate. The results showed that the moisture content (wet basis) of pineapple was reduced from about 87.5 to 14.5 per cent (equilibrium moisture content) in about 29 hours in the bottom tray and 32 hours in the top tray. The thermal efficiency of the solar air heater was also reviewed. 9 refs., 5 figs.
Forced convection heat transfer in structure packed beds: packing form and particle shape
International Nuclear Information System (INIS)
A forced convection heat transfer inside micro pores of structure packed beds with spherical or ellipsoidal particles are numerically studied in this paper. Three-dimensional Navier Stokes equations and RNG k-? turbulent model with scalable wall function are adopted for present computations. The effects of packing form and particle shape are investigated in detail. The macroscopic hydrodynamic and heat transfer results are obtained from micro pore cells by using integrating method. The results show that, with the same physical parameters, the pressure drops in structure packed beds are much lower than those in randomly packed beds while the overall heat transfer efficiencies (except SC packing) are much higher. The traditional correlations of flow and heat transfer extracted from randomly packings are unavailable for structured packings, and some modified correlations are obtained. Furthermore, it finds that, with the same particle shape (sphere), the overall heat transfer performance of SC packing is the best and with the same packing form (FCC), the overall heat transfer performance of packing with long ellipsoidal particles is the best
Forced convection heat transfer in structure packed beds: packing form and particle shape
Energy Technology Data Exchange (ETDEWEB)
Jian, Yang; Min, Zeng; Qiuwang, Wang [Xi' an Jiaotong University, Xi' an (China); Xiao, Yan [Nuclear Power Institute of China, Chengdu (China)
2009-07-01
A forced convection heat transfer inside micro pores of structure packed beds with spherical or ellipsoidal particles are numerically studied in this paper. Three-dimensional Navier Stokes equations and RNG k-{epsilon} turbulent model with scalable wall function are adopted for present computations. The effects of packing form and particle shape are investigated in detail. The macroscopic hydrodynamic and heat transfer results are obtained from micro pore cells by using integrating method. The results show that, with the same physical parameters, the pressure drops in structure packed beds are much lower than those in randomly packed beds while the overall heat transfer efficiencies (except SC packing) are much higher. The traditional correlations of flow and heat transfer extracted from randomly packings are unavailable for structured packings, and some modified correlations are obtained. Furthermore, it finds that, with the same particle shape (sphere), the overall heat transfer performance of SC packing is the best and with the same packing form (FCC), the overall heat transfer performance of packing with long ellipsoidal particles is the best.
Reliability comparison of forced and natural convection residual heat removal in the GCFR
International Nuclear Information System (INIS)
Loss of cooling consequences for all breeder cores require an increased reliability of the engineered Residual Heat Removal (RHR) systems provided to assure abundant cooling of the core at decay heat levels. An upflow GCFR core design offers the capability for pressurized decay heat removal by natural convection, thus enhancing core cooling reliability and diversity. A quantitative assessment is presented for the Residual Heat Removal reliability achievable with and without natural convection. The reliability gains due to natural convection are limited by the demand frequency for PCRV depressurization and by the equipment which has to change state in order to establish natural convection. The coolant circulation diversity accomplished with natural convection is a major advantage
Evaluating a tobacco-curing oven using a forced-convection heat exchanger USCO — MADR
Directory of Open Access Journals (Sweden)
Néstor Enrique Cerquera Peña
2010-05-01
Full Text Available A traditional oven for curing tobacco leaves was redesigned (based on existing infrastructure; a forced-convection heat exchan- ger system was implemented in it which worked with coffee hulls as fuel. This oven (called a forced-convection tobacco leaf curing oven was evaluated during the harvesting season. It was found that temperature and relative humidity inside the furnace could be controlled with this assembly during the three stages involved in curing tobacco leaves. The equipment used performed excellently when using coffee hulls as fuel, having the following approximate consumption during curing: 8.92 kilograms per hour during the yellowing stage, 17.75 kilograms per hour during the leaf drying and color fixation phase and 19.29 kilograms per hour during the stem drying stage. Comparative analysis of the oven’s operating costs along with the proposed adjustments to be made to it would allow its implementation as a promising alternative in the existing tobacco chain.
LAMINAR FORCED CONVECTION HEAT TRANSFER OVER STAGGERED CIRCULAR TUBE BANKS: A CFD APPROACH
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Tahseen A. Tahseen
2013-06-01
Full Text Available This paper presents the numerical study of two-dimensional forced convection heat transfer for staggered tube banks in cross flow under incompressible, steady-state conditions. This system is solved for body-fitted coordinates using the finite volume method for flow over a bundle of cylindrical tubes. A constant heat flux is imposed on the surface of the tubes as the thermal boundary condition. The type of arrangement considered is a set of staggered tubes. Ratios of longitudinal pitch to tube diameter (ST/D of 1.25, 1.5, and 2 are considered. Reynolds numbers are varied from 25 to 250 and the Prandtl number is taken as 0.71. Velocity field vectors, temperature contours, and the local and average Nusselt numbers are analyzed in this paper. It can be seen that the predicted results are in good agreement with experimental and numerical results obtained previously. The obtained results show that the heat transfer rate increases with a reduction in the step of the longitudinal tube diameter. The local heat transfer depends strongly on the Reynolds number. The highest values are obtained at the surface opposite to the direction of flow. The heat transfer rate is insignificant in the areas of recycling.
An assessment of correlations of forced convection heat transfer to water at supercritical pressure
International Nuclear Information System (INIS)
Highlights: • Evaluates 26 correlations of heat transfer to supercritical water. • Compiles 3220 experimental data points for the evaluation. • Evaluates based on both entire database and three heat transfer regimes. • Provides a guide to choosing the best correlation for engineering design. - Abstract: The heat transfer of supercritical water is essential for supercritical water-cooled nuclear reactors. Many empirical correlations for heat transfer to supercritical water were proposed over the past few decades. Some evaluations of the correlations were conducted, and inconsistent conclusions appeared owing to limited correlations or experimental data. This work presents an extensive survey of the literature of correlations and experiments of forced convection heat transfer to water flowing upward in vertical tubes at supercritical pressure. There are 26 correlations found, and an experimental database containing 3220 data points from vertical tubes are compiled from nine independent laboratories. All available correlations are assessed against the experimental database. The results show that the best correlation has a mean absolute deviation of 12.8%, predicting 82.3% of the database within ±20%. The entire database is divided into three categories, and the correlations which can give the most accurate predictions of the experimental data from different categories are also identified. The results provide a guide to choosing a proper correlation for engineering practice. Some topics worthy of attention for future studies are indicated
Directory of Open Access Journals (Sweden)
P. MOHAJERI KHAMENEH,
2010-11-01
Full Text Available Three dimensional simulations of the single-phase laminar flow and forced convective heat transfer of water in round tube and straight microchannel heat exchangers were investigated numerically. This numerical method was developed to measure heat transfer parameters of round tube and straight microchannel tube geometries. Then, similarities and differences were compared between different geometries. The geometries and operating conditions of those indicated heat exchangers were created using a finite volume-based computational fluid dynamics technique. In this article, at each Z-location variation of dimensionless local temperature, nondimensional local heat flux variation and dimensionless local Nusselt number distribution along the tube length were compared between round tube and straight microchannel heat exchangers. Consequently, averaged computational Nusselt number was obtained for those indicated models and then validation study was performed for round tube counter flow type heat exchanger model. Finally, all of these numerical results for both kind of geometries in counter flow heat exchangers were discussed in details.
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 be performed to develop probability of confidence in what is measured in the test loop. Overall, the testing loop will allow development of needed heat transfer related thermophysical parameters for all the salts, validate existing correlations, validate measuring instruments under harsh environment, and have extensive corrosion testing of materials of construction.
International Nuclear Information System (INIS)
In the present study, spray cooling heat transfer was experimentally investigated for the case in which water is sprayed onto the surfaces of micro-fins in forced convection and nucleate boiling regions. The experimental results show that an increase in the droplet flow rate improves heat transfer due to forced convection and nucleate boiling in the both case of smooth surface and surfaces of micro-fins. However, the effect of subcooling for fixed droplet flow rate is very weak. Micro-fins surfaces enhance the spray cooling heat transfer significantly. In the dilute spray region, the micro-fin structure has a significant effect on the spray cooling heat transfer. However, this effect is weak in the dense spray region. A previously determined correlation between the Nusselt number and Reynolds number shows good agreement with the present experimental data for a smooth surface
International Nuclear Information System (INIS)
Three-dimensional simulations of the single-phase laminar flow and forced convective heat transfer of water in microchannels with small rectangular sections having specific hydraulic diameters and distinct geometric configurations were investigated numerically. The numerical results indicated that the laminar heat transfer was to be dependent upon the aspect ratio and the ratio of the hydraulic diameter to the center to center distance of the microchannels. The geometries and operating conditions of that indicated microchannel were created using a finite volume-based computational fluid dynamics technique. The aim of this paper is to obtain computational Nusselt number in laminar flow using Fluent CFD Solver and to validate it with available experimental studies. Acquired numerical results have an admissible compatibility with available experimental studies. In addition, conceivable temperature profiles and pressure drops have been obtained at each Z-location in this model. Then, pressure drop values in the present model were explored for each Reynolds number. Finally, the effects of geometric parameters on the average Nusselt number in the laminar flow were illustrated numerically.
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.
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.
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 test sections provides the Joule heating required to heat up the fluid to supercritical conditions. A high-temperature dielectric gasket isolates the current carrying parts of the test section from the rest of the assembly. Temperature and pressure drop data are collected at the inlet and outlet, and along the heated length of the test section. The test loops and test sections are designed according to American Society of Mechanical Engineers (ASME) Pressure Piping B31.1, and Boiler and Pressure Vessel Code, Section VIII-Division 1 rules. The final test loops and test sections assemblies are certified by Technical Standards and Safety Authority (TSSA). Every attempt is made to use off-the-shelf components where possible in order to streamline the design process and reduce costs. Following a rigorous selection process, stainless steel Types 316 and 316H are selected as the construction materials for the test loops, and Inconel 625 is selected as the construction material for the test sections. This thesis describes the design of the SCW and R-134a loops along with the three test-section geometries (i.e., tubular, annular and bundle designs).
Transient convective heat transfer
Scientific Electronic Library Online (English)
J., Padet.
2005-03-01
Full Text Available In nature, as well as within the human-made thermal systems, the time-variable regimes are more commonly encountered, if not always, than the permanent regimes. Nevertheless, studies in convection are still more frequent in the permanent regimes, undoubtedly due to the related difficulties in calcul [...] ation in terms of time and cost of computation. One may distinguish two categories of time-dependent transfers: those which are due to external causes (variable boundary conditions) and those that are due to internal causes (sources of variable power, instabilities, turbulence), and the combination of these two types may also be encountered. In this presentation, we shall analyze some situations which belong to the first category. These are concerned with: - a group of boundary layer flows in forced, natural or mixed convection, where the wall is subjected to time-variable conditions in temperature or flux. - another group of fluid flows within ducts, in laminar mixed convection regime, where the entry conditions (mass flow rate, temperature) are time-dependent. The techniques of analysis are mainly extensions to the differential method and to the integral method of Karman-Polhausen in boundary layer flows, and the finite differences solution of the vorticity and energy equations for internal flows. The results presented in the transient state are caused by steps of temperature, heat flux or velocity, and in particular show the time evolution of the dynamic and thermal boundary layers, as well of the heat transfer coefficients. Three examples of applications will then be treated: the active control of convective transfers, the measurement of heat transfer coefficients, and the analysis of heat exchangers. The main idea in the active control is that of managing the temperatures or heat fluxes by employing a variable regime. Under certain conditions, this procedure may reveal itself quite interesting. The measurement of transfer coefficients by the photothermal impulse method possesses a great interest since it is performed in a non-intrusive way without contact. However, in order to be precise, it needs to account for the thermal boundary layer perturbation due to the radiative flux sent over the surface, which means to know the evolution of the transfer coefficient during the measurement. Previous studies therefore provide essential information. Within the domain of heat exchangers, we shall present a different global method, which allows for the evaluation of the time constant of an equipment in response to sample variations of temperature or mass flow rates at the entrance. In conclusion, a brief balance of the ICHMT Symposium "Transient heat and mass transfer", Cesme, Turkey, August 2003, will be presented.
Burnout in boiling heat transfer. Part II: subcooled and low quality forced-convection systems
International Nuclear Information System (INIS)
Recent experimental and analytical developments regrading burnout in subcooled and low quality forced-convection systems are reviewed. Much data have been accumulated which clarify the parametric trends and lead to new design correlations for water and a variety of other coolants in both simple and complex geometries. A number of critical experiments and models have been developed to attempt to clarify the burnout mechanism(s) in simpler geometries and power transients
Kakac, Sadik; Pramuanjaroenkij, Anchasa
2014-01-01
Intended for readers who have taken a basic heat transfer course and have a basic knowledge of thermodynamics, heat transfer, fluid mechanics, and differential equations, Convective Heat Transfer, Third Edition provides an overview of phenomenological convective heat transfer. This book combines applications of engineering with the basic concepts of convection. It offers a clear and balanced presentation of essential topics using both traditional and numerical methods. The text addresses emerging science and technology matters, and highlights biomedical applications and energy technologies. What’s New in the Third Edition: Includes updated chapters and two new chapters on heat transfer in microchannels and heat transfer with nanofluids Expands problem sets and introduces new correlations and solved examples Provides more coverage of numerical/computer methods The third edition details the new research areas of heat transfer in microchannels and the enhancement of convective heat transfer with nanofluids....
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.
International Nuclear Information System (INIS)
An experimental work is conducted on combined (free and forced) convection to study the local and average heat transfer for hydrodynamically fully developed and thermally developing laminar air flow in a horizontal circular cylinder. The experimental setup uses an aluminum cylinder as test section with 30 mm inside diameter and 900 mm heated length (L/D = 30) subjected to a constant wall heat flux boundary condition. The investigation covers the Reynolds number range from 400 to 1600 and the heat flux range from 60 W/m2 to 400 W/m2. The fully developed condition is achieved by using aluminum entrance section pipes (calming sections) having the same inside diameter as the test section pipe but with variable lengths. The entrance sections included two long calming sections, one with length of 180 cm (L/D = 60), and the other with length of 240 cm (L/D = 80), and two short calming sections with lengths of 60 cm (L/D = 20) and of 120 cm (L/D = 40). The surface temperature distribution along the cylinder surface and the local and average Nusselt number distributions with dimensionless axial distance Z+ were presented. For all entrance sections, an increase in the Nusselt number values was found as the heat flux increases. It was concluded that the free convection effects tended to decrease the heat transfer results at low Re number and to increase the heat transfer results for high Re number. The combined convection regime could be bounded by a suitable selection of Re number ranges and heat flux ranges. The obtained Richardson numbers (Ri) ranged approximately from 0.1171 to 7.125. The average Nusselt numbers were correlated with the Rayleigh numbers/Reynolds numbers
Burnout in boiling heat transfer. II. Subcooled and low-quality forced-convection systems
International Nuclear Information System (INIS)
Recent experimental and analytical developments regarding burnout in subcooled and low-quality forced-convection systems are reviewed. Many data have been accumulated which clarify the parametric trends and lead to new design correlations for water and a variety of other coolants in both simple and complex geometries. A number of critical experiments and models have been developed to attempt to clarify the burnout mechanism(s) in simpler geometries. Other topics discussed include burnout with power transients and techniques to augment burnout. 86 references
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)
International Nuclear Information System (INIS)
The heat transfer resistance of a porous deposit can be expressed as the sum of two components: one associated with conduction through the fluid-filled deposit and a second associated with surface roughness. This simple relationship appears to hold under both single-phase forced-convection and boiling heat-transfer conditions. The conductive component of the total deposit resistance is always positive, whereas the roughness component is negative. Values for ? and Rroughness measured in this investigation are as follows. Under single-phase forced-convection heat-transfer conditions, ? = 1.3 ± 0.2 W/mK and Rroughness = -4 x 10-6 m2K/W for magnetite deposits. Under flow-boiling heat-transfer conditions, ? = 0.2 to 0.9 W/mK and Rroughness = -36 x 10-6 m2K/W for magnetite deposits, whereas ? = 2.0 W/mK and Rroughness = -43 x 10-6 m2K/W for deposits composed of approximately equal proportions of copper and magnetite. (author)
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.
International Nuclear Information System (INIS)
Turbulent mixed and forced convection were investigated experimentally in a horizontal duct which modeled both thermally and hydraulically a nuclear waste repository. Reynolds numbers ranged from 6,000 to 180,000. The maximum Grashof number achieved was 1.5 109. The experimental apparatus consisted of a horizontal duct, 30.5 cm wide by 45.2 cm high (12 inches by 18 inches), and 4.42 m (14.5 feet) in length. The duct had 29 heater sections in the axial direction, each 15.2 cm (6 inches) long. Heat flux profiles were imposed on the four walls of the duct by a network of resistance heater pads. The opposing sidewall heaters were connected by pairs in parallel, giving five independently controlled sets of heaters; floor, lower side, middle side, upper side, and ceiling. Since the power inputs to these five heater sets were controlled separately, a wide range of heat flux profiles could be investigated
Bejan, Adrian
2013-01-01
Written by an internationally recognized authority on heat transfer and thermodynamics, this second edition of Convection Heat Transfer contains new and updated problems and examples reflecting real-world research and applications, including heat exchanger design. Teaching not only structure but also technique, the book begins with the simplest problem solving method (scale analysis), and moves on to progressively more advanced and exact methods (integral method, self similarity, asymptotic behavior). A solutions manual is available for all problems and exercises.
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)
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.
International Nuclear Information System (INIS)
In this paper we present new experimental results of investigation on average heat transfer characteristics of a forced air-flow through a rectangular channel with the lower and upper surfaces roughened by ribs; data for a rectangular channel with flat surfaces are presented for comparison as well. The channel cross-section is 120 mm wide and 12 mm high; the channel is operated with the lower and upper walls kept at fixed temperature whereas the sides are adiabatic. The ribs have a square cross section and are mounted 60° parallel-tilted (the angle is respect to main stream) in astaggered arrangement. The tested configurations differ each other for the rib side dimension, namely, 2 or 4 mm, and for their pitch-to-side ratio equal to 10, 20 and 40. Upstream the test channel, there is an entry-section consisting of a 800 mm long, rectangular duct with the same transverse dimensions as the test section but with flat and adiabatic walls. Air flow rates have been varied in order to have Reynolds numbers, based on the duct hydraulic diameter, ranging between 700 and 7500. The average Nusselt numbers are evaluated on the basis of the air-flow bulk-temperature at entrance and exit from the heated zone, as well as of the surface temperature measured by eight T-type thermocouples plugged into the heated walls. The test section is also equipped with static pressure taps placed at the heated zone ends. Results show an increase of the average Nusselt number, calculated as the ratio Nu/Nu0, for the all tested ribbed channels ranging between 1.0 and 5.0.
Cucchi, M.; Fustinoni, D.; Gramazio, P.; Colombo, L. P. M.; Niro, A.
2014-04-01
In this paper we present new experimental results of investigation on average heat transfer characteristics of a forced air-flow through a rectangular channel with the lower and upper surfaces roughened by ribs; data for a rectangular channel with flat surfaces are presented for comparison as well. The channel cross-section is 120 mm wide and 12 mm high; the channel is operated with the lower and upper walls kept at fixed temperature whereas the sides are adiabatic. The ribs have a square cross section and are mounted 60° parallel-tilted (the angle is respect to main stream) in astaggered arrangement. The tested configurations differ each other for the rib side dimension, namely, 2 or 4 mm, and for their pitch-to-side ratio equal to 10, 20 and 40. Upstream the test channel, there is an entry-section consisting of a 800 mm long, rectangular duct with the same transverse dimensions as the test section but with flat and adiabatic walls. Air flow rates have been varied in order to have Reynolds numbers, based on the duct hydraulic diameter, ranging between 700 and 7500. The average Nusselt numbers are evaluated on the basis of the air-flow bulk-temperature at entrance and exit from the heated zone, as well as of the surface temperature measured by eight T-type thermocouples plugged into the heated walls. The test section is also equipped with static pressure taps placed at the heated zone ends. Results show an increase of the average Nusselt number, calculated as the ratio Nu/Nu0, for the all tested ribbed channels ranging between 1.0 and 5.0.
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 designs in Gen-IV supercritical water nuclear reactors (SCWR). The main objective of this study has been to validate the performance of the current approach as a tool for the analysis of large-scale systems.
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.
Aris, M.S.; McGlen, R.; Owen, I.; Sutcliffe, C.J.
2011-01-01
Forced air convection heat pipe cooling systems play an essential role in the thermal management of electronic and power electronic devices such as microprocessors and IGBT's (Integrated Gate Bipolar Transistors). With increasing heat dissipation from these devices, novel methods of improving the thermal performance of fin stacks attached to the heat pipe condenser section are required. The current work investigates the use of a wing type surface protrusions in the form of 3-D delta wing tabs...
International Nuclear Information System (INIS)
Highlights: • Thermal conductivity is assumed a linear function of temperature. • It occurs due to diffusion-like radiation or a high temperature gradient. • Nusselt number and temperature profiles are obtained based on perturbation method. • Variable conductivity reveals enhancement obtained by porous media more clearly. • Nusselt number changes linearly with a linear change in conductivity. - Abstract: Effects of variation of the thermal conductivity on forced convection in a parallel-plates channel heat exchanger occupied by a fluid saturated porous medium are investigated analytically based on the perturbation methods. Walls of the channel are kept at a constant heat flux. Thermal conductivity of the medium is assumed to be a linear function of temperature (due to moderate radiation heat transfer in cellular foams or temperature dependent conductivity of the material). The Brinkman–Forchheimer–extended Darcy model for the flow field is used. Relations representing the temperature profile and Nusselt number as functions of porous medium shape parameter and thermal conductivity variation parameter are derived. Obtained Nusselt number and temperature profile are studied parametrically. No analytical investigation based on a variable conductivity approach for Brinkman–Forchheimer–extended Darcy model has been previously performed. Results show that a linear increase in the thermal conductivity of the medium results in a semi-linear increase in the Nusselt number
International Nuclear Information System (INIS)
An investigation of the thermal hydraulic characteristics in the passive residual heat removal system of the System integrated Modular Advanced ReacTor-P (SMART-P) has been carried out using the MARS code, which is a best estimate system analysis code. The SMART-P is designed to cool the system during accidental conditions by a natural convection. The dominant heat transfer in the steam generator is a boiling mode under a forced convection condition, and it is a single-phase liquid and a boiling heat transfer under a natural convection condition. Most of the heat is removed in the heat exchanger of the passive residual heat removal system by a condensation heat transfer. The passive residual heat removal system can remove the energy from the primary side as long as the heat exchanger is submerged in the refueling water tank. The mass flow is stable under a natural circulation condition though it oscillates periodically with a small amplitude. The parameter study is performed by considering the effects of an effective height between the steam generator and the heat exchanger, a hydraulic resistance, an initial pressure, a non-condensable gas fraction in the compensating tank, and a valve actuation time, which are useful for the design of the passive residual heat removal system. The mass flow in the passive residual heat removal system has been affected by the height between the steam generator and the heat exchanger, and the hydraulic resistance of the loop
Ahmed, Mahmoud; Eslamian, Morteza
2015-12-01
Laminar natural convection in differentially heated (? = 0°, where ? is the inclination angle), inclined (? = 30° and 60°), and bottom-heated (? = 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 ? = 90° (bottom-heated) and the maximum heat transfer rate occurs in an inclination angle which varies with the Ra number. PMID:26183389
Simulation of MHD CuO-water nanofluid flow and convective heat transfer considering Lorentz forces
Sheikholeslami, Mohsen; Bandpy, Mofid Gorji; Ellahi, R.; Zeeshan, A.
2014-11-01
Magnetic field effect on CuO-water nanofluid flow and heat transfer in an enclosure which is heated from below is investigated. Lattice Boltzmann method is applied to solve the governing equations. The effective thermal conductivity and viscosity of nanofluid are calculated by KKL (Koo-Kleinstreuer-Li) correlation. In this model effect of Brownian motion on the effective thermal conductivity is considered. Effect of active parameter such as: Hartmann number, heat source length, nanoparticle volume fraction and Rayleigh numbers on the flow and heat transfer characteristics have been examined. The results reveal that the enhancement in heat transfer increases as Hartmann number and heat source length increase but it decreases with increase of Rayleigh number. Also it can be found that effect of Hartmann number and heat source length is more pronounced at high Rayleigh number.
Study of the equivalent diameter concept for heat transfer by forced convection in annular channels
International Nuclear Information System (INIS)
This work describes a comparative analysis between experimental values of heat transfer coefficients in fully developed turbulent flow for a concentric annular channel, and those calculated with the empirical correlations obtained for tubes by Dittus-Boelter, Sieder and Tate, a modified Colburn equation, and that proposed by Gnielinski which applies the analogy between friction and heat transfer. The coefficients were calculated by means of two different equivalent diameters: 1) The hydraulic equivalent diameter; and 2) The heated equivalent diameter. It was concluded that the hydraulic equivalent diameter gives much better results than the heated equivalent diameter. (Author)
Heterogeneous nanofluids: natural convection heat transfer enhancement
Bennacer Rachid; Oueslati Fakhreddine
2011-01-01
Abstract Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account th...
Simulation of MHD CuO–water nanofluid flow and convective heat transfer considering Lorentz forces
International Nuclear Information System (INIS)
Magnetic field effect on CuO–water nanofluid flow and heat transfer in an enclosure which is heated from below is investigated. Lattice Boltzmann method is applied to solve the governing equations. The effective thermal conductivity and viscosity of nanofluid are calculated by KKL (Koo–Kleinstreuer–Li) correlation. In this model effect of Brownian motion on the effective thermal conductivity is considered. Effect of active parameter such as: Hartmann number, heat source length, nanoparticle volume fraction and Rayleigh numbers on the flow and heat transfer characteristics have been examined. The results reveal that the enhancement in heat transfer increases as Hartmann number and heat source length increase but it decreases with increase of Rayleigh number. Also it can be found that effect of Hartmann number and heat source length is more pronounced at high Rayleigh number. - Highlights: • This paper analyses the magnetic effect on CuO–water nanofluid. • Koo–Kleinstreuer–Li correlation and Lattice Boltzmann method are used. • Effects of pertinent parameters are presented through tables and graphs
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.
Indian Academy of Sciences (India)
V Vasu; K Rama Krishna; A C S Kumar
2007-09-01
Nano?uids are a new class of heat transfer ?uids developed by suspending nanosized solid particles in liquids. Larger thermal conductivity of solid particles compared to the base ?uid such as water, ethylene glycol, engine oil etc. signi?cantly enhances their thermal properties. Several phenomenological models have been proposed to explain the anomalous heat transfer enhancement in nano?uids. This paper presents a systematic literature survey to exploit the characteristics of nano?uids, viz., thermal conductivity, speci?c heat and other thermal properties. An empirical correlation for the thermal conductivity of Al2O3 + water and Cu + water nano?uids, considering the effects of temperature, volume fraction and size of the nanoparticle is developed and presented. A correlation for the evaluation of Nusselt number is also developed and presented and compared in graphical form. This enhanced thermophysical and heat transfer characteristics make ?uids embedded with nanomaterials as excellent candidates for future applications.
Energy Technology Data Exchange (ETDEWEB)
Bu, S.S.; Yang, J.; Zhou, M.; Li, S.Y. [Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049 (China); Wang, Q.W., E-mail: wangqw@mail.xjtu.edu.cn [Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049 (China); Guo, Z.X. [Department of Mechanical and Aerospace Engineering, Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, NJ 08854 (United States)
2014-04-01
Highlights: • A systematical study on contact modifications is performed for structured packed beds. • The bridges modification is found to give the most reasonable macroscopic results. • The overlaps and bridges methods are suitable for predicting local heat transfer. • Reasonable bridge diameter is found in a range from 16% d{sub p} to 20% d{sub p}. - Abstract: The present paper systematically investigated the appropriateness of different contact point modification approaches for forced convective heat transfer analysis in structured packed beds of spheres. The three-dimensional Navier–Stokes equations and RNG k–? turbulence model with scalable wall function are adopted to model the turbulent flow inside the pores. Both macroscopic and local flow and heat transfer characteristics for different packing forms (simple cubic, body center cubic and face center cubic packing forms) and contact treatments (gaps, overlaps, bridges and caps modifications) are carefully examined. In particular, the effects caused by the bridge size for the bridges treatment are discussed, and the numerical results are compared with available experiments in literature. It is found that the effects of contact treatments on the pressure drops are remarkable for different structured packing forms, especially when the porosity is relatively low, while such effects on the Nusselt numbers are relatively small. Among the four different contact modifications, the bridges method would give the most reasonable pressure drops for all the structured packing forms studied and this method is also proved to be suitable for predicting the Nusselt numbers. The local flow and heat transfer characteristics in the structured packed bed are sensitive to the methodology of contact modifications. The gaps and caps treatments would distort the local flow and temperature distributions in the packed bed, especially near the contact zones. While the local flow and temperature distributions from the overlaps and bridges treatments would be more reasonable and close to those in the original packing with points contact. Based on both the macroscopic and local flow and heat transfer analyses, the bridges treatment is recommended. The effects caused by the bridge size in the bridges treatment are also remarkable. It is noted that too small or too large bridge size would lead to unreasonable results for both the macroscopic and local flow and heat transfer analyses. A reasonable range of bridge diameter is found to be from 16% d{sub p} to 20% d{sub p}.
Experimental investigation of forced convective heat transfer in rectangular micro-channels
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R. Kalaivanan
2010-05-01
Full Text Available This paper investigates the experimental program on the study of heat transfer characteristics in micro-channels. The two test sections used are of 47 and 50 micro-channels in rectangular cross-section of equivalent diameters 387 and 327 µm respectively. Each channel of length 192 mm is fabricated on a 304 stainless steel substrate (230 mm x 160 mm x 1.6 mm by photo chemical etching process. Covering the top with another plate of 0.5 mm thickness forms the channels by vacuum brazing. Experiments cover laminar region using the fluids ethanol, methanol and an ethanol-methanol mixture. The heat transfer coefficient is evaluated based on the heat carried away by the coolant and an average wall to mean fluid temperature difference. The Nusselt number is correlated through empirical correlations involving Reynolds number and Prandtl number with length parameter, the hydraulic diameter.
Sensitivity studies of heat transfer: forced convection across a cylindrical pipe and duct flow
Ferrantelli, Andrea; Viljanen, Martti
2013-01-01
We consider two common heat transfer processes and perform a through sensitivity study of the variables involved. We derive and discuss analytical formulas for the heat transfer coefficient in function of film velocity, air temperature and pipe diameter. The according plots relate to a qualitative analysis of the multi-variable function $h$, according to functional optimization. For each process, we provide with graphs and tables of the parameters of interest, such as the Reynolds number. This method of study and the specific values can constitute a useful reference for didactic purposes.
Experimental study of micro-particle fouling under forced convective heat transfer
S. M. Peyghambarzadeh; A. Vatani; M. Jamialahmadi
2012-01-01
Particulate fouling studies of a hydrocarbon based suspension containing 2 µm alumina particles were performed in an annular heat exchanger having a hydraulic diameter of 14.7 mm. During fouling experiments, the classical asymptotical behavior was observed. It is shown that particle concentration, fluid velocity, and wall temperature have strong influences on the fouling curve and the asymptotic fouling resistance. Furthermore, a mathematical model is developed to formulate the asymptotic fou...
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Lazarus Godson Asirvatham
2009-03-01
Full Text Available The present work is an experimental study of steady state convective heat transfer of de-ionized water with a low volume fraction (0.003% by volume of copper oxide (CuO nanoparticles dispersed to form a nanofluid that flows through a copper tube. The effect of mass flow rate ranging from (0.0113 kg/s to 0.0139 kg/s and the effect of inlet temperatures at 100C and 17 0C on the heat transfer coefficient are studied on the entry region under laminar flow condition. The results have shown 8% enhancement of the convective heat transfer coefficient of the nanofluid even with a low volume concentration of CuO nanoparticles. The heat transfer enhancement was increased considerably as the Reynolds number increased. Possible reasons for the enhancement are discussed. Nanofluid thermo-physical properties and chaotic movement of ultrafine particles which accelerate the energy exchange process are proposed to be the main reasons for the observed heat transfer enhancement. A correlation for convective heat transfer coefficient of nanofluids, based on transport property and D/x for 8 mm tube has been evolved. The correlation predicts variation in the local Nusselt number along the flow direction of the nanofluid. A good agreement (±10% is seen between the experimental and predicted results.
Effect of confinement on forced convection from a heated sphere in Bingham plastic fluids
Das, Pradipta K.; Gupta, Anoop K.; Nirmalkar, Neelkanth; Chhabra, Raj P.
2015-05-01
In this work, the momentum and heat transfer characteristics of a heated sphere in tubes filled with Bingham plastic fluids have been studied. The governing differential equations (continuity, momentum and thermal energy) have been solved numerically over wide ranges of conditions as: Reynolds number, 1 ? Re ? 100; Prandtl number, 1 ? Pr ? 100; Bingham number, 0 ? Bn ? 100 and blockage ratio,0 ? ? ? 0.5 where ? is defined as the ratio of the sphere to tube diameter. Over this range of conditions, the flow is expected to be axisymmetric and steady. The detailed flow and temperature fields in the vicinity of the surface of the sphere are examined in terms of the streamline and isotherm contours respectively. Further insights are developed in terms of the distribution of the local Nusselt number along the surface of the sphere together with their average values in terms of mean Nusselt number. Finally, the wall effects on drag are present only when the fluid-like region intersects with the boundary wall. However, heat transfer is always influenced by the wall effects. Also, the flow domain is mapped in terms of the yielded- (fluid-like) and unyielded (solid-like) sub-regions. The fluid inertia tends to promote yielding whereas the yield stress counters it. Furthermore, the introduction of even a small degree of yield stress imparts stability to the flow and therefore, the flow remains attached to the surface of the sphere up to much higher values of the Reynolds number than that in Newtonian fluids. The paper is concluded by developing predictive correlations for drag and Nusselt number.
Experimental study of micro-particle fouling under forced convective heat transfer
Scientific Electronic Library Online (English)
S. M., Peyghambarzadeh; A., Vatani; M., Jamialahmadi.
2012-12-01
Full Text Available Particulate fouling studies of a hydrocarbon based suspension containing 2 µm alumina particles were performed in an annular heat exchanger having a hydraulic diameter of 14.7 mm. During fouling experiments, the classical asymptotical behavior was observed. It is shown that particle concentration, f [...] luid velocity, and wall temperature have strong influences on the fouling curve and the asymptotic fouling resistance. Furthermore, a mathematical model is developed to formulate the asymptotic fouling resistance in terms of the mass transfer coefficient, thermophoresis velocity, and fluid shear rate. The results demonstrate that the prediction of the new model is in good agreement with the experimental observations.
Energy Technology Data Exchange (ETDEWEB)
Ahmed, Sameh E., E-mail: sameh_sci_math@yahoo.com [Mathematics Department, Faculty of Sciences, South Valley University, 83523 Qena (Egypt); Mansour, M.A. [Department of Mathematics, Assuit University, Faculty of Science, Assuit (Egypt); Mahdy, A., E-mail: mahdy4@yahoo.com [Mathematics Department, Faculty of Sciences, South Valley University, 83523 Qena (Egypt)
2013-12-15
Highlights: • We model MHD mixed convection in an inclined lid-driven cavity. • Increasing the Hartmann number leads to increase the heat transfer rate. • Increasing the inclination angle leads to the increase of the heat transfer rate. • Nusselt number at the left wall, for forced convection case, increases as the amplitude ratio increases. - Abstract: A numerical study of laminar magnetohydrodynamic mixed convection in an inclined lid-driven square cavity with opposing temperature gradients is presented. The vertical sidewalls are assumed to have non-uniform temperature variation while the top and bottom walls are kept insulated with the top surface moving at a constant speed. The transport equations are given in terms of the stream functions-vorticity formulation and are non-dimensionalized and then solved numerically by an accurate finite-volume method. The computation is carried out for wide ranges of the inclination angle (0 ? ? ? ?/2), the Richardson number (0.01 ? Ri ? 100), the Hartmann number (0 ? Ha ? 100), the amplitude ratio (0 ? ? ? 1) and the phase deviation (0 ? ? ? ?). The results indicate that the rate of heat transfer along the heated walls is enhanced on increasing either Hartmann number or inclination angle. Average Nusselt number is also, increased with increasing of the amplitude ratio for all values of the phase deviation. The non-uniform heating on both walls provides higher heat transfer rate than non-uniform heating of one wall.
International Nuclear Information System (INIS)
Highlights: • We model MHD mixed convection in an inclined lid-driven cavity. • Increasing the Hartmann number leads to increase the heat transfer rate. • Increasing the inclination angle leads to the increase of the heat transfer rate. • Nusselt number at the left wall, for forced convection case, increases as the amplitude ratio increases. - Abstract: A numerical study of laminar magnetohydrodynamic mixed convection in an inclined lid-driven square cavity with opposing temperature gradients is presented. The vertical sidewalls are assumed to have non-uniform temperature variation while the top and bottom walls are kept insulated with the top surface moving at a constant speed. The transport equations are given in terms of the stream functions-vorticity formulation and are non-dimensionalized and then solved numerically by an accurate finite-volume method. The computation is carried out for wide ranges of the inclination angle (0 ? ? ? ?/2), the Richardson number (0.01 ? Ri ? 100), the Hartmann number (0 ? Ha ? 100), the amplitude ratio (0 ? ? ? 1) and the phase deviation (0 ? ? ? ?). The results indicate that the rate of heat transfer along the heated walls is enhanced on increasing either Hartmann number or inclination angle. Average Nusselt number is also, increased with increasing of the amplitude ratio for all values of the phase deviation. The non-uniform heating on both walls provides higher heat transfer rate than non-uniform heating of one wall
International symposium on transient convective heat transfer: book of abstracts
International Nuclear Information System (INIS)
The international symposium on convective heat transfer was held on 19-23 August 1996, in Cesme, Izmir, Turkey. The spesialists discussed forced convection, heat exchangers, free convection and multiphase media and phase change at the meeting. Almost 53 papers were presented in the meeting
Energy Technology Data Exchange (ETDEWEB)
Budzynski, R.; Sobanski, R. [Politechnika Szczecinska, Szczecin (Poland)
1993-12-31
Results of an experimental investigation of mixed convection in laminar air flow across a tungsten wire are presented. Results are correlated by a 3-D surface equation of heat transfer, and compared with traditional equations for mixed convection that have been suggested by several investigators. Experimental results are given for Reynolds numbers in the range of 0.8 {<=} Re {<=} 17.5, Grashof numbers in the range of 1.5*10{sup -3} {<=} Gr {<=} 25 and buoyancy in the range of 6.10{sup -5} {<=} Gr/Re{sup 2} {<=} 11.5. (Authors). 7 refs., 5 figs.
International Nuclear Information System (INIS)
Some correlations of forced convection burn-out data are based on the approximate linearity of the relationship between burn-out heat flux and the channel-averaged quality at the burn-out point. These correlations perform satisfactorily on data obtained from uniformly heated configurations. Therefore the further inference is sometimes made that the burn-out heat flux is uniquely related to the quality, and that the burn-out in non-uniformly heated configurations can be calculated from measurements made with uniform heating. This report presents burn-out data for Freon 12 flowing vertically upwards through both uniformly and non-uniformly heated round tubes. This data shows that the quality at burn-out does depend on the heat flux profile, and that the inference mentioned above is not justified. (author)
International Nuclear Information System (INIS)
In the heat transfer studies by forced convection, we have few data about behavior of the fluids in an annular channel heated by a concentric pipe, such date is necessary to know the heat transfer coefficient that establish the interchange of energy and the thermic properties of the fluid with the geometry of the flow. In this work the objective, was to compare some empirical correlations that we needed for determinate the heat transfer coefficient for annular channels, where we obtained similar at the theoretical results of an experiment made by Miller and Benforado. It is important to know such coefficients because we can determinate the heat quantity transmitted to a probe zone, in which we simulate a nuclear fuel element that developed huge heat quantity that must be dispersed in short time. We give theoretical data of the heat forced transfer convection and we analyzed the phenomena in annular channels given some empirical correlations employed by some investigators and we analyzed each one. (Author)
S.P. Anjali Devi; J. Wilfred Samuel Raj
2014-01-01
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 s...
Lazarus Godson Asirvatham; Nandigana Vishal; Senthil Kumar Gangatharan; Dhasan Mohan Lal
2009-01-01
The present work is an experimental study of steady state convective heat transfer of de-ionized water with a low volume fraction (0.003% by volume) of copper oxide (CuO) nanoparticles dispersed to form a nanofluid that flows through a copper tube. The effect of mass flow rate ranging from (0.0113 kg/s to 0.0139 kg/s) and the effect of inlet temperatures at 100C and 17 0C on the heat transfer coefficient are studied on the entry region under laminar flow condition. The results have shown 8% e...
Heterogeneous nanofluids: natural convection heat transfer enhancement
Directory of Open Access Journals (Sweden)
Bennacer Rachid
2011-01-01
Full Text Available Abstract Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account the thermal conductivity, viscosity versus nanofluids type and concentration and the spatial heterogeneous concentration induced by the Soret effect is presented. The obtained results, by solving numerically the full governing equations, are found to be in good agreement with the developed solution based on the scale analysis approach. The resulting convective flows are found to be dependent on the local particle concentration ? and the corresponding solutal to thermal buoyancy ratio N. The induced nanofluid heterogeneity showed a significant heat transfer modification. The heat transfer in natural convection increases with nanoparticle concentration but remains less than the enhancement previously underlined in forced convection case.
Heterogeneous nanofluids: natural convection heat transfer enhancement.
Oueslati, Fakhreddine Segni; Bennacer, Rachid
2011-01-01
Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account the thermal conductivity, viscosity versus nanofluids type and concentration and the spatial heterogeneous concentration induced by the Soret effect is presented. The obtained results, by solving numerically the full governing equations, are found to be in good agreement with the developed solution based on the scale analysis approach. The resulting convective flows are found to be dependent on the local particle concentration ? and the corresponding solutal to thermal buoyancy ratio N. The induced nanofluid heterogeneity showed a significant heat transfer modification. The heat transfer in natural convection increases with nanoparticle concentration but remains less than the enhancement previously underlined in forced convection case. PMID:21711755
Heterogeneous nanofluids: natural convection heat transfer enhancement
2011-01-01
Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account the thermal conductivity, viscosity versus nanofluids type and concentration and the spatial heterogeneous concentration induced by the Soret effect is presented. The obtained results, by solving numerically the full governing equations, are found to be in good agreement with the developed solution based on the scale analysis approach. The resulting convective flows are found to be dependent on the local particle concentration ? and the corresponding solutal to thermal buoyancy ratio N. The induced nanofluid heterogeneity showed a significant heat transfer modification. The heat transfer in natural convection increases with nanoparticle concentration but remains less than the enhancement previously underlined in forced convection case. PMID:21711755
International Nuclear Information System (INIS)
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 103Re5). 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.
Internally heated convection and Rayleigh-B\\'enard convection
Goluskin, David
2015-01-01
This work reviews basic features of both internally heated (IH) convection and Rayleigh-B\\'enard (RB) convection, along with findings on IH convection from laboratory experiments and numerical simulations. In the first chapter, six canonical models of convection are described: three configurations of IH convection driven by constant and uniform volumetric heating, and three configurations of RB convection driven by the boundary conditions. The IH models are distinguished by differing pairs of thermal boundary conditions: top and bottom boundaries of equal temperature, an insulating bottom with heat flux fixed at the top, and an insulating bottom with temperature fixed at the top. The RB models also are distinguished by whether temperatures or heat fluxes are fixed at the top and bottom boundaries. Integral quantities important to heat transport are discussed, including the mean fluid temperature, the mean temperature difference between the boundaries, and the mean convective heat transport. Integral relations...
Terminal project heat convection in thin cylinders
International Nuclear Information System (INIS)
Heat convection in thin cylinders and analysis about natural convection for straight vertical plates, and straight vertical cylinders submersed in a fluid are presented some works carry out by different authors in the field of heat transfer. In the part of conduction, deduction of the equation of heat conduction in cylindrical coordinates by means of energy balance in a control volume is presented. Enthalpy and internal energy are used for the outlining of the equation and finally the equation in its vectorial form is obtained. In the convection part development to calculate the Nusselt number for a straight vertical plate by a forces analysis, an energy balance and mass conservation over a control volume is outlined. Several empiric correlations to calculate the Nusselt number and its relations with other dimensionless numbers are presented. In the experimental part the way in which a prototype rode is assembled is presented measurements of temperatures attained in steady state and in free convection for working fluids as air and water are showed in tables. Also graphs of Nusselt numbers obtained in the experimental way through some empiric correlations are showed (Author)
Directory of Open Access Journals (Sweden)
Tasawar Hayat
2011-09-01
Full Text Available In this paper we analyse the effects of internal heat generation, thermal radiation and buoyancy force on the laminar boundary layer about a vertical plate in a uniform stream of fluid under a convective surface boundary condition. In the analysis, we assumed that the left surface of the plate is in contact with a hot fluid whilst a stream of cold fluid flows steadily over the right surface; the heat source decays exponentially outwards from the surface of the plate. The similarity variable method was applied to the steady state governing non-linear partial differential equations, which were transformed into a set of coupled non-linear ordinary differential equations and were solved numerically by applying a shooting iteration technique together with a sixth-order Runge–Kutta integration scheme for better accuracy. The effects of the Prandtl number, the local Biot number, the internal heat generation parameter, thermal radiation and the local Grashof number on the velocity and temperature profiles are illustrated and interpreted in physical terms. A comparison with previously published results on similar special cases showed excellent agreement.
Scientific Electronic Library Online (English)
Philip O., Olanrewaju; Jacob A., Gbadeyan; Tasawar, Hayat; Awatif A., Hendi.
2011-10-01
Full Text Available In this paper we analyse the effects of internal heat generation, thermal radiation and buoyancy force on the laminar boundary layer about a vertical plate in a uniform stream of fluid under a convective surface boundary condition. In the analysis, we assumed that the left surface of the plate is in [...] contact with a hot fluid whilst a stream of cold fluid flows steadily over the right surface; the heat source decays exponentially outwards from the surface of the plate. The similarity variable method was applied to the steady state governing non-linear partial differential equations, which were transformed into a set of coupled non-linear ordinary differential equations and were solved numerically by applying a shooting iteration technique together with a sixth-order Runge-Kutta integration scheme for better accuracy. The effects of the Prandtl number, the local Biot number, the internal heat generation parameter, thermal radiation and the local Grashof number on the velocity and temperature profiles are illustrated and interpreted in physical terms. A comparison with previously published results on similar special cases showed excellent agreement.
Suppression of saturated nucleate boiling by forced convective flow
International Nuclear Information System (INIS)
Tube-side forced convective boiling nitrogen and oxygen and thin film shell-side forced convective boiling R-11 data demonstrate a reduction in the heat transfer coefficient associated with nucleate boiling as the two-phase friction pressure drop increases. Techniques proposed in the literature to account for nucleate boiling during forced convective boiling are discussed. The observed suppression of nucleate boiling for the tube-side data is compared against the Chen correlation. Although general agreement is exhibited, supporting the interactive heat transfer mechanism theory, better agreement is obtained by defining a bubble growth region within the thermal boundary layer. The data suggests that the size of the bubble growth region is independent of the friction drop, but is only a function of the physical properties of the boiling liquid. 15 refs
International Nuclear Information System (INIS)
Forced air convection heat pipe cooling systems play an essential role in the thermal management of electronic and power electronic devices such as microprocessors and IGBT's (Integrated Gate Bipolar Transistors). With increasing heat dissipation from these devices, novel methods of improving the thermal performance of fin stacks attached to the heat pipe condenser section are required. The current work investigates the use of a wing type surface protrusions in the form of 3-D delta wing tabs adhered to the fin surface, thin wings punched-out of the fin material and TiNi shape memory alloy delta wings which changed their angles of attack based on the fin surface temperature. The longitudinal vortices generated from the wing designs induce secondary mixing of the cooler free stream air entering the fin stack with the warmer fluid close to the fin surfaces. The change in angle of the attack of the active delta wings provide heat transfer enhancement while managing flow pressure losses across the fin stack. A heat transfer enhancement of 37% compared to a plain fin stack was obtained from the 3-D tabs in a staggered arrangement. The punched-out delta wings in the staggered and inline arrangements provided enhancements of 30% and 26% respectively. Enhancements from the active delta wings were lower at 16%. However, as these devices reduce the pressure drop through the fin stack by approximately 19% in the de-activate position, over the activated position, a reduction in fan operating cost may be achieved for systems operating with inlet air temperatures below the maximum inlet temperature specification for the device. CFD analysis was also carried out to provide additional detail of the local heat transfer enhancement effects. The CFD results corresponded well with previously published reports and were consistent with the experimental findings. - Highlights: ? Heat transfer enhancements of heat pipe fin stacks was successfully achieved using fixed and active delta wing vortex generators. ? The active vortex generators, made from Ti-Ni, protruded into the flow stream at high temperatures and resume a low profile position at a low temperature set point. ? By considering wing spacing and the distance from the leading edge of the fin stack, heat transfer enhancements of up to 37%, compared to plane fin stacks, were achieved. ? By replacing the fixed delta wings with the active vortex generators, heat transfer enhancements of up to 16% was achieved and the pressure loss associated with flow obstructions was effectively managed.
International Nuclear Information System (INIS)
This paper provides information on heat transfer enhancement due to jet mixing inside a cylindrical enclosure. The work addresses conservative heat transfer assumptions regarding mixing and condensation that have typically been incorporated into passive containment design analyses. The current research presents an interesting possibility for increasing decay heat removal of passive containment systems under combined natural and forced convection. Eliminating these conservative assumptions could provide the basis for a change of containment design and reduce the construction cost. It is found that the ratio of forced- and free convection Nusselt numbers can be predicted as a function of the Archimedes number and a correlated factor accounting for jet orientation and enclosure geometry. To use the small-scale tests for large containment design, scale-up methods and criteria are important for matching the key governing parameters and fluid properties. In the present experiment, a cylindrical enclosure was constructed with a vertical wall of 2.29-m diameter and 0.8-m height and a vertically adjustable ceiling. A horizontal copper plate was installed at the bottom to provide an isothermal heating surface. Cold air was injected at several positions with varying pipe diameters and injecting orientations and was removed from the top of the enclosure. The experiment was performed with an extensive set of tests to study the combined natural- and forced convection heat transfer in a cylindrical enclosure mixed by an injected jet. The goals are to evaluate the key parameters governing the heat transfer augmentation by a forced jet and to investigate the effect of geometric factors, including jet diameter, jet injection orientation, and enclosure geometry (aspect ratio). Flow velocity measurement further provides a better understanding of the flow patterns developed inside the enclosure, which will determine the effectiveness of the whole volume mixing process. An additional experiment with a vertically cooled wall, similar to the condition of a passive containment cooling surface, is under construction. It will be used for further investigation of similar phenomena for steam condensation in the presence of non-condensable gases. From the current study with the cylindrical enclosure, it was found that the augmentation of natural-convection heat transfer by a forced jet is primarily determined by the jet Reynolds number, while the injecting nozzle diameter has a relatively weak effect. The jet orientation also plays an important role in determining the augmentation ratio, and of the four different jet orientations studied here, vertical downward injection at the center of the enclosure gives the highest augmentation, while azimuthal injection gives the lowest. The enclosure geometry and aspect ratio are also important factors, depending on the jet orientation. A combining rule is employed with a weighted relation to balance the contributions from separate heat-transfer correlations representing natural and forced dominated convections. It was found that under natural convection without the jet, the mean Nusselt number inside the large enclosure can be correlated by the enclosure Rayleigh number, RaD1/3, and under forced convection with a strong jet, the data are well represented as a function of the jet Reynolds number, Rej2/3. According to these relationships, a correlation form predicting mixed convection inside the enclosure was developed by a function that uses the ratio of forced- and free-convection heat transfer coefficients. This form can then be further reduced to predict the augmentation ratio as a function of the Archimedes number and a correcting factor accounting for jet orientation and enclosure geometry. Figure 1 presents the heat transfer augmentation as the Nusselt number ratio of mixed convection to natural convection (NuD/Nunc) versus the Archimedes number. The data, including four injecting orientations and three different jet diameters, are well correlated for each injection mode. It is found that the injection or
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.
Approximate physical burnout model for forced convection of saturated fluid
International Nuclear Information System (INIS)
Approximate physical burnout model for forced convection of saturated fluid is considered. Relationships for determining critical heat flux are presented. They correspond satisfactorily with experimental data. Experimental data on burnout in two-phase flow for various fluids including water and helium are presented
International Nuclear Information System (INIS)
Within the project 'Convection in a Cylinder' (CiC) heat transfer enhancement is studied for the case of two concentric, vertically aligned cylinders. The cylindrical gap is filled with a dielectric liquid, which viscosity is just few times higher than that of water. The inner cylinder is heated and the outer one is cooled. This setup in a gravitational buoyancy field leads to a fluid movement in a single convective cell with hot fluid rising at the inner boundary and cold fluid sinking at the outer boundary. The top and bottom part of the system shows horizontal movement, again in boundary layers. The strengthening of temperature gradient induces instabilities of that convective motion. If we vary the buoyancy force by means of electro-hydrodynamic effects, the patterns of convection differ from those instabilities rising only from variation of the temperature gradient.
Directory of Open Access Journals (Sweden)
Rajesh Khatri
2012-03-01
Full Text Available In this paper heat transfer and fluid flow characteristics in a channel has been theoretically investigated. In this study, FEM is employed to analyze a fluid flow inside a channel and then solve for the heat flow transfer through the same channel. The fluid flow is expressed by partial differential equation (Poisson’s equation.While, heat transfer is analyzed using the energy equation. The Navier Stokes equations along with the energy equation have been solved by using simple technique. The domain is discretized using 2626 elements and that corresponds to a total number of nodes 2842. The channel has a constant heat flux at the two walls and the threedimensional numerical simulations. Numerical solutions were obtained using commercial software Ansys Fluent. The working fluid was air (Pr=0.7. The local Nusselt numbers are obtained, which can be used inestimation of flow and heat transfer performance in a channel In addition, local Nusselt numbers, velocity magnitude and temperature profiles, and pressure profiles are analyzed. Results showed that both fluid flow and temperature flow are influenced significantly with changing entrance velocity. The overall objective of thispaper is to study the flow characteristics and heat transfer analysis inside a channel while increasing entrance velocity.
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.
International Nuclear Information System (INIS)
Liquid nitrogen was used as working fluid in a tube heated in cosine distribution to study burn-out phenomena in the present experiment. Two types of burn-out were observed. One occurred when flow pattern changed from churn flow or slug flow to annular flow. Another one is DNB phenomena when the flow was unstable. (author)
Free convection heat transfer to supercritical helium
International Nuclear Information System (INIS)
The study of cryogenic free convective heat transfer from a sphere to supercritical helium is reported. The free convective heat transfer coefficient has been measured within the region of 4.2 to 25 K and 3 to 35 atmospheres. Measurements were made for sphere to helium temperature difference of 0.1 to 7 K. (author)
Internally heated convection and Rayleigh-Bénard convection
Goluskin, David
2016-01-01
This Brief describes six basic models of buoyancy-driven convection in a fluid layer: three configurations of internally heated convection and three configurations of Rayleigh-Bénard convection. The author discusses the main quantities that characterize heat transport in each model, along with the constraints on these quantities. This presentation is the first to place the various models in a unified framework, and similarities and differences between the cases are highlighted. Necessary and sufficient conditions for convective motion are given. For the internally heated cases only, parameter-dependent lower bounds on the mean fluid temperature are proven, and results of past simulations and laboratory experiments are summarized and reanalyzed. The author poses several open questions for future study.
Effects of rib size and arrangement on forced convective heat transfer in a solar air heater channel
Skullong, Sompol; Thianpong, Chinaruk; Promvonge, Pongjet
2015-10-01
The article presents an experimental investigation on turbulent heat transfer and friction loss behaviors of airflow through a constant heat-fluxed solar air heater channel fitted with rib turbulators. The experiment was conducted for the airflow rate in terms of Reynolds numbers based on the hydraulic diameter of the channel in a range of 5000-24,000. In the present work, a comparative study between square and thin ribs (90°-rib) with three rib arrangements, namely, one ribbed wall (or single rib), in-line and staggered ribs on two opposite walls was first introduced. The study shows a significant effect of the presence of the ribs on the heat transfer rate and friction loss over the smooth wall channel. The comparison made at a single rib pitch and height also revealed that the thin rib performs better than the corresponding square one. Among the three arrangements, the in-line rib array provides higher heat transfer and friction loss than the staggered and the single one. However, the staggered thin rib provides the highest thermal performance. With this reason, only the staggered thin ribs at four different relative heights (BR = 0.1, 0.2, 0.3 and 0.4) and three relative pitches (PR = 0.5, 0.75 and 1.33) are investigated further. It is found that the staggered rib at BR = 0.4 and PR = 0.5 yields the highest heat transfer and friction factor but the maximum thermal performance is at BR = 0.2 and PR = 0.75.
Scientific Electronic Library Online (English)
Néstor Enrique, Cerquera Peña; Yaneth Liliana, Ruiz Osorio; Eduardo, Pastrana Bonilla.
2010-04-01
Full Text Available Teniendo como base la infraestructura existente de un horno tradicional de curado de tabaco, se rediseño e implementó en él un sistema de intercambio de calor por convección forzada que funciona con cisco de café como combustible. Este horno de curado de tabaco por convección forzada USCO-MADR fue e [...] valuado durante el periodo de cosecha, lográndose un manejo controlado de las variables de temperatura y humedad relativa dentro de él durante las tres etapas del curado de la hoja de tabaco; el equipo utilizado tuvo un excelente desempeño al emplear cisco de café como combustible con los siguientes consumos durante el proceso de curado: en la fase de “amarillamiento”, 8,92 kilogramos por hora; en la de “secado de paño y fijación de color”, 17,75 kilogramos por hora; y en la de “secado de vena”, 19,29 kilogramos por hora; el análisis comparativo de los costos operativos del horno evaluado, con los ajustes propuestos a éste, permiten presentarlo a la cadena de tabaco como una alternativa promisoria. Abstract in english A traditional oven for curing tobacco leaves was redesigned (based on existing infrastructure); a forced-convection heat exchanger system was implemented in it which worked with coffee hulls as fuel. This oven (called a forced-convection tobacco leaf curing oven) was evaluated during the harvesting [...] season. It was found that temperature and relative humidity inside the furnace could be controlled with this assembly during the three stages involved in curing tobacco leaves. The equipment used performed excellently when using coffee hulls as fuel, having the following approximate consumption during curing: 8.92 kilograms per hour during the yellowing stage, 17.75 kilograms per hour during the leaf drying and color fixation phase and 19.29 kilograms per hour during the stem drying stage. Comparative analysis of the oven´s operating costs along with the proposed adjustments to be made to it would allow its implementation as a promising alternative in the existing tobacco chain.
Mechanistic modeling of CHF in forced-convection subcooled boiling
International Nuclear Information System (INIS)
Because of the complexity of phenomena governing boiling heat transfer, the approach to solve practical problems has traditionally been based on experimental correlations rather than mechanistic models. The recent progress in computational fluid dynamics (CFD), combined with improved experimental techniques in two-phase flow and heat transfer, makes the use of rigorous physically-based models a realistic alternative to the current simplistic phenomenological approach. The objective of this paper is to present a new CFD model for critical heat flux (CHF) in low quality (in particular, in subcooled boiling) forced-convection flows in heated channels
Directory of Open Access Journals (Sweden)
Amnart Boonloi
2014-01-01
Full Text Available The influences of modified V-shaped baffle in a square channel for heat transfer and thermal performance enhancement are presented numerically in three Dimensional (3D. The V-shaped baffles are modified in order to comfortable to installation in the square channel. The plates are used for clamping on both the upper and lower V-shaped baffles resulting the modified V-shaped baffle like orifice plate called “V-shaped orifice tubulators, VOT”. The effects of Blockage Ratios (BR = 0.05-0.20, flow attack angles (? = 20°, 30° and 45° and flow directions (V-Downstream and V-Upstream with a single Pitch Ratio (PR = 1 are investigated for Reynolds number based on the hydraulic diameter of the square channel (Dh, Re = 100-2000. The fully developed periodic flow and heat transfer are applied for the computational domain. The SIMPLE algorithm and the finite volume method are used in the current study. The numerical results show that the use of VOT not only increasing heat transfer rate, but also rise up very enlarge pressure loss due to reducing the flow area of the cross sectional area. In addition, the maximum thermal enhancement factors are found around 2.4 and 2.5 for BR = 0.10, ? = 30° at the highest Reynolds number of V-Downstream and V-Upstream, respectively.
Energy Technology Data Exchange (ETDEWEB)
Sridharan, Kumar; Anderson, Mark; Allen, Todd; Corradini, Michael
2012-01-30
The goal of this NERI project was to perform research on high temperature fluoride and chloride molten salts towards the long-term goal of using these salts for transferring process heat from high temperature nuclear reactor to operation of hydrogen production and chemical plants. Specifically, the research focuses on corrosion of materials in molten salts, which continues to be one of the most significant challenges in molten salts systems. Based on the earlier work performed at ORNL on salt properties for heat transfer applications, a eutectic fluoride salt FLiNaK (46.5% LiF-11.5%NaF-42.0%KF, mol.%) and a eutectic chloride salt (32%MgCl2-68%KCl, mole %) were selected for this study. Several high temperature candidate Fe-Ni-Cr and Ni-Cr alloys: Hastelloy-N, Hastelloy-X, Haynes-230, Inconel-617, and Incoloy-800H, were exposed to molten FLiNaK with the goal of understanding corrosion mechanisms and ranking these alloys for their suitability for molten fluoride salt heat exchanger and thermal storage applications. The tests were performed at 850Ã?Â?Ã?Â?Ã?Â?Ã?Â?C for 500 h in sealed graphite crucibles under an argon cover gas. Corrosion was noted to occur predominantly from dealloying of Cr from the alloys, an effect that was particularly pronounced at the grain boundaries Alloy weight-loss due to molten fluoride salt exposure correlated with the initial Cr-content of the alloys, and was consistent with the Cr-content measured in the salts after corrosion tests. The alloysÃ?Â?Ã?Â¢Ã?Â?Ã?Â?Ã?Â?Ã?Â? weight-loss was also found to correlate to the concentration of carbon present for the nominally 20% Cr containing alloys, due to the formation of chromium carbide phases at the grain boundaries. Experiments involving molten salt exposures of Incoloy-800H in Incoloy-800H crucibles under an argon cover gas showed a significantly lower corrosion for this alloy than when tested in a graphite crucible. Graphite significantly accelerated alloy corrosion due to the reduction of Cr from solution by graphite and formation 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 (0.083Ã?Â?Ã?Â¢Ã?Â?Ã?Â?Ã?Â
Implications of some of moist convection's other paths to heating.
Stevens, Bjorn
2015-04-01
Atmospheric moist convection plays a fundamental role in governing atmospheric circulation system and the response of the atmosphere to forcing. Although the role of deep convective heating in balancing the atmospheric energy budget and driving circulations has long been appreciated, other ways in which convection can be diabatic have been less extensively studied. For instance, the coupling between convection and radiative heating in the atmosphere appears to play an important role in determining the position of the ITCZ, and the strength of tropical variability, both on inter-seasonal (MJO) and inter-annual (ENSO) timescales. Likewise precipitation from very shallow convection is found, under some circumstances to be very efficient and to regulate the structure of the marine boundary layer in the trades. This may have implications for convective aggregation and the large-scale organization of convection, as well as for the susceptibility of clouds to forcing. In addition to addressing the above ideas, new frameworks (observations and simulation) for exploring the interplay of convection with large-scale circulations will be presented.
International Nuclear Information System (INIS)
Full text of publication follows: The prediction of the Critical Heat Flux (CHF) in a heat flux controlled boiling heat exchanger is important to assess the maximal thermal capability of the system. In the case of a nuclear reactor, CHF margin gain (using improved mixing vane grid design, for instance) can allow power up-rate and enhanced operating flexibility. In general, current nuclear core design procedures use quasi-1D approach to model the coolant thermal-hydraulic conditions within the fuel bundles coupled with fully empirical CHF prediction methods. In addition, several CHF mechanistic models have been developed in the past and coupled with 1D and quasi-1D thermal-hydraulic codes. These mechanistic models have demonstrated reasonable CHF prediction characteristics and, more remarkably, correct parametric trends over wide range of fluid conditions. However, since the phenomena leading to CHF are localized near the heater, models are needed to relate local quantities of interest to area-averaged quantities. As a consequence, large CHF prediction uncertainties may be introduced and 3D fluid characteristics (such as swirling flow) cannot be accounted properly. Therefore, a fully mechanistic approach to CHF prediction is, in general, not possible using the current approach. The development of CHF-enhanced fuel assembly designs requires the use of more advanced 3D coolant properties computations coupled with a CHF mechanistic modeling. In the present work, the commercial CFD code CFX-5 is used to compute 3D coolant conditions in a vertical heated tube with upward flow. Several CHF mechanistic models at low quality available in the literature are coupled with the CFD code by developing adequate models between local coolant properties and local parameters of interest to predict CHF. The prediction performances of these models are assessed using CHF databases available in the open literature and the 1995 CHF look-up table. Since CFD can reasonably capture 3D fluid flow characteristics in fuel rod bundles, this will eventually allow for numerical assessment of CHF performance of newly developed fuel assembly designs for scoping purposes before actual CHF testing. (authors)
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.
Comparison of Thermal Comfort by Radiant Heating and Convective Heating
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Shigeru Imai
2015-01-01
Full Text Available Currently, convective heating with a heat-pump system, which has high energy efficiency, is popular for room heating. However, it is possible that energy savings using convective heating can be further improved using heat pumps that service both occupied and unoccupied spaces. Moreover, convective heating increases vertical temperature gradients in a room; thus, it is hard to say whether occupants are being provided with sufficient thermal comfort. The purpose of this study is to compare the thermal comfort provided by both radiant and convective heating systems. In this study, a small office room was modeled, and then temperature and airflow distributions in the room were calculated by Computational Fluid Dynamics (CFD simulations using ESP-r (Environmental research simulation software. Furthermore, distributions of Standard Effective Temperatures (SET* were calculated using the air temperature distributions obtained from the CFD simulations, which allows us to compare the thermal comfort provided by convective heating with that provided by radiant heating. The results show that radiant heating can provide satisfactory thermal comfort, even when the room air temperature is low. However, thermal comfort also depends on the temperature of blowing air, and blowing air must reach occupied regions; thus, only radiant heating cannot circulate sufficient air. In contrast, convective heating increases vertical temperature gradients in a room. Therefore, rather than using only radiant or convective heating, it may be more effective to combine them efficiently.
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.
EXPERIMENTAL AND NUMERICAL STUDY OF FORCED CONVECTION IN ENGINE BLOCK
Directory of Open Access Journals (Sweden)
BELSARE S.N.
2014-04-01
Full Text Available Experiments were conducted to study forced convection in three different engine blocks. Current study uses three engine blocks machined from plain Aluminium. Plain Cylinder Block, Cylinder Block with fins & Square Block with fins is used. Inside the cylinder block fine heaters were fitted to mimic heating at different locations. Temperatures at various locations were measured. Few experimental data is compared with CFD results by Fluent.
Cryogenic forced convection refrigerating system
International Nuclear Information System (INIS)
This patent describes the method of refrigerating products by contact with a refrigerating gas which comprises introducing product into a refrigeration zone, contacting the product with the refrigerating gas for a sufficient time to refrigerate it to the appropriate extent and removing the refrigerated product. The improvement for producing the refrigeration gas from a liquid cryogen such that essentially all of the liquid cryogen is fully vaporized before contacting the product comprises: (a) introducing the liquid cryogen, selected from the group consisting of liquid air and liquid nitrogen, at elevated pressure into an ejector as the motive fluid to accelerate a portion of a warm refrigerating gas through the ejector while mixing the cryogen and gas to effect complete vaporization of the liquid cryogen and substantial cooling of the portion of the refrigerating gas resulting in a cold discharge gas which is above the liquefaction temperature of the cryogen; (b) introducing the cold discharge gas into a forced circulation pathway of refrigerating gas and producing a cold refrigerating gas which contacts and refrigerates product and is then at least partially recirculated; (c) sensing the temperature of the refrigerating gas in the forced circulation pathway and controlling the introduction of liquid cryogen with regard to the sensed temperature to maintain the temperature of the discharge gas above the liquefacton temperature of the cryogen utilized
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.
Pattern formation in spatially forced thermal convection
Weiss, S.; Seiden, G.; Bodenschatz, E.
2012-05-01
In this paper, we present experimental results on the interplay between two different symmetry breaking mechanisms in a pattern forming system, namely inclined layer convection (ILC) with a spatially modulated heated plate. By varying the relative strength and relative orientation, we explored in detail the interplay of these symmetry breaking mechanisms. We found a stabilization of spatio-temporal chaos and resonant interactions that led to superlattice patterns. The fundamental mechanisms observed should be equally applicable to other pattern forming systems.
Pattern formation in spatially forced thermal convection
International Nuclear Information System (INIS)
In this paper, we present experimental results on the interplay between two different symmetry breaking mechanisms in a pattern forming system, namely inclined layer convection (ILC) with a spatially modulated heated plate. By varying the relative strength and relative orientation, we explored in detail the interplay of these symmetry breaking mechanisms. We found a stabilization of spatio-temporal chaos and resonant interactions that led to superlattice patterns. The fundamental mechanisms observed should be equally applicable to other pattern forming systems. (paper)
Latent Heating Processes within Tropical Deep Convection
van den Heever, S. C.; Mcgee, C. J.
2013-12-01
It has been suggested that latent heating above the freezing level plays an important role in reconciling Riehl and Malkus' Hot Tower Hypothesis (HTH) with observational evidence of diluted tropical deep convective cores. In this study, recent modifications to the HTH have been evaluated through the use of Lagrangian trajectory analysis of deep convective cores simulated using the Regional Atmospheric Modeling System (RAMS), a cloud-resolving model (CRM) with sophisticated microphysical, surface and radiation parameterization schemes. Idealized, high-resolution simulations of a line of tropical convective cells have been conducted. A two-moment microphysical scheme was utilized, and the initial and lateral boundary grid conditions were obtained from a large-domain CRM simulation approaching radiative convective equilibrium. As the tropics are never too far from radiative convective equilibrium, such a framework is useful for investigating the relationships between radiation, thermodynamics and microphysics in tropical convection. Microphysical impacts on latent heating and equivalent potential temperature (?e) have been analyzed along trajectories ascending within convective regions. Changes in ?e along backward trajectories are partitioned into contributions from latent heating due to ice processes and a residual term that is shown to be an approximate representation of mixing. It is apparent from the CRM simulations that mixing with dry environmental air decreases ?e along ascending trajectories below the freezing level, while latent heating due to freezing and vapor deposition increase ?e above the freezing level. The along-trajectory contributions to latent heating from cloud nucleation, condensation, evaporation, freezing, deposition, and sublimation have also been quantified. Finally, the source regions of trajectories reaching the upper troposphere have been identified. The analysis indicates that while much of the air ascending within convective updrafts originates from above the lowest 2 km AGL, the strongest updrafts are composed of air from closer to the surface. Thus, both the boundary layer and mid-level inflow appears to play an important role in deep convection developing within moist environments.
Double tube heat exchanger with novel enhancement: Part II - single phase convective heat transfer
Energy Technology Data Exchange (ETDEWEB)
Tiruselvam, R.; Chin, W.M.; Raghavan, Vijay R. [OYL Sdn. Bhd., Research and Application Department, Kuala Lumpur (Malaysia)
2012-08-15
The study is conducted to evaluate the heat transfer characteristics of two new and versatile enhancement configurations in a double tube heat exchanger annulus. The novelty is that they are usable in single phase forced convection, evaporation and condensation. Heat transfer coefficients are determined by the Wilson Plot technique in laminar and turbulent flow and correlations are proposed for Nusselt numbers. Comparisons are then made between heat transfer and flow friction. (orig.)
Convective Heat Transfer and Infrared Thermography (IRTh
Directory of Open Access Journals (Sweden)
J.M. Buchlin
2010-01-01
Full Text Available The paper deals with the application of the infrared thermography to the determination of the convective heat transfer coefficient in complex flow configurations. The fundamental principles upon which the IRTh relies are reviewed. The different methods developed to evaluate the heat exchange are described and illustrated through applications to the aerospace and aeronautical field as well as to the industrial processes.
Convective Heat Transfer and Infrared Thermography (IRTh)
J.M. Buchlin
2010-01-01
The paper deals with the application of the infrared thermography to the determination of the convective heat transfer coefficient in complex flow configurations. The fundamental principles upon which the IRTh relies are reviewed. The different methods developed to evaluate the heat exchange are described and illustrated through applications to the aerospace and aeronautical field as well as to the industrial processes.
Boiling water reactor with forced convection coolant
International Nuclear Information System (INIS)
The forced convection of the primary coolant of the BWR is done via the wheels of circulating pumps. The wheels are connected via pump waves with the motors arranged outside the pressure vessel. The pump waves are placed in three radial bearings to avoid bending oscillations. The lubrication of these hydrostatic bearings is done with lubricating water from the primary water cleaning. This contains the primary pump, a regeneration preheater, an after-cooler, a mechanical filter and a mixed-bed filter. (DG)
Natural convection in a horizontal fluid layer periodically heated from above and below
Hossain, M. Z.; Floryan, J. M.
2015-08-01
Natural convection in a horizontal slot heated from above and from below has been considered. Each heating has a certain spatial distribution. It has been demonstrated that a wide variety of convection patterns can be generated by changing the relative position of both heating patterns. A significant intensification of convection, compared to convection resulting from heating applied at one wall only, results if there is no phase shift between both patterns, while a significant reduction of convection results from the phase shift corresponding to half of the heating wavelength. The system generates a nonzero mean shear stress at each wall for all phase shifts except shifts corresponding to half of and one full heating wavelength. This effect, which is generated within one convection cell, gives rise to a global force which may lead to a thermally induced drift of the walls if such a drift was allowed.
An assessment on forced convection in metal foams
International Nuclear Information System (INIS)
Metal foams are a class of cellular structured materials with open cells randomly oriented and mostly homogeneous in size and shape. In the last decade, several authors have discussed the interesting heat transfer capabilities of these materials as enhanced surfaces for air conditioning, refrigeration, and electronic cooling applications. This paper reports an assessment on the forced convection through metal foams presenting experimental and analytical results carried out during air heat transfer through twelve aluminum foam samples and nine copper foam samples. The metal foam samples present different numbers of pores per linear inch (PPI), which vary between 5 and 40 with a porosity ranging between 0.896–0.956; samples of different heights have been studied. From the experimental measurements two correlations for the heat transfer coefficient and pressure drop calculations have been developed. These models can be successfully used to optimize different foam heat exchangers for any given application.
An assessment on forced convection in metal foams
Mancin, S.; Rossetto, L.
2012-11-01
Metal foams are a class of cellular structured materials with open cells randomly oriented and mostly homogeneous in size and shape. In the last decade, several authors have discussed the interesting heat transfer capabilities of these materials as enhanced surfaces for air conditioning, refrigeration, and electronic cooling applications. This paper reports an assessment on the forced convection through metal foams presenting experimental and analytical results carried out during air heat transfer through twelve aluminum foam samples and nine copper foam samples. The metal foam samples present different numbers of pores per linear inch (PPI), which vary between 5 and 40 with a porosity ranging between 0.896-0.956 samples of different heights have been studied. From the experimental measurements two correlations for the heat transfer coefficient and pressure drop calculations have been developed. These models can be successfully used to optimize different foam heat exchangers for any given application.
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
Convective intitiation over a heated mountain: mechanisms and predictability
Kirshbaum, D.
2010-09-01
In conditionally unstable flows over orography, the strong horizontal convergence generated by elevated heating locally weakens convective inhibition and increases the likelihood of convective initiation. This generally serves to enhance the predictability of deep convection, except when the associated uplift lies just at the margin of the forcing needed for convective initiation. In such marginal cases, airflows with very small initial differences may experience substantially different evolutions. To investigate the processes that govern cloud development in such cases, this study analyzes ensembles of idealized, high-resolution 2d simulations of the diurnal cycle in conditionally unstable flow over a mountain ridge. The case considered is based on a well-observed event from the Convective and Orographic Precipitation Study (COPS) that has proven highly difficult to predict in NWP models. This event was characterized by strong conditional instability but also large convective inhibition and a very dry mid-troposphere that presented a hostile environment for ascending clouds. Within each ensemble, the members differ only in their random seeds of low-amplitude, white-noise thermal perturbations added to the initial flow (0600 local time). The members of each ensemble experience similar mesoscale evolution, with convective inhibition (CIN) eroding completely and large CAPE developing over the high terrain by noon. Shallow orographic cumuli form predictably in response, but only in some cases do these transition to deep cumulonimbi. The dynamical and microphysical mechanisms that determine the cloud evolution in these simulations are examined through parcel trajectory analysis and an entraining thermal model.
Convective Instability in a Fluid Mixture Heated from Above
Energy Technology Data Exchange (ETDEWEB)
La Porta, A.; Surko, C.M. [Department of Physics, University of California, San Diego, La Jolla, California 92093 (United States)
1998-04-01
Convection patterns in ethanol-water mixtures with negative {psi} are studied when the fluid is heated from above. Although the linear analysis predicts that the instability occurs at zero wave number, a large wave number pattern is observed. The onset is supercritical with a threshold that is experimentally indistinguishable from zero. The convection amplitude exhibits damped oscillations for sudden change in the forcing parameter. At the constant Rayleigh number the patterns first coarsen, then exhibit growth of narrow plumes. The instability appears to be related to salt fingering. {copyright} {ital 1998} {ital The American Physical Society}
Natural Convective Heat Transfer from Narrow Plates
Oosthuizen, Patrick H
2013-01-01
Natural Convective Heat Transfer from Narrow Plates deals with a heat transfer situation that is of significant practical importance but which is not adequately dealt with in any existing textbooks or in any widely available review papers. The aim of the book is to introduce the reader to recent studies of natural convection from narrow plates including the effects of plate edge conditions, plate inclination, thermal conditions at the plate surface and interaction of the flows over adjacent plates. Both numerical and experimental studies are discussed and correlation equations based on the results of these studies are reviewed.
International Nuclear Information System (INIS)
The Muon Ionization Cooling Experiment (MICE) has three 350-mm long liquid hydrogen absorbers to reduce the momentum of 200 MeV muons in all directions. The muons are then re-accelerated in the longitudinal direction by 200 MHz RF cavities. The result is cooled muons with a reduced emittance. The energy from the muons is taken up by the liquid hydrogen in the absorber. The hydrogen in the MICE absorbers is cooled by natural convection to the walls of the absorber that are in turn cooled by helium gas that enters at 14 K. This report describes the MICE liquid hydrogen absorber and the heat exchanger between the liquid hydrogen and the helium gas that flows through passages in the absorber wall
Modelling of convective heat and mass transfer in rotating flows
Shevchuk, Igor V
2016-01-01
This monograph presents results of the analytical and numerical modeling of convective heat and mass transfer in different rotating flows caused by (i) system rotation, (ii) swirl flows due to swirl generators, and (iii) surface curvature in turns and bends. Volume forces (i.e. centrifugal and Coriolis forces), which influence the flow pattern, emerge in all of these rotating flows. The main part of this work deals with rotating flows caused by system rotation, which includes several rotating-disk configurations and straight pipes rotating about a parallel axis. Swirl flows are studied in some of the configurations mentioned above. Curvilinear flows are investigated in different geometries of two-pass ribbed and smooth channels with 180° bends. The author demonstrates that the complex phenomena of fluid flow and convective heat transfer in rotating flows can be successfully simulated using not only the universal CFD methodology, but in certain cases by means of the integral methods, self-similar and analyt...
Numerical Investigation of Turbulent Forced Convection Nanofluids Inside an Annulus
Directory of Open Access Journals (Sweden)
Farhad Vahidinia
2015-09-01
Full Text Available Turbulent forced convection heat transfer of Al2O3-water nanofluid has been studied numerically under uniform heat flux on the inner and outer walls in an annulus with rough tubes. Solid nanoparticles diameter were considered to be 18, 32, and 67 nm. Two-dimensional elliptic governing equations were used and the second-order upstream difference scheme and finite volume method were used for the discretization of governing equations. SIMPLEC algorithm has been established the relationship between pressure and velocity. The results demonstrated that the surface friction coefficient increases both in the inner and outer wall of the heat exchanger by increasing the diameter of nanoparticles. On the other side, the Nusselt number of nanofluid is greater than the base fluid for a given Reynolds number and nanoparticle volume fraction. Also, the Nusselt number decreases in the inner and outer wall with increasing the diameter of nanoparticles.
Free convection film flows and heat transfer
Shang, Deyi
2010-01-01
Presents development of systematic studies for hydrodynamics and heat and mass transfer in laminar free convection, accelerating film boiling and condensation of Newtonian fluids, and accelerating film flow of non-Newtonian power-law fluids. This book provides a system of analysis models with a developed velocity component method.
Heat Flux Sensors for Infrared Thermography in Convective Heat Transfer
Directory of Open Access Journals (Sweden)
Giovanni Maria Carlomagno
2014-11-01
Full Text Available This paper reviews the most dependable heat flux sensors, which can be used with InfraRed (IR thermography to measure convective heat transfer coefficient distributions, and some of their applications performed by the authors’ research group at the University of Naples Federico II. After recalling the basic principles that make IR thermography work, the various heat flux sensors to be used with it are presented and discussed, describing their capability to investigate complex thermo-fluid-dynamic flows. Several applications to streams, which range from natural convection to hypersonic flows, are also described.
Heat Flux Sensors for Infrared Thermography in Convective Heat Transfer
Giovanni Maria Carlomagno; Luigi de Luca; Gennaro Cardone; Tommaso Astarita
2014-01-01
This paper reviews the most dependable heat flux sensors, which can be used with InfraRed (IR) thermography to measure convective heat transfer coefficient distributions, and some of their applications performed by the authors’ research group at the University of Naples Federico II. After recalling the basic principles that make IR thermography work, the various heat flux sensors to be used with it are presented and discussed, describing their capability to investigate complex thermo-fluid-dy...
Energy Technology Data Exchange (ETDEWEB)
Ramis, M.K.; Jilani, G.; Jahangeer, S. [Department of Mechanical Engineering, National Institute of Technology Calicut, Kerala 673 601 (India)
2008-02-15
The main objective of this paper is to present a comparative study of uniform and non-uniform volumetric energy generation in a rectangular nuclear fuel element washed by upward moving stream of liquid sodium. Employing finite difference schemes, the boundary layer equations governing the flow and thermal fields in the fluid domain are solved simultaneously with two-dimensional energy equation in the solid domain by satisfying the continuity of temperature and heat flux at the solid-fluid interface. Numerical results are presented for a wide range of aspect ratio, A{sub r}, conduction-convection parameter, N{sub cc}, total energy generation parameter, Q{sub t}, and flow Reynolds number, Re{sub H}. It is concluded that for the same total energy generation, a somewhat realistic non-uniform volumetric energy generation puts greater restriction on the thermal power generation as compared to the idealistic uniform volumetric energy generation. Further, it is found that despite the total energy generation being the same for two cases, the non-uniform volumetric energy generation within the fuel element results in considerably higher energy dissipation rate. (author)
Penetrative internally heated convection in two and three dimensions
Goluskin, David
2015-01-01
Convection of an internally heated fluid, confined between top and bottom plates of equal temperature, is studied by direct numerical simulation in two and three dimensions. The unstably stratified upper region drives convection that penetrates into the stably stratified lower region. The fraction of produced heat escaping across the bottom plate, which is one half without convection, initially decreases as convection strengthens. Entering the turbulent regime, this decrease reverses in two dimensions but continues monotonically in three dimensions. The mean fluid temperature, which grows proportionally to the heating rate ($H$) without convection, grows like $H^{4/5}$ when convection is strong in both two and three dimensions. The ratio of the heating rate to the fluid temperature is likened to the Nusselt number of Rayleigh-B\\'enard convection. Simulations are reported for Prandtl numbers between 0.1 and 10 and for Rayleigh numbers (defined in terms of the heating rate) up to $5\\times10^{10}$.
International Nuclear Information System (INIS)
Thermal hydraulic analysis for the helically coiled steam generator has been carried out by means of the MARS code for a full range of reactor operating conditions. A selected event is an increased main steam flow under nominal conditions. Under forced convection condition, a dominant heat transfer in the steam generator is a nucleate boiling mode, which transfers 72% of total generated heat. For natural convection condition, 40% and 60% of the total energy is extracted in the steam generator by single-phase liquid and a nucleate boiling heat transfer, respectively. And 80% and 20% of the extracted energy is exhausted at the heat exchanger by a condensation and single-phase liquid heat transfer. The flow shows an oscillating behavior due to instability in the two-phase natural convection
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 operating conditions of the AP1000 pressurized water reactor. The effects of both axial and lateral nonuniform power distributions inside reactor fuel elements are accounted for. Boiling flows are simulated for three different computational domains of increasing complexity: a quarter-subchannel bordering a single fuel pin, two subchannels surround by an array of 2 by 3 fuel pins, and in four subchannels surrounded by an array of 3 by 3 fuel pins. The predicted behavior is consistent with expectations. In the 3 by 3 array, the two-phase coolant is predicted to flow from the hot channels to the cold channels, enhancing heat exchange between subchannels. This, in turn, demonstrates that the new model is capable of capturing the turbulence- and buoyancy-induced coolant mixing across the neighboring channels.
Free surface deformation and heat transfer by thermocapillary convection
Fuhrmann, Eckart; Dreyer, Michael; Basting, Steffen; Bänsch, Eberhard
2015-06-01
Knowing the location of the free liquid/gas surface and the heat transfer from the wall towards the fluid is of paramount importance in the design and the optimization of cryogenic upper stage tanks for launchers with ballistic phases, where residual accelerations are smaller by up to four orders of magnitude compared to the gravity acceleration on earth. This changes the driving forces drastically: free surfaces become capillary dominated and natural or free convection is replaced by thermocapillary convection if a non-condensable gas is present. In this paper we report on a sounding rocket experiment that provided data of a liquid free surface with a nonisothermal boundary condition, i.e. a preheated test cell was filled with a cold but storable liquid in low gravity. The corresponding thermocapillary convection (driven by the temperature dependence of the surface tension) created a velocity field directed away from the hot wall towards the colder liquid and then in turn back at the bottom towards the wall. A deformation of the free surface resulting in an apparent contact angle rather different from the microscopic one could be observed. The thermocapillary flow convected the heat from the wall to the liquid and increased the heat transfer compared to pure conduction significantly. The paper presents results of the apparent contact angle as a function of the dimensionless numbers (Weber-Marangoni and Reynolds-Marangoni number) as well as heat transfer data in the form of a Nusselt number. Experimental results are complemented by corresponding numerical simulations with the commercial software Flow3D and the inhouse code Navier.
Mixed convection in a horizontal porous duct with a sudden expansion and local heating from below
International Nuclear Information System (INIS)
Results are reported for an experimental and numerical study of forced and mixed convective heat transfer in a liquid-saturated, horizontal porous duct. The cross section of the duct has a sudden expansion with a heated region on the lower surface downstream and adjacent to the expansion. Calculated and measured Nusselt numbers for 0.1 1.5 and Ra/Pe1.5. Calculated Nusselt numbers are very close to those for the bottom-heated flat duct, and this result has several important implications for convective heat and mass transfer in geophysical systems and porous matrix heat exchangers
Evaluation of heat removal from vertical cylinder by natural convection
International Nuclear Information System (INIS)
We have studied a cooling system in a maintenance facility to store low level radioactive wastes by using natural draft without forced air ventilation. A fundamental study of natural convection around the vertical cylindrical heaters was carried out experimentally and numerically, and the ambient air was used as a cooling fluid. It was found that the velocity and temperature of air at the center of the flow channel surrounded by 4 heaters was high. And also it was shown that natural heat transfer formula for system design was conservative in the interior temperature of heater due to the increase of flow rate by chimney effect. (author)
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.
International Nuclear Information System (INIS)
Among the topics discussed are: prediction of turbulent heat transfer in flows past a cylindrical cavity; turbulent heat transfer in rotating rectangular ducts; and visualization of secondary flow patterns in an isothermally heated curved pipe. Consideration is also given to: laminar mixed convection in the entrance region of a horizontal annulus; two-dimensional mixed convection along a flat plate; mixed convection flow about slender bodies of revolution; and measurement of laminar mixed convection from an inclined surface. Additional topics discussed include: the effects of opposing buoyancy on the flow of free and wall jets; mixed convection heat transfer around a rotating heated cylinder; and combined buoyancy effects of thermal and mass diffusion on laminar forced convection heat transfer in a vertical tube
Two-phase forced-convective fouling under steam generator operating conditions
International Nuclear Information System (INIS)
Two-phase forced-convective fouling can occur in adiabatic two-phase flow and in diabatic two-phase flow, where it can be a significant contributor to fouling under flow-boiling conditions. For recirculating steam generators (SGs), it is, therefore, of significance to steam separators, tube support plates, tubesheet and the tube bundle. Loop test data are presented on forced-convective fouling rate of iron corrosion products under a range of conditions relevant to the secondary-side of recirculating SGs. The measurements were performed using a number of corrosion products (magnetite, hematite and lepidocrocite) under a range of water chemistry conditions, with several different amines. The measurements were limited to the straight-tube geometry. Comparable fouling data are given for flow-boiling conditions. A SG artefact was examined to corroborate the loop data. The rate constants for the forced-convective fouling measurements are compared with those for flow-boiling fouling. Their relative magnitudes can vary greatly, depending on the chemistry and thermohydraulic conditions. Boiling fouling dominated over forced-convection fouling for hematite and lepidocrocite particles, likely because of particle-bubble interactions. Forced-convective fouling rate was only slightly lower than boiling fouling for magnetite. For the region of cross-flow (upper tube bundle), deposits show significant thickness variation. Four or five deposit thickness peaks are noted, approximately equally spaced circumferentially. It is hypothesized that the fouling pattern is developed due to the cross-flow pattern present in the tube bundle. The possible interactions between the force-convective and nucleate-boiling fouling streams are briefly discussed. A method is presented for the superposition of the forced-convective and nucleate boiling fouling components. This method is based on the Chen heat transfer correlation. (author)
Energy Technology Data Exchange (ETDEWEB)
Bories, S.; Mojtabi, A.; Prat, M.; Quintard, M. [Institut de Mecanique des Fluides de Toulouse, 31 (France)
2008-10-15
Multiple physico-chemical and transport phenomena take place in porous media. The study of these phenomena requires the knowledge of fluid storage, transfer and mechanical properties of these media. Like all polyphasic heterogenous systems, these properties depend on the morphology of the matrix and of the phenomena interacting in the different phases. This makes the heat transfers in porous media a particularly huge field of researches. This article makes a synthesis of these researches. Content: 1 - classification and characterization of porous media; 2 - modeling of transfer phenomena; 3 - heat transfer by conduction: concept of equivalent thermal conductivity (ETC), modeling of conduction heat transfer, ETC determination; 4 - heat transfer by convection: modeling of convection heat transfer, natural convection (in confined media, along surfaces or impermeable bodies immersed in a saturated porous medium), forced and mixed convection; 5 - radiant heat transfer: energy status equation, approximate solutions of the radiant transfer equation, use of the approximate solutions: case of fibrous insulating materials; 6 - conclusion. (J.S.)
International Nuclear Information System (INIS)
To continue with the equipment of the thermal hydraulics laboratory, it was designed thermal and mechanically an heat exchanger, to satisfy the requirements to have circuit that allows to carry out heat transfer experiments. The heat exchanger was manufactured and proven in the workshops of the Prototypes and Models Management, and it is expected that to obtain the foreseen results once completely installed the circuit, in the laboratory of thermal hydraulics of the Management of Nuclear Systems. (Author)
Endwall convective heat transfer for bluff bodies
DEFF Research Database (Denmark)
Wang, Lei; Salewski, Mirko; Sundén, Bengt; Borg, Andreas; Abrahamsson, Hans
2012-01-01
The endwall heat transfer characteristics of forced flow past bluff bodies have been investigated using liquid crystal thermography (LCT). The bluff body is placed in a rectangular channel with both its ends attached to the endwalls. The Reynolds number varies from 50,000 to 100,000. In this study, a single bluff body and two bluff bodies arranged in tandem are considered. Due to the formation of horseshoe vortices, the heat transfer is enhanced appreciably for both cases. However, for the case ...
Convection zone origins of solar atmospheric heating
Schatten, Kenneth H.; Mayr, Hans G.
1986-10-01
Spicules are examined as a means for supplying the corona with mass, energy, and magnetic field. It is suggested that spicules form from the supersonic upward expansion of material on nearly evacuated network flux tubes embedded within the sun's convection zone. This allows supersonic but subescape velocities to be attained by the material as it flows outward through the photosphere. Although supersonic, the kinetic energy (subescape) of the spicule material, as observed, is insufficient for coronal heating. It is suggested that, through buoyancy changes on evacuated flux tubes, the magnetic field first 'wicks' material flow into the solar atmosphere. Subsequently, the magnetic field energizes the gaseous material to form the conventional hot, dynamically expanding, solar corona. This occurs through momentum and energy transport by Alfven waves and associated Maxwell stresses concurrently flowing upward on these 'geysers' (spicules). The vertical momentum equation governing fluid flow is examined, and a particular equipartition solution is presented for the flow velocity along a simple field geometry.
Turbulent forced convective flow in an an-isothermal channel
International Nuclear Information System (INIS)
The influence of variable viscosity effects on momentum and heat transfer of a non-isothermal turbulent forced convective flow is studied using thermal large-eddy simulation (LES). LES of bi-periodic channel flow with significant heat transfer at a low Mach number was performed to study the modulation in the near-wall turbulence structure due to anisotropic viscosity. The temperature ratio (R? = Thot/Tcold) is varied from 1.01 to 5 to study the isolated effect of variable viscosity with (Thot) and (Tcold) as a wall temperature. It is shown that average and turbulent fields undergo significant changes in a broad range of Reynolds number, compared to isothermal flow with constant viscosity, we observe enhanced turbulence on the cold side of the channel, characterized by locally lower viscosity whereas a decrease of turbulent kinetic energy is found at the hot wall. The turbulent structures via H criteria of high vorticity shows very short and densely populated vortices near cold wall whereas long streaky structure or large elongated vortices at the hot wall. Q invariant totally eradicate all the streaky structure at the hot wall as a consequence of re-laminarization. To further clarify this issue spectral study is conducted that reveals complete suppression of turbulence at the hot side of the channel at large temperature ratio because no inertial zone (i.e. index of Kolmogorov scaling law is zero) is obtained on the spectra in these region. (authors)
Performance of a convective, infrared and combined infrared- convective heated conveyor-belt dryer.
El-Mesery, Hany S; Mwithiga, Gikuru
2015-05-01
A conveyor-belt dryer was developed using a combined infrared and hot air heating system that can be used in the drying of fruits and vegetables. The drying system having two chambers was fitted with infrared radiation heaters and through-flow hot air was provided from a convective heating system. The system was designed to operate under either infrared radiation and cold air (IR-CA) settings of 2000 W/m(2) with forced ambient air at 30 °C and air flow of 0.6 m/s or combined infrared and hot air convection (IR-HA) dryer setting with infrared intensity set at 2000 W/m(2) and hot at 60 °C being blown through the dryer at a velocity of 0.6 m/s or hot air convection (HA) at an air temperature of 60 °C and air flow velocity 0.6 m/s but without infrared heating. Apple slices dried under the different dryer settings were evaluated for quality and energy requirements. It was found that drying of apple (Golden Delicious) slices took place in the falling rate drying period and no constant rate period of drying was observed under any of the test conditions. The IR-HA setting was 57.5 and 39.1 % faster than IR-CA and HA setting, respectively. Specific energy consumption was lower and thermal efficiency was higher for the IR-HA setting when compared to both IR-CA and HA settings. The rehydration ratio, shrinkage and colour properties of apples dried under IR-HA conditions were better than for either IR-CA or HA. PMID:25892769
International Nuclear Information System (INIS)
The vertical upward flow of water in a heated tube at a supercritical pressure was numerically simulated by means of a commercially available computational fluid dynamics code, FLUENT. To examine the reliability of the embedded turbulence models at a supercritical pressure, a series of simulations was performed with several two-equation turbulence models: a representative low-Reynolds number k-? model, k-? model, RNG k-? model as well as a standard k-? model. The turbulence models were evaluated by comparing the results of the simulations with the experimental data published by Yamagata et al. (1972). In normal heat transfer enhancement regions, the RNG k-? model with an enhanced wall treatment option reproduces best the experiment. But, in heat transfer deterioration regions, the difference between the simulations and the experiment are remarkable. The heat transfer trend predicted by the calculation with the k-? model and low-Reynolds number k-? model is qualitatively similar to that of the experiment. Therefore, in the heat transfer deterioration regions, the k-? models could be a candidate for a reasonable solution if some modifications are made. (author)
Analysis of natural convection in volumetrically-heated melt pools
Energy Technology Data Exchange (ETDEWEB)
Sehgal, B.R.; Dinh, T.N.; Nourgaliev, R.R. [Royal Inst. of Tech., Stockholm (Sweden). Div. of Nuclear Power Safety
1996-12-01
Results of series of studies on natural convection heat transfer in decay-heated core melt pools which form in a reactor lower plenum during the progression of a core meltdown accident are described. The emphasis is on modelling and prediction of turbulent heat transfer characteristics of natural convection in a liquid pool with an internal energy source. Methods of computational fluid dynamics, including direct numerical simulation, were applied for investigation. Refs, figs, tabs.
Analysis of natural convection in volumetrically-heated melt pools
International Nuclear Information System (INIS)
Results of series of studies on natural convection heat transfer in decay-heated core melt pools which form in a reactor lower plenum during the progression of a core meltdown accident are described. The emphasis is on modelling and prediction of turbulent heat transfer characteristics of natural convection in a liquid pool with an internal energy source. Methods of computational fluid dynamics, including direct numerical simulation, were applied for investigation
Endwall convective heat transfer for bluff bodies
DEFF Research Database (Denmark)
Wang, Lei; Salewski, Mirko
2012-01-01
The endwall heat transfer characteristics of forced flow past bluff bodies have been investigated using liquid crystal thermography (LCT). The bluff body is placed in a rectangular channel with both its ends attached to the endwalls. The Reynolds number varies from 50,000 to 100,000. In this study, a single bluff body and two bluff bodies arranged in tandem are considered. Due to the formation of horseshoe vortices, the heat transfer is enhanced appreciably for both cases. However, for the case of two bluff bodies in tandem, it is found that the presence of the second bluff body decreases the heat transfer as compared to the case of a single bluff body. In addition, the results show that the heat transfer exhibits Reynolds number similarity. For a single bluff body, the Nusselt number profiles collapse well when the data are scaled by Re0.55; for two bluff bodies arranged in tandem, the heat transfer scaling is changed to Re0.51, indicating that the power index of Reynolds number is flow dependent.
Time evolution simulation of heat removal in a small water tank by natural convection
International Nuclear Information System (INIS)
One of the cooling modes for any source of heat such as in a shutdown nuclear core is the natural convection. The design specifications of any cooling pool can only be done when the removal heat rate and the corresponding mass flow rate is reasonably established. In our simulation scheme, we assumed that the body forces acting in the cubic water cell are: the weight, the drag force and the integrated pressure forces on the horizontal surfaces, the viscosity shear forces on the vertical surfaces and also a special viscosity drag force due to the mass dislocation along a Bernoulli type current tube outside the motive region. For a suitable time step, the uprising convection velocity is determined by an implicit and also by an explicit solution algorithm. The resulting differential equation depends on updating specific mass, dynamic viscosity and constant pressure heat coefficient with the last known temperature in the cell that absorbed heat. Numerical calculation software was performed using MATLAB’s technical computing language and then applied for a heat generation plate simulating a spent fuel assembler from a shutdown nuclear core. The results show time evolution of convection, terminal velocity and water temperature distribution. Pool dimension as well as pool level decrement are also determined for various air exhausting system conditions and heat rate of the spent fuel plate being cooled. (author)
Forced Convection and Sedimentation Past a Flat Plate
Pelekasis, Nikolaos A.; Acrivos, Andreas
1995-01-01
The steady laminar flow of a well-mixed suspension of monodisperse solid spheres, convected steadily past a horizontal flat plate and sedimenting under the action of gravity, is examined. It is shown that, in the limit as Re approaches infinity and epsilon approaches 0, where Re is the bulk Reynolds number and epsilon is the ratio of the particle radius a to the characteristic length scale L, the analysis for determining the particle concentration profile has several aspects in common with that of obtaining the temperature profile in forced-convection heat transfer from a wall to a fluid stream moving at high Reynolds and Prandtl numbers. Specifically, it is found that the particle concentration remains uniform throughout the O(Re(exp -1/2)) thick Blasius boundary layer except for two O(epsilon(exp 2/3)) thin regions on either side of the plate, where the concentration profile becomes non-uniform owing to the presence of shear-induced particle diffusion which balances the particle flux due to convection and sedimentation. The system of equations within this concentration boundary layer admits a similarity solution near the leading edge of the plate, according to which the particle concentration along the top surface of the plate increases from its value in the free stream by an amount proportional to X(exp 5/6), with X measuring the distance along the plate, and decreases in a similar fashion along the underside. But, unlike the case of gravity settling on an inclined plate in the absence of a bulk flow at infinity considered earlier, here the concentration profile remains continuous everywhere. For values of X beyond the region near the leading edge, the particle concentration profile is obtained through the numerical solution of the relevant equations. It is found that, as predicted from the similarity solution, there exists a value of X at which the particle concentration along the top side of the plate attains its maximum value phi(sub m) and that, beyond this point, a stagnant sediment layer will form that grows steadily in time. This critical value of X is computed as a function of phi(sub s), the particle volume fraction in the free stream. In contrast, but again in conformity with the similarity solution, for values of X sufficiently far removed from the leading edge along the underside of the plate, a particle-free region is predicted to form adjacent to the plate. This model, with minor modifications, can be used to describe particle migration in other shear flows, as, for example, in the case of crossflow microfiltration.
Boiling of subcooled water in forced convection
International Nuclear Information System (INIS)
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/cm2), 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)
A numerical study of Li-SF6 wick combustion - Forced and mixed convective burning
Damaso, R. C.; Chen, L.-D.
1992-01-01
A numerical study is conducted to study Li-SF6 wick diffusion flames under mixed convective burning conditions at a pressure of 0.01 MPa. Both planar and cylindrical wicks are considered. The model is based on a conserved scalar approach. The objective of this study is to assess the effects of particular parameters on the burning rate and heat transfer. The flat-plate solution yields a fuel mass burning rate per unit surface area following the x exp -1/2 dependence of the classical similarity solution, where x is the streamwise distance. Cylindrical wick geometries yield enhanced burning rates over planar wicks. For the case of mixed convective burning, the burning rate results approach either the forced or natural convective burning limits as ambient streamwise velocity is changed. Critical Richardson numbers specifying these burning limits are determined for a given condition. Reducing gravity results in a lower burning rate because the influence of natural convection is diminished. Under reduced gravity of 1/1000 of the sea-level value, mixed convective burning nearly resembles forced convection.
International Nuclear Information System (INIS)
The practical objective of research on 'burn-out' is a reliable method giving the maximum safe rating for any water cooled reactor. Experimental work, which began at numerous centres about 10 years ago, has been concerned principally with endeavouring to understand the phenomenon as it applies to simple geometries such as round and rectangular channels. Many millions of pounds have been spent on this work and several thousand separate experimental results obtained. This considerable effort has achieved little real success in providing an explanation of 'burn-out' however. Many conflicting views have arisen and correlations so far developed have been shown to give calculated 'burn-out' heat fluxes varying by a factor of the order of 5> when applied to a typical reactor situation. While some uncertainty may be due to experimental variations, inadequate analytical effort is considered to be the primary cause of the present confused situation. To overcome this various analytical studies are being initiated by the Reactor Development Division at Winfrith and a detailed plan is being evolved for bringing effort to bear on certain fundamental aspects of boiling which have been neglected and which in some oases will require the development of special experimental techniques. This report describes the result of some work already carried out. It concerns an initial examination made on a large group of 'burn-out' data and describes the development of a correlation which predicts 'burn-out' heat fluxes to within an R.M.S. error of less than 10% over a very wide range of operating conditions including pressure. (author)
Heat removal by natural convection in a RPR reactor
International Nuclear Information System (INIS)
In this paper natural convection in RPR reactor is analysed. The effect of natural convection valves size on cladding temperature is studied. The reactor channel heat transfer problem is solved using finite elements in a two-dimensional analysis. Results show that two valves with ? = 0.16 m are suited to keep coolant and cladding temperatures below 730C. (author)
Natural and forced convection film boiling over axisymmetric bodies at high temperatures
International Nuclear Information System (INIS)
Natural and forced convection film boiling over axisymmetric bodies is analysed in the case of high surface temperatures. In these conditions, the global heat transfer is no more driven by simple conduction through the vapour film, an assumption that is commonly made in film boiling modelling for simplicity reasons, but rather by convection through this film. Therefore a mathematical method was developed which enables a full description of the vapour flow by including the inertia and convection terms of the momentum and energy equations. It is based on classical two-phase boundary layer integral methods where polynomial functions of order 5 are used to describe the velocity and the temperature profiles in the vapour flow. Also, a simple scaling analysis is described in order to understand when inertial and convective effects in the vapour flow become important. It is shown for example that for any given fluid, these effects will become predominant even at a low surface superheat when the fluid pressure is increased near its critical pressure. The developed models are then compared with three simpler models: a similar model which uses order 3 polynomial functions, a model where the convective effects are modelled by just using an effective latent heat, and an even simpler model where convection and inertia in the vapour film are not considered at all. It is shown on some examples with very high surface temperatures that if convective and inertial effects are totally neglected, the global heat transfer is clearly underestimated compared to the ones calculated with the two other models, which are quite similar. However, if other important parameters such as vapour production or vapour film thickness at the front stagnation point are calculated, the results given by the model with the effective latent heat diverge from those given by the developed models. (author)
Convection zone origins of solar atmospheric heating
International Nuclear Information System (INIS)
Spicules are examined as a means for supplying the corona with mass, energy, and magnetic field. It is suggested that spicules form from the supersonic upward expansion of material on nearly evacuated network flux tubes embedded within the sun's convection zone. This allows supersonic but subescape velocities to be attained by the material as it flows outward through the photosphere. Although supersonic, the kinetic energy (subescape) of the spicule material, as observed, is insufficient for coronal heating. It is suggested that, through buoyancy changes on evacuated flux tubes, the magnetic field first wicks material flow into the solar atmosphere. Subsequently, the magnetic field energizes the gaseous material to form the conventional hot, dynamically expanding, solar corona. This occurs through momentum and energy transport by Alfven waves and associated Maxwell stresses concurrently flowing upward on these geysers (spicules). The vertical momentum equation governing fluid flow is examined, and a particular equipartition solution is presented for the flow velocity along a simple field geometry. 39 references
Nadia Potoceanu
2007-01-01
The paper presented the most aspects of convective circulate mode of heat transfer : heat transfer through the boundary layer formed at the surface of the heat generator; heat transfer in the heat carrier and heat transfer through the boundary layer formed at the heated surface
Numerical solution of staggered circular tubes in two-dimensional laminar forced convection
Scientific Electronic Library Online (English)
Carlos Henrique, Marchi; Maykel Alexandre, Hobmeir.
2007-03-01
Full Text Available This paper aims to demonstrate the importance of adequately estimating the discretization error intrinsic in the result of any numerical simulation. The problem under consideration is forced convection in a staggered circular tube heat exchanger. The problem is solved to analyze the effect of the di [...] stance between the tubes, aiming to optimize the heat exchanger’s geometrical configuration by two Reynolds numbers (50 and 100). The present work did not confirm the existence of an optimal geometrical point for the operation of staggered circular tube heat exchangers, as claimed in a numerical study published in the literature.
Laminar forced convection inside externally finned tubes
International Nuclear Information System (INIS)
Analytical solutions are obtained for thermal entry region problems inside ducts with axially varying heat transFer coefficient, by making use of the ideas in the recently advanced generalized integral transform technique. The analysis is applied to stepwise variations of Biot number that simulate intermitent rows of external fins. Numerical results are obtained for different fin arrangements, in a systematic manner, so as to critically examine the relative marits of wider and/or more numerous fin rows. (author)
Reynolds stress and heat flux in spherical shell convection
Käpylä, P. J.; Mantere, M. J.; Guerrero, G.; Brandenburg, A; Chatterjee, P.
2010-01-01
(abridged) Context. Turbulent fluxes of angular momentum and heat due to rotationally affected convection play a key role in determining differential rotation of stars. Here we perform a systematic comparison between Cartesian and spherical geometries as a function of the rotation rate. Aims. We extend the earlier studies by using spherical wedges to obtain turbulent angular momentum and heat transport as functions of the rotation rate from stratified convection. We compare ...
Analytical Solution of Forced-Convective Boundary-Layer Flow over a Flat Plate
DEFF Research Database (Denmark)
Mirgolbabaei, H.; Barari, Amin
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 and those of numerical solution shows excellent agreement, illustrating the effectiveness of the method. The solution obtained by ADM gives an explicit expression of temperature distribution and velocity distribution over a flat plate.
Analytical Solution of Forced-Convective Boundary-Layer Flow over a Flat Plate
DEFF Research Database (Denmark)
Mirgolbabaei, H.; Barari, Amin; Ibsen, Lars Bo; Sfahani, M. G.
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 and those of numerical solution shows excellent agreement, illustrating the effectiveness of the method. The solution obtained by ADM gives an explicit expression of temperature distribution and velocity ...
Solution of heat removal from nuclear reactors by natural convection
Directory of Open Access Journals (Sweden)
Zitek Pavel
2014-03-01
Full Text Available This paper summarizes the basis for the solution of heat removal by natural convection from both conventional nuclear reactors and reactors with fuel flowing coolant (such as reactors with molten fluoride salts MSR.The possibility of intensification of heat removal through gas lift is focused on. It might be used in an MSR (Molten Salt Reactor for cleaning the salt mixture of degassed fission products and therefore eliminating problems with iodine pitting. Heat removal by natural convection and its intensification increases significantly the safety of nuclear reactors. Simultaneously the heat removal also solves problems with lifetime of pumps in the primary circuit of high-temperature reactors.
SIMULATION OF MIXED CONVECTIVE HEAT TRANSFER USING LATTICE BOLTZMANN METHOD
Directory of Open Access Journals (Sweden)
A. R. M. Rosdzimin
2010-12-01
Full Text Available In this paper, mixed (forced–natural convective heat transfer around a heated square cylinder located inside a lid driven cavity has been studied numerically using the lattice Boltzmann method in the range of 100? Re ? 1000 with the corresponding Richardson number 0.01?Ri?10. The double-population lattice Boltzmann formulation is used as the governing equation. Two dimensional nine-velocity models are used for the computation of the velocity field while a four-velocity model is used for the computation of the temperature field. We found that the combination of nine- and four-velocity models can be applied to the calculation without losing its accuracy. The results are presented in the form of streamline and isotherm plots as well as the variation of local Nusselt number at the top surface of the heated square. The computational results demonstrate that the flow pattern, formation of vortex and also the Nusselt number are influence by the Reynolds number and Richardson number.
International Nuclear Information System (INIS)
An experimental study was performed to obtain local fluid velocity and temperature measurements in the mixed (combined free and forced) convection regime for specific flow coastdown transients. A brief investigation of steady-state flows for the purely free-convection regime was also completed. The study was performed using an electrically heated 2 x 6 rod bundle contained in a flow housing. In addition a transient data base was obtained for evaluating the COBRA-WC thermal-hydraulic computer program
D'Orazio, A.; Karimipour, A.; Nezhad, A. H.; Shirani, E.
2014-11-01
Laminar mixed convective heat transfer in two-dimensional rectangular inclined driven cavity is studied numerically by means of a double population thermal Lattice Boltzmann method. Through the top moving lid the heat flux enters the cavity whereas it leaves the system through the bottom wall; side walls are adiabatic. The counter-slip internal energy density boundary condition, able to simulate an imposed non zero heat flux at the wall, is applied, in order to demonstrate that it can be effectively used to simulate heat transfer phenomena also in case of moving walls. Results are analyzed over a range of the Richardson numbers and tilting angles of the enclosure, encompassing the dominating forced convection, mixed convection, and dominating natural convection flow regimes. As expected, heat transfer rate increases as increases the inclination angle, but this effect is significant for higher Richardson numbers, when buoyancy forces dominate the problem; for horizontal cavity, average Nusselt number decreases with the increase of Richardson number because of the stratified field configuration.
Thermal Performance of Convective-Radiative Heat Transfer in Porous Fins
Directory of Open Access Journals (Sweden)
Majid SHAHBABAEI
2014-01-01
Full Text Available Forced and natural convection in porous fins with convective coefficient at the tips under radiation and convection effects are investigated in this paper. Aluminum and copper as fin materials are investigated. In forced and natural convection, air and water are applied as working fluids, respectively. In order to solve this nonlinear equation, Homotopy Perturbation Method (HPM and Variational Iteration Method (VIM are used. To verify the accuracy of the methods, a comparison is made to the exact solution (BVP. In this work, the effects of porosity parameter (, Radiation parameter (? and Temperature-Ratio parameter (µ on non-dimensional temperature distribution for both of the flows are shown. The results show that the effects of (? and (µ on temperature distribution in natural convection are based on porosity and in forced convection are uniform, approximately. Also, it is shown that both VIM and HPM are capable of being used to solve this nonlinear heat transfer equation.doi:10.14456/WJST.2014.64
Validation of PARET for the modeling of heat transfer under natural convection core cooling
International Nuclear Information System (INIS)
The PARET code is a one-dimensional, coupled thermal-hydraulic and point-kinetics code, which was originally developed for the analysis of SPERT-I transients and later adapted for the analysis of transient behavior in research reactors. Due to its ease of transportability and relative simplicity of input preparation, it is widely used internationally and is particularly attractive for research reactors with limited computational facilities. The thermal-hydraulic modeling of the current version of PARET accounts for buoyancy forces in the core and external pressure gradients that may arise from density differences between the core inlet and outlet. This feature of PARET makes it a useful tool for the analysis of research reactors cooled by natural convection as well as those cooled by forced convection. Since PARET has been applied to the analysis of the International Atomic Energy Agency 10-MW benchmark cores for protected and unprotected transients and also for the analysis of SPERT-I transients, its forced convection heat-removal model is reliable. However, there has been little experience with the capability of PARET to model heat removal in cores cooled by natural convection. This paper reports the results of some experiments performed at the Malaysian PUSPATI reactor to compare PARET predictions for power increases under natural convection core cooling to measured data
Numerical simulations on natural convective heat transfer and active cooling of IFMIF Test Cell
Energy Technology Data Exchange (ETDEWEB)
Chen, Yuming, E-mail: yuming.chen@kit.edu [Institute for Neutron Physics and Reactor Technology, Karlsruhe Institute of Technology (Germany); Arbeiter, Frederik; Heinzel, Volker; Kondo, Keitaro [Institute for Neutron Physics and Reactor Technology, Karlsruhe Institute of Technology (Germany); Mittwollen, Martin [Institute for Material Handling and Logistics, Karlsruhe Institute of Technology (Germany); Tian, Kuo [Institute for Neutron Physics and Reactor Technology, Karlsruhe Institute of Technology (Germany)
2014-10-15
Highlights: •Thermo-hydraulic simulations of the IFMIF Test-Cell were carried out with Ansys-CFX V.14 on a reference case. •The current simulation model includes the natural convection inside the TC, several forced convective water flows in the pipelines attached on the steel liners and the helium-cooled HFTM (High Flux Test Module). •A kind of CFX Beta feature was used; in which multiple fluid domains associated with individual turbulence (laminar) models were defined and solved in a single simulation. •The simulations provide the key information on the flow and heat transfer of the Test-Cell components. -- Abstract: The International Fusion Materials Irradiation Facility (IFMIF) is designated to generate a materials irradiation database for the future fusion reactors. The Test Cell (TC) accommodates the Test Modules and the lithium target assembly. Due to the nuclear heat generation, all the Test Modules inside the TC will be actively cooled. Other components like supporting structures, pipelines, cables etc., will be passively cooled by natural convection. The heat will be removed from the steel liners surrounding the TC by active water cooling. This paper concerns the thermo-hydraulic simulations of the Test Cell using Ansys-CFX. The current simulation model includes the natural convection inside the TC, several forced convective water flows in the pipelines attached on the steel liners and the helium-cooled HFTM (High Flux Test Module). The simulations provide the only means for validating the design before the construction and operation.
International Nuclear Information System (INIS)
In construction, the use of Phase Change Materials (PCM) allows the storage/release of energy from solar radiation and internal loads. The application of such materials for lightweight construction (e.g., a wood house) makes it possible to improve thermal comfort and reduce energy consumption. The heat transfer process between the wall and the indoor air is convection. In this paper, we have developed a numerical model to evaluate several convective heat transfer correlations from the literature for natural, mixed and forced convection flows. The results show that the convective heat transfer highly influences the storage/release process in case of PCM walls. For the natural convection, the numerical results are highly dependent on the correlation used and the results may vary up to 200%. In the case of mixed and forced convection flows, the higher is the velocity, the more important is the storage capacity. - Highlights: ? We study effect of inside convection correlation on energy stored in PCM wall. ? We developed a 1D conduction model for multilayer walls, with phase change material. ? Correlations have been constructed for mixed convection in all flow regimes. ? Up to 200% variation of energy stored in PCM layer, depending on convection correlation. ? Ventilation can increase the energy stored in the PCM layer.
SRS reactor control rod cooling without normal forced convection cooling
International Nuclear Information System (INIS)
This paper describes an analytical study of the coolability of the control rods in the Savannah River site (SRS) K production reactor under conditions of loss of normal forced convection cooling. The study was performed as part of the overall safety analysis of the reactor supporting its restart. The analysis addresses the buoyancy-driven boiling flow over the control rods that occurs when forced cooling is lost. The objective of the study was to demonstrate that the control rods will remain cooled (i.e., no melting) at powers representative of those anticipated for restart of the reactor
International Nuclear Information System (INIS)
Highlights: • Convective–radiative radial fins with base convective heating were analyzed. • Homogeneous material and functionally graded material fins were investigated. • Fin efficiency and the effects of dimensionless parameters in fins were analyzed. - Abstract: This paper studies a radial fin of uniform thickness with convective heating at the base and convective–radiative cooling at the tip. The fin is assumed to experience uniform internal heat generation. The exposed surfaces of the fin lose heat by simultaneous convection and radiation to the surroundings. Two types of fin materials are investigated: homogeneous material and functionally graded material (FGM). For the homogeneous material, the thermal conductivity is assumed to be a linear function of temperature, while for the FGM fin the thermal conductivity is modeled as a linear function of the dimensionless radial coordinate. The analysis is conducted using the differential transformation method (DTM). The accuracy of DTM is verified by comparing the results for the simplified versions of the present model with an exact analytical solution derived here. Once the accuracy of DTM is authenticated, the method is used to generate results for the general problem formulated here. These results illustrate the effects of various dimensionless parameters on the thermal performance of homogeneous material fins and FGM fins
Slow forced and free convection in inclined channels
International Nuclear Information System (INIS)
Mixed convection at low Reynolds numbers in tilted rectangular channels of intermediate aspect ratios is dealt with. A laminar flow enters into an inclined enclosure whose upper plate is adiabatic whereas a constant heat flux is supplied to the bottom plate. The analytic solution of the linearised problem is first obtained. Subsequently the numerical solution of the pertinent equations is carried on by a computer's time saving iterative procedure. (Author)
Energy Technology Data Exchange (ETDEWEB)
Ghalambaz, M.; Noghrehabadi, A.; Ghanbarzadeh, A., E-mail: m.ghalambaz@gmail.com, E-mail: ghanbarzadeh.a@scu.ac.ir [Department of Mechanical Engineering, Shahid Chamran University of Ahvaz, Ahvaz (Iran, Islamic Republic of)
2014-04-15
In this paper, the natural convective flow of nanofluids over a convectively heated vertical plate in a saturated Darcy porous medium is studied numerically. The governing equations are transformed into a set of ordinary differential equations by using appropriate similarity variables, and they are numerically solved using the fourth-order Runge-Kutta method associated with the Gauss-Newton method. The effects of parametric variation of the Brownian motion parameter (Nb), thermophoresis parameter (Nt) and the convective heating parameter (Nc) on the boundary layer profiles are investigated. Furthermore, the variation of the reduced Nusselt number and reduced Sherwood number, as important parameters of heat and mass transfer, as a function of the Brownian motion, thermophoresis and convective heating parameters is discussed in detail. The results show that the thickness of the concentration profiles is much lower than the temperature and velocity profiles. For low values of the convective heating parameter (Nc), as the Brownian motion parameter increases, the non-dimensional wall temperature increases. However, for high values of Nc, the effect of the Brownian motion parameter on the non-dimensional wall temperature is not significant. As the Brownian motion parameter increases, the reduced Sherwood number increases and the reduced Nusselt number decreases. (author)
Scientific Electronic Library Online (English)
M., Ghalambaz; A., Noghrehabadi; A., Ghanbarzadeh.
2014-06-01
Full Text Available In this paper, the natural convective flow of nanofluids over a convectively heated vertical plate in a saturated Darcy porous medium is studied numerically. The governing equations are transformed into a set of ordinary differential equations by using appropriate similarity variables, and they are [...] numerically solved using the fourth-order Runge-Kutta method associated with the Gauss-Newton method. The effects of parametric variation of the Brownian motion parameter (Nb), thermophoresis parameter (Nt) and the convective heating parameter (Nc) on the boundary layer profiles are investigated. Furthermore, the variation of the reduced Nusselt number and reduced Sherwood number, as important parameters of heat and mass transfer, as a function of the Brownian motion, thermophoresis and convective heating parameters is discussed in detail. The results show that the thickness of the concentration profiles is much lower than the temperature and velocity profiles. For low values of the convective heating parameter (Nc), as the Brownian motion parameter increases, the non-dimensional wall temperature increases. However, for high values of Nc, the effect of the Brownian motion parameter on the non-dimensional wall temperature is not significant. As the Brownian motion parameter increases, the reduced Sherwood number increases and the reduced Nusselt number decreases.
An experimental investigation of forced convection flat plate solar air heater with storage material
Aissa Walid; El-Sallak Mostafa; Elhakem Ahmed
2012-01-01
Solar air heater (SAH) is a heating device that uses the heated air in the drying of agriculture products and many engineering applications. The purpose of the present work is to study a forced convection flat plate solar air heater with granite stone storage material bed under the climatic conditions of Egypt-Aswan. Experiments are performed at different air mass flow rates ; varying from 0.016 kg/s to 0.08 kg/s, for five hot summer days of July 2008. Hourly values of global solar radi...
Reynolds stress and heat flux in spherical shell convection
Käpylä, P J; Guerrero, G; Brandenburg, A; Chatterjee, P
2010-01-01
Context. Turbulent fluxes of angular momentum and heat due to rotationally affected convection play a key role in determining differential rotation of stars. Aims. We compute turbulent angular momentum and heat transport as functions of the rotation rate from stratified convection. We compare results from spherical and Cartesian models in the same parameter regime in order to study whether restricted geometry introduces artefacts into the results. Methods. We employ direct numerical simulations of turbulent convection in spherical and Cartesian geometries. In order to alleviate the computational cost in the spherical runs and to reach as high spatial resolution as possible, we model only parts of the latitude and longitude. The rotational influence, measured by the Coriolis number or inverse Rossby number, is varied from zero to roughly seven, which is the regime that is likely to be realised in the solar convection zone. Cartesian simulations are performed in overlapping parameter regimes. Results. For slow ...
Convective heat transfer in building simulation; Convectieve warmteoverdracht in gebouwsimulatie
Energy Technology Data Exchange (ETDEWEB)
Peeters, L.F.R. [Departement Mechanica, Vrije Universiteit Brussel, Brussels (Belgium)
2012-05-15
The sensitivity of the convective heat transfer correlations to air movements related to the normal use of a room is discussed. Also the parameters determining and the control variables directing the selection of the convection coefficient correlation during an energy calculation are discussed. The current approach for modelling convective heat transfer in common building simulation programs is discussed. As is the additional research required to further improve convection modelling. [Dutch] Er zijn drie vormen van warmteoverdracht: straling, geleiding en convectie. Terwijl de eerste twee accuraat en relatief eenvoudig analytisch kunnen worden uitgeschreven, is dat voor convectie minder evident. Een convectieve stroming is afhankelijk van een hele reeks parameters, die in min of meerdere mate relevant zijn voor de energieberekeningen in gebouwsimulatie. Het is dus van belang om te weten waar die sensitiviteiten liggen en hoe die worden gemodelleerd in gebouwsimulatie. Dit artikel geeft een analyse van de relevante parameters voor modellering van convectie in twee veel gebruikte simulatiecodes.
Khaled, M.; Garnier, B.; Harambat, F.; Peerhossaini, H.
2010-02-01
A new experimental technique is presented that allows simultaneous measurement of convective and radiative heat flux in the underhood. The goal is to devise an easily implemented and accurate experimental method for application in the vehicle underhood compartment. The new method is based on a technique for heat-flux measurement developed by the authors (Heat flow (flux) sensors for measurement of convection, conduction and radiation heat flow 27036-2, © Rhopoint Components Ltd, Hurst Green, Oxted, RH8 9AX, UK) that uses several thermocouples in the thickness of a thermal resistive layer (foil heat-flux sensor). The method proposed here uses a pair of these thermocouples with different radiative properties. Measurements validating this novel technique are carried out on a flat plate with a prescribed constant temperature in both natural- and forced-convection flow regimes. The test flat plate is instrumented by this new technique, and also with a different technique that is intrusive but very accurate, used as reference here (Bardon J P and Jarny Y 1994 Procédé et dispositif de mesure transitoire de température et flux surfacique Brevet n°94.011996, 22 February). Discrepancies between the measurements by the two techniques are less than 10% for both convective and radiative heat flux. Error identification and sensitivity analysis of the new method are also presented.
Convective Heat Transfer In Porous Ceramic Materials
BÜYÜKALACA, Orhan
1999-01-01
In this study heat transfer in porous ceramic materials, which offer a potential as an alternative heat transfer medium in a number of systems in which heat transfer takes place, is investigated experimentally. Experiments were performed for five different specimens at various air flow rates and specimen temperatures. The volumetric heat transfer coefficient was determined using the results of the experiments. A characteristic length obtained from the pressure drop data was used in ...
Heating of matter by microwaves without convection
Draškovi?, Draško
2012-01-01
This thesis considers heating of matter by means of microwaves. The most common device that uses micro waves for the heating of matter is the microwave oven. Because of the microwave ovens ubiquity and accessibility there are many articles that address the way it works and in how it heats matter. However, it appears that articles contradict each other in their claims that the heating of the matter is either from the inside or from the outside. What actually means »heating the material in the...
Natural convection heat transfer within horizontal spent nuclear fuel assemblies
Energy Technology Data Exchange (ETDEWEB)
Canaan, R.E.
1995-12-01
Natural convection heat transfer is experimentally investigated in an enclosed horizontal rod bundle, which characterizes a spent nuclear fuel assembly during dry storage and/or transport conditions. The basic test section consists of a square array of sixty-four stainless steel tubular heaters enclosed within a water-cooled rectangular copper heat exchanger. The heaters are supplied with a uniform power generation per unit length while the surrounding enclosure is maintained at a uniform temperature. The test section resides within a vacuum/pressure chamber in order to subject the assembly to a range of pressure statepoints and various backfill gases. The objective of this experimental study is to obtain convection correlations which can be used in order to easily incorporate convective effects into analytical models of horizontal spent fuel systems, and also to investigate the physical nature of natural convection in enclosed horizontal rod bundles in general. The resulting data consist of: (1) measured temperatures within the assembly as a function of power, pressure, and backfill gas; (2) the relative radiative contribution for the range of observed temperatures; (3) correlations of convective Nusselt number and Rayleigh number for the rod bundle as a whole; and (4) correlations of convective Nusselt number as a function of Rayleigh number for individual rods within the array.
Natural convection heat transfer within horizontal spent nuclear fuel assemblies
International Nuclear Information System (INIS)
Natural convection heat transfer is experimentally investigated in an enclosed horizontal rod bundle, which characterizes a spent nuclear fuel assembly during dry storage and/or transport conditions. The basic test section consists of a square array of sixty-four stainless steel tubular heaters enclosed within a water-cooled rectangular copper heat exchanger. The heaters are supplied with a uniform power generation per unit length while the surrounding enclosure is maintained at a uniform temperature. The test section resides within a vacuum/pressure chamber in order to subject the assembly to a range of pressure statepoints and various backfill gases. The objective of this experimental study is to obtain convection correlations which can be used in order to easily incorporate convective effects into analytical models of horizontal spent fuel systems, and also to investigate the physical nature of natural convection in enclosed horizontal rod bundles in general. The resulting data consist of: (1) measured temperatures within the assembly as a function of power, pressure, and backfill gas; (2) the relative radiative contribution for the range of observed temperatures; (3) correlations of convective Nusselt number and Rayleigh number for the rod bundle as a whole; and (4) correlations of convective Nusselt number as a function of Rayleigh number for individual rods within the array
Heat transfer by natural convection into an horizontal cavity
International Nuclear Information System (INIS)
At this thesis it is studied the heat transfer by natural convection in an horizontal cavity, it is involved a boiling's part that is described the regimes and correlations differences for boiling's curve. It is designed a horizontal cavity for realize the experimental part and it's mention from equipment or instrumentation to succeed in a experimentation that permits to realize the analysis of heat transfer, handling as water fluid at atmospheric pressure and where it's present process from natural convection involving part boiling's subcooled. The system consists of heater zone submerged in a horizontal cavity with water. Once part finished experimental with information to obtained it's proceeded to obtain a correlation, realized starting from analysis dimensionless such as: Jakob, Bond and Grasoft (Boiling) besides of knows in natural convection: Prandtl and Nusselt. The mathematical model explains the behavior for natural convection continued part boiling's subcooled. It is realize analysis graphics too where it's show comparing with Globe Dropkin and Catton equations by natural convection with bottom heating. (Author)
Investigating Convective Heat Transfer with an Iron and a Hairdryer
Gonzalez, Manuel I.; Lucio, Jesus H.
2008-01-01
A simple experimental set-up to study free and forced convection in undergraduate physics laboratories is presented. The flat plate of a domestic iron has been chosen as the hot surface, and a hairdryer is used to generate an air stream around the plate. Several experiments are proposed and typical numerical results are reported. An analysis and…
M. A. Mansour
2013-01-01
The effects of magnetic force, acting vertically downward on natural convection within a nanofluid filled tilted trapezoidal enclosure saturated with an electrically conducting fluid have been investigated numerically. The bottom wall of the enclosure is subjected to a constant cold temperature and the top wall experiences a heat source whereas the remaining sidewalls are kept adiabatic. The physical problems are represented mathematically by different sets of governing equations along with t...
A multiple-relaxation-time lattice Boltzmann model for convection heat transfer in porous media
Liu, Qing; He, Ya-Ling; Li, Qing; Tao, Wen-Quan
2013-01-01
In this paper, a two-dimensional (2D) multiple-relaxation-time (MRT) lattice Boltzmann (LB) model is developed for simulating convection heat transfer in porous media at the representative elementary volume scale. In the model, a MRT-LB equation is used to simulate the flow field, while another MRT-LB equation is employed to simulate the temperature field. The effect of the porous media is considered by introducing the porosity into the equilibrium moments, and adding a forc...
Three-dimensional convection planforms with internal heat generation
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Travis, B. (Los Alamos National Laboratory, NM (USA)); Weinstein, S.; Olson, P. (Johns Hopkins Univ. (USA))
1990-03-01
Thermal convection planforms in an infinite Prandtl number, isoviscous fluid layer with basal and internal heating are determined using three-dimensional finite difference calculations. With basal heating only, at Rayleigh number Ra{sub T}=1.5{times}10{sup 5} the planform consists of a weakly time-dependent spoke pattern, with connected networks of equally buoyant up- and downwellings. With volumetric heat sources the planform is strongly time variable. It consists of negatively buoyant flow concentrated in thermal trenches (discrete, arcuate sheets) and spots (cylindrical columns), separated by diffuse, nonbuoyant upwellings. Rapid planform changes occur primarily by trench propagation, as observed experimentally by Weinstein and Olson (this issue). The analogy between thermal trenches in internally heated convection and convergent plate boundaries on Earth suggests that the global pattern of subduction may evolve by propagation of Wadati-Benioff zones along the strike of the slab.
Non intrusive measurement of the convective heat transfer coefficient
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Rebay, M.; Mebarki, G.; Padet, J. [Reims Univ., Reims (France). Faculty of Science, GRESPI Thermomechanical Lab; Arfaoui, A. [Reims Univ., Reims (France). Faculty of Science, GRESPI Thermomechanical Lab; Tunis Univ., Tunis (Tunisia). Faculty of Science, EL MANAR, LETTM; Maad, B.R. [Tunis Univ., Tunis (Tunisia). Faculty of Science, EL MANAR, LETTM
2010-07-01
The efficiency of cooling methods in thermal systems such as radiators and heat exchangers must be improved in order to enhance performance. The evaluation of the heat transfer coefficients between a solid and a fluid is necessary for the control and the dimensioning of thermal systems. In this study, the pulsed photothermal method was used to measure the convective heat transfer coefficient on a solid-fluid interface, notably between an air flow and a heated slab mounted on a PVC flat plate. This configuration simulated the electronic air-cooling inside enclosures and racks. The influence of the deflector's inclination angle on the enhancement of heat transfer was investigated using 2 newly developed identification models. The first model was based on a constant heat transfer coefficient during the pulsed experiment, while the second, improved model was based on a variable heat transfer coefficient. The heat transfer coefficient was deduced from the evolution of the transient temperature induced by a sudden deposit of a luminous energy on the front face of the slab. Temperature evolutions were derived by infrared thermography, a camera for cartography and a detector for precise measurement in specific locations. The results show the improvement of measurement accuracies when using a model that considers the temporal evolution of the convective heat transfer coefficient. The deflection of air flow on the upper surface of the heated slab demonstrated better cooling of the slab by the deflection of air flow. 11 refs., 1 tab., 8 figs.
Natural convection heat transfer in moderate aspect ratio enclosures
International Nuclear Information System (INIS)
Local and average heat transfer coefficients for natural convection between parallel plates separated by slats to create enclosures of moderate aspect ratio have been experimentally determined using an interferometric technique. The effects of Rayleigh number, tilt and slat angle, and aspect ratio on the Nusselt number have been determined. The Rayleigh number range tested was up to 7 x 104, and the aspect ratio (ratio of enclosure length to plate spacing) varied between 0.25 and 4. The angles of tilt of the enclosure with respect to the horizontal were 45, 60 and 90 deg. Slat angles of 45, 60 90 and 135 deg were studied. The results obtained in a previous investigation [1] for aspect ratios of 9 to 36 are included to show continuity. The results indicate that the convective heat transfer is a strong function of the aspect ratio for aspect ratios less than 4. For aspect ratios in the range of 0.5 to 4, spacers between the plates increase, rather than decrease, natural convection heat transfer compared to that for long enclosures. Slat angles less than 90 deg (i.e., oriented downward) reduce convective heat transfer
Heat convection in a set of three vertical cylinders
International Nuclear Information System (INIS)
Experimental results on temperature and heat flow in a set of three vertical cylinders with internal generation of heat, water submerged and in free convection are presented in this work . Temperature distribution, Nusselt number and convective coefficient (h) for each rod, developed for the distance between the axis of cylinders in vertical position, as a consequence of the application of power in its outside, are analyzed. Experimental information about heat transfer by free convection in vertical cylinders and surfaces is analyzed. Information of the several author who have carried out studies about the heat transfer on vertical cylinders was compiled, and the proposed equations with the experimental data obtained in the thermo fluids laboratory of National Institute of Nuclear Research (ININ) were tested. The way in which separation distance, s, distribution temperature array, Nusselt number, and convective coefficient calculated for the proposed channel with the Keyhani, Dutton and experimental equations are tabulated and they are plotted for each power value and for each separation between rods. The scheme of the used equipment and the experimentation description as well as the observations of tests and graphical results are included. (Author)
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…
Optimisation of convective heat dissipation from ventilated brake discs
Galindo-Lopez, Carlos Hannover
2009-01-01
Fast heat dissipation from brake discs is sought in current vehicles, where high power braking duties demand harmonic combination of strength, (undamped) disc mass and cooling abilities for a wide speed range. This work analyses the convective heat dissipation from ventilated brake discs and proposes means for its optimisation. The focus of research is the ventilation geometry of a standard brake disc with an outer diameter of 434mm and radial channels of 101mm in length. After...
Scaling of heat transport near onset in rapidly rotating convection
Ecke, Robert E.
2015-10-01
We consider the scaling of heat transport in the geostrophic regime of rotating Rayleigh-Bénard convection near onset for small Ekman number Ek from the perspective of weakly nonlinear theory. We show that available heat transport data from numerical simulation [1] for Ek law slope depends sensitively on the magnitude of the coefficients a and b. Similar arguments apply to Pr = 7 numerical data although the weakly nonlinear expansion appears valid for a smaller range of ? than in the Pr = 1 case.
MHD forced and free convection boundary layer flow near the leading edge
International Nuclear Information System (INIS)
Magnetohydrodynamic forced and free convection flow of an electrically conducting viscous incompressible fluid past a vertical flat plate with uniform heat flux in the presence of a magnetic field acting normal to the plate that moves with the fluid has been studied near the leading edge of the plate. The coupled non-linear equations are solved by the method of superposition for the values of the Prandtl number ranges from 0.01 to 10.0. The velocity and the temperature profiles are presented graphically and the values of the wall shear-stress as well as the heat transfer rate are presented in tabular form showing the effect of the buoyancy force and the applied magnetic field. To show the accuracy of the present method some typical values are compared with the available one. (author). 17 refs, 3 figs, 2 tabs
Theoretical Convective Heat Transfer Model Developement of Cold Storage Using Taguchi Analysis.
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Dr.N.Mukhopadhyay
2015-01-01
Full Text Available Energy crisis is one of the most important problems the world is facing now-a-days. With the increase of cost of electrical energy operating cost of cold storage storing is increasing which forces the increased cost price of the commodities that are kept. In this situation if the maximum heat energy(Q is absorbed by the evaporator inside the cold room through convective heat transfer process in terms of –heat transfer due to convection and heat transfer due to condensation, more energy has to be wasted to maintain the evaporator space at the desired temperature range of 2- 8 degree centigrade. In this paper we have proposed a theoretical heat transfermodel of convective heat transfer incold storage using Taguchi L9 orthogonal array. Velocity of air(V, Temperature difference(dT, RelativeHumidity(RHare the basic variable and three ranges are taken each of them in the model development. Graphical interpretations from the model justifies the reality
International Nuclear Information System (INIS)
Natural convective heat transfer from a wide heated vertical isothermal plate with adiabatic surfaces above and below the heated surface has been considered. There are a series of equally spaced vertical thin, flat surfaces (termed 'slats') near the heated surface, these surfaces being, in general, inclined to the heated surface. There is, in general, a uniform heat generation in the slats. The slats are pivoted about their centre-point and thus as their angle is changed, the distance of the tip of the slat from the plate changes. The situation considered is an approximate model of a window with a vertical blind, the particular case where the window is hotter than the room air being considered. The heat generation in the slats in this situation is the result of solar radiation passing through the window and falling on and being absorbed by the slats of the blind. The flow has been assumed to be laminar and steady. Fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces. The governing equations have been written in dimensionless form and the resulting dimensionless equations have been solved using a commercial finite-element package. The solution has the following parameters: (1) the Rayleigh number (2) the Prandtl number (3) the dimensionless heat generation rate in the slats per unit frontal area (4) the dimensionless distance of the slat center point (the pivot point) from the surface (5) the dimensionless slat size (6) the dimensionless slat spacing (7) the angle of inclination of the slats. Because of the application that motivated the study, results have only been obtained for a Prandtl number of 0.7. The effect of the other dimensionless variables on the mean dimensionless heat transfer rate from the heated vertical surface has been examined. (author)
Convective heat transfer with buoyancy effects from thermal sources on a flat plate
Tewari, S. S.; Jaluria, Y.
1991-06-01
An experimental study is carried out on the thermal interaction between two finite-size heat sources, located on a flat plate that is well insulated on the back. Both the horizontal and the vertical orientations of the surface are studied by measuring the flow velocities, the temperature field, and the local heat flux. The investigation is directed at the pure natural convection circumstance (no forced flow velocity) and the buoyancy-dominated mixed-convection circumstance (presence of a relatively small forced flow velocity). Large temperature gradients occur in the vicinity of the heat sources, resulting in a substantial diffusion of heat along the plate length. However, the effect of conduction is found to be highly localized. The orientation of the surface has a very strong effect on the interaction of the wakes from the heat sources for the circumstances considered. An upstream source is found to have a very strong influence on the temperature of a downstream source in the vertical surface orientation but has a much weaker influence in the horizontal orientation. In the latter circumstance the presence of a small forced flow velocity may actually increase the temperature of a downstream source by tilting the wake from the upstream source toward the downstream source.
International Nuclear Information System (INIS)
In order to understand the influence of a semispherical crucible geometry combined with different convection modes as a thermocapillary convection, natural convection and forced convection, induced by crystal rotation, on melt flow pattern in silicon Czochralski crystal growth process, a set of numerical simulations are conducted using Fluent Software. We solve the system of equations of heat and momentum transfer in classical geometry as cylindrical and modified crystal growth process geometry as cylindro-spherical. In addition, we adopt hypothesis adapted to boundary conditions near the interface and calculations are executed to determine temperature, pressure and velocity fields versus Grashof and Reynolds numbers. The analysis of the obtained results led to conclude that there is advantage to modify geometry in comparison with the traditional one. The absence of the stagnation regions of fluid in the hemispherical crucible corner and the possibility to control the melt flow using the crystal rotation enhances the quality of the process comparatively to the cylindrical one. The pressure field is strongly related to the swirl velocity.
Energy Technology Data Exchange (ETDEWEB)
Mokhtari, F [Physics Department, Faculty of Science, University of Mouloud Mammeri, Tizi Ouzou (Algeria); Bouabdallah, A; Zizi, M [LTSE Laboratory, University of Science and Technology USTHB. BP 32 Elalia, Babezzouar, Algiers (Algeria); Hanchi, S [UER Mecanique/ E.M.P B.P 17, Bordj El Bahri, Algiers (Algeria); Alemany, A, E-mail: abouab2002@yahoo.f [Laboratoire EPM, CNRS, Grenoble (France)
2010-03-01
In order to understand the influence of a semispherical crucible geometry combined with different convection modes as a thermocapillary convection, natural convection and forced convection, induced by crystal rotation, on melt flow pattern in silicon Czochralski crystal growth process, a set of numerical simulations are conducted using Fluent Software. We solve the system of equations of heat and momentum transfer in classical geometry as cylindrical and modified crystal growth process geometry as cylindro-spherical. In addition, we adopt hypothesis adapted to boundary conditions near the interface and calculations are executed to determine temperature, pressure and velocity fields versus Grashof and Reynolds numbers. The analysis of the obtained results led to conclude that there is advantage to modify geometry in comparison with the traditional one. The absence of the stagnation regions of fluid in the hemispherical crucible corner and the possibility to control the melt flow using the crystal rotation enhances the quality of the process comparatively to the cylindrical one. The pressure field is strongly related to the swirl velocity.
Stafford, J.; Walsh, E.; Egan, V.
2009-01-01
Convective heat transfer, due to axial flow fans impinging air onto a heated flat plate, is investigated with infrared thermography to assess the heated-thin-foil technique commonly used to quantify two-dimensional heat transfer performance. Flow conditions generating complex thermal profiles have been considered in the analysis to account for dominant sources of error in the technique. Uncertainties were obtained in the measured variables and the influences on the resultant heat transfer dat...
Coupled Convective and Radiative Heat Transfer Simulation for Urban Environments
Gracik, Stefan; Sadeghipour, Mostapha; Pitchurov, George; Liu, Jiying; Heidarinejad, Mohammad; Srebric, Jelena; Building Science Group, Penn State Team
2013-11-01
A building's surroundings affect its energy use. An analysis of building energy use needs to include the effects of its urban environment, as over half of the world's population now lives in cities. To correctly model the energy flow around buildings, an energy simulation needs to account for both convective and radiative heat transfer. This study develops a new model by coupling OpenFOAM and Radiance, open source packages for simulating computational fluid dynamics (CFD) and solar radiation, respectively. The model currently provides themo-fluid parameters including convective heat transfer coefficients, pressure coefficients, and solar heat fluxes that will be used as inputs for building energy simulations in a follow up study. The model uses Penn State campus buildings immersed in the atmospheric boundary layer flow as a case study to determine the thermo-fluid parameters around buildings. The results of this case study show that shadows can reduce the solar heat flux of a building's surface by eighty percent during a sunny afternoon. Convective heat transfer coefficients can vary by around fifty percent during a windy day.
Free convection from a constant heat flux elliptic tube
International Nuclear Information System (INIS)
An experimental investigation is presented on free convection of air around the outer surface of a constant heat flux elliptic tube. The local and average Nusselt number distribution is reported for different values of Rayleigh number and different tube inclination angles. The test Rayleigh number, based on input heat flux, ranges from 1.1x107 to 8x107. Average Nusselt numbers are evaluated and correlated with Rayleigh number for the elliptic tube with vertical major axis. Comparison between the convection characteristics of isothermal and constant heat flux elliptic tubes has been presented. Also, the effect of elliptic tube orientation on the average Nusselt number is shown. It is found that a higher value of average Nusselt number is achieved when the major axis of the tube is vertical
Convective Heat Transfer in Nanofluids: A Computational Approach
Giraldo, Mauricio; Sanín, Daniel; Flórez, Whady F.
2010-09-01
Nanofluids are a novel strategy to increase heat transfer characteristics of fluids by the addition of solid particles with diameters below 100 nm. Experimental measurements have shown that this approach can greatly increase heat conductivity, even above that predicted by Maxwell's theory. This paper shows a direct numerical simulation of the flow and thermal behaviour of a nanofluid loaded with alumina nanoparticles. The Boundary Element Method is used given its capabilities to deal with moving boundary problems. Results showed strong convective currents caused by the presence of the nanoparticles, which in time increase total heat flow in the cavity.
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.
Natural Convection Heat Transfer Experiments on an Inclined Helical Coil
International Nuclear Information System (INIS)
Research interests for the compact heat exchanger increase with growing needs on compact nuclear systems. Accordingly, the heat exchangers of helical coil types are adopted replacing the once-through type and the U-tube type ones. When the helical coil type heat exchangers are used for nuclear propulsions, the heat transfer of inclined helical coil becomes an important problem due to the shaking of the ship. This study measured the natural convection heat transfer from the outside surface of the helical coil in a circular duct varying the coil inclination and turn number. It is also conducted using a circular duct having same height of the helical coil. Based upon the analogy concept, a mass transfer system was used instead of a heat transfer system. A cupric acid-copper sulfate electroplating system was employed as the mass transfer systems
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Sumon Saha
2006-10-01
Full Text Available Combined free convection and forced convection from a flush-mounted uniform heat source on the bottom of a horizontal rectangular enclosure with side openings is studied numerically. The inlet opening allows an externally induced air stream at the ambient temperature to flow through the cavity and exits from another two openings placed top of the both side walls. Two-dimensional forms of Navier-Stokes equations are solved by using control volume based finite element technique. Three typical values of the Reynolds numbers, based on the enclosure height, are chosen as Re = 50, 100 and 200, and steady, laminar results are obtained in the range of Richardson number as 0 = Ri = 10 and a fixed Prandtl number of 0.71. The parametric studies for a wide range of governing parameters show consistent performance of the present numerical approach to obtain as stream functions and temperature profiles. Heat transfer rates at the heated walls are presented in terms of average Nusselt numbers. The computational results indicate that the heat transfer coefficient is strongly affected by Reynolds number and Richardson number. An empirical correlation is developed by using Nusselt number, Reynolds number and Richardson number.
Dendrite growth under forced convection: analysis methods and experimental tests
International Nuclear Information System (INIS)
An analysis is given of the nonisothermal growth of a dendrite crystal under forced fluid flow in a binary system. The theoretical model utilized employs a free moving crystal–liquid interface and makes use of the Oseen approximation for the equations of motion of the liquid. A criterion for the stable growth of two-dimensional and three-dimensional parabolic dendrites is derived under the assumption of an anisotropic surface tension at the crystal–liquid interface, which generalizes the previous known results for the stable growth of a dendrite with convection in a one-component fluid and for the growth of a dendrite in a two-component system at rest. The criterion obtained within the Oseen hydrodynamic approximation is extended to arbitrary Peclet numbers and dendrite growth with convection in a nonisothermal multicomponent system. Model predictions are compared with experimental data on crystal growth kinetics in droplets processed in electromagnetic and electrostatic levitation facilities. Theoretical and simulation methods currently being developed are applied to crystallization processes under earthly and reduced gravity conditions. (reviews of topical problems)
Modelling Marangoni convection in laser heat treatment
Drezet, Jean-Marie; Pellerin, S.; Bezençon, Cyrille; Mokadem, Sélim
2004-01-01
Epitaxial Laser Metal Forming (E-LMF) consists in impinging a jet of metallic powder onto a molten pool formed by controlled laser heating and thereby, generating epitaxially a single crystal deposit onto the damaged component. This new technique aims to be used for the repair and reshape single crystal gas turbine components. Because of the very localised melting pool, the high temperature gradients produced during the process must be carefully controlled in order to avoid both the columnar-...
Evaporation of a binary liquid film by forced convection
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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.
Extinction transition in bacterial colonies under forced convection
Neicu, T; Larochelle, D A; Kudrolli, A
2000-01-01
We report the spatio-temporal response of {\\it Bacillus subtilis} growing on a nutrient-rich layer of agar to ultra-violet (UV) radiation. Below a crossover temperature, the bacteria are confined to regions that are shielded from UV radiation. A forced convection of the population is effected by rotating a UV radiation shield relative to the petri dish. The extinction speed at which the bacterial colony lags behind the shield is found to be qualitatively similar to the front velocity of the colony growing in the absence of the hostile environment as predicted by the model of Dahmen, Nelson and Shnerb. A quantitative comparison is not possible without considering the slow dynamics and the time-dependent interaction of the population with the hostile environment.
Origin of Knudsen forces on heated microbeams
Zhu, Taishan
2010-09-09
The presented work probes the fundamentals of Knudsen forces. Using the direct simulation Monte Carlo (DSMC) method, the flows induced by temperature inhomogeneity within a representative configuration and the Knudsen force acting on a heated microbeam are captured as functions of Knudsen number in the entire flow regime. Both flow strength and Knudsen force peak in the transition regime and negative Knudsen force absent in experimental data is observed. The mechanisms of the thermally induced flows and Knudsen forces are studied. It has been found that thermal edge flow is the main driven source for the formation of the Knudsen force on microbeams and domain configuration plays an important role in the process.
Free and forced convective-diffusion solutions by finite element methods
International Nuclear Information System (INIS)
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
A correlation for natural convection heat transfer from inclined plate-finned heat sinks
International Nuclear Information System (INIS)
Steady-state natural convection heat transfer from inclined plate-finned heat sinks to air is numerically investigated by using an experimentally validated model. The heat sinks with parallel arrangement of uniform rectangular cross section plate fins are inclined from the vertical in both forward and backward directions in order to investigate the effect of inclination on convection. Our previously validated numerical model for vertically oriented heat sinks is directly used without changing any model parameters, but only by varying the direction of the gravitational acceleration to create the effect of inclination. The flow and temperature fields are resolved using a finite volume computational fluid dynamics code. Performing a large number of simulations for the heat sink base inclination angles of ±4°, ±10°, ±20°, ±30°, ±45°, ±60°, ?65°, ?70°, ±75°, ±80°, ±85°, ±90° from the vertical, the dependence of the convective heat-transfer rate to the inclination angle and Rayleigh number is investigated. Scale analyses are performed in order to generalize estimates for the convection heat-transfer rates. A single correlation is suggested and shown to be valid for a very wide range of angles from ?60° (upward) to +80° (downward) in a wide range of Rayleigh numbers from 0 to 2 × 108. -- Highlights: ? Natural convection heat transfer from inclined plate-finned heat sinks is investigated. ? A correlation for estimating convection heat-transfer rates is suggested. ? The correlation is shown to be valid in a very wide range of angles, ?60° ? ? ? +80°. ? The correlation is verified with all available experimental data in literature. ? Flow separation and fin height play the most significant roles at high inclinations
International Nuclear Information System (INIS)
The electro-thermo-convective motion in a plane horizontal dielectric liquid layer subjected to simultaneous action of electric field and thermal gradient is numerically investigated. We consider the case of a strong unipolar charge injection C = 10 from above or below. Therefore in this context, we only take into account the Coulomb force, disregarding the dielectric one. The effect of the electric field on the heat transfer is analyzed through the characterization of the time history of the Nusselt number as well as its evolution according to the characteristic dimensionless electric parameter T. It is demonstrated that the electric effects dominate the buoyancy ones resulting in an electrically induced convection which significantly enhance the heat transfer.
Natural convection in wavy enclosures with volumetric heat sources
Energy Technology Data Exchange (ETDEWEB)
Oztop, H.F.; Varol, Y. [Department of Mechanical Engineering, Technology Faculty, Firat University, TR-23119 Elazig (Turkey); Abu-Nada, E. [Institut fur Technische Verbrennung, Leibniz Universitat, Hannover, Welfengarten 1a, 30167 Hannover (Germany); Department of Mechanical Engineering, King Faisal University, Al-Ahsa 31982 (Saudi Arabia); Chamkha, A. [Manufacturing Engineering Department, The Public Authority for Applied, Education and Training, Shuweikh 70654 (Kuwait)
2011-04-15
In this paper, the effects of volumetric heat sources on natural convection heat transfer and flow structures in a wavy-walled enclosure are studied numerically. The governing differential equations are solved by an accurate finite-volume method. The vertical walls of enclosure are assumed to be heated differentially whereas the two wavy walls (top and bottom) are kept adiabatic. The effective governing parameters for this problem are the internal and external Rayleigh numbers and the amplitude of wavy walls. It is found that both the function of wavy wall and the ratio of internal Rayleigh number (Ra{sub I}) to external Rayleigh number (Ra{sub E}) affect the heat transfer and fluid flow significantly. The heat transfer is predicted to be a decreasing function of waviness of the top and bottom walls in case of (IRa/ERa)>1 and (IRa/ERa)<1. (authors)
MHD mixed convective heat transfer flow about an inclined plate
Ayd?n, Orhan; Kaya, Ahmet
2009-11-01
Mixed convection heat transfer about a semi-infinite inclined plate in the presence of magneto and thermal radiation effects is studied. The fluid is assumed to be incompressible and dense. The nonlinear coupled parabolic partial differential equations governing the flow are transformed into the non-similar boundary layer equations, which are then solved numerically using the Keller box method. The effects of the mixed convection parameter R i, the angle of inclination ?, the magnetic parameter M and the radiation-conduction parameter R d on the velocity and temperature profiles as well as on the local skin friction and local heat transfer parameters. For some specific values of the governing parameters, the results are compared with those available in the literature and a fairly good agreement is obtained.
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Schumacher, Courtney
2012-12-13
Heating associated with tropical cloud systems drive the global circulation. The overall research objectives of this project were to i) further quantify and understand the importance of heating in tropical convective cloud systems with innovative observational techniques, and ii) use global models to determine the large-scale circulation response to variability in tropical heating profiles, including anvil and cirrus cloud radiative forcing. The innovative observational techniques used a diversity of radar systems to create a climatology of vertical velocities associated with the full tropical convective cloud spectrum along with a dissection of the of the total heating profile of tropical cloud systems into separate components (i.e., the latent, radiative, and eddy sensible heating). These properties were used to validate storm-scale and global climate models (GCMs) and were further used to force two different types of GCMs (one with and one without interactive physics). While radiative heating was shown to account for about 20% of the total heating and did not have a strong direct response on the global circulation, the indirect response was important via its impact on convection, esp. in how radiative heating impacts the tilt of heating associated with the Madden-Julian Oscillation (MJO), a phenomenon that accounts for most tropical intraseasonal variability. This work shows strong promise in determining the sensitivity of climate models and climate processes to heating variations associated with cloud systems.
Silk cocoon drying in forced convection type solar dryer
International Nuclear Information System (INIS)
The thin layer silk cocoon drying was studied in a forced convection type solar dryer. The drying chamber was provided with several trays on which the cocoons loaded in thin layer. The hot air generated in the solar air heater was forced into drying chamber to avoid the direct exposure of sunlight and UV radiation on cocoons. The drying air temperature varied from 50 to 75 oC. The cocoon was dried from the initial moisture content of about 60-12% (wb). The drying data was fitted to thin layer drying models. Drying behaviour of the silk cocoon was best fitted with the Wang and Singh drying model. Good agreement was obtained between predicted and experimental values. Quality of the cocoons dried in the solar dryer was at par with the cocoons dried in the conventional electrical oven dryer in term of the silk yield and strength of the silk. Saving of electrical energy was about 0.75 kWh/kg cocoons dried. Economic analysis indicated that the NPV of the solar dryer was higher and more stable (against escalation rate of electricity) as compare to the same for electrical oven dryer. Due to simplicity in design and construction and significant saving of operational electrical energy, solar cocoon dryer seems to be a viable option.
Turbulent convective heat transfer around a horizontal rectangular cylinder
International Nuclear Information System (INIS)
A procedure for numerical analysis of turbulent convective heat transfer around a horizontal rectangular cylinder is presented. The ?-? turbulent model is used and the staggered grid system is introduced for the velocity components. Finite difference equations are derived from elliptic partial differential equations using hybrid scheme. TDMA and line-by-line SIMPLE algorithms are used to solve finite difference equations. The variation of mean velocity, turbulent kinetic energy, dissipation rate of turbulent kinetic energy and temperature behind the cylinder are presented. The heat transfer coefficient from a rectangular cylinder in cross flow is represented by the relation Nu=0.0282 Re0.766 Pr1/3 for 1035. (Author)
Convective Heat Transfer Analysis in Fluid Flow with Turbulence Promoters with Heat Pipes
Directory of Open Access Journals (Sweden)
Theodor Mateescu
2007-01-01
Full Text Available The present paper proposes the analysis and the simulation of the convection heat transfer into the fluid flow with turbulence promoters utilizing heat pipes. The study is based on the necesity of the unconventional energy forms capitalization, increasing of the energy efficiency and leads to the energy consumtion decrease in concordance with the sustainable development concept.
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.
Neshat, E.; Hossainpour, S.; Bahiraee, F.
2014-06-01
Both of experimental and numerical investigations were performed to understand unsteady natural convection from outer surface of helical coils. Four helical coils with two different curvature ratios were used. Each coil was mounted in the shell both vertically and horizontally. The cold water was entered the coil and the hot water in the shell was cooling by unsteady natural convection. A CFD code was developed to simulate natural convection heat transfer. Equations of tube and shell are solved simultaneously. Statistical analyses have been done on data points of temperature and natural convection Nusselt number. It was revealed that shell-side fluid temperature and the Nusselt number of the outer surface of coils are functions of in-tube fluid mass flow rate, specific heat of fluids and geometrical parameters including length, inner diameter of the tube and the volume of the shell, and time.
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.
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. Th...
Optimization of a Radial Flow Heat Sink Under Natural Convection
Bhowmik, Himangshu
2014-01-01
A steady-state three-dimensional numerical model is developed to predict natural convection heat transfer from a radial flow heat sink. The considered medium is air. The effect of several design parameters, such as the fin length and height, number of fins, and the heat sink base radius, on heat transfer is investigated. The Taguchi method, known to be a very useful tool for selecting the best levels of control factors, is employed. Five factors and four levels for each factor are chosen. Sixteen kinds of models are analyzed, and the total heat transfer for each model is obtained. The results are used to estimate the optimum design values of the parameters affecting the heat sink performance. The reliability of these values is verified. The average heat transfer rate of the optimum model is shown to increase by 60% as compared to the reference model. Finally, the heat transfer data at different outer radii of the radial flow heat sink are correlated.
Validating The Goddard Convective-Stratiform Heating Algorithm for PMM
Lang, S. E.; Tao, W. K.; Takayabu, Y. N.; Shige, S.; Johnson, R. H.; Ciesielski, P. E.
2014-12-01
RMS errors from 2007 through 2008 are computed between the equivalent rainfall obtained from integrating the cloud heating retrieved by the latest Goddard Convective-Stratiform Heating (CSH) algorithm (i.e., the TRMM 3G31 daily gridded product) and the observed surface rainfall obtained from the TRMM 2B31 Combined Algorithm at various time and space scales as well as for varying rain intensities both over land and ocean areas. RMS errors are significantly reduced by spatial and temporal averaging, noticeably lower over ocean than over land, and decrease with increasing rain intensity. There is a mean global positive heating bias of ~24%, which is much higher over land (~41%) than over ocean (~16%); these biases are reduced to (~5%) globally, ~23% over land and a negative bias of just ~2% over ocean when the residual background heating in far from rain regions is not included.
Directory of Open Access Journals (Sweden)
M. A. Mansour
2013-09-01
Full Text Available The effects of magnetic force, acting vertically downward on natural convection within a nanofluid filled tilted trapezoidal enclosure saturated with an electrically conducting fluid have been investigated numerically. The bottom wall of the enclosure is subjected to a constant cold temperature and the top wall experiences a heat source whereas the remaining sidewalls are kept adiabatic. The physical problems are represented mathematically by different sets of governing equations along with the corresponding boundary conditions. By using approximations of finite difference method, the non-dimensional governing equations are discritized. For natural convection the influential parameters are Rayleigh number Ra, the rotational angle of the enclosure? and the Hartmann number Ha, through which different thermo-fluid characteristics inside the enclosure are obtained. In the present study, the obtained results are presented in terms of streamlines, isotherms and average Nusselt number along the heat source. The result shows that with increasing Ha, the diffusive heat transfer become prominent even though Rayleigh number increases. Optimum heat transfer rate is obtained at higher values of Ra in the absence of magnetic force.
Scientific Electronic Library Online (English)
J. N. N., QUARESMA; E. N., MACÊDO.
1998-03-01
Full Text Available The thermal entry region in laminar forced convection of Herschel-Bulkley fluids is solved analytically through the integral transform technique, for both circular and parallel-plates ducts, which are maintained at a prescribed wall temperature or at a prescribed wall heat flux. The local Nusselt nu [...] mbers are obtained with high accuracy in both developing and fully-developed thermal regions, and critical comparisons with previously reported numerical results are performed.
Moh'd A. Al-Nimr; Renhe A. Damseh; Mohammed Q. Al-Odat
2004-01-01
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 en...
Directory of Open Access Journals (Sweden)
S. Venkatachalapathy
2010-01-01
Full Text Available Natural convection cooling using air as a fluid is commonly used in the cooling of electronic equipment and many other devices. In this work, a three-dimensional numerical study of natural convection heat transfer from multiple protruding heat sources simulating electronic components is conducted. Computational fluid dynamics (CFD software, FLUENT is used in this analysis. A 4 by 5 array of heat sources are embedded in the bottom wall of an adiabatic square enclosure. The heat sources with a constant heat flux source at the bottom are of square cross-section and arranged in an in-line manner. Each heat source is attached with one thermocouple, which is connected to a data acquisition system and a computer. The steady state temperatures of heat sources, air inlet, outlet and enclosure walls are measured. The analysis is carried out by varying the heat fluxes and outlet areas. The heat transfer coefficient, Nusselt number and Grashof number are obtained. Results indicate that the heat sources inside the array are hotter and the heat transfer coefficient increases almost linearly with heat source surface temperatures. Grashof number and outlet opening areas strongly influence the Nusselt number. The heat transfer coefficient for the inner heat sources in a row is lower than those near the enclosure walls. The results of numerical analysis are compared with the experiments and there is a good agreement between the two.
DORE, VALENTINA
2010-01-01
The motion of buoyancy driven plumes is, on all scales, the most common heat and momentum transfer mechanism in geophysical flows, well known as Free Convection. Similarly, density stratification due to heating inequalities is also an ordinary scenario in nature. Free Convection phenomenon coupled with a density stratified fluid setting leads to the so-called Penetrative Free Convection (PFC). When a fluid, in static equilibrium, is stably stratified a thermal forcing can produce an unstable ...
Natural convection heat transfer below downward facing horizontal surfaces
International Nuclear Information System (INIS)
The laminar steady-state natural convection below an infinite strip and below a circular plate, heated at their bottom sides, has been calculated analytically for a uniform surface temperature as well as for a uniform surface heat flux. This convection is driven by the non-uniform temperature distribution near the edges of the plate. This particular feature makes the problem a basically elliptic one, a fact that was not taken into account in earlier studies. In contrast to the flow near inclined heated plates the horizontal situation cannot be described by a boundary layer theory alone. Similarity solutions have been obtained employing the method of matched asymptotic expansions. A potential flow or a Stokes' flow with an unknown upstream velocity is taken as the outer expansion and a boundary layer flow as the inner expansion. Both expansions are matched by an energy balance in the region near the stagnation point at the plate center. In this way the unknown upstream velocity is fixed. This iterative procedure is first restricted to the limiting cases of low and high Prandtl numbers. Then an interpolation of these asymptotes yields heat transfer correlations for arbitrary Prandtl numbers. In comparison to earlier approximations these results are independent of any empirical boundary layer profiles and are based on experimental data to a very limited extend. (orig.)
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.
Natural convection of ferrofluids in partially heated square enclosures
International Nuclear Information System (INIS)
In this study, natural convection of ferrofluid in a partially heated square cavity is numerically investigated. The heater is located to the left vertical wall and the right vertical wall is kept at constant temperature lower than that of the heater. Other walls of the square enclosure are assumed to be adiabatic. Finite element method is utilized to solve the governing equations. The influence of the Rayleigh number (104?Ra?5×105), heater location (0.25H?yh?0.75H), strength of the magnetic dipole (0???2), horizontal and vertical location of the magnetic dipole (?2H?a??0.5H, 0.2H?b?0.8H) on the fluid flow and heat transfer characteristics are investigated. It is observed that different velocity components within the square cavity are sensitive to the magnetic dipole source strength and its position. The length and size of the recirculation zones adjacent to the heater can be controlled with magnetic dipole strength. Averaged heat transfer increases with decreasing values of horizontal position of the magnetic dipole source. Averaged heat transfer value increases from middle towards both ends of the vertical wall when the vertical location of the dipole source is varied. When the heater location is changed, a symmetrical behavior in the averaged heat transfer plot is observed and the minimum value of the averaged heat transfer is attained when the heater is located at the mid of vertical wall. - Highlights: • Free convection of ferrofluids in a partially heated cavity is numerically studied. • Velocities are sensitive to the magnetic dipole source strength and its position. • Averaged Nu increases with decreasing x-location values of the magnetic dipole. • Averaged Nu increases from middle towards ends when y-location of dipole changes
Wang, Liang-Bi; Zhang, Qiang; Li, Xiao-Xia
2009-01-01
This paper aims to contribute to a better understanding of convective heat transfer. For this purpose, the reason why thermal diffusivity should be placed before the Laplacian operator of the heat flux, and the role of the velocity gradient in convective heat transfer are analysed. The background to these analyses is that, when the energy…
Convective heat transfer for viscoelastic fluid in a curved pipe
Energy Technology Data Exchange (ETDEWEB)
Norouzi, M.; Kayhani, M.H. [Shahrood University of Technology, Mechanical Engineering Department, Shahrood (Iran); Nobari, M.R.H. [Amirkabir University of Technology, Mechanical Engineering Department, Tehran (Iran); Joneidi, A.A. [Eindhoven University of Technology, Mechanical-Polymer Technology Group, Eindhoven (Netherlands)
2010-10-15
In this paper, fully developed convective heat transfer of viscoelastic flow in a curved pipe under the constant heat flux at the wall is investigated analytically using a perturbation method. Here, the curvature ratio is used as the perturbation parameter and the Oldroyd-B model is applied as the constitutive equation. In the previous studies, the Dirichlet boundary condition for the temperature at the wall has been used to simplify the solution, but here exactly the non-homogenous Neumann boundary condition is considered to solve the problem. Based on this solution, the non-axisymmetric temperature distribution of Dean flow is obtained analytically and the effect of flow parameters on the flow field is investigated in detail. The current analytical results indicate that increasing the Weissenberg number, viscosity ratio, curvature ratio, and Prandtl number lead to the increase of the heat transfer in the Oldroyd-B fluid flow. (orig.)
Convective Heat Transfer in Impinging- Gas- Jet Arrangements
Directory of Open Access Journals (Sweden)
J.M. Buchlin
2011-01-01
Full Text Available The paper deals with heat transfer by convection between impinging gas jets and solid surfaces. It considers both single and multiple jet systems. It emphasizes the flow and geometrical parameters as well as the environment conditions at which the jet emerges. In particular, it points out the effect of the jet tilting, thermal entrainment and jet confinement. ASN and ARN schemes are illustrated through industrial and aeronautical applications. Design correlations are proposed. Experimental data obtained from infrared thermography are compared to CFD simulations.
Convective Heat Transfer of Magnetic Nanofluids in a Microtube
Directory of Open Access Journals (Sweden)
Kuo Jung Lo
2015-05-01
Full Text Available This paper conducts an analysis of convective heat transfer of magnetic nanofluids in an isothermally heated microtube. The main purpose is to investigate the influences of particle volume fraction and external magnetic field strength on the fluid velocity, temperature, pressure, pressure drop, flow drag, and heat transfer rate. Firstly, a flow and heat transfer model is built. A water-based magnetite (Fe3O4 nanofluid is then pre-pared, and a thermal flow test system is further de-signed, so as to verify the theoretical model with experimental data. Finally, the thermal flow fields and the corresponding characteristics are numerically analyzed by using the marching implicit (MI procedure. The results reveal that when the particle volume fraction is increased, the average flow drag also increases while the average heat transfer rate goes down. Furthermore, as the external magnetic field strength is increased, the average flow drag rises and the average heat transfer rate also rises in the general case.
Numerical simulation of combined natural and forced convection during thermal-hydraulic transients
International Nuclear Information System (INIS)
The single-phase COMMIX (COMponent MIXing) computer code performs fully three-dimensional, transient, thermal-hydraulic analyses of liquid-sodium LMFBR components. It solves the conservation equations of mass, momentum, and energy as a boundary-value problem in space and as an initial-value problem in time. The concepts of volume porosity, surface permeability and distributed resistance, and heat source have been employed in quasi-continuum (rod-bundle) applications. Results from three transient simulations involving forced and natural convection are presented: (1) a sodium-filled horizontal pipe initially of uniform temperature undergoing an inlet velocity rundown transient, as well as an inlet temperature transient; (2) a 19-pin LMFBR rod bundle undergoing a velocity transient; and, (3) a simulation of a water test of a 1/10-scale outlet plenum undergoing both velocity and temperature transients
Development and performance evaluation of forced convection potato solar dryer
International Nuclear Information System (INIS)
This research paper deals with the design development and testing of a forced convection solar dryer, for drying and converting to flour of high moisture content vegetables like potatoes. The angle of solar collector was made adjustable for the absorption of maximum solar radiation by the absorber plate. The air flow rate was controlled by adjustable gate valve to find the optimum flow rate for dehydration of the product. The penetration of solar radiation raised the temperature of the absorber plate of the dryer to 110 deg. C during the operation under stagnation or no load conditions. The maximum air temperature attained in the solar air heater, under this condition was 80 deg. C. The dryer was loaded with 12 Kg of blanched potato chips having an initial moisture content of 89.75%, and the final desired moisture content of 6.95% was achieved within five hours without losing the color of potato chips, while the moisture contents reduction was from 89.75% to 33.75% for five hours in open sun drying under shade. The drying cost for 1 Kg of potatoes was calculated as Rs. 245 and it was Rs. 329 in the case of an electric dryer. The life span of the solar dryer was assumed to be 20 years. The cumulative present worth of annual savings over the life of the solar dryer was calculated for blanched potato chips drying, and it turned out be Rs.163177.67/- which was much higher than the capital cost of the dryer (Rs. 25000). The payback period was calculated as 0.89 years, which was also very small considering the life of the system (20 years). (author)
Yano, Ryosuke
2014-01-01
We discuss the thermal conduction and convection of thermally relativistic fluids between two parallel walls under the gravitational force, both theoretically and numerically. In the theoretical discussion, we assume that the Lorentz contraction is ignored and spacetime is flat. For understanding of the thermal conduction and convection of thermally relativistic fluids between two parallel walls under the gravitational force, we solve the relativistic Boltzmann equation usin...
Directory of Open Access Journals (Sweden)
Zeinali Heris Saeed
2011-01-01
Full Text Available Abstract In this article, laminar flow-forced convective heat transfer of Al2O3/water nanofluid in a triangular duct under constant wall temperature condition is investigated numerically. In this investigation, the effects of parameters, such as nanoparticles diameter, concentration, and Reynolds number on the enhancement of nanofluids heat transfer is studied. Besides, the comparison between nanofluid and pure fluid heat transfer is achieved in this article. Sometimes, because of pressure drop limitations, the need for non-circular ducts arises in many heat transfer applications. The low heat transfer rate of non-circular ducts is one the limitations of these systems, and utilization of nanofluid instead of pure fluid because of its potential to increase heat transfer of system can compensate this problem. In this article, for considering the presence of nanoparticl: es, the dispersion model is used. Numerical results represent an enhancement of heat transfer of fluid associated with changing to the suspension of nanometer-sized particles in the triangular duct. The results of the present model indicate that the nanofluid Nusselt number increases with increasing concentration of nanoparticles and decreasing diameter. Also, the enhancement of the fluid heat transfer becomes better at high Re in laminar flow with the addition of nanoparticles.
Grooms, Ian
2014-01-01
The non-hydrostatic, quasigeostrophic approximation for rapidly rotating Rayleigh-B\\'enard convection admits a class of exact `single mode' solutions. These solutions correspond to steady laminar convection with a separable structure consisting of a horizontal planform characterized by a single wavenumber multiplied by a vertical amplitude profile, with the latter given as the solution of a nonlinear boundary value problem. The heat transport associated with these solutions is studied in the regime of strong thermal forcing (large reduced Rayleigh number $\\widetilde{Ra}$). It is shown that the Nusselt number $Nu$, a nondimensional measure of the efficiency of heat transport by convection, for this class of solutions is bounded below by $Nu\\gtrsim \\widetilde{Ra}^{3/2}$, independent of the Prandtl number, in the limit of large reduced Rayleigh number. Matching upper bounds include only logarithmic corrections, showing the accuracy of the estimate. Numerical solutions of the nonlinear boundary value problem for ...
Experimental study of natural convection heat transfer in a volumetrically heated semicircular pool
International Nuclear Information System (INIS)
Highlights: • Natural convection tests were performed to study high modified Rayleigh number. • The relations between the Nu and Ra? were determined for boundary conditions. • The upward and downward natural convection heat transfer rates were affected by Pr. • From the test, the Rayleigh numbers was obtained from 5.7 × 106 up to 7.0 × 1011. • The resultant correlations can be applied to determine the reactor vessel failure. - Abstract: SIGMA CP (Simulant Internal Gravitated Material Apparatus Circular Pool) tests were performed to study natural convection characterized by a high modified Rayleigh number (Ra?) in a semicircular pool. The test apparatus had a 500 mm diameter, 250 mm height and 100 mm width. Two thin cartridge heaters, which had a sheath diameter of 6 mm and a length of 2000 mm, were used to simulate internal heating in the pool. They were uniformly distributed in the semicircular pool to supply a maximum heating power of 3 kW to the pool. The Ra? was varied from 106 to 1011 based on the power input. The working fluid Prandtl number (Pr) ranged between 4 and 8 for water and was 0.7 for air. Particular attention was paid to the influence of Pr on the natural convection heat transfer in the pool. A number of relations between the Nusselt number (Nu) and Ra? were determined for the boundary conditions in the semicircular pool. The boundary condition is that the upper and lower walls are cooled isothermally. The experimental results indicated that both the upward and downward natural convection heat transfer rates were affected by Pr. The Rayleigh numbers were determined up to 7 × 1011 from this test. The resultant engineering correlations can be applied to simulations of a spectrum of loss-of-coolant accidents to determine the reactor vessel failure time
International Nuclear Information System (INIS)
The paper deals with the investigation of thermo-magnetic convection in a paramagnetic liquid subjected to a non-uniform magnetic field in weightlessness conditions. Indeed, in zero-g space conditions such as realized in International Space Station (ISS), or in artificial satellite, or in free-flight space vessels, the classical thermo-gravitational convection in fluid disappears. In any cases, it may be useful to restore the convective thermal exchange inside fluids such as liquid oxygen. In this paper, the restoration of heat exchange by the way of creation of magnetic convection is numerically studied.
Scientific Electronic Library Online (English)
Cláudia R., Andrade; Edson L., Zaparoli.
Full Text Available This work studies the forced convection problem in internal flow between concentric annular ducts, with radial fins at the internal tube surface. The finned surface heat transfer is analyzed by two different approaches. In the first one, it is assumed one-dimensional heat conduction along the intern [...] al tube wall and fins, with the convection heat transfer coefficient being a known parameter, determined by an uncoupled solution. In the other way, named conjugated approach, the mathematical model (continuity, momentum, energy and K-epsilon equations) applied to tube annuli problem was numerically solved using finite element technique in a coupled formulation. At first time, a comparison was made between results obtained for the conjugated problem and experimental data, showing good agreement. Then, the temperature profiles under these two approaches were compared to each other to analyze the validity of the one-dimensional classical formulation that has been utilized in the heat exchanger design.
Bhaskar Kalita
2012-01-01
This paper is devoted for the study of effects influences by heat source on unsteady free convection flow and heat transfer characteristic of a viscous incompressible and electrically conducting fluid between two heated vertical plates in the presence of a uniform magnetic field applied transversely to the flow. The leading momentum and energy equations are solved by the Laplace transform technique and solutions are presented through graphs for velocity and temperature distribution.
Shang, De-Yi
2012-01-01
This book presents recent developments in our systematic studies of hydrodynamics and heat and mass transfer in laminar free convection, accelerating film boiling and condensation of Newtonian fluids, as well as accelerating film flow of non-Newtonian power-law fluids (FFNF). These new developments provided in this book are (i) novel system of analysis models based on the developed New Similarity Analysis Method; (ii) a system of advanced methods for treatment of gas temperature- dependent physical properties, and liquid temperature- dependent physical properties; (iii) the organically combined models of the governing mathematical models with those on treatment model of variable physical properties; (iv) rigorous approach of overcoming a challenge on accurate solution of three-point boundary value problem related to two-phase film boiling and condensation; and (v) A pseudo-similarity method of dealing with thermal boundary layer of FFNF for greatly simplifies the heat-transfer analysis and numerical calculati...
Experimental investigation of turbulent mixed convection in the wake of a heated sphere
International Nuclear Information System (INIS)
The axisymmetric wake of a heated sphere under conditions of turbulent mixed convection is investigated in the water test section FLUTMIK. The sphere is located in a vertical channel with forced convective upward flow. The influence of buoyancy forces to the flow field is studied by comparison with the unheated wake. The theoretical fundamentals describing turbulent flows and different versions of the k-? turbulence model extended by buoyancy terms are described in detail. The quantities to be determined experimentally are derived. The temperature and the components of the velocity vector in axial and radial directions are measured simultaneously by means of a thermocouple probe and a two component, two color laser Doppler anemometer. The flow quantities are determined at axial distances between 5 and 106 sphere diameters. The functional principle and the basis of the laser Doppler anemometer are explained. The mean velocity, the mean temperature, the intensities of their fluctuations and the turbulent exchange quantities of momentum and heat transport are calculated. The decay laws of the quantities along the axis of the channel and the radial profiles are indicated and discussed. The applicability of the experimental results of the axisymmetric buoyancy influenced turbulent wake with respect to the turbulence models presented are shown. (orig.)
Convective Heating of the LIFE Engine Target During Injection
Energy Technology Data Exchange (ETDEWEB)
Holdener, D S; Tillack, M S; Wang, X R
2011-10-24
Target survival in the hostile, high temperature xenon environment of the proposed Laser Inertial Fusion Energy (LIFE) engine is critical. This work focuses on the flow properties and convective heat load imposed upon the surface of the indirect drive target while traveling through the xenon gas. While this rarefied flow is traditionally characterized as being within the continuum regime, it is approaching transition where conventional CFD codes reach their bounds of operation. Thus ANSYS, specifically the Navier-Stokes module CFX, will be used in parallel with direct simulation Monte Carlo code DS2V and analytically and empirically derived expressions for heat transfer to the hohlraum for validation. Comparison of the viscous and thermal boundary layers of ANSYS and DS2V were shown to be nearly identical, with the surface heat flux varying less than 8% on average. From the results herein, external baffles have been shown to reduce this heat transfer to the sensitive laser entrance hole (LEH) windows and optimize target survival independent of other reactor parameters.
International Nuclear Information System (INIS)
This thesis presents the results of an experimental investigation of natural convection heat transfer in a staggered array of heated cylinders, oriented horizontally within a rectangular enclosure. The main purpose of this research was to extend the knowledge of heat transfer within enclosed bundles of spent nuclear fuel rods sealed within a shipping or storage container. This research extends Canaan's investigation of an aligned array of heated cylinders that thermally simulated a boiling water reactor (BWR) spent fuel assembly sealed within a shipping or storage cask. The results are presented in terms of piecewise Nusselt-Rayleigh number correlations of the form Nu = C(Ra)n, where C and n are constants. Correlations are presented both for individual rods within the array and for the array as a whole. The correlations are based only on the convective component of the heat transfer. The radiative component was calculated with a finite-element code that used measured surface temperatures, rod array geometry, and measured surface emissivities as inputs. The correlation results are compared to Canaan's aligned array results and to other studies of natural convection in horizontal tube arrays
Energy Technology Data Exchange (ETDEWEB)
Triplett, C.E.
1996-12-01
This thesis presents the results of an experimental investigation of natural convection heat transfer in a staggered array of heated cylinders, oriented horizontally within a rectangular enclosure. The main purpose of this research was to extend the knowledge of heat transfer within enclosed bundles of spent nuclear fuel rods sealed within a shipping or storage container. This research extends Canaan`s investigation of an aligned array of heated cylinders that thermally simulated a boiling water reactor (BWR) spent fuel assembly sealed within a shipping or storage cask. The results are presented in terms of piecewise Nusselt-Rayleigh number correlations of the form Nu = C(Ra){sup n}, where C and n are constants. Correlations are presented both for individual rods within the array and for the array as a whole. The correlations are based only on the convective component of the heat transfer. The radiative component was calculated with a finite-element code that used measured surface temperatures, rod array geometry, and measured surface emissivities as inputs. The correlation results are compared to Canaan`s aligned array results and to other studies of natural convection in horizontal tube arrays.
International Nuclear Information System (INIS)
The helically coiled tube of heat exchanger is used for the evaporator of prototype fast breeder reactor 'Monju'. This paper aims at the grasp of two-phase flow phenomena of forced convective boiling of water inside helical coiled tube, especially focusing on oscillation phenomena of dryout point. A glass-made helically coiled tube was used to observe the inside water boiling behavior flowing upward, which was heated by high temperature oil outside the tube. This oil was also circulated through a glass made tank to provide the heat source for water evaporation. The criterion for oscillation of dryout point was found to be a function of inlet liquid velocity and hot oil temperature. The observation results suggest the mechanism of dryout point oscillation mainly consists of intensive nucleate boiling near the dryout point and evaporation of thin liquid film flowing along the helical tube. In addition, the oscillation characteristics were experimentally confirmed. As inlet liquid velocity increases, oscillation amplitude also increases but oscillation cycle does not change so much. As hot oil temperature increases, oscillation amplitude and cycle gradually decreases. (author)
Natural Convection Heat and Mass Transfer from Falling Films in Vertical Channels
Buck, Gregory Allen
1990-01-01
In the design of solar collector/regenerators for use in open cycle absorption refrigeration (OCAR) units, the problem of predicting evaporation rates and solution temperatures is of paramount importance in determining overall cycle performance. This transport of heat and mass is dominated by natural convection with buoyant forces primarily generated as a result of film heating by the solar flux, but aided by the evaporation of water (the lighter species) into the rising moist air stream. In order to better understand the mechanism of these combined buoyant interactions, the governing equations for natural convection flow in a vertical channel bounded by a heated falling film (simulating a glazed collector/regenerator) were solved using several different finite difference techniques. The numerical results were validated against existing experimental and numerical results for simplified boundary conditions. The appropriate nondimensionalization for the falling film boundary condition was established, ostensibly for the first time, and a parametric study for an air-water vapor mixture has been presented. Curve fits to the numerical results were determined for engineering design applications. To further confirm the validity of the numerical solutions, an experimental apparatus was constructed using electric resistance heat to simulate the constant heat flux of the solar source. Water was introduced at the top of this heated vertical surface at various flow rates and under various supplied heat fluxes, and a natural convection channel flow generated between the heated falling film and a parallel, plexiglass surface. Film temperatures and moist air velocity profiles were measured at various streamwise (vertical) locations for comparison with the numerical results. In general, measured film temperatures were 15 to 20 percent lower than the predicted values, but came to within 3 percent of the predictions when experimental uncertainty was incorporated into the numerical inputs. Photographic smoke-wire measurements of the induced moist air velocity were about 20 percent higher than the numerical predictions for small channel gap spacing, and about 50 percent higher for large gap spacing. These trends in the data indicate that a redistribution of the supplied heat flux from the film to the moist air is required to lower predicted film temperatures and raise predicted gas velocities. Physically plausible arguments to explain this redistribution and suggestions for improving the numerical predictions and the experimental measurements are offered.
Energy Technology Data Exchange (ETDEWEB)
Moon, Hee Gon; Wang, Sem Yung; Shim, Ho kyoung [Kwangju Institute of Science and Technology, Kwangju (Korea, Republic of)
2004-07-01
This paper presents the adjoint variable Design Sensitivity Analysis (DSA) for thermal systems considering both conduction and convection heat transfer. Both nodal temperature and total heat flow are considered to be objective functions and design sensitivity formulas are derived for each case. For the case of convection heat transfer, the adjoint analysis is carefully proceeded to obtain a precise result. A topology optimization example is examined for a simple planar square plate in order to design a heat exchanger as verification.
Convective heat transfer in vee-trough linear concentrators
Meyer, B. A.; Mitchell, J. W.; El-Wakil, M. M.
1982-01-01
Natural convection heat-transfer coefficients have been experimentally determined for trough-type collectors. The effects of Rayleigh number, tilt angle, and ideal concentration ratio on the Nusselt number have been experimentally determined over ranges representative of collector operation. The Rayleigh number range tested was up to 10,000,000, the tilt angle was varied from 30-90 deg and ideal concentration ratios of 2, 3, 4 and 5 were tested. The experimental results are supported by finite element solutions to the governing equations. A method of extending the results to truncated trough collectors and CPC collectors is suggested. The convective losses are compared to those for conventional flat plate collectors. The critical Rayleigh number increases with concentration ratio. Nusselt number increases more rapidly at high concentration ratio than at low values. Correlation equations are developed over the range of parameters tested. These results are useful to designers of collectors of this type, and allow estimation of the thermal losses during operation.
Negative Knudsen force on heated microbeams
Zhu, Taishan
2011-11-18
Knudsen force acting on a heated microbeam adjacent to a cold substrate in a rarefied gas is a mechanical force created by unbalanced thermal gradients. The measured force has its direction pointing towards the side with a lower thermal gradient and its magnitude vanishes in both continuum and free-molecule limits. In our previous study, negative Knudsen forces were discovered at the high Knudsen regime before diminishing in the free-molecule limit. Such a phenomenon was, however, neither observed in experiment [A. Passian et al., Phys. Rev. Lett. 90, 124503 (2003)], nor captured in the latest numerical study [J. Nabeth et al., Phys. Rev. E 83, 066306 (2011)]. In this paper, the existence of such a negative Knudsen force is further confirmed using both numerical simulation and theoretical analysis. The asymptotic order of the Knudsen force near the collisionless limit is analyzed and the analytical expression of its leading term is provided, from which approaches for the enhancement of negative Knudsen forces are proposed. The discovered phenomenon could find its applications in novel mechanisms for pressure sensing and actuation.
Neely, AJ; Ireland, PT; Harper, LR
1997-01-01
An experimental investigation of the performance of extended fin surfaces for the forced convective cooling of a range of engine component geometries in crossflow is reported. The experiments were undertaken to measure the surface heat transfer coefficient distributions of external finning around non-cylindrical geometries for use in aviation gas turbines in which the cooling performance/mass ratio must be maximized. The geometries examined were a box (square with rounded corners), a flute (r...
Energy Technology Data Exchange (ETDEWEB)
Surducan, E.; Surducan, V.; Neamtu, C., E-mail: camelia.neamtu@itim-cj.ro [National Institute for Research and Development of Isotopic and Molecular Technologies (INCDTIM), 67-103 Donat St., 400293, Cluj?Napoca (Romania); Limare, A.; Di Giuseppe, E. [Institut de Physique du Globe de Paris (IPGP), Univ. Paris Diderot, UMR CNRS 7154, 1 rue Jussieu, 75005, Paris (France)
2014-12-15
We report the design, construction, and performances of a microwave (MW) heating device for laboratory experiments with non-contact, homogeneous internal heating. The device generates MW radiation at 2.47 GHz from a commercial magnetron supplied by a pulsed current inverter using proprietary, feedback based command and control hardware and software. Specially designed MW launchers direct the MW radiation into the sample through a MW homogenizer, devised to even the MW power distribution into the sample's volume. An adjustable MW circuit adapts the MW generator to the load (i.e., the sample) placed in the experiment chamber. Dedicated heatsinks maintain the MW circuits at constant temperature throughout the experiment. Openings for laser scanning for image acquisition with a CCD camera and for the cooling circuits are protected by special MW filters. The performances of the device are analyzed in terms of heating uniformity, long term output power stability, and load matching. The device is used for small scale experiments simulating Earth's mantle convection. The 30 × 30 × 5 cm{sup 3} convection tank is filled with a water?based viscous fluid. A uniform and constant temperature is maintained at the upper boundary by an aluminum heat exchanger and adiabatic conditions apply at the tank base. We characterize the geometry of the convective regime as well as its bulk thermal evolution by measuring the velocity field by Particle Image Velocimetry and the temperature field by using Thermochromic Liquid Crystals.
International Nuclear Information System (INIS)
We report the design, construction, and performances of a microwave (MW) heating device for laboratory experiments with non-contact, homogeneous internal heating. The device generates MW radiation at 2.47 GHz from a commercial magnetron supplied by a pulsed current inverter using proprietary, feedback based command and control hardware and software. Specially designed MW launchers direct the MW radiation into the sample through a MW homogenizer, devised to even the MW power distribution into the sample's volume. An adjustable MW circuit adapts the MW generator to the load (i.e., the sample) placed in the experiment chamber. Dedicated heatsinks maintain the MW circuits at constant temperature throughout the experiment. Openings for laser scanning for image acquisition with a CCD camera and for the cooling circuits are protected by special MW filters. The performances of the device are analyzed in terms of heating uniformity, long term output power stability, and load matching. The device is used for small scale experiments simulating Earth's mantle convection. The 30 × 30 × 5 cm3 convection tank is filled with a water?based viscous fluid. A uniform and constant temperature is maintained at the upper boundary by an aluminum heat exchanger and adiabatic conditions apply at the tank base. We characterize the geometry of the convective regime as well as its bulk thermal evolution by measuring the velocity field by Particle Image Velocimetry and the temperature field by using Thermochromic Liquid Crystals
Surducan, E.; Surducan, V.; Limare, A.; Neamtu, C.; Di Giuseppe, E.
2014-12-01
We report the design, construction, and performances of a microwave (MW) heating device for laboratory experiments with non-contact, homogeneous internal heating. The device generates MW radiation at 2.47 GHz from a commercial magnetron supplied by a pulsed current inverter using proprietary, feedback based command and control hardware and software. Specially designed MW launchers direct the MW radiation into the sample through a MW homogenizer, devised to even the MW power distribution into the sample's volume. An adjustable MW circuit adapts the MW generator to the load (i.e., the sample) placed in the experiment chamber. Dedicated heatsinks maintain the MW circuits at constant temperature throughout the experiment. Openings for laser scanning for image acquisition with a CCD camera and for the cooling circuits are protected by special MW filters. The performances of the device are analyzed in terms of heating uniformity, long term output power stability, and load matching. The device is used for small scale experiments simulating Earth's mantle convection. The 30 × 30 × 5 cm3 convection tank is filled with a water-based viscous fluid. A uniform and constant temperature is maintained at the upper boundary by an aluminum heat exchanger and adiabatic conditions apply at the tank base. We characterize the geometry of the convective regime as well as its bulk thermal evolution by measuring the velocity field by Particle Image Velocimetry and the temperature field by using Thermochromic Liquid Crystals.
Fast Prediction Method for Steady-State Heat Convection
Wáng, Yì
2012-03-14
A reduced model by proper orthogonal decomposition (POD) and Galerkin projection methods for steady-state heat convection is established on a nonuniform grid. It was verified by thousands of examples that the results are in good agreement with the results obtained from the finite volume method. This model can also predict the cases where model parameters far exceed the sample scope. Moreover, the calculation time needed by the model is much shorter than that needed for the finite volume method. Thus, the nonuniform POD-Galerkin projection method exhibits high accuracy, good suitability, and fast computation. It has universal significance for accurate and fast prediction. Also, the methodology can be applied to more complex modeling in chemical engineering and technology, such as reaction and turbulence. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Mixed convection heat transfer in rotating vertical elliptic ducts
Scientific Electronic Library Online (English)
Olumuyiwa A., Lasode.
2007-06-01
Full Text Available This paper presents an investigation into the solution of laminar mixed convective heat transfer in vertical elliptic ducts containing an upward flowing fluid rotating about a parallel axis. The coupled system of normalized conservation equations are solved using a power series expansion in ascendin [...] g powers of rotational Rayleigh Number, Ratau - a measure of the rate of heating and rotation as the perturbation parameter. The results show the influence of rotational Rayleigh number, Ratau and modified Reynolds number, Re m on the temperature and axial velocity fields. The effect of Prandtl number, Pr, in the range 1 to 5, and eccentricity, e on the peripheral local Nusselt number are also reported. The mean Nusselt number is observed to be highest at duct eccentricity, e=0 for a given Prandtl number. However, results indicate insensitivity of peripheral local Nusselt number to Prandtl number at eccentricity, e=0.866, which is an important result to a designer of rotating vertical heat exchanger. The effect of eccentricity on the friction coefficient is also presented. The parameter space for the overall validity of the results presented is Ratau Re mPr
Heat Transfer Characteristics on Toroidal Convection Loop with Nanofluids
International Nuclear Information System (INIS)
Experimental studies on single-phase toroidal circulation loop(thermosyphon) have been performed in the present study with Ag-nanofluids as a working fluids. The present paper deals with an experimental study on the heat transfer behavior of single-phase toroidal loop. Toroidal loop charged with nanofluid has been constructed and a number of tests have been carried out. Different geometric parameter, e.g., orientation has been investigated. The tests were conducted employing two fluids: distilled water and Ag-nanofluid of various volume concentrations. The experiments at Rayleigh number from 105 to 106 showed a systematic and slight deterioration in natural convective heat transfer. It was observed that the deterioration due to the particle concentration was in the range of 5-10%. At a given particle concentration of 0.05%, abrupt decrease in the Nusselt number and the Raleigh number was observed. The present study with toroidal loop shows that the application of nanofluids for heat transfer intensification should not be decided only by the effective thermal conductivity with increasing particle concentration
Optimal Heat Transport in Rayleigh-B\\'enard Convection
Sondak, David; Waleffe, Fabian
2015-01-01
Steady flows that optimize heat transport are obtained for two-dimensional Rayleigh-B\\'enard convection with no-slip horizontal walls for a variety of Prandtl numbers $Pr$ and Rayleigh number up to $Ra\\sim 10^9$. Power law scalings of $Nu\\sim Ra^{\\gamma}$ are observed with $\\gamma\\approx 0.31$, where the Nusselt number $Nu$ is a non-dimensional measure of the vertical heat transport. Any dependence of the scaling exponent on $Pr$ is found to be extremely weak. On the other hand, the presence of two local maxima of $Nu$ with different horizontal wavenumbers at the same $Ra$ leads to the emergence of two different flow structures as candidates for optimizing the heat transport. For $Pr \\lesssim 7$, optimal transport is achieved at the smaller maximal wavenumber. In these fluids, the optimal structure is a plume of warm rising fluid which spawns left/right horizontal arms near the top of the channel, leading to downdrafts adjacent to the central updraft. For $Pr > 7$ at high-enough Ra, the optimal structure is a...
Describing the Heat Transport of Turbulent Rayleigh--B\\'enard Convection by POD methods
Lülff, Johannes
2015-01-01
Rayleigh--B\\'enard convection, which is the buoyancy-induced motion of a fluid enclosed between two horizontal plates, is an idealised setup to study thermal convection. We analyse the modes that transport the most heat between the plates by computing the proper orthogonal decomposition (POD) of numerical data. Instead of the usual POD ansatz of finding modes that describe the energy best, we propose a method that is optimal in describing the heat transport. Thereby, we can determine the modes with the major influence on the heat transport and the coherent structures in the convection cell. We also show that in lower-dimensional projections of numerical convection data, the newly developed modes perform consistently better than the standard modes. We then use this method to analyse the main modes of three-dimensional convection in a cylindrical vessel as well as two-dimensional convection with varying Rayleigh number and varying aspect ratio.
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.
Modeling a forced to natural convection boiling test with the program LOOP-W
International Nuclear Information System (INIS)
Extensive testing has been conducted in the Simulant Boiling Flow Visualization (SBFV) loop in which water is boiled in a vertical transparent tube by circulating hot glycerine in an annulus surrounding the tube. Tests ranged from nonboiling forced convection to oscillatory boiling natural convection. The program LOOP-W has been developed to analyze these tests. This program is a multi-leg, one-dimensional, two-phase equilibrium model with slip between the phases. In this study, a specific test, performed at low power where non-boiling forced convection was changed to boiling natural convection and then to non-boiling again, has been modeled with the program LOOP-W
Heat Transfer Convection in The Cooking of Apple Using a Solar Cooker Box-Type
Terres, H.; Chávez, S.; Lizardi, A.; López, R.; Vaca, M.; Flores, J.; Salazar, A.
2015-01-01
In this work, experimental results to determine the convection heat transfer coefficient in the cooking process of apple using a solar cooker box-type are presented. Experimental data of temperatures for water, surface and central point of the apple were used. To determine the convection coefficient, the apple was modelled as a sphere. The temperatures evolution was defined using thermocouples located at water, surface and central point in the vegetables. Using heat transfer convection equations in transitory state and the temperatures measured, the Biot number and the convection coefficient were determined.
International Nuclear Information System (INIS)
Graphical abstract: Photograph of the experimental set-up. - Highlights: • Thermal performance of an indirect-mode solar dryer is investigated. • Mathematical models are obtained for thin layer drying of thymus and mint. • Both thymus and mint show the constant and falling rate drying periods. - Abstract: An indirect-mode forced convection solar dryer was designed and fabricated. The thermal performance of the solar dryer under Tanta (latitude, 30° 47? N and longitude, 31° E) prevailing weather conditions was experimentally investigated. The system consists of a double pass v-corrugated plate solar air heater connected to a drying chamber. A blower was used to force the heated air to the drying chamber. Drying experiments were performed for thymus (initial moisture content 95% on wet basis) and mint (initial moisture content 85% on wet basis) at an initial temperature of 29 °C. The final moisture contents for thymus and mint were reached after 34 and 5 h, respectively. Fourteen mathematical models of thin layer drying were tested to specify the suitable model for describing the drying behavior of the studied products. It was found that, Midilli and Kucuk model is convenient to describe the thin layer solar drying of mint. However, the Page and modified Page models were found to be the best among others for describing the drying curves of thymus
Energy Technology Data Exchange (ETDEWEB)
Ma, R.Y. [California State Polytechnic Univ., Pomoma, CA (United States). Dept. of Mechanical Engineering
1993-09-01
Tests were performed to determine the convective heat loss characteristics of a cavity receiver for a parabolid dish concentrating solar collector for various tilt angles and wind speeds of 0-24 mph. Natural (no wind) convective heat loss from the receiver is the highest for a horizontal receiver orientation and negligible with the reveler facing straight down. Convection from the receiver is substantially increased by the presence of side-on wind for all receiver tilt angles. For head-on wind, convective heat loss with the receiver facing straight down is approximately the same as that for side-on wind. Overall it was found that for wind speeds of 20--24 mph, convective heat loss from the receiver can be as much as three times that occurring without wind.
Energy Technology Data Exchange (ETDEWEB)
Hayat, T. [Department of Mathematics, Quaid-i-Azam University 45320, Islamabad 44000 (Pakistan); Department of Mathematics, Faculty of Science, King Abdulaziz University, P. O. Box 80257, Jeddah 21589 (Saudi Arabia); Iqbal, Z., E-mail: zahidiqbal_qau@yahoo.com [Department of Mathematics, Quaid-i-Azam University 45320, Islamabad 44000 (Pakistan); Mustafa, M. [Research Centre for Modeling and Simulation, National University of Sciences and Technology, Sector H-12, Islamabad 44000 (Pakistan); Alsaedi, A. [Department of Mathematics, Faculty of Science, King Abdulaziz University, P. O. Box 80257, Jeddah 21589 (Saudi Arabia)
2012-11-15
Highlights: Black-Right-Pointing-Pointer Boundary layer flow of an upper-convected Maxwell (UCM) fluid over a moving surface. Black-Right-Pointing-Pointer Convective boundary conditions have been used. Black-Right-Pointing-Pointer Series solutions are obtained by homotopy analysis method (HAM). Black-Right-Pointing-Pointer Graphical results for various interesting parametric values. - Abstract: This study discusses the flow and heat transfer in an upper-convected Maxwell (UCM) fluid over a moving surface in the presence of a free stream velocity. The convective boundary conditions have been handled. Similarly transformations are invoked to convert the partial differential equations governing the steady flow of a Maxwell fluid into an ordinary differential system. This system is solved by a homotopic approach. The effects of influential parameters such as Deborah number ({beta}), Prandtl number (Pr), Eckert number (Ec), suction parameter (S) and ratio ({lambda}) have been thoroughly examined.
Free convection heat transfer across rectangular-celled diathermanous honeycombs
International Nuclear Information System (INIS)
Experimental obtained Nusselt number-Rayleigh number plots are presented for free convective heat transfer across inclined honeycomb panels filled with air. The honeycomb cells were rectangular in shape with very long cell dimensions across the slope and comparatively short dimensions up the slope. Elevation aspect ratios, A/sub E/, investigated were 3, 5 and 10; angles of inclination, theta, measured from the horizontal, were 0, 30, 60, 75 and 90 deg. The effect on the Nusselt number, of the emissivities of the plates bounding the honeycomb, and of the emissivity of honeycomb material, was also investigated. The measurements confirmed that the critical Rayleigh number and the post-critical heat transfer depend on the radiant properties of the honeycomb cells. The critical Rayleigh numbers at theta=0 were well predicted by the methods of Sun and Edwards. For 030 deg. The theta=90 deg data were found to be closely correlated by an equation of the form recently proposed by Bejan and Tien
Coupled heat and mass transfer in a convective tunnel dryer
International Nuclear Information System (INIS)
The mechanism of drying in a convective tunnel dryer with air heated in solar collectors was approached first experimentally with a pilot laboratory unit, then numerically taking into account the coupled heat and mass transfers. In the present study, several experimental essays were conducted followed by the adoption of a simulation tool describing the opening conditions of the tunnel dryer and a behavioural model that can be of great interest in the design and the automation of such industrial units. Indeed, behavioural models of thermodynamic system are characterised by the interactions of a large number of complex phenomenon, which call for various types of energy. This dynamic feature requires a modeling approach, using physical phenomenon such as energy storage. energy transformation and energy dissipation as data. The pseudo-bond graph methodology was used in modelling the drying system. This methodology was very suitable for thermo fluid process. It accepts the use of elements that do not exist in the traditional bond graph methods. An explicit pseudo-bond graph model who describes the process of water evaporation under the tray is studies in this paper and the governing equations are determined using bond graph properties.(Author)
A note on the heat transfer in convective fins
International Nuclear Information System (INIS)
In this paper a generalized approach to the problem of heat transfer through convective fins is given. The proper dimensionless variables, which specify the general problem are identified, and upper bounds of the values of the dimensionless number Nsub(r) defined as 'the ratio of the heat transferred by the fin to that of the corresponding bare surface' are derived. It was shown that these limiting values of the Nsub(r) are 1/?B1 and ?2/B1 for longitudinal fins and spines respectively, where B1 is the Biot number hb/k, while for annular fins of constant thickness and hyperbolic profile, Nsub(r) 1, where K(?) is a number determined by the profile of the fin and the ratio ? = x2/x1 of the outside to the inside radii. It was also shown that for longitudinal fins and spinces the possible adverse insulating effect by the use of the fin is avoided, if one selects the value of ?hA/kC < 1, which is a rather stricter criterion than the one reported in the literature, namely that of hA/kC < 1 [2-5]. An example is given to show how one may utilize the appropriate value of Nsub(r) and the fin effectiveness e, to obtain the dimensions of the fin. (orig.)
An experimental investigation of forced convection flat plate solar air heater with storage material
Directory of Open Access Journals (Sweden)
Aissa Walid
2012-01-01
Full Text Available Solar air heater (SAH is a heating device that uses the heated air in the drying of agriculture products and many engineering applications. The purpose of the present work is to study a forced convection flat plate solar air heater with granite stone storage material bed under the climatic conditions of Egypt-Aswan. Experiments are performed at different air mass flow rates ; varying from 0.016 kg/s to 0.08 kg/s, for five hot summer days of July 2008. Hourly values of global solar radiation and some meteorological data (temperature, pressure, relative humidities, etc. for measuring days are obtained from the Egyptian Meteorological Authority, Aswan station. Inlet and outlet temperatures of air from a SAH have been recorded. In this work, attempt has been made to present the temperature distribution in non dimensional form that makes it useable for any region and not restricted to local conditions. The variation of solar radiation, air heater efficiency, Nusselt number and temperature distribution along the air heater are discussed. Comparisons between the calculated values of outlet air temperatures, average air temperatures and storage material temperatures and the corresponding measured values showed good agreement. Comparison between current work and those in previous investigations showed fair agreement.
Scientific Electronic Library Online (English)
V. C., Mariani; L. S., Coelho.
2007-09-01
Full Text Available A numerical study was conducted to investigate steady heat transfer and flow phenomena of natural convection of air in enclosures, with three aspect ratios (H/W = 1, 2, and 4), within which there is a local heat source on the bottom wall at three different positions, Wh. This heat source occupies 1% [...] of the total volume of the enclosure. The vertical walls in the enclosures are insulated and there is an opening on the right wall. The natural convection is influenced by the difference in temperature between the left and right walls, represented by a Rayleigh number (Ra e), and by local heat source, represented by a Rayleigh number (Ra i). Numerical simulations were performed for several values of the Rayleigh number ranging between 10³ and 10(6), while the intensity of the two effects - the difference in temperature on the vertical walls and the local heat source - was evaluated based on the Ra i/Ra e ratio in the range between 0 and 2500. The analysis proceeds by observing variations in the streamlines and isotherms with respect to the different Ra e, R ratios, aspect ratios, of the radius and positions of the local heat source. The average Nusselt numbers on the hot and cold walls are influenced by different values of the parameters R, Ra e, Wh, and H/W. Results show the presence of different flow patterns in the enclosures studied. Thus, the flow and heat transfer can be controlled by external heating, and local heat source.
Effects of Control on the Onset of Marangoni-Benard Convection with Uniform Internal Heat Generation
Norfifah Bachok; Norihan Md. Arifin; Fadzilah Md Ali
2008-01-01
The effect of control on the onset of Marangoni-Benard convection in a horizontal layer of fluid with internal heat generation heated from below and cooled from above is investigated. The resulting eigenvalue problem is solved exactly. The effects of control are studied by examining the critical Marangoni numbers and wave numbers. It is found that the onset of Marangoni-Benard convection with internal heat generation can be delayed through the use of control.
Directory of Open Access Journals (Sweden)
Ziyaddin RECEBL?
2008-01-01
Full Text Available In some studies, the effect of magnetic field on heat convection has been investigated given that physical properties are constant regardless of temperature. The effect of magnetic field on heat convection and fluids whose physical properties change by temperature has been investigated in this study as physical properties of fluids change by the effect of temperature. Momentum, continuity and energy equations including electromagnetic force affecting the fluid were used in the solution. Temperatures at axial and radial directions and Nusselt numbers were calculated depending on magnetic field intensity and other physical properties of fluid by solving the equation system written in cylindrical coordinates system by means of one of the numerical methods which is finite difference method. According to results, velocity and temperature of the cooled fluid decreased following an increase in the intensity of magnetic field placed vertically to flow direction. As determined in the previous one, this study also indicated that the increase in Reynolds number increases Nusselt number, and increasing the effect of magnetic field decreases Nusselt number. The theoretical results of the present study are in conformity with the results of our previous one.
Yano, Ryosuke
2015-01-01
We discuss the thermal conduction and convection of thermally relativistic fluids between two parallel walls under the gravitational force, both theoretically and numerically. In the theoretical discussion, we assume that the Lorentz contraction is ignored and spacetime is flat. For understanding of the thermal conduction and convection of thermally relativistic fluids between two parallel walls under the gravitational force, we solve the relativistic Boltzmann equation using the direct simulation Monte Carlo method. Numerical results indicate that strongly nonequilibrium states are formed in vicinities of two walls, which do not allow us to discuss the transition of the thermal conduction to the thermal convection of thermally relativistic fluids under the gravitational force in the framework of the relativistic Navier-Stokes-Fourier equation, when the flow-field is under the transition regime between the rarefied and continuum regimes, whereas such strongly nonequilibrium states are not formed in vicinities...
International Nuclear Information System (INIS)
This paper is concerned with an unsteady, laminar, free convective flow over a heated sphere with the effect of internal heat generation/absorption. The dimensionless governing equations have been solved employing the finite difference method as well as a perturbation method for short time and an asymptotic method for long time. We examine the effects of the physical parameters, such as, the Prandtl number, Pr, and the heat generation/absorption parameter, ?, on the friction factor and heat transfer rate as well as the velocity and temperature profiles. It is observed that when the Prandtl number, Pr, is increased, the friction factor decreases while the heat transfer rate increases. In the presence of internal heat generation, the friction factor increases while the heat transfer rate reduces. The reverse pattern is found with the heat absorption parameter. The momentum and thermal boundary layers become thicker with an increase of the heat generation parameter. A comparison among the numerical solutions, the perturbation solutions for short time and the asymptotic solutions for long time has been presented which provides a good agreement among the solutions. (authors)
International Nuclear Information System (INIS)
A new version of the ENERGY series code, ENERGY-IV, was written for predicting coolant temperature distributions in wire-wrapped rod assemblies used in the Liquid Metal Fast Breeder Reactor. The ENERGY-IV Code is applicable to both steady-state forced and mixed convection operation for a single isolated assembly. (The SUPERENERGY Code, [Basehore (1980)] is applicable to core wide forced convection analysis.) ENERGY-IV is an empirical code designed to be fast running. Hence the core designer can use it as an inexpensive thermal hydraulic design or diagnosis tool
Modeling the Effect of Internal Convection Currents on Heat Transfer Coefficient of Liquid Foods
SAJID ALI; RASHID ALI
2013-01-01
The internal convection currents generated during the cooling process affect convective heat transfer coefficient from the surface of the container, these convection currents may increase the effective value of the surface film conductance (h). Therefore, in such situation the Nu-Re correlations, which are generally used to predict h-values, may not yield realistic results. In the present work, this effect has been investigated by using the empirical correlation developed through Temperature-...
Cebeci, Tuncer
1989-01-01
This book is designed to accompany Physical and Computational Aspects of Convective Heat Transfer by T Cebeci and P Bradshaw and contains solutions to the exercises and computer programs for the numerical methods contained in that book Physical and Computational Aspects of Convective Heat Transfer begins with a thorough discussion of the physical aspects of convective heat transfer and presents in some detail the partial differential equations governing the transport of thermal energy in various types of flows The book is intended for senior undergraduate and graduate students of aeronautical, chemical, civil and mechanical engineering It can also serve as a reference for the practitioner
International Nuclear Information System (INIS)
Natural convective heat transfer from a wide heated vertical isothermal plate with adiabatic surfaces above and below the heated surface has been considered. There are a series of equally spaced vertical thin, flat adiabatic surfaces (termed 'slats') near the heated surface, these surfaces being, in general, inclined to the heated surface. The slats are pivoted about their center-point and thus as their angle is changed, the distance of the tip of the slat from the plate changes. The situation considered is an approximate model of a window with a vertical blind, the particular case where the window is hotter than the room air, i.e. where air-conditioning is being used, being considered. The flow has been assumed to be laminar and steady. Fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces, this being treated by means of the Biuniqueness type approximation. Although the flow is in general three-dimensional, the flow over each slat is assumed to be the same and attention can therefore be restricted to flow over a single slat by using repeating boundary conditions. The governing equations have been written in dimensionless form and the resulting dimensionless equations have been solved using a commercial finite-element package. The solution has the following parameters: (1) the Rayleigh number (2) the Prandtl number (3) the dimensionless distance of the slat center point (the pivot point) from the surface (4) the dimensionless slat size (5) the dimensionless slat spacing (6) the angle of inclination of the slats. Because of the application that motivated the study, results have only been obtained for a Prandtl number of 0.7. The effect of the other dimensionless variables on the mean dimensionless heat transfer rate from the heated surface has been examined. (author)
Development of the finite analytic method for turbulent forced and free convection
Bernatz, Richard Allen
The finite analytic (FA) method is used to solve turbulent forced and free convective flows. The governing partial differential equations are the primitive variable, ensemble averaged Navier-Stokes and energy equations for two dimensional, incompressible, and constant property fluids. Turbulent characteristics of the flow are simulated using the k-epsilon - Eddy turbulence model. Solutions are found on non-staggered grids using the momentum weighted interpolation method (MWIM). The turbulent flow near solid walls is modeled using a wall function. The square cavity laminar flow problem is solved for Reynolds (Re) numbers of 10(exp 2) and 10(exp 3) to verify the accuracy of the solution procedure incorporating the FA method and MWIM. Model results for both the velocity and thermal fields are consistent with previous experimental and numerical results. Turbulent flow within the square cavity is solved for Re = 200,000, extending the MWIM to turbulent flow problems for the first time. Results for the mean velocity field, and the turbulent fluctuations of the mean velocity field, compare favorably with experimental results. The thermal field predictions for this problem represent new findings for the high Reynolds number square cavity flow. The turbulent, free convective sea breeze flow is solved for a 24 hour heating and cooling cycle using the FA method and MWIM. The model predicts a spatial structure and magnitude for the mean circulation that is in agreement with experimental studies. In terms of turbulent quantities, the model results compare favorably with experimental findings that show the magnitudes of mean velocities and their turbulent fluctuations to be of the same order. Also, the model predicts the rate of turbulent kinetic energy dissipation to be on the order of 100 sq cm/cu sec, which is consistent with experimental measurements. The dynamics of the advancing sea breeze front is correctly simulated by the k-epsilon - Eddy turbulence model as its rate of inland penetration quickens as the land surface begins to cool in the late afternoon.
Experimental transient natural convection heat transfer from a vertical cylindrical tank
International Nuclear Information System (INIS)
In this paper heat transfer experimental data is presented and compared to general correlations proposed in the literature for transient laminar free convection from a vertical cylindrical tank. The experimental data has been obtained from heating and cooling experiments carried out with a cylindrical full-scale hot water storage tank working under real operating conditions. The experimental device and the data acquisition system are described. The calculation procedures established to obtain the experimental values of the heat transfer coefficients, as well as the data reduction process, are detailed. The local convection and radiation heat transfer coefficients are obtained from different heating power conditions for local Rayleigh numbers within the range of 1x105-3x108. The great quantity of available experimental data allows a detailed analysis with a reliable empirical base. The experimental local convection heat transfer coefficients are correlated and compared to correlations proposed in open literature for engineering calculations. - Highlights: ? Experimental data of transient local convection heat transfer coefficients from a cylindrical tank for heating and cooling processes is obtained. ? The transient behaviour of the convection coefficients is dependent on temperature difference evolutions between the surface and the air. ? The Nu.Ra-1/4 ratio decreases proportionally in (Ts-T?)-0.9. ? A new correlation based on the semi-infinite region theory for laminar transient free convection is proposed.
International Nuclear Information System (INIS)
It is observed that the temperature distribution in the boundary layer changes qualitatively and the heat transfer in the upper part of a plate intensifies substantially in comparison with a surface with a continuous heat flux. For the case of substantial heat flux density in the initial stage of formation of a free convective flow a two-dimensional vortex is found to appear
Numerical Modeling of Dendritic Growth in Alloy Solidification with Forced Convection
Sun, Dongke; Zhu, Mingfang; Pan, Shiyan; Raabe, Dierk
A two dimensional (2D) cellular automaton (CA) - lattice Boltzmann (LB) model is presented to investigate the effects of forced melt convection on the solutal dendritic growth. In the model, the CA approach of simulating the dendritic growth is incorporated with the kinetic-based lattice Boltzmann method (LBM) for numerically solving the melt flow and solute transport. Two sets of distribution functions are used in the LBM to model the convective-diffusion phenomena during dendritic growth. After validating the model by comparing the numerical results with the theoretical solutions, it is applied to simulate the single and multi dendritic growth of Al-Cu alloys without and with a forced convection. The typical asymmetric growth features of convective dendrite are reproduced and the dendritic morphology is strongly influenced by melt convection. The simulated convective multi dendritic features by the present model are also compared with that by the CA-NS model. The present model is found to be more computationally efficient and numerically stable than the CA-NS model.
International Nuclear Information System (INIS)
Paper describes design and peculiar features of application of heat flux measuring sensor, operation of which is based on the Seebeck transverse effect. Sensor capabilities are shown using investigation into free-convection heat transfer as an example. Paper present the results of measuring of instantaneous value of heat flux on a heated vertical surface under various air flow regimes within free-convection boundary layer
Short-wave convective turbulence and anomalous electron heat conduction of a plasma
International Nuclear Information System (INIS)
A turbulent mechanism of anomalous electron transfer in a magnetized plasma due to excitation in the plasma of short-wave convective oscillations is suggested. Stationary spectra of short-wave convective turbulence are found in the framework of a general static approach and anomalous electron heat conduction of the plasma is calculated
Convection-radiation heat transfer to steam in rod bundle geometry/
International Nuclear Information System (INIS)
An engineering heat transfer model was developed to predict the total heat transfer coefficients above the froth region in a nuclear reactor core undergoing a slow core uncovering. The model consists of a new heat transfer correlation for convection to steam and a one-dimensional thermal radiation equation. Above the froth region, large wall-to-bulk temperature ratios can take place; therefore, variable property effects on flow and heat transfer were examined because they can affect the heat transfer conditions to a considerable extent. The convective heat transfer coefficients and rod surface temperatures were calculated by using various correlations. The comparison of the results showed that the new correlation accurately predicts the convective heat transfer coefficients and, when combined with the radiation equation, the wall temperatures. The use of this model should be of value in modeling small-break loss-of-coolant accidents and preliminary design work
Simulation of Convective Heat-Transfer Coefficient in a Buried Exchanger
Directory of Open Access Journals (Sweden)
Taoufik Mnasri
2008-01-01
Full Text Available This study presents analytical models allowing to study a forced convection laminar flow in non-established dynamic and thermic regimes. We treated a flow in a bitubular exchanger in permanent thermal contact with a semi-infinite medium, such as the ground. The wall temperature as well as the wall heat flux evolve in the course of time until a quasi-steady mode. The theoretical method is original because it uses Green's functions method to determine the analytical solutions of the heat propagation equation on the wall during the heating phase. These analytical solutions allow to identify the temperature distribution versus time. The complexity of the system geometry as well as the infinity of the medium surrounding the exchanger make the traditional methods of numerical resolution unable to solve the problem. We used, to solve it, the finite volume method coupled with the finite element method at the boundary. We studied the effect of Reynolds number, the fluid entry temperature and the transfer duration on the axial evolution of the heat transfer coefficient. We illustrated also the profile of the temperature field in the fluid medium.
Directory of Open Access Journals (Sweden)
Sourtiji Ehsan
2012-01-01
Full Text Available A numerical study of natural convection heat transfer through an alumina-water nanofluid inside L-shaped cavities in the presence of an external magnetic field is performed. The study has been carried out for a wide range of important parameters such as Rayleigh number, Hartmann number, aspect ratio of the cavity and solid volume fraction of the nanofluid. The influence of the nanoparticle, buoyancy force and the magnetic field on the flow and temperature fields have been plotted and discussed. The results show that after a critical Rayleigh number depending on the aspect ratio, the heat transfer in the cavity rises abruptly due to some significant changes in flow field. It is also found that the heat transfer enhances in the presence of the nanoparticles and increases with solid volume fraction of the nanofluid. In addition, the performance of the nanofluid utilization is more effective at high Rayleigh numbers. The influence of the magnetic field has been also studied and deduced that it has a remarkable effect on the heat transfer and flow field in the cavity that as the Hartmann number increases the overall Nusselt number is significantly decreased specially at high Rayleigh numbers.
Method of cleaning convective heating surfaces from deposits
Energy Technology Data Exchange (ETDEWEB)
Keylin, I.I.; Dedikov, S.N.; Nikitin, A.I.
1982-01-01
According to the main USSR cert. of auth. 767500, sprayed water with temperature 70-100/sup 0/C is supplied to the heating surface to be cleaned having temperature of 150-350/sup 0/C. In this case the sprayed water turns into steam on the hot deposits, they are loosened and are washed away by technical water. However, in cleaning the surfaces with vertical pipes, only the front coils of the first 2-3 rows are exposed to the effect of the hot technical water. According to the new proposal, when the temperature in the gas line reaches 350/sup 0/C, exhaust fans are turned on, the slide valves are closed and sprayed water with temperature of 70-100/sup 0/C is supplied to the cleaning section, which coming into contact with the hot heating surfaces evaporates, forming a steam bath in the gas line. The deposits are separated and washed away by water from the frontal coils. Within 45 minutes after the beginning of cleaning, the slide valves are opened, and because of the thrust created by the smokestack, the boiler is ventilated for 5-15 minutes. Water passes through the entire packet of cleanable surface, sprinkling the inner coils. After this the slide valves are closed, and cleaning lasts for another 45 minutes. Then air is supplied into the convective shaft to ventilate the vertical coils in the opposite direction. Cleaning is done until the temperature in the gas line drops to 150/sup 0/C, i.e., 5-6 h. Then the surfaces are flushed with technical water from hoses. The proposed method excludes the need for deslagging and cleaning of the vertical packets by manual tool.
International Nuclear Information System (INIS)
Two-phase flow heat transfer has been exhaustively studied over recent years. However, in this field several questions remain unanswered. Heat transfer coefficient prediction related to nucleate and convective boiling have been studied using different approaches, numerical, analytical and experimental. In this work, an experimental analysis, data representation and heat transfer coefficient prediction on two-phase heat transfer on nucleate and convective boiling are presented. An empirical correlation is obtained based on genetic algorithms search engine over a dimensional analysis of the two-phase flow heat transfer problem. (author)
Heat transfer in the thermo-electro-hydrodynamic convection under microgravity conditions
Tadie Fogaing, Mireille; Yoshikawa, Harunori,; Crumeyrolle, Olivier; Mutabazi, Innocent
2014-01-01
This article deals with the thermal convection in a dielectric fluid confined in a finite-length plane capacitor with a temperature gradient under microgravity conditions. The dielectrophoretic force resulting from differential polarization of the fluid plays the role of buoyancy force associated with an electric effective gravity. It induces the convection when the Rayleigh number based on this electric gravity exceeds a critical value. Two-dimensional numerical simulation for a geometry wit...
International Nuclear Information System (INIS)
Numerical analysis of natural convection inside a heat generated fluid was performed for four different spherical geometries that match the experimental vessels used by Asfia et al. [5-7]. The transient calculations were performed with the CFX 5.7 fluid dynamic software. The simulations show that the highest heat flux is just below the rim of the cavity and it can be 50 times higher than at the bottom. Based on the numerical results, the local values of heat transfer coefficient and the distributions of global Nusselt number were calculated. The present, three-dimensional simulation results were compared with the numerical results of Mayinger et al. [3] and Reineke et al. [4], and with the experimental data of Asfia et al. [5-7]. The agreement between the results that is well inside the experimental scatter verifies the selected modeling approach. (author)
Fang, Pingping
1998-12-01
An extended numerical investigation of fully developed, forced convective laminar flows with heat transfer in eccentric annuli has been carried out. Both Newtonian and non-Newtonian (power-law or Ostwald-de Waele) fluids are studied, representing typical applications in petrochemical, bio-chemical, personal care products, polymer/plastic extrusion and food industries. For the heat transfer problem, with an insulated outer surface, two types of thermal boundary conditions have been considered: Constant wall temperature (T), and uniform axial heat flux with constant peripheral temperature (H1) on the inner surface of the annulus. The governing differential equations for momentum and energy conservation are solved by finite-difference methods. Velocity and temperature distributions in the flow cross section, the wall shear-stress distribution, and isothermal f Re, Nu i,T and Nu i,H1 values for different eccentric annuli (0/le?/*/le0.6,/ 0.2/le r/sp/*/le0.8) are presented. In Newtonian flows, the eccentricity is found to have a very strong influence on the flow and temperature fields. In an annulus with relatively large inner cylinder eccentricity, the flow tends to stagnate in the narrow section and has higher peak velocities in the wide section of the annulus. There is considerable flow maldistribution in the azimuthal direction, which in turn produces greater nonuniformity in the temperature field and a consequent degradation in the average heat transfer. Also, the H1 wall condition sustains higher heat transfer coefficients relative to the T boundary condition on the inner surface. For viscous, power-law type non-Newtonian flows, both shear thinning (n1) fluids are considered. Here, the non-linear shear behavior of the fluid is found to further aggravate the flow and temperature maldistribution, and once again the eccentricity is seen to exhibit a very strong influence on the friction and heat transfer behavior. Finally, the hydrodynamic characteristics of fully developed axial laminar flow of Newtonian fluids in eccentric annuli with a rotating inner cylinder are investigated. These are of significant importance to the design and operation of oil and gas drilling wells. Using finite-difference method to solve the governing flow equations in bipolar coordinates, computational results for a wide range of annulus geometry (0/le r/sp/*/le1,/ 0/le/varepsilon/sp/*/le0.8), and rotational Reynolds number (0/le Rer/le150) are presented, where the rotational speeds are restricted to the sub-critical Taylor number regime. The results delineate the effects of annuli r/sp/* and ?sp/*, and inner cylinder rotation speed on the flow structure and frictional losses.
Forced convection along a wall. Liquid metals application
International Nuclear Information System (INIS)
From the experimental results in pipes, heated with constant wall heat flux, the dynamical and thermal structure of the wall region of a turbulent flow is studied. We can show that, for high values of Reynolds and Peclet numbers, logarithmic profiles of velocity and temperature exist. A continuous description of the wall is obtained with the use of simple modelisation. The study of the thermal wall region structure is then made in the case of a liquid metal flow
Predicting Turbulent Convective Heat Transfer in Fully Developed Duct Flows
Rokni, Masoud; Gatski, Thomas B.
2001-01-01
The performance of an explicit algebraic stress model (EASM) is assessed in predicting the turbulent flow and forced heat transfer in both straight and wavy ducts, with rectangular, trapezoidal and triangular cross-sections, under fully developed conditions. A comparison of secondary flow patterns. including velocity vectors and velocity and temperature contours, are shown in order to study the effect of waviness on flow dynamics, and comparisons between the hydraulic parameters. Fanning friction factor and Nusselt number, are also presented. In all cases. isothermal conditions are imposed on the duct walls, and the turbulent heat fluxes are modeled using gradient-diffusion type models. The formulation is valid for Reynolds numbers up to 10(exp 5) and this minimizes the need for wall functions that have been used with mixed success in previous studies of complex duct flows. In addition, the present formulation imposes minimal demand on the number of grid points without any convergence or stability problems. Criteria in terms of heat transfer and friction factor needed to choose the optimal wavy duct cross-section for industrial applications among the ones considered are discussed.
Efficiency of Heat Transfer in Turbulent Rayleigh-Benard Convection.
Czech Academy of Sciences Publication Activity Database
Urban, Pavel; Musilová, V?ra; Skrbek, L.
2011-01-01
Ro?. 107, ?. 1 (2011), 014302:1-4. ISSN 0031-9007 R&D Projects: GA AV ?R KJB200650902 Institutional research plan: CEZ:AV0Z20650511 Keywords : natural convection * thermal convection Subject RIV: BK - Fluid Dynamics Impact factor: 7.370, year: 2011
International Nuclear Information System (INIS)
The model laws for the initial film boiling at forced convection are realized in vertical tubes. The local conditions in the investigated area were regarded to be most effective and sufficient for the description. The theory was confirmed by experimental data. (orig.)
Tri–Dimensional Numerical Analysis for Forced Convection over a Forward Facing Step
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J.A. Jiménez–Berna
2010-01-01
Full Text Available A finite volume discretization technique is used to develop a numerical code to simulate the flow structures and forced convection in a forward facing step channel. The velocity field and pressure distribution inside the computacional domain are linked by the SIMPLE algorithm. The duct dimensions are defined in terms of the step height, such that the aspect and expansion ratios are four and two respectivelly. The total length in the streamflow direction is sixty times the step height, while the step edge is located twenty times the step height after the channel inlet. The boundary conditions at the channel inlet correspond to a fully developed flow at a constant temperature T0. The heating conditions are those of considereing the bottom wall at a high constant temperature (Tw>T0 and the other walls as adiabatic ones. Results for the location and size of the re–circulating zones, as well as the flow structures and temperature distributions at different planes inside the computational domain for three different Reynolds parameters are presented.
Convection initiation along a dryline under conditions of weak synoptic forcing
Trier, S.; Chen, F.; Manning, K.
2003-04-01
During the spring and early summer a dryline characterized by relatively weak temperature (density) gradients but large moisture contrasts is a common feature over the southern Great Plains (SGP) region of the United States. In the spring, when strong vertical wind shear is present, the dryline often serves as a focus for severe convective storms. In the current study we examine a quasi-stationary dryline present over the SGP for a five-day period from 19 to 23 June 1998. In contrast to the more widely reported spring cases, this dryline episode and its associated deep convection occurred in the absence of strong vertical wind shear and organized synoptic-scale ascent. The detailed mechanisms of convection initiation along drylines are not well understood. The current case allows a unique opportunity for the assessment of planetary boundary layer (PBL)-based deep convection initiation mechanisms, which are likely to be of general importance in dryline convection cases, without the complicating effects of strong large-scale forcing. Numerical simulations with the Pennsylvania State/NCAR Mesoscale Model Version 5 (MM5), at both deep convection resolving resolutions and at coarser resolution over regional scale domains, have accurately reproduced the initiation and short term (3-6 h) evolution of deep convection, as revealed by satellite, radar, and surface rainfall observations. Convection initiation was both observed and simulated at the eastern edge of the dryline, which later retrogressed westward during the lifecycle of the slowly eastward moving convection. During the five-day episode there was little severe weather reported, however, localized heavy rainfall amounts were common in the large-CAPE environment along and slightly east of the dryline. Trajectory analysis with model output elucidates the thermodynamic destabilization of the PBL air that feeds the updrafts of the developing convection. Model-based sensitivity studies are planned to examine the role of land-surface atmosphere interactions (particularly the impact of soil moisture and temperature), which are likely to be especially important for convection initiation in the weakly forced synoptic environment. Results from this analysis will be discussed at the conference.
Preliminary Numerical Analysis of Convective Heat Transfer Loop Using MARS Code
Energy Technology Data Exchange (ETDEWEB)
Lee, Yongjae; Seo, Gwang Hyeok; Jeun, Gyoodong; Kim, Sung Joong [Hanyang Univ., Seoul (Korea, Republic of)
2014-05-15
The MARS has been developed adopting two major modules: RELAP5/MOD3 (USA) for one-dimensional (1D) two-fluid model for two-phase flows and COBRA-TF code for a three-dimensional (3D), two-fluid, and three-field model. In addition to the MARS code, TRACE (USA) is a modernized thermal-hydraulics code designed to consolidate and extend the capabilities of NRC's 3 legacy safety code: TRAC-P, TRAC-B and RELAP. CATHARE (French) is also thermal-hydraulic system analysis code for Pressurized Water Reactor (PWR) safety. There are several researches on comparing experimental data with simulation results by the MARS code. Kang et al. conducted natural convection heat transfer experiments of liquid gallium loop, and the experimental data were compared to MARS simulations. Bang et al. examined the capability of the MARS code to predict condensation heat transfer experiments with a vertical tube containing a non-condensable gas. Moreover, Lee et al. adopted MELCOR, which is one of the severe accident analysis codes, to evaluate several strategies for the severe accident mitigation. The objective of this study is to conduct the preliminary numerical analysis for the experimental loop at HYU using the MARS code, especially in order to provide relevant information on upcoming experiments for the undergraduate students. In this study, the preliminary numerical analysis for the convective heat transfer loop was carried out using the MARS Code. The major findings from the numerical simulations can be summarized as follows. In the calculations of the outlet and surface temperatures, the several limitations were suggested for the upcoming single-phase flow experiments. The comparison work for the HTCs shows validity for the prepared input model. This input could give useful information on the experiments. Furthermore, the undergraduate students in department of nuclear engineering, who are going to be taken part in the experiments, could prepare the program with the input, and will be provided with expected results for the single-phase and forced convective phenomena. For the future study, different materials for the heating part are considered, such as other metals or silicon carbide (SiC) tube, which is a candidate material of fuel claddings for current and next-generation reactors.
Directory of Open Access Journals (Sweden)
Prasad Kerehalli
2015-01-01
Full Text Available An analysis is carried out to study the effects of temperature-dependent transport properties on the fully developed free and forced MHD convection flow in a vertical channel. In this model, viscous and Ohmic dissipation terms are also included. The governing nonlinear equations (in non-dimensional form are solved numerically by a second order finite difference scheme. A parametric study is performed in order to illustrate the interactive influences of the model parameters; namely, the magnetic parameter, the variable viscosity parameter, the mixed convection parameter, the variable thermal conductivity parameter, the Brinkmann number and the Eckert number. The velocity field, the temperature field, the skin friction and the Nusselt number are evaluated for several sets of values of these parameters. For some special cases, the obtained numerical results are compared with the available results in the literature: Good agreement is found. Of all the parameters, the variable thermo-physical transport property has the strongest effect on the drag, heat transfer characteristics, the stream-wise velocity, and the temperature field.
Transition to finger convection in double-diffusive convection
Kellner, M
2014-01-01
Finger convection is observed experimentally in an electrodeposition cell in which a destabilizing gradient of copper ions is maintained against a stabilizing temperature gradient. This double-diffusive system shows finger convection even if the total density stratification is unstable. Finger convection is replaced by an ordinary convection roll if convection is fast enough to prevent sufficient heat diffusion between neighboring fingers, or if the thermal buoyancy force is less than 1/30 of the compositional buoyancy force. At the transition, the ion transport is larger than without an opposing temperature gradient.
Influence of Pr on Natural Convection Heat Transfer of an Open Channel Finned Plate
International Nuclear Information System (INIS)
The finned plate provides the extended heat transfer area and improves the heat transfer. However when the fin spacing becomes small, the pressure drop increases due to frictional loss, heat transfer is impaired. Thus there is an optimal fin spacing. For the natural convection heat transfer, the heated thermal boundary layer drives the flow and the influence of the Prandtl number on the heat transfer will be very important as the thickness of the thermal boundary layer depends on it. This study aims at investigating the influence of the Prandtl number on the natural convection heat transfer of the finned plate. Numerical analyses were performed by varying the Pr from 2 to 2,014. Numerical analysis was performed for the natural convection heat transfer of a finned plate in an open channel. In order to investigate the influence of the Prandtl number on the heat transfer, four different values of Prandtl numbers were simulated and compared. As expected, the velocity profiles were almost similar except for the fact that the boundary layer develops earlier for smaller Prandtl number fluid. However the temperature profiles varied drastically depending on the values of the Prandtl number. As the Prandtl number increases, the thermal boundary layer reduces. The comparisons of the results with Le Fevre natural convection heat transfer correlation for vertical plate shows that as the Pr increases, the NuL of the finned plate becomes similar to that of the flat plate of the same heat transfer area
Jiao, Anjun; Zhang, Yuwen; Ma, Hongbin; Critser, John
2009-01-01
Heat and mass transfer in a circular tube subject to the boundary condition of the third kind is investigated. The closed form of temperature and concentration distributions, the local Nusselt number based on the total external heat transfer and convective heat transfer inside the tube, as well as the Sherwood number were obtained. The effects of Lewis number and Biot number on heat and mass transfer were investigated.
Scale/Analytical Analyses of Freezing and Convective Melting with Internal Heat Generation
Energy Technology Data Exchange (ETDEWEB)
Ali S. Siahpush; John Crepeau; Piyush Sabharwall
2013-07-01
Using a scale/analytical analysis approach, we model phase change (melting) for pure materials which generate constant internal heat generation for small Stefan numbers (approximately one). The analysis considers conduction in the solid phase and natural convection, driven by internal heat generation, in the liquid regime. The model is applied for a constant surface temperature boundary condition where the melting temperature is greater than the surface temperature in a cylindrical geometry. The analysis also consider constant heat flux (in a cylindrical geometry).We show the time scales in which conduction and convection heat transfer dominate.
A reassessment of the heat transport by variable viscosity convection with plates and lids
International Nuclear Information System (INIS)
The heat transport by a viscous fluid with temperature dependent viscosity has been studied numerically. As opposed to previous models, the top surface of the fluid clearly defines a tectonic plate with horizontally uniform velocity and subduction. Past studies failed to incorporate plates, the heat transport is as efficient as Rayleigh-Benard convection with constant viscosity; there is a strong buffering between internal temperature and heat loss. Past studies of parameterized convection which incorporated parameters indicative of strong buffering between internal temperature and total heat output still provide the most physically plausible representation of the Earth's thermal evolution
Convective heat transfer from a heated elliptic cylinder at uniform wall temperature
Directory of Open Access Journals (Sweden)
Kaprawi S., Dyos Santoso
2013-01-01
Full Text Available This study is carried out to analyse the convective heat transfer from a circular and an elliptic cylinders to air. Both circular and elliptic cylinders have the same cross section. The aspect ratio of cylinders range 0-1 are studied. The implicit scheme of the finite difference is applied to obtain the discretized equations of hydrodynamic and thermal problem. The Choleski method is used to solve the discretized hydrodynamic equation and the iteration method is applied to solve the discretized thermal equation. The circular cylinder has the aspect ratio equal to unity while the elliptical cylinder has the aspect ratio less than unity by reducing the minor axis and increasing the major axis to obtain the same cross section as circular cylinder. The results of the calculations show that the skin friction change significantly, but in contrast with the elliptical cylinders have greater convection heat transfer than that of circular cylinder. Some results of calculations are compared to the analytical solutions given by the previous authors.
Convective heat transfer from a heated elliptic cylinder at uniform wall temperature
Energy Technology Data Exchange (ETDEWEB)
Kaprawi, S.; Santoso, Dyos [Mechanical Department of Sriwijaya University, Jl. Raya Palembang-Prabumulih Km. 32 Inderalaya 50062 Ogan Ilir (Indonesia)
2013-07-01
This study is carried out to analyse the convective heat transfer from a circular and an elliptic cylinders to air. Both circular and elliptic cylinders have the same cross section. The aspect ratio of cylinders range 0-1 are studied. The implicit scheme of the finite difference is applied to obtain the discretized equations of hydrodynamic and thermal problem. The Choleski method is used to solve the discretized hydrodynamic equation and the iteration method is applied to solve the discretized thermal equation. The circular cylinder has the aspect ratio equal to unity while the elliptical cylinder has the aspect ratio less than unity by reducing the minor axis and increasing the major axis to obtain the same cross section as circular cylinder. The results of the calculations show that the skin friction change significantly, but in contrast with the elliptical cylinders have greater convection heat transfer than that of circular cylinder. Some results of calculations are compared to the analytical solutions given by the previous authors.
International Nuclear Information System (INIS)
Natural convection in low aspect ratio rectangular enclosures is considered along with three-dimensional convection within rectangular boxes, natural convection flow visualization in irradiated water cooled by air flow over the surface, free convection in vertical slots, the stratification in natural convection in vertical enclosures, the flow structure with natural convection in inclined air-filled enclosures, and natural convection across tilted, rectangular enclosures of small aspect ratio. Attention is given to the effect of wall conduction and radiation on natural convection in a vertical slot with uniform heat generation of the heated wall, a numerical study of thermal insulation enclosure, free convection in a piston-cylinder enclosure with sinusoidal piston motion, natural convection heat transfer between bodies and their spherical enclosure, an experimental study of the steady natural convection in a horizontal annulus with irregular boundaries, three-dimensional natural convection in a porous medium between concentric inclined cylinders, a numerical solution for natural convection in concentric spherical annuli, and heat transfer by natural convection in porous media between two concentric spheres
Onset of nuclear boiling in forced convection (Method of detection)
International Nuclear Information System (INIS)
Local onset of boiling in any pressure water cooling systems, as a PWR for instance, can mean a possible dangerous mismatch between the produced heat and the cooling capabilities. Its consequences can lead to serious accidental conditions and a reliable technique to detect such a phenomenon is therefore of particular need. Most techniques used up to now rely basically on local measurements and assume therefore usually the previous knowledge of the actual hot or boiling spot. The method proposed here based on externally located accelerometers appears to be sensitive to the global behaviour of the mechanical structure and is therefore not particularly bound to any exact localization of the sensors. The vibrations produced in the mechanical structure of the heated assembly are measured by accelerometers placed on the external surfaces that are easily accessible. The onset of the boiling, the growth and condensation of the bubbles on the heated wall, induces a resonance in the structure and an excitation at its particular eigen frequencies. Distinctive peaks are clearly observed in the spectral density function calculated from the accelerometer signal as soon as bubbles are produced. The technique is shown to be very sensitive even at the earliest phase of boiling and quite independent on sensor position. A complete hydrodynamic analysis of the experimental channels have been performed in order to assess the validity of the method both in steady conditions and during rapid power transients
Natural convection heat transfer in vertical triangular subchannel in Zirconia-water nanofluid
Tandian, N. P.; Alkharboushi, A. A. K.; Kamajaya, K.
2015-09-01
Natural convection heat transfer in vertical triangular sub-channel has important role in cooling mechanism of the APWR and the PHWR nuclear reactors. Unfortunately, natural convection correlation equations for such geometry are scarcely available. Recent studies showed that ZrO2-water nanofluid has a good prospect to be used in the nuclear reactor technology due to its low neutron absorption cross section. Although several papers have reported transport properties of ZrO2-water nanofluids, practically there is no correlation equation for predicting natural convection heat transfer in a vertical triangular sub-channel in ZrO2-water nanofluid. Therefore, a study for finding such heat transfer correlation equation has been done by utilizing Computational Fluid Dynamics software and reported in this paper. In the study, natural convection heat transfer in a vertical triangular sub-channel has been simulated at several values of heat transfer flux within 9.1 to 30.9 kW/m2 range and ZrO2 concentrations of 0 (pure water), 0.27, and 3 volume-% of ZrO2. The study shows that the ZrO2 concentration has no significant influence to the natural convection heat transfer at those concentration levels. The obtained theoretical heat transfer correlation equations were verified through experiment, and they showed very similar results. The correlation equations are reported in this paper.
Modified Laser Flash Method for Thermal Properties Measurements and the Influence of Heat Convection
Lin, Bochuan; Zhu, Shen; Ban, Heng; Li, Chao; Scripa, Rosalia N.; Su, Ching-Hua; Lehoczky, Sandor L.
2003-01-01
The study examined the effect of natural convection in applying the modified laser flash method to measure thermal properties of semiconductor melts. Common laser flash method uses a laser pulse to heat one side of a thin circular sample and measures the temperature response of the other side. Thermal diffusivity can be calculations based on a heat conduction analysis. For semiconductor melt, the sample is contained in a specially designed quartz cell with optical windows on both sides. When laser heats the vertical melt surface, the resulting natural convection can introduce errors in calculation based on heat conduction model alone. The effect of natural convection was studied by CFD simulations with experimental verification by temperature measurement. The CFD results indicated that natural convection would decrease the time needed for the rear side to reach its peak temperature, and also decrease the peak temperature slightly in our experimental configuration. Using the experimental data, the calculation using only heat conduction model resulted in a thermal diffusivity value is about 7.7% lower than that from the model with natural convection. Specific heat capacity was about the same, and the difference is within 1.6%, regardless of heat transfer models.
The role of a convective surface in models of the radiative heat transfer in nanofluids
Energy Technology Data Exchange (ETDEWEB)
Rahman, M.M., E-mail: mansurdu@yahoo.com; Al-Mazroui, W.A.; Al-Hatmi, F.S.; Al-Lawatia, M.A.; Eltayeb, I.A.
2014-08-15
Highlights: • The role of a convective surface in modelling with nanofluids is investigated over a wedge. • Surface convection significantly controls the rate of heat transfer in nanofluid. • Increased volume fraction of nanoparticles to the base-fluid may not always increase the rate of heat transfer. • Effect of nanoparticles solid volume fraction depends on the types of constitutive materials. • Higher heat transfer in nanofluids is found in a moving wedge rather than in a static wedge. - Abstract: Nanotechnology becomes the core of the 21st century. Nanofluids are important class of fluids which help advancing nanotechnology in various ways. Convection in nanofluids plays a key role in enhancing the rate of heat transfer either for heating or cooling nanodevices. In this paper, we investigate theoretically the role of a convective surface on the heat transfer characteristics of water-based nanofluids over a static or moving wedge in the presence of thermal radiation. Three different types of nanoparticles, namely copper Cu, alumina Al{sub 2}O{sub 3} and titanium dioxide TiO{sub 2} are considered in preparation of nanofluids. The governing nonlinear partial differential equations are made dimensionless with the similarity transformations. Numerical simulations are carried out through the very robust computer algebra software MAPLE 13 to investigate the effects of various pertinent parameters on the flow field. The obtained results presented graphically as well as in tabular form and discussed from physical and engineering points of view. The results show that the rate of heat transfer in a nanofluid in the presence of thermal radiation significantly depends on the surface convection parameter. If the hot fluid side surface convection resistance is lower than the cold fluid side surface convection resistance, then increased volume fraction of the nanoparticles to the base fluid may reduces the heat transfer rate rather than increases from the surface of the wedge to the nanofluid. This finding is new and has not been reported in any open literature.
The role of a convective surface in models of the radiative heat transfer in nanofluids
International Nuclear Information System (INIS)
Highlights: • The role of a convective surface in modelling with nanofluids is investigated over a wedge. • Surface convection significantly controls the rate of heat transfer in nanofluid. • Increased volume fraction of nanoparticles to the base-fluid may not always increase the rate of heat transfer. • Effect of nanoparticles solid volume fraction depends on the types of constitutive materials. • Higher heat transfer in nanofluids is found in a moving wedge rather than in a static wedge. - Abstract: Nanotechnology becomes the core of the 21st century. Nanofluids are important class of fluids which help advancing nanotechnology in various ways. Convection in nanofluids plays a key role in enhancing the rate of heat transfer either for heating or cooling nanodevices. In this paper, we investigate theoretically the role of a convective surface on the heat transfer characteristics of water-based nanofluids over a static or moving wedge in the presence of thermal radiation. Three different types of nanoparticles, namely copper Cu, alumina Al2O3 and titanium dioxide TiO2 are considered in preparation of nanofluids. The governing nonlinear partial differential equations are made dimensionless with the similarity transformations. Numerical simulations are carried out through the very robust computer algebra software MAPLE 13 to investigate the effects of various pertinent parameters on the flow field. The obtained results presented graphically as well as in tabular form and discussed from physical and engineering points of view. The results show that the rate of heat transfer in a nanofluid in the presence of thermal radiation significantly depends on the surface convection parameter. If the hot fluid side surface convection resistance is lower than the cold fluid side surface convection resistance, then increased volume fraction of the nanoparticles to the base fluid may reduces the heat transfer rate rather than increases from the surface of the wedge to the nanofluid. This finding is new and has not been reported in any open literature
Open Channel Natural Convection Heat Transfer on a Vertical Finned Plate
Energy Technology Data Exchange (ETDEWEB)
Park, Joo Hyun; Heo, Jeong Hwan; Chung, Bum Jin [Kyung Hee Univ., Yongin (Korea, Republic of)
2013-10-15
The natural convection heat transfer of vertical plate fin was investigated experimentally. Heat transfer systems were replaced by mass-transfer systems, based on the analogy concept. The experimental results lie within the predictions of the existing heat transfer correlations of plate-fin for the natural convections. An overlapped thermal boundary layers caused increasing heat transfer, and an overlapped momentum boundary layers caused decreasing heat transfer. As the fin height increases, heat transfer was enhanced due to increased inflow from the open side of the fin spacing. When fin spacing and fin height are large, heat transfer was unaffected by the fin spacing and fin height. Passive cooling by natural convection becomes more and more important for the nuclear systems as the station black out really happened at the Fukushima NPPs. In the RCCS (Reactor Cavity Cooling System) of a VHTR (Very High Temperature Reactor), natural convection cooling through duct system is adopted. In response to the stack failure event, extra cooling capacity adopting the fin array has to be investigated. The finned plate increases the surface area and the heat transfer increases. However, the plate of fin arrays may increase the pressure drop and the heat transfer decreases. Therefore, in order to enhance the passive cooling with fin arrays, the parameters for the fin arrays should be optimized. According to Welling and Wooldridge, a natural convection on vertical plate fin is function of Gr, Pr, L, t, S, and H. The present work investigated the natural convection heat transfer of a vertical finned plate with varying the fin height and the fin spacing. In order achieve high Rayleigh numbers, an electroplating system was employed and the mass transfer rates were measured using a copper sulfate electroplating system based on the analogy concept.
Forced turbulent convection in tube bundles with longitudinal attack
International Nuclear Information System (INIS)
A numerical pattern is presented for forecasting the pressure loss and heat transfer coefficients in a tubular bundle exchanger with longitudinal attack, which is based on a differential turbulence pattern. The work already achieved is reviewed, then the principle of the pattern is presented: use of transport equations for the components of the Reynolds tensor, choice of the discretisation grid, boundary conditions. The results of computer calculations lead to expressions for the loss of pressure coefficient and the Nusselt number for different bundle configurations. The use of a simplifying method, hydraulic diameter, equivalent-annular space, is shown to be no longer satisfactory for liquid metals
International Nuclear Information System (INIS)
Convective heat transfer at exterior building surfaces has an impact on the design and performance of building components such as double-skin facades, solar collectors, solar chimneys and ventilated photovoltaic arrays, and also affects the thermal climate and cooling load in urban areas. In this study, an overview is given of existing correlations of the exterior convective heat transfer coefficient (CHTC) with the wind speed, indicating significant differences between these correlations. As an alternative to using existing correlations, the applicability of CFD to obtain forced CHTC correlations is evaluated, by considering a cubic building in an atmospheric boundary layer. Steady Reynolds-averaged Navier-Stokes simulations are performed and, instead of the commonly used wall functions, low-Reynolds number modelling (LRNM) is used to model the boundary-layer region for reasons of improved accuracy. The flow field is found to become quasi independent of the Reynolds number at Reynolds numbers of about 105. This allows limiting the wind speed at which the CHTC is evaluated and thus the grid resolution in the near-wall region, which significantly reduces the computational expense. The distribution of the power-law CHTC-U10 correlation over the windward and leeward surfaces is presented (U10 = reference wind speed at 10 m height). It is shown that these correlations can be accurately determined by simulations with relatively low wind speed values, which avoids the use of excessively fine grids for LRNM, and by using only two or three discrete wind speed values, which limits the required number of CFD simulations.
Lopez, Jose M; Avila, Marc
2015-01-01
The flow of fluid confined between a heated rotating cylinder and a cooled stationary cylinder is a canonical experiment for the study of heat transfer in engineering. The theoretical treatment of this system is greatly simplified if the cylinders are assumed to be of infinite length or periodic in the axial direction, in which cases heat transfer occurs only through conduction as in a solid. We here investigate numerically heat transfer and the onset of turbulence in such flows by using both periodic and no-slip boundary conditions in the axial direction. We obtain a simple linear criterion that determines whether the infinite-cylinder assumption can be employed. The curvature of the cylinders enters this linear relationship through the slope and additive constant. For a given length-to-gap aspect ratio there is a critical Rayleigh number beyond which the laminar flow in the finite system is convective and so the behaviour is entirely different from the periodic case. The criterion does not depend on the Pra...
Scientific Electronic Library Online (English)
I. K., Adegun; F. L., Bello-Ochende.
2004-09-01
Full Text Available A numerical study of steady state laminar forced and free convective and radiative heat transfer in an inclined rotating rectangular duct with a centered circular tube is reported for an hydrodynamically fully developed flow. The two heat transfer mechanisms of convection and radiation are treated i [...] ndependently and simultaneously. The coupled equations of momentum and energy transports are solved using Gauss-Seidel iteration technique subject to given boundary constraints. A thermal boundary condition of uniform wall temperature in the flow direction is considered. A special discritization method is employed to solve the problem associated with near boundary grid points. Results for mean and total mean Nusselt numbers for various values of Reynolds number ,Re; Rayleigh number , Ra ; Geometric ratio ,r g ; Aspect ratio, rA ; Radiation-Conduction parameter , . ; Optical thickness, J ; Rotational Reynolds number ,Ro and Emissivity, epsilon ; are presented. For the range of parameters considered, results show that radiation and rotation enhance heat transfer. It is also indicated in the results that heat transfer from the surface of the circle exceeds that of the rectangle. Optimum heat transfer and fluid bulk temperature are attained when the duct is vertically positioned. The Parameter ranges of 0.2 # r g # 0.84, 0 # PeRa # 7.3 x 10(5) and r g rA # 1 demarcate the extent of the validity of the numerical solution.
International Nuclear Information System (INIS)
Coupled buoyancy and thermo-capillary convection lead to a convective motion of the interface liquid/gas, which changes drastically the heat and mass transfer across the liquid layer. Two experiments are 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 looking like petals or rays appears when the aspect ratio length/depth is small, and like concentric rings in the case of large values of the aspect ratio. The lateral confinement selects the azimuthal length wave. 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 * 105). In the case of mercury, the thermo-capillary effect is reduced to zero, due to impurities at the surface, which have special trajectories we describe and compare to a simpler experiment. The only buoyancy forces induces an un-stationary, weakly turbulent flow as soon as the heating power exceeds 4 W (? 4.5 * 103, calculated with h = 1 mm). The last part concerns the analysis of the effect on the flow of the boundary conditions, the geometry, the Prandtl number, the buoyancy force, with the help of the literature. Results concerning heat transfer, especially the exponent of the law Nusselt number vs. heating power, are compared with available data. (author)
Natural convection in water along a vertical plate with constant surface heat flux
International Nuclear Information System (INIS)
An experimental investigation of natural convection in water along a vertical plate with constant surface heat flux is described. The results show the effects of the variation of the physical properties on heat transfert especially in the transition regime. In this regime, all the exprimental datas are correlated by a single law which includes the variation of viscosity and the critical Rayleigh number
A LABORATORY SIMULATION OF TURBULENT CONVECTION OVER AN URBAN HEAT ISLAND
A systematic experimental study of the heat-island-induced circulation under turbulent conditions was conducted in the laboratory for an idealized, circular heat isalnd in an initially thermally stratified fluid (water) in a convection tank with no ambient flow. he primary object...
Heat output to the medium of supercritical parameters with mixed convection in horizontal tubes
Energy Technology Data Exchange (ETDEWEB)
Robakidze, L.V.; Khasanov-Agayev, L.R.; Miropolskiy, Z.L.
1983-01-01
Results are presented of experimental study of heat emission in a horizontal tube with supply of heat (1 approx. = 4,000 mm, d = 16 mm) with flow of carbon dioxide of supercritical pressure under conditions of mixed convection in the region of significant influence of the thermal-gravitation effects.
Convective Heat Transfer Augmentation by Flexible fins in Laminar Channel Pulsating flow
Joshi, Rakshitha U; Bhardwaj, Rajneesh
2015-01-01
Fluid-structure interaction (FSI) of thin flexible fins coupled with convective heat transfer has applications in energy harvesting and in understanding functioning of several biological systems. We numerically investigate FSI of the thin flexible fins involving large-scale flow-induced deformation as a potential heat transfer enhancement technique. An in-house, strongly-coupled fluid-structure interaction (FSI) solver is employed in which flow and structure solvers are based on sharp-interface immersed boundary and finite element method, respectively. We consider twin flexible fins in a heated channel with laminar pulsating cross flow. The vortex ring past the fin sweep higher sources of vorticity generated on the channel walls out into the downstream - promoting the mixing of the fluid. The moving fin assists in convective mixing, augmenting convection in bulk and at the walls; and thereby reducing thermal boundary layer thickness and improving heat transfer at the channel walls. The thermal augmentation is...
Wood, Toby S; Stellmach, Stephan
2012-01-01
Regions of stellar and planetary interiors that are unstable according to the Schwarzschild criterion, but stable according to the Ledoux criterion, are subject to a form of oscillatory double-diffusive (ODD) convection often called "semi-convection". In this series of papers, we use an extensive suite of three-dimensional (3D) numerical simulations to quantify the transport of heat and composition by ODD convection, and ultimately propose a new 1D prescription that can be used in stellar and planetary structure and evolution models. The first paper in this series demonstrated that under certain conditions ODD convection spontaneously transitions from an initially homogeneously turbulent state into a staircase of convective layers, which results in a substantial increase in the transport of heat and composition. Here, we present simulations of ODD convection in this layered regime, we describe the dynamical behavior of the layers, and we derive empirical scaling laws for the transport through layered convecti...
International Nuclear Information System (INIS)
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 ?, Da?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
El-Amin, Mohamed
2013-01-01
In this paper, the effects of thermal dispersion and variable viscosity on the non-Darcy free, mixed, and forced convection heat transfer along a vertical flat plate embedded in a fluid-saturated porous medium are investigated. Forchheimer extension is employed in the flow equation to express the non-Darcy model. The fluid viscosity varies as an inverse linear function of temperature. The coefficient of thermal diffusivity has been assumed to be the sum of the molecular diffusivity and the dynamic diffusivity due to mechanical dispersion. Similarity solutions of the governing equations, for an isothermally heated plate, are obtained. Effects of the physical parameters, which govern the problem, on the rate of heat transfer in terms of Nusselt number, the slip velocity, and the boundary layer thickness, for the two cases Darcy and non-Darcy, are shown on graphs or entered in tables. © 2013 by Begell House, Inc.
Specialists' meeting on evaluation of decay heat removal by natural convection
International Nuclear Information System (INIS)
Decay heat removal by natural convection (DHRNC) is essential to enhancing the safety of liquid metal fast reactors (LMFRs). Various design concepts related to DHRNC have been proposed and experimental and analytical studies have been carried out in a number of countries. The purpose of this Specialists' Meeting on 'Decay Heat Removal by Natural Convection' organized by the International Working Group on Fast Reactors IAEA, is to exchange information about the state of the art related to methodologies on evaluation of DHRNC features (experimental studies and code developments) and to discuss problems which need to be solved in order to evaluate DHRNC properly and reasonably. The following main topical areas were discussed by delegates: Overview; Experimental studies and code validation; Design study. Two main DHR systems for LMFR are under consideration: (i) direct reactor auxiliary cooling system (DRACS) with immersed DFIX in main vessel, intermediate sodium loop and sodium-air heat exchanger; and (ii) auxiliary cooling system which removes heat from the outside surface of the reactor vessel by natural convection of air (RVACS). The practicality and economic viability of the use of RVACS is possible up to a modular type reactor or a middle size reactor based on current technology. For the large monolithic plant concepts DRACS is preferable. The existing experimental results and the codes show encouraging results so that the decay heat removal by pure natural convection is feasible. Concerning the objective, 'passive safety', the DHR by pure natural convection is essential feature to enhance the reliability of DHR
Numerical simulation of turbulent forced convection in liquid metals
Vodret, S.; Vitale Di Maio, D.; Caruso, G.
2014-11-01
In the frame of the future generation of nuclear reactors, liquid metals are foreseen to be used as a primary coolant. Liquid metals are characterized by a very low Prandtl number due to their very high heat diffusivity. As such, they do not meet the so-called Reynolds analogy which assumes a complete similarity between the momentum and the thermal boundary layers via the use of the turbulent Prandtl number. Particularly, in the case of industrial fluid-dynamic calculations where a resolved computation near walls could be extremely time consuming and could need very large computational resources, the use of the classical wall function approach could lead to an inaccurate description of the temperature profile close to the wall. The first aim of the present study is to investigate the ability of a well- established commercial code (ANSYS FLUENT v.14) to deal with this issue, validating a suitable expression for the turbulent Prandtl number. Moreover, a thermal wall-function developed at Universite Catholique de Louvain has been implemented in FLUENT and validated, overcoming the limits of the solver to define it directly. Both the resolved and unresolved approaches have been carried out for a channel flow case and assessed against available direct numerical and large eddy simulations. A comparison between the numerically evaluated Nusselt number and the main correlations available in the literature has been also carried out. Finally, an application of the proposed methodology to a typical sub-channel case has been performed, comparing the results with literature correlations for tube banks.
Heat transfers in a double-skin roof ventilated by natural convection in summer time
Biwole, Pascal; Woloszyn, Monika; Pompeo, C.
2013-01-01
The double-skin roofs investigated in this paper are formed by adding a metallic screen on an existing sheet metal roof. The system enhances passive cooling of dwellings and can help diminishing power costs for air conditioning in summer or in tropical and arid countries. In this work, radiation, convection and conduction heat transfers are investigated. Depending on its surface properties, the screen reflects a large amount of oncoming solar radiation. Natural convection in...
NUMERICAL SIMULATION OF LAMINAR FORCED CONVECTION AIR FLOW IN A RECTANGULAR VENTURI CHANNEL
IGO SERGE WENDSIDA; KOKOU N'WUITCHA; ISSAKA OUEDRAOGO; DIEUDONNE JOSEPH BATHIEBO; BELKACEM ZEGHMATI
2014-01-01
In this work, a laminar forced convection air flow in a rectangular vertical venturi has been numerically simulated. Mathematical transformation has been used to transform the irregular profile of the venturi wall into straight line. Transfers equations are solved using finite volume method, Gauss and Thomas algorithms. A computing algorithm has been generated for the problem simulation. Hydrodynamics and thermals effects are investigated in detail. Results are presented as velocity and strea...
'Butterfly effect' in porous Bénard convection heated from below
International Nuclear Information System (INIS)
Transition from steady to chaos for the onset of Bénard convection in porous medium was analyzed. The governing equation is reduced to ordinary differential equation and solved using built in MATLAB ODE45. The transition from steady to chaos take over from a limit cycle followed by homoclinic explosion
Second law analysis of laminar forced convection in a rotating curved duct
Directory of Open Access Journals (Sweden)
Razavi Esmail Seyed
2015-01-01
Full Text Available In this paper, flow characteristics, heat transfer and entropy generation in a rotating curved duct are studied numerically. The continuity, Navier-Stokes and energy equations are solved using control volume method. The effects of Dean number, non-dimensional wall heat flux, and force ratio (the ratio of Coriolis to centrifugal forces on the entropy generation due to friction and heat transfer irreversibility and also overall entropy generation are presented. Optimal thermal operating conditions (based on dimensionless parameters are determined from the viewpoint of thermodynamics second law. The comparison of numerical results at different force ratios indicates that for any fixed Dean number or non-dimensional heat flux, the minimal frictional entropy generation occurs when the Coriolis and centrifugal forces have the same value but in the opposite direction. For a specific non-dimensional heat flux, there is a force ratio with maximum heat transfer irreversibility which depends on Dean number. Based on optimal analysis, the optimal force ratio with minimal total entropy generation depends on heat flux and Dean number.
Numerical studies of convective heat transfer in an inclined semiannular enclosure
Wang, Lin-Wen; Yung, Chain-Nan; Chai, An-Ti; Rashidnia, Nasser
1989-01-01
Natural convection heat transfer in a two-dimensional differentially heated semiannular enclosure is studied. The enclosure is isothermally heated and cooled at the inner and outer walls, respectively. A commercial software based on the SIMPLER algorithm was used to simulate the velocity and temperature profiles. Various parameters that affect the momentum and heat transfer processes were examined. These parameters include the Rayleigh number, Prandtl number, radius ratio, and the angle of inclination. A flow regime extending from conduction-dominated to convection-dominated flow was examined. The computed results of heat transfer are presented as a function of flow parameter and geometric factors. It is found that the heat transfer rate attains a minimum when the enclosure is tilted about +50 deg with respect to the gravitational direction.
Numerical studies of convective heat transfer in an inclined semi-annular enclosure
Wang, L.-W.; Chai, A.-T.; Yung, C.-N.; Rashidnia, N.
1989-01-01
Natural convection heat transfer in a two-dimensional differentially heated semiannular enclosure is studied. The enclosure is isothermally heated and cooled at the inner and outer walls, respectively. A commercial software based on the SIMPLER algorithm was used to simulate the velocity and temperature profiles. Various parameters that affect the momentum and heat transfer processes were examined. These parameters include the Rayleigh number, Prandtl number, radius ratio, and the angle of inclination. A flow regime extending from conduction-dominated to convection-dominated flow was examined. The computed results of heat transfer are presented as a function of flow parameter and geometric factors. It is found that the heat transfer rate attains a minimum when the enclosure is tilted about +50 deg with respect to the gravitational direction.
Energy Technology Data Exchange (ETDEWEB)
Bose, Sumantra; Palo, Daniel R.; Paul, Brian
2007-07-24
Diffusion bonding cycle times can be a large cost factor in the production of metal microchannel devices. The challenge is to significantly minimize this cost by reducing the bonding cycle time through rapid and uniform heating and cooling within the bonding process. Heating rates in diffusion bonding processes are typically limited by the need to minimize thermal gradients during bonding. A novel method is described which takes advantage of the internal flow passages within microchannel devices for convective heat transfer during the bonding process. The internal convective heating (ICH) technique makes use of heated inert gas to provide the microchannel assembly with rapid and uniform heat input. This paper will demonstrate the ability to effectively diffusion bond microchannel laminae using the ICH method by investigating the leakage rates.
Indian Academy of Sciences (India)
Ke Wu; Le Wang; Yi-Bo Yu; Zhi-Yi Huang; Pei Liang
2013-12-01
Heat dissipation enhancement of LED luminaries is of great significance to the large-scale application of LED. Luminaries-level structure improvement by the method of boring through-hole is adopted to intensify heat dissipation. Furthermore, the natural convection heat transfer process of LED luminaries is simulated by computational fluid dynamics (CFD) model before and after the structural modification. As shown by computational results, boring through-hole is beneficial to develop bottomto-top natural convection, eliminate local circumfluence, and finally form better flow pattern. Analysis based on field synergy principle shows that boring through-hole across LED luminaries improves the synergy between flow field and temperature field, and effectively decreases the thermal resistance of luminaries-level heat dissipation structure. Under the same computational conditions, by luminaries-level structure improvement the highest temperature of heat sink is decreased by about 8° C and the average heat transfer coefficient is increased by 45.8%.
International Nuclear Information System (INIS)
A boundary condition dissection method is developed for the solution of boundary value heat conduction problems with position-varying convective coefficients imposed at the boundaries. The method is postulated on the fact that a condition imposed on the boundary of a heat conduction problem may be realized in practice by using conditions that are not of the imposed kind. Thus, a Robin condition with position varying convective coefficient can be dissected as a linear combination of boundary heat flux and temperature, and either of these conditions can be used in place of the Robin condition in formulating an auxiliary problem. This auxiliary problem can be solved for temperature, and finally this temperature is forced to satisfy the actual Robin conditions imposed at the boundaries. Three basic problems are solved in this paper, and by means of two examples the boundary condition dissection method is shown to yield temperature results that are more accurate than those by the finite difference method, which has been conventionally used in solving the heat conduction problems. (orig.)
Local Convective Boiling Heat Transfer And Pressure Drop Of Nanofluid In Narrow Rectangular Channels
Boudouh, Mounir; Gualous, Hasna Louahlia; De Labachelerie, Michel
2010-01-01
Abstract This paper reports an experimental study on convective boiling heat transfer of nanofluids and de-ionized water flowing in a multichannels. The test copper plate contains 50 parallel rectangular minichannels of hydraulic diameter 800 ?m. Experiments were performed to characterize the local heat transfer coefficients and surface temperature using copper-water nanofluids with very small nanoparticles concentration. Axial distribution of local heat transfer is estimated using...
International Nuclear Information System (INIS)
Using perturbations in electron density and temperature induced by small Helium gas puffs in TFTR (Tokamak Fusion Test Reactor) the dominance of convective heat transport in the core (r/a < 0.4) of supershot plasmas has been demonstrated in a new way. The TRANSP transport code was used to calculate the time-dependent particle and heat fluxes. Perturbations in the calculated convective and total electron heat fluxes were compared. They demonstrate that the conductive component decreases moving into the supershot core, and the convective component dominates in the supershot core. These results suggest a different transport drive in the supershot core compared to that in the rest of the supershot plasma
Energy Technology Data Exchange (ETDEWEB)
Kissick, M.W.; Efthimion, P.C.; Mansfield, D.K.; Callen, J.D.; Bush, C.E.; Park, H.K.; Schivell, J.; Synakowski, E.J.; Taylor, G.
1993-08-01
Using perturbations in electron density and temperature induced by small Helium gas puffs in TFTR (Tokamak Fusion Test Reactor) the dominance of convective heat transport in the core (r/a < 0.4) of supershot plasmas has been demonstrated in a new way. The TRANSP transport code was used to calculate the time-dependent particle and heat fluxes. Perturbations in the calculated convective and total electron heat fluxes were compared. They demonstrate that the conductive component decreases moving into the supershot core, and the convective component dominates in the supershot core. These results suggest a different transport drive in the supershot core compared to that in the rest of the supershot plasma.
Kissick, M. W.; Efthimion, P. C.; Mansfield, D. K.; Callen, J. D.; Bush, C. E.; Park, H. K.; Schivell, J.; Synakowski, E. J.; Taylor, G.
1993-10-01
Using perturbations in electron density and temperature induced by small helium gas puffs in TFTR (Tokamak Fusion Test Reactor) [Plasma Phys. Controlled Nucl. Fusion Res. 1, 51 (1986)], the dominance of convective heat transport in the core (r/a<0.4) of supershot plasmas has been demonstrated in a new way. The transp [J. Comput. Phys. 43, 61 (1981)] transport code was used to calculate the time-dependent particle and heat fluxes. Perturbations in the calculated convective and total electron heat fluxes were compared. They demonstrate that the conductive component decreases moving into the supershot core, and the convective component dominates in the supershot core. These results suggest a different transport drive in the supershot core compared to that in the rest of the supershot plasma.
International Nuclear Information System (INIS)
For the long operation of secondary passive cooling system, however, water level goes down by evaporation in succession at emergency cooling tank. At the end there would be no place to dissipate heat from condensation heat exchanger. Therefore, steam cooling heat exchanger is put on the top of emergency cooling tank to maintain appropriate water level by collecting evaporating steam. Steam cooling heat exchanger is installed inside an air chimney and evaporated steam is cooled down by air natural convection. In this study, thermal sizing of steam cooling heat exchanger under air natural convection was conducted by TSCON program for the design of experimental setup as shown in Fig. 2. Thermal sizing of steam cooling heat exchanger tube under air natural convection was conducted by TSCON program for the design of experimental setup. 25 - 1' tubes which has a length 1687 mm was determined as steam cooling heat exchanger at 2 kW heat load and 100 liter water pool in emergency cooling tank (experimental limit condition). The corresponding width of two tubes is 50 mm and has 5 by 5 tube array for heat exchanger
DEFF Research Database (Denmark)
Le Dreau, Jerome; Heiselberg, Per
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 models for convection. In a full-scale test room, the heat transfer was investigated during 12 h of discharge by night-time ventilation. A total of 34 experiments have been performed, with different ventilation types (mixing and displacement), air change rates, temperature differences between the inlet 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. For mixing ventilation, existing correlations did not predict accurately the convective heat transfer at the ceiling due to differences in the experimental conditions. But the use of local parameters of the air flow showed interesting results to obtain more adaptive CHTC correlations. For displacement 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.
International Nuclear Information System (INIS)
An experimental study was performed to obtain local fluid velocity and temperature measurements in the mixed (combined free and forced) convection regime for specific flow coastdown transients in an electrically heated 2*6 rod bundle cooled by water. The objective of the study was to develop an understanding of the thermal-hydraulic phenomena at low flows in a rod bundle subjected to lateral power skews and to develop a transient data base for evaluating themal-hydraulic computer programs. Test conditions included 150 second and 45 second flow coastdown transients, a Reynolds number range of 300 to 1300, and power skews of 1:0 and 2:1. A model incorporating the enhanced turbulent mixing due to thermal plume interactions was used to improve the COBRA-WC code predictions at large ratios of Gr/Re/sup 2/. A preliminary criterion was developed for the inception of recirculation within the bundle. 10 refs
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.
Planform structure and heat transfer in turbulent free convection over horizontal surfaces
Theerthan, S. Ananda; Arakeri, Jaywant H.
2000-04-01
This paper deals with turbulent free convection in a horizontal fluid layer above a heated surface. Experiments have been carried out on a heated surface to obtain and analyze the planform structure and the heat transfer under different conditions. Water is the working fluid and the range of flux Rayleigh numbers (Ra) covered is 3×107-2×1010. The different conditions correspond to Rayleigh-Bénard convection, convection with either the top water surface open to atmosphere or covered with an insulating plate, and with an imposed external flow on the heated boundary. Without the external flow the planform is one of randomly oriented line plumes. At large Rayleigh number Ra and small aspect ratio (AR), these line plumes seem to align along the diagonal, presumably due to a large scale flow. The side views show inclined dyelines, again indicating a large scale flow. When the external flow is imposed, the line plumes clearly align in the direction of external flow. The nondimensional average plume spacing, Ra?1/3, varies between 40 and 90. The heat transfer rate, for all the experiments conducted, represented as Ra?T-1/3, where ?T is the conduction layer thickness, varies only between 0.1-0.2, showing that in turbulent convection the heat transfer rates are similar under the different conditions.
Roberts, J. H.; Nimmo, F.
2007-12-01
Rapid strike-slip motion is predicted to be a consequence of diurnal tidal stresses in most satellites of the outer solar system with short orbital timescales [1]. Such motion can lead to near-surface heating through friction or viscous dissipation [2]. Here we discuss the effect of near-surface shear heating on convection in the underlying ice shells of icy satellites [3], with a focus on Enceladus and a possible origin of the south polar thermal anomaly [4]. We present models of convection in spherical ice shells including both spatially variable volumetric tidal heating [5] and regional shear heating localized in the top 5 km at either the pole or the equator. We observe that the presence of the near-surface heating strongly controls the convective pattern, increasing the wavelength, and promoting the formation of a hot upwelling beneath the shear zone. Our results suggest that localized near- surface heating may result in a degree-1 convective planform in an ice shell of a thickness that may be appropriate for a differentiated Enceladus (d Tobie, G., A. Mocquet, C. Sotin, Icarus, 177 534-549. [6] Nimmo, F., R. T. Pappalardo, Nature, 441, 614-616.
Experimental study of cooling BIPV modules by forced convection in the air channel
International Nuclear Information System (INIS)
Highlights: • An experimental setup for studying the effects of forced convection on cell temperature. • The induced velocity within the forced convection channel significantly affects the PV cooling. • Correlations for the Ross coefficient, module temperature, efficiency, and power output. • Prediction of the thermal behavior of the PV module in BIPV configurations. - Abstract: The efficiency of photovoltaic systems depends mainly on the cell temperature. Frequently, the PV collectors are installed on the top of the building. One cost effective method to regulate the temperature of rooftop integrated photovoltaic panels is to provide an open air channel beneath the panel. The cell temperature of these PV modules is very much influenced by the capability of ventilating this channel. The ventilation may be modified by different factors such as the wind velocity, the air gap size, and the forced convection induced by a fan or by a conventional air conditioning system. This paper describes an experimental setup to study the influence of the air gap size and the forced ventilation on the cell temperature (and consequently on the electrical efficiency of the PV module) of a BIPV configuration, for different values of the incident solar radiation, ambient temperatures, and aspect ratios, as well as for several forced ventilation conditions. Semi empirical correlations for the Ross coefficient, module temperature, electrical efficiency, and power output are proposed, showing a good agreement with respect to experimental measurements. A critical channel aspect ratio close to 0.11 can be considered to minimize overheating of PV devices. For a duct velocity Vv = 6 m/s, a power output increase of 19% is observed over the natural ventilation case (Vv = 0.5 m/s)
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.
AN INTERNAL CONVECTIVE HEATING TECHNIQUE FOR DIFFUSION BONDING ARRAYED MICROCHANNEL ARCHITECTURES
Energy Technology Data Exchange (ETDEWEB)
Paul, Brian; Bose, Sumantra; Palo, Daniel R.
2010-01-08
Diffusion bonding cycle times can be a large factor in the production cost of metal microchannel devices. The challenge is to significantly minimize bonding cycle times through rapid heating and cooling within the bonding process. A novel method is described which takes advantage of the internal flow passages within microchannel devices for convective heat transfer during the bonding process. The internal convective heating (ICH) technique makes use of heated inert gas to provide the microchannel assembly with rapid and uniform heat input. Results demonstrate that the ICH technique is feasible, capable of producing microchannels with higher dimensional integrity and shorter bonding cycle times than traditional vacuum hot press methods. Results suggest that this may be due to smaller thermal gradients within microchannel devices during the ICH bonding cycle.
Forced convection to laminar flow of liquid egg yolk in circular and annular ducts
Scientific Electronic Library Online (English)
M., Bernardi; V., Silveira Jr.; V. R. N., Telis; A. L., Gabas; J., Telis-Romero.
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 secti [...] ons 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.
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.
Chakraborty, Swastika; Saha, Upal; Maitra, Animesh
2015-07-01
The response of atmospheric heat fluxes and sea surface temperatures on the convective precipitation over the tropics has been an important area of research in recent decades. A long-term observation (1979-2008) of the increase in convective precipitation in relation to the latent and sensible heat fluxes on a tropical location, Kolkata, has been investigated in the present study. Invigoration of convective precipitation has been caused by vertically integrated divergence of moisture flux, rise in sea surface temperatures, convective cloud cover and surface evaporation rate over the tropical region. A convective precipitation estimation (CPE) index is proposed, considering the Bowen ratio, surface evaporation rate, sea surface temperature and temperatures at 500 hpa pressure level during the pre-monsoon season (March-May), to estimate the amount of convective precipitation over the tropics using multiple linear regression technique is also another aim of this study. A good agreement is obtained between the results from the proposed model and the MERRA observations during the years 2009-2013.
Wåhlin, A. K.; Johansson, A. M.; Aas, E.; Broström, G.; Weber, J. E. H.; Grue, J.
2010-03-01
An experimental study of horizontal convection with a free surface has been conducted. Fresh water was heated from above by an infrared lamp placed at one end of a tank, and cooled by evaporation as the water moved away from the heat source. The heat radiated from the lamp was absorbed in a thin (less than 1 mm) layer next to the surface, and then advected and diffused away from the lamp region. Latent heat loss dominated the surface cooling processes and accounted for at least 80% of the energy loss. The velocity and temperature fields were recorded with PIV technology, thermometers and an infrared camera. In similarity with previous horizontal convection experiments the measurements showed a closed circulation with a gradually cooling surface current moving away from the lamp. Below the surface current the water was stably stratified with a comparatively thick and slow return current. The thickness and speed, and hence the mass transport, of the surface- and the return current increased with distance from the lamp. The latent cooling at the free surface gives a heat flux which increases with the temperature difference between the surface water and the air above it. Hence the surface temperature relaxes towards an equilibrium value, for which the heat flux is zero. The main new result is a scaling law, taking into account this relaxation boundary condition for the surface temperature. The new scaling includes a (relaxation) length scale for the surface temperature, equivalent to the distance the surface current travels before it has lost the heat that was gained underneath the lamp. The length scale increases with the forcing strength and the (molecular) thermal diffusivity but decreases with the strength of the relaxation. Numerical simulations of this problem for a shallow tank have also been performed. The velocity and temperature in the laboratory and numerical experiments agree with the scaling laws in the upper part of the tank, but not in the lower.
DEFF Research Database (Denmark)
Hosseini, R.; Kolaei, Alireza Rezania; Alipour, M.; Rosendahl, Lasse
2012-01-01
In this work, the natural convection heat transfer from a long vertical electrically heated cylinder to an adjacent air gap is experimentally studied. The aspect and diameter ratios of the cylinder are 55.56 and 6.33, respectively. The experimental measurements were obtained for a concentric condition and six eccentricities from 0.1 to 0.92 at five different heat fluxes. The surface temperature of the heated rod is measured at different heights, and the Nusselt number is calculated at the temper...
Analysis of a convection loop for GFR post-LOCA decay heat removal
International Nuclear Information System (INIS)
A computer code (LOCA-COLA) has been developed at MIT for steady state analysis of convective heat transfer loops. In this work, it is used to investigate an external convection loop for decay heat removal of a post-LOCA gas-cooled fast reactor (GFR). The major finding is that natural circulation cooling of the GFR is feasible under certain circumstances. Both helium and CO2 cooled system components are found to operate in the mixed convection regime, the effects of which are noticeable as heat transfer enhancement or degradation. It is found that CO2 outdoes helium under identical natural circulation conditions. Decay heat removal is found to have a quadratic dependence on pressure in the laminar flow regime and linear dependence in the turbulent flow regime. Other parametric studies have been performed as well. In conclusion, convection cooling loops are a credible means for GFR decay heat removal and LOCA-COLA is an effective tool for steady state analysis of cooling loops. (authors)
International Nuclear Information System (INIS)
The main objectives of this thesis are the direct numerical simulation of natural convection in a vertical differentially heated slot and the improvements of second-order turbulence modelling. A three-dimensional direct numerical simulation code has been developed in order to gain a better understanding of turbulence properties in natural convection flows. This code has been validated in several physical configurations: non-stratified natural convection flows (conduction solution), stratified natural convection flows (double boundary layer solution), transitional and turbulent Poiseuille flows. For the conduction solution, the turbulent regime was reached at a Rayleigh number of 1*105 and 5.4*105. A detailed analysis of these results has revealed the principal qualities of the available models but has also pointed our their shortcomings. This data base has been used in order to improve the triple correlations transport models and to select the turbulent time scales suitable for such flows. (author). 122 refs., figs., tabs., 4 appends
Determining convective heat transfer coefficient using phoenics software package
Energy Technology Data Exchange (ETDEWEB)
Kostikov, A.; Matsevity, Y. [Institute of Mechanical Engineering Problems of National Academy of Sciences of Ukraine, Kharkov (Ukraine)
1997-12-31
The two methods of determination of such important quantity of heat exchange on a body surface using PHOENICS are suggested in the presentation. The first method consists in a post-processing of results of conjugate heat transfer problem solved by PHOENICS. The second one is solving an inverse heat conduction problem for solid body using PHOENICS. Comparative characteristic of these two methods is represented. (author) 4 refs.
Energy Technology Data Exchange (ETDEWEB)
Liang Zhao; Liejin Guo; Bofeng Bai; Yucheng Hou; Ximin Zhang [Xi' an Jiaotong Univ. (China). State Key Lab. of Multiphase Flow in Power Engineering
2003-12-01
The pressure drop and boiling heat transfer characteristics of steam-water two-phase flow were studied in a small horizontal helically coiled tubing once-through steam generator. The generator was constructed of a 9-mm ID 1Cr18Ni9Ti stainless steel tube with 292-mm coil diameter and 30-mm pitch. Experiments were performed in a range of steam qualities up to 0.95, system pressure 0.5-3.5 MPa, mass flux 236-943 kg/m{sup 2}s and heat flux 0-900 kW/m{sup 2}. A new two-phase frictional pressure drop correlation was obtained from the experimental data using Chisholm's B-coefficient method. The boiling heat transfer was found to be dependent on both of mass flux and heat flux. This implies that both the nucleation mechanism and the convection mechanism have the same importance to forced convective boiling heat transfer in a small horizontal helically coiled tube over the full range of steam qualities (pre-critical heat flux qualities of 0.1-0.9), which is different from the situations in larger helically coiled tube where the convection mechanism dominates at qualities typically >0.1.Traditional single parameter Lockhart-Martinelli type correlations failed to satisfactorily correlate present experimental data, and in this paper a new flow boiling heat transfer correlation was proposed to better correlate the experimental data. (author)
DEFF Research Database (Denmark)
Bhattacharyya, S.; Singh, Ashok
2010-01-01
The influence of surface heating of a circular cylinder on the wake structure and heat transfer in the range of Reynolds number (Re) for which parallel vortex shedding occurs, is investigated numerically for different values of the buoyancy parameter, Gr. The role of buoyancy induced baroclinic vorticity on the wake formation is addressed in the present study. The variation of Strouhal number and Nusselt number with the 'effective Reynolds number', is analyzed for different values of cylinder to free stream temperature ratio. Both Strouhal number and the rate of heat transfer increases monotonically with the increase of the effective Reynolds number. The validity of the correlations, which have been established by several authors, between the effective Reynolds number and Strouhal/ Nusselt number for forced convection, is examined in the mixed convection regime. The curves between the effective Reynolds number and the computed data for Strouhal number and Nusselt number do not collapse for the range of temperature ratio considered here. The flow field is found to be asymmetric and the cylinder experiences a negative lift. The drag coefficient increases steadily with the rise of surface temperature. © 2010 Elsevier Ltd. All rights reserved.
Singh, P.; Queeny, A. M.; Sharma, R. N.
The effect of lateral mass flux on mixed convection heat and mass transfer in a saturated porous medium adjacent to an inclined permeable surface is analyzed. A similarity solution is obtained when surface temperature and concentration, free stream velocity and injection/suction velocity of fluid are prescribed as power functions of distance from the leading edge. The cases when the flow and buoyancy forces are in the same and opposite directions are discussed both for aiding and opposing buoyancy effects. The governing parameters are the mixed convection parameter Gr, the Lewis number Le, the buoyancy ratio N, the lateral mass flux parameter fw, representing the effects of injection or withdrawal of fluid at the wall, and ? which specifies three cases of the inclined plate. The interactive effect of these parameters on heat and mass transfer rates are presented. It is observed that the diffusion ratio (Le) has a more pronounced effect on concentration field than on flow and temperature fields. It is found that the rates of heat and mass transfer increase with suction and decrease with injection of the fluid.
International Nuclear Information System (INIS)
Experimental studies were made on burnout heat flux in highly subcooled forced-convection boiling of water for the design of beam dumps of a high power neutral beam injector for Japan Atomic Energy Research Institute Tokamak-60. These dumps are composed of many circular tubes with two longitudinal fins. The tube was irradiated with nonuniformly distributed hydrogen ion beams of 120 to 200 kW for as long as 10 s. The coolant water was circulated at flow velocities of 3 to 7.5 m/s at exit pressures of 0.4 to 0.9 MPa. The burnout and film-boiling data were obtained at local heat fluxes of 8 to 15 MW/m2. These values were as high as 2.5 times larger than those for the circumferentially uniform heat flux case with the same parameters. These data showed insensitivity to local subcooling as well as to pressure, and simple burnout correlations were derived. From these results, the beam dumps have been designed to receive energetic beam fluxes of as high as 5 MW/m2 with a margin of a factor of 2 for burnout
Dynamos driven by weak thermal convection and heterogeneous outer boundary heat flux
Sahoo, Swarandeep; Sreenivasan, Binod; Amit, Hagay
2016-01-01
We use numerical dynamo models with heterogeneous core-mantle boundary (CMB) heat flux to show that lower mantle lateral thermal variability may help support a dynamo under weak thermal convection. In our reference models with homogeneous CMB heat flux, convection is either marginally supercritical or absent, always below the threshold for dynamo onset. We find that lateral CMB heat flux variations organize the flow in the core into patterns that favour the growth of an early magnetic field. Heat flux patterns symmetric about the equator produce non-reversing magnetic fields, whereas anti-symmetric patterns produce polarity reversals. Our results may explain the existence of the geodynamo prior to inner core nucleation under a tight energy budget. Furthermore, in order to sustain a strong geomagnetic field, the lower mantle thermal distribution was likely dominantly symmetric about the equator.
Convective Heat Transfer over a Wall Mounted Cube Using Large Eddy Simulation
Directory of Open Access Journals (Sweden)
Habibollah Heidarzadeh
2012-06-01
Full Text Available Fluid flow and convective heat transfer over wall mounted cube have been studied numerically using Large Eddy Simulation. Surface of wall mounted cube and plane floor has a constant heat flux. Two subgrid scale models were used in this study; Wall-Adapting Eddy viscosity (WALE and Dynamic Smagorinsky (DS. The numerical results were compared with the experimental data of Nakamura et al [2] that showed DS model has better results toward WALE model. Results contain the plots of time averaged normalized streamwise velocity and Reynolds stress in different positions, Temperature contours, local Nusselt number over the surfaces of cube and some characteristics of flow field and heat transfer. The local convective heat transfer from the surfaces of the cube and plate are directly related to the complexity of flow field.
Heat Transfer of Forced Fluid Flow in a Channel with Parallel Fillisters
Directory of Open Access Journals (Sweden)
Hai-Ping Hu
2013-09-01
Full Text Available This study analyzes heat transfer and fluid dynamics in a forced convection laminar flow in a channel with parallel fillisters. The problem is solved by the point-matching method. The influence of the height and width of the fillisters on the thermal-fluid characteristics of a channel flow is discussed in the present research. The local dimensionless velocity, f Re values, local dimensionless temperature and mean Nusselt number of the fluid flow are all obtained for a channel flow under the influence of parallel fillisters.
Henkes, R. A. W. M.; Hoogendoorn, C. J.
1989-01-01
A numerical code for solving the boundary-layer equations is used to evaluate the performance of various turbulence models for the natural convection boundary layer along a heated vertical plate. The Cebeci-Smith (1974) model yields wall-heat transfer and turbulent viscosity values that are lower than the experimental values, while the standard k-epsilon model with wall functions for k and epsilon yields high wall-heat transfer values and resonable velocity and temperature profiles. Low-Reynolds-number k-epsilon models provide accurate wall-heat transfer results.
Energy Technology Data Exchange (ETDEWEB)
Aksenova, A.E.; Chudanov, V.V.; Strizhov, V.F.; Vabishchevich, P.N. [Institute of Nuclear Safety Russian Academy Science, Moscow (Russian Federation)
1995-09-01
Unsteady natural convection of a heat-generating fluid with phase transitions in the enclosures of a square section with isothermal rigid walls is investigated numerically for a wide range of dimensionless parameters. The quasisteady state solutions of conjugate heat and mass transfer problem are compared with available experimental results. Correlation relations for heat flux distributions at the domain boundaries depending on Rayleigh and Ostrogradskii numbers are obtained. It is shown that generally heat transfer is governed both by natural circulation and crust formation phenomena. Results of this paper may be used for analysis of experiments with prototypic core materials.
Comments on heat trasfer efficiency in cryogenic helium turbulent Rayleigh-Bénard convection.
Czech Academy of Sciences Publication Activity Database
Urban, Pavel; Musilová, V?ra; Králík, Tomáš; Skrbek, L.
2011-01-01
Ro?. 318, ?. 8 (2011), s. 082012. ISSN 1742-6588 R&D Projects: GA AV ?R KJB200650902 Institutional research plan: CEZ:AV0Z20650511 Keywords : Rayleigh-Benard convection * heat transfer efficiency Subject RIV: BK - Fluid Dynamics
Directory of Open Access Journals (Sweden)
Vohmin V. S.
2011-06-01
Full Text Available The influence of convective induction heating on methane producing bacteria vital functions at various temperature modes is studied. The intensive way of recycling of animal farms waste in a unified process at the same time with obtaining alternative energy is offered
Simulation of Free Convection from an Inclined HeatedThin Plate in a Square Enclosure
Nora M. Sahib; Ahmed N. Mehdy; Qahtan Abdul Zahra
2009-01-01
Simulation of free convection heat transfer in a square enclosure induced by heated thin plate is represented numerically. All the enclosure walls have constant temperature lower than the plate?s temperature. The flow is assumed to be two-dimensional. The discretized equations were solved stream function, vorticity, and energy equations by finite difference method using explicit technique and Successive Over- Relaxation method. The study was performed for different values of Rayleigh number r...
Thermal performance of a porus radial fin with natural convection and radiative heat losses
Directory of Open Access Journals (Sweden)
Darvishi M.T.
2015-01-01
Full Text Available An analytic (series solution is developed to describe the thermal performance of a porous radial fin with natural convection in the fluid saturating the fin and radiation heat loss from the top and bottom surfaces of the fin. The HAM results for the temperature distribution and base heat flux are compared with the direct numerical results and found to be very accurate.
CFD numerical simulation of air natural convection over a heated cylindrical surface
Flori, M.; Vîlceanu, L.
2015-06-01
In this study a CFD numerical simulation is used to describe the fluid flow and heat transfer in air surrounding a heated horizontal cylinder. The model is created in 2D space dimension involving a finite element solver of Navier-Stokes equations. As natural convection phenomenon is induced by a variable fluid density field with temperature rising, the Boussinesq approximation was coupled to the model.
Two-phase numerical model for thermal conductivity and convective heat transfer in nanofluids
Kondaraju Sasidhar; Lee Joon Sang
2011-01-01
Abstract Due to the numerous applications of nanofluids, investigating and understanding of thermophysical properties of nanofluids has currently become one of the core issues. Although numerous theoretical and numerical models have been developed by previous researchers to understand the mechanism of enhanced heat transfer in nanofluids; to the best of our knowledge these models were limited to the study of either thermal conductivity or convective heat transfer of nanofluids. We have develo...
Single phase laminar convective heat transfer of nanofluids in a micro-tube
Lumbreras Basagoiti, Itziar
2011-01-01
Nanofluids are homogeneous mixture of dispersed solid particles in base fluids. These solid particles are usually smaller than 100nm. Suspended nanoparticles modify the properties of based fluids. It is claimed, in some literature, for nanofluids to have greater than expected heat transfer performance. Due to this, nanofluids have gained great attention from both research and development and industries active in cooling systems. This thesis reports several measurements of convective heat tran...
International Nuclear Information System (INIS)
Convective heat transfer at subchannel in vertical cylinder arranged is very useful in many engineering application, include the design and operation of heat exchanger, steam generator and nuclear reactor safety. It is important to learn characteristic of fluid flow in subchannel before learn convective heat transfer in subchannel. In this research, theoretical study of flow characteristic in subchannel has been carried out by using CFD code. The subchannel is square arrangement and consist of nine cylinder heater with 2.54 cm diameter and P/D ratio of 1.5. For the inlet velocity are 0.01 m/s, 0.02 m/s and 0.03 m/s, the result of CFD analysis indicated that fully developed region is formed at 0.2 m below the reference axis. The velocity of coolant in the center of subchannel is faster than in the edge of subchannel. (author)
Wilson, G. R.
1994-01-01
We report here the results of modeling work aimed at understanding the development of ionospheric O(+) field-aligned upflows that develop in response to high-latitude E x B drift induced frictional heating. The model used is a collisional semikinetic model which includes ion-neutral resonant charge exchange and polarization collisions as well as Coulomb self-collisions. It also includes the process of chemical removal of O(+) as well as all of the macroscopic forces: ambipolar electric, gravity, magnetic mirror, and centripetal. Model results show the development of several types of non-Maxwellian velocity distributions including toroids at low altitude, distributions with large heat flow in the perpendicular component at intermediate altitudes, and distributions with a separate upflowing population or upward superthermal tail at high altitudes. Whenever the convection electric field increases from a small value (less than 25 mV/m) to a large value (100-200 mV/m) in 6 min or less large upflows develop with parallel drift speeds which peak (below 1000 km) at values between 500 m/s and 2 km/s, parallel fluxes which peak between 6.0 x 10(exp 8) and 3.2 x 10(exp 9)/sq cm/s, and parallel per particle heat flows which peak between 8.0 x 10(exp -9) and 8.0 x 10(exp -8) ergs cm/s. The higher values in these ranges occur for a cooler neutral atmosphere, with a larger convection electric field that is turned on quickly. The model produces field-aligned O(+) flow speeds that are larger than those produced by a 20-moment generalized transport model but smaller then those produced by an isotropic hydrodynamic model for comparable values of the convection turn on times. The model results compare favorably with some topside satellite and radar data.
Solar drying of whole mint plant under natural and forced convection
Sallam, Y.I.; M. H Aly; Nassar, A.F.; E. A. Mohamed
2013-01-01
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 dryi...
International Nuclear Information System (INIS)
Highlights: ? Tenoxicam electrochemical oxidation was studied from aqueous solution with a CPE. ? Both stagnant and forced convection conditions were considered. ? We found tenoxicam electrochemical oxidation is a mass transfer-controlled process. ? An EC mechanism was found where the electrodic and chemical kinetics are fast. ? It was found that in this case n = 2 and E1/2 = 0.770 V. ? Calculated D was 4.09 × 10?6 cm2 s?1 which compares with theoretically estimated. - Abstract: From potentiostatic current transients and voltammetry studies, carried out under both stagnant and forced convection conditions, the tenoxicam electrochemical behavior on a bare carbon paste rotating disk electrode was assessed in an aqueous solution (pH = 0.403). It was found that tenoxicam's electrochemical oxidation is a mass transfer-controlled process where a current peak is clearly formed at around 0.74 V when the potential scan was varied in the positive direction. However, when the potential was switched to the negative direction, up to the initial potential value, no reduction peak was formed. Tenoxicam's electrochemical oxidation follows an EC mechanism where the electrodic and chemical kinetics are fast. From sample-current voltammetry both the number of electrons, n, that tenoxicam losses during its electro-oxidation and its half-wave potential, E1/2, were determined to be 2 and 0.770 V vs. Ag/AgCl, respectively. Moreover, from differential pulse voltammetry plots it was confirmed that effectively in this case n = 2. Considering 2 electrons and both the Randles-Sevcik and Cotrell equations, the tenoxicam's diffusion coefficient, D, was determined to be (3.745 ± 0.077) × 10?6 and (4.116 ± 0.086) × 10?6 cm2 s?1, respectively. From linear sweep voltammetry plots recorded under forced convection conditions, it was found that Levich's equation describes adequately the limiting current recorded as a function of the electrode rotation rate, from where the D value was also found to be (4.396 ± 0.058) × 10?6 cm2 s?1. Therefore, the average D value was (4.09 ± 0.33) × 10?6 cm2 s?1. Furthermore, from the radius of the tenoxicam molecule, previously optimized at M052X/6-31 + G(d,p) level of theory, and using the Stokes–Einstein approach, D was also estimated to be 4.54 × 10?6 cm2 s?1 which is similar to the experimentally estimated values, under both stagnant and forced convection hydrodynamic conditions.
International Nuclear Information System (INIS)
Previous studies showed that, under free or forced convection, the application of a non-uniform electric field with a sufficient intensity at the neighbourhood of the heating elements resulted in a significant improvement of the thermal exchange coefficient under some flow rate and pressure conditions. But these improvements first required the creation of ions in the gas (shock ionization and existence of a corona effect above a sufficient voltage). This document therefore report further studies performed at the neighbourhood of the Melusine atomic pile which would provide ionizing radiations of much greater intensity. The objectives were, on the one hand, a global verification of work hypotheses imagined for the exchange improvement mechanism, and, on the other hand, to obtain data on the conditions under which this exchange improvement mechanism could be used in channels of a reactor. The authors present the instrumentation (measurement cells, gas circuit, heating circuit, high voltage circuit), describe how physical values are measured (pressure, gas flow rate, temperature of the heating element, gas temperature, high voltage). They describe how they compare powers exchanged under forced convection between dioxide carbon and a heated wire with or without external ionizing radiations, and report the various performed calculations (power supplied to the gas, temperature shift, Nusselt and Reynolds numbers). They report and discuss experimental results obtained outside the pile without ionizing radiations, and within the pile in presence of a ionizing radiation
Thermal modeling of the forced convection Sandwich Greenhouse drying system for rubber sheets
International Nuclear Information System (INIS)
Highlights: • Sandwich Greenhouse is designed for better quality and efficiency of rubber sheet drying. • Thermal models are developed to predict the convection heat transfer coefficient. • The models are validated and show good agreement with the actual experimental data. • The proposed greenhouse can maintain 40–60 °C, suitable for rubber sheet drying. • This greenhouse can bring down the moisture content to 2.8% in fewer than 2 days. - Abstract: In this paper, a novel “Sandwich Greenhouse” for rubber sheet drying is proposed. Using solar energy as the only heat source instead of traditional smoke house that requires firewood, it eliminates shortcomings such as skilled labor monitoring requirement, possible fire hazard, and darken-color rubber sheets due to soot particle contamination. Our greenhouse is specially designed to retain solar energy within, while minimizing the heat loss to the outside environment. The mathematical models are developed to predict the convection mass transfer coefficient and to study the thermal behavior during the drying of rubber sheets under our proposed greenhouse design. Validated with experimental observations, the models show good agreement with the actual experimental data. The experiment demonstrates an effectiveness of our proposed Sandwich Greenhouse, as the temperature of the rubber sheet is 15 °C and 5 °C higher than the ambient temperature during the daytime and nighttime, respectively. As a result, the moisture content of the rubber sheets can decrease from 36.4% to 2.8% in fewer than 2 days
Scientific Electronic Library Online (English)
A., Mahdy; R.A., Mohamed; F.M., Hady.
2009-12-01
Full Text Available The problem of combined heat and mass transfer in buoyancy-induced MHD natural convection flow of an electrically conducting fluid along a vertical wavy plate with power-law variation of both heat and mass flux was investigated. The resulting transformed governing equations are solved numerically by [...] an implicit finite-difference scheme. The results are presented for the major parameters including the wave amplitude a, the magnetic parameter Mn, the buoyancy ratio between species and thermal diffusion Br, the Lewis number Le, and the power-law parameter ?. A systematic study on the effects of the various parameters on flow, heat and mass transfer characteristics is carried out.
Rai, R.; Roy, G. G.; DebRoy, T.
2007-03-01
Computationally efficient heat transfer models of keyhole mode laser welding ignore fluid flow in the gas, liquid, and the two phase solid-liquid regions. These models cannot be applied to high Peclet number systems where convective heat transfer affects weld pool geometry, cooling rate, and other weld attributes. Here we show that by synthesizing features of an existing model to determine keyhole shape and size with rigorous fluid flow and heat transfer calculations in the liquid and the two phase solid-liquid regions, important features of both high and low Peclet number systems can be satisfactorily simulated. The geometry of the keyhole is calculated by assuming thermal equilibrium at the gas/liquid interface and point by point heat balance at the keyhole wall. The heat transfer outside the vapor cavity is calculated by numerically solving the equations of conservation of mass, momentum, and energy. A vorticity based turbulence model is used to estimate the values of effective viscosity and effective thermal conductivity of the liquid metal in the weld pool. It is shown that the temperature profile and the weld pool shape and size depend strongly on the convective heat transfer for low thermal conductivity alloys like stainless steel. For high thermal conductivity aluminum alloys, on the other hand, convection does not play a significant role in determining the shape and size of the weld pool. The computed solidification parameters indicated that the solidification structure becomes less dendritic and coarser with the decrease in welding velocity. The results demonstrate that a numerically efficient convective heat transfer model of keyhole mode laser welding can significantly improve the current understanding of weld attributes for different materials with widely different thermal properties.
Guo, L. J.; Feng, Z. P.; Chen, X. J.; Thomas, N. H.
1996-07-01
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 3.5 MPa. Three kinds of oscillation are reported: density waves; pressure drop excursions; 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.
Convection in a nematic liquid crystal with homeotropic alignment and heated from below
Energy Technology Data Exchange (ETDEWEB)
Ahlers, G. [Univ. of California, Santa Barbara, CA (United States)
1995-12-31
Experimental results for convection in a thin horizontal layer of a homeotropically aligned nematic liquid crystal heated from below and in a vertical magnetic field are presented. A subcritical Hopf bifurcation leads to the convecting state. There is quantitative agreement between the measured and the predicted bifurcation line as a function of magnetic field. The nonlinear state near the bifurcation is one of spatio-temporal chaos which seems to be the result of a zig-zag instability of the straight-roll state.
Natural Convection Heat Transfer in Concentric Horizontal Annuli Containing a Saturated Porous Medi
Directory of Open Access Journals (Sweden)
Ahmed F. Alfahaid, R.Y. Sakr
2012-10-01
Full Text Available Natural convection in horizontal annular porous media has become a subject receiving increasing attention due to its practical importance in the problem of insulators, such as ducting system in high temperature gas-cooled reactors, heating systems, thermal energy storage systems, under ground cable systems, etc. This paper presents a numerical study for steady state thermal convection in a fully saturated porous media bounded by two horizontal concentric cylinders, the cylinders are impermeable to fluid motion and maintained at different, uniform temperatures. The solution scheme is based on two-dimensional model, which is governed by Darcy-Oberbeck-Boussinesq equations. The finite element method using Galerkin technique is developed and employed to solve the present problem. A numerical simulation is carried out to examine the parametric effects of Rayleigh number and radius ratio on the role played by natural convection heat transfer in the porous annuli. The numerical results obtained from the present model were compared with the available published results and good agreement is observed. The average Nusselt number at the heating surface of the inner cylinder is correlated to Rayleigh number and radius ratio.Keywords: Natural convection, numerical investigation, saturated porous media, finite element method, concentric horizontal annuli.
Mixed convection in horizontal duct flow with transverse magnetic field and heating of side wall
Lv, Xinyuan; Zikanov, Oleg
2014-09-01
Computational study of mixed convection in a horizontal duct with very strong (Hartmann numbers Ha up to 800) horizontal transverse magnetic field and electrically insulated walls is conducted. Strong constant-rate heating (Grashof numbers Gr up to 109) is applied to one vertical wall, while the other walls are thermally insulated. It is found that in a wide range of Ha and Gr the flow is two-dimensional (uniform in the streamwise direction) and steady-state. No instability to convection rolls aligned with the magnetic field detected recently in a similar duct and pipe with bottom heating is observed. The velocity field is a combination of convection-modified streamwise flow and convection-induced circulation in the transverse plane. The circulation has strong jets near the heated and horizontal walls as prominent features. It is demonstrated that, at a constant Reynolds number, integral characteristics and spatial structure of the flow can be approximately considered as determined by Gr/Ha2 as a single parameter. We also present results of a systematic comparison study of several commonly used grid clustering schemes in their application to wall-bounded flows with strong magnetic fields.
Bibliography on augmentation of convective heat and mass transfer
Energy Technology Data Exchange (ETDEWEB)
Bergles, A.E.; Webb, R.L.; Junkhan, G.H.; Jensen, M.K.
1979-05-01
Heat transfer augmentation has developed into a major specialty area in heat transfer research and development. A bibliography of world literature on augmentation is presented. The literature is classified into passive augmentation techniques, which require no external power, and active techniques, which do require external power. The fourteen techniques are grouped in terms of their application to the various modes of heat transfer. Mass transfer is included for completeness. Key words are included with each citation for technique/mode identification. The total number of publications cited is 1,967, including 75 surveys of various techniques and 42 papers on performance evaluation of passive techniques. Patents are not included as they will be the subject of a future topical report.
Bibliography on augmentation of convective heat and mass transfer
International Nuclear Information System (INIS)
Heat transfer augmentation has developed into a major specialty area in heat transfer research and development. A bibliography of world literature on augmentation is presented. The literature is classified into passive augmentation techniques, which require no external power, and active techniques, which do require external power. The fourteen techniques are grouped in terms of their application to the various modes of heat transfer. Mass transfer is included for completeness. Key words are included with each citation for technique/mode identification. The total number of publications cited is 1,967, including 75 surveys of various techniques and 42 papers on performance evaluation of passive techniques. Patents are not included as they will be the subject of a future topical report
Turbulent convective heat transfer in an inclined tube filled with sodium
Vasil'ev, A. Yu.; Kolesnichenko, I. V.; Mamykin, A. D.; Frick, P. G.; Khalilov, R. I.; Rogozhkin, S. A.; Pakholkov, V. V.
2015-09-01
Turbulent free convection of liquid sodium in a straight thermally insulated tube with a length equal to 20 diameters and with end heat exchangers ensuring a fixed temperature drop is investigated experimentally. The experiments are performed for a fixed Rayleigh number Ra = 2.4 × 106 and various angles of inclination of the tube relative to the vertical. A strong dependence of the power transferred along the tube on the angle of inclination is revealed: the Nusselt number in the angular range under investigation changes by an order of magnitude with a maximum at the angle of 65° with the vertical. The characteristics of large-scale circulation and turbulent temperature pulsations show that convective heat transfer is mainly determined by the velocity of large-scale circulation of sodium. Turbulent pulsations are maximal for small angles of inclination (? = 20°-30°) and reduce the heat flux along the channel, although in the limit of small angles (vertical tube), there is no large-scale circulation, and the convective heat flux, which is an order of magnitude larger than the molecular heat flux, is ensured only by small scale (turbulent) flow.
Heat transfers in a double-skin roof ventilated by natural convection in summer time
Biwole, Pascal; Pompeo, C
2013-01-01
The double-skin roofs investigated in this paper are formed by adding a metallic screen on an existing sheet metal roof. The system enhances passive cooling of dwellings and can help diminishing power costs for air conditioning in summer or in tropical and arid countries. In this work, radiation, convection and conduction heat transfers are investigated. Depending on its surface properties, the screen reflects a large amount of oncoming solar radiation. Natural convection in the channel underneath drives off the residual heat. The bi-dimensional numerical simulation of the heat transfers through the double skin reveals the most important parameters for the system's efficiency. They are, by order of importance, the sheet metal surface emissivity, the screen internal and external surface emissivity, the insulation thickness and the inclination angle for a channel width over 6 cm. The influence of those parameters on Rayleigh and Nusselt numbers is also investigated. Temperature and air velocity profiles on seve...
Study on the natural convection heat transfer characteristics in the air duct
Energy Technology Data Exchange (ETDEWEB)
Kim, Y. K.; Lee, Y. B.; Choi, S. K.; Hwang, J. S.; Nam, H. Y. [Korea Atomic Energy Research Institute, Taejon (Korea, Republic of)
1997-12-31
Temperature distribution measurements in the mockup apparatus of reactor vessel were performed to determine the effective thermal conductivity of porous media with different geometry and to obtain the experimental data for the heat transfer processes by natural convection occurring in the air duct. The temperature distributions at four separated sections with different arrangements of porous media have different slopes according to the geometrical configuration. From the measured temperature distribution, effective thermal conductivity have been derived using the least square fitting method. The test at air duct was performed to the high heat removal at 3.4kW/m{sup 2} by the natural convection from the outer wall to the air. And also the temperature distributions in the sir duct agree well with the 1/7th power-law turbulent temperature distribution. The obtained heat transfer data have been compared with the Shin`s and Sieger`s correlations. 10 refs., 6 figs. (Author)
Study of natural convection heat transfer characteristics. (1) Influence of ventilation duct height
International Nuclear Information System (INIS)
Natural cooling system has been investigated in waste storage. It is important to evaluate the flow by natural draft enough to removal the decay heat from the waste. In this study, we carried out the fundamental experiment of ventilation duct height effect for natural convection on vertical cylindrical heater in atmospheric air. The scale of test facility is about 4m height with single heater. The heating value is varied in the range of 33-110W, where Rayleigh number is over 1010. Natural convection flow rate were calculated by measured velocity with thermo anemometer in the inlet duct. The temperature of the cylindrical heater wall and fluid were measured with thermocouples. It was found that the heat transfer coefficient difference between long duct and short duct is small in this experiment. (author)
International Nuclear Information System (INIS)
The ability to control melter convection patterns may provide a means of mitigating gas layer buildup under the cold cap, enhancing the heat transfer to the batch, and possible accelerating batch reactions, thereby increasing melt rate and glass throughput. Other operational benefits could result from such control. Convective patterns in an electrically heated melter are dominated by the distribution of Joule heat and thermal boundary conditions for a given melter design and geometry. We believe that control of electrical driving, in particular control of the interaction of electrical fields connected to distinct electrode pairs, can be used to vary the distribution of Joule heat generation. The under-investigated aspect of electrical driving control is the effect of waveform ''overlap'' of the driving voltages, the ''overlap'' in the case of harmonic driving being determined by the relative phase. For electrical driving using waveforms chopped by Silicon Controlled Rectifiers (SCRs), the chopping influences the ''overlap.'' This control can provide a means of controlling melt convection. The objective of the present investigation is to test that hypothesis, verifying that such control can be observed for a numerical model of a simple melter geometry
Modeling of heat transfer in cooling towers with natural convection.
Czech Academy of Sciences Publication Activity Database
Zuniga-Gonzalez, Israel; Maršík, František
Gdansk : IFFM Publishers, 2005 - (Mikielewicz, J.; Butrymowicz, D.; Trela, M.; Cie?li?ski, J.), s. 585-592 ISBN 83-88237-90-X. [ HEAT 2005 : International Conference on Transport Phenomena in Multiphase Systems. Gdansk (PL), 26.06.2005-30.06.2005] R&D Projects: GA ?R GA101/02/0364; GA ?R(CZ) GA101/05/2536 Institutional research plan: CEZ:AV0Z20760514 Keywords : cooling tower * heat transfer * evaporative cooling Subject RIV: BK - Fluid Dynamics
Khan, Masood; Malik, Rabia; Munir, Asif
2015-08-01
In this article, the mixed convective heat transfer to Sisko fluid over a radially stretching surface in the presence of convective boundary conditions is investigated. The viscous dissipation and thermal radiation effects are also taken into account. The suitable transformations are applied to convert the governing partial differential equations into a set of nonlinear coupled ordinary differential equations. The analytical solution of the governing problem is obtained by using the homotopy analysis method (HAM). Additionally, these analytical results are compared with the numerical results obtained by the shooting technique. The obtained results for the velocity and temperature are analyzed graphically for several physical parameters for the assisting and opposing flows. It is found that the effect of buoyancy parameter is more prominent in case of the assisting flow as compared to the opposing flow. Further, in tabular form the numerical values are given for the local skin friction coefficient and local Nusselt number. A remarkable agreement is noticed by comparing the present results with the results reported in the literature as a special case.
International Nuclear Information System (INIS)
This book contains the short papers from the International Symposium on convective heat and Mass Transfer in sustainable Energy ( conv-09), organized on behalf of the International Centre for Heat and Mass Transfer, it was held on April 26- 1st May, In Hammamet, Tunisia. The objective of this conference is to bring together researchers in a forum to exchange innovative ideas, methods and results, and visions of the future related to the general theme of convective heat and mass transfer
International Nuclear Information System (INIS)
This book contains the short papers from the International Symposium on Convective heat and Mass Transfer in sustainable Energy ( Conv-09), organized on behalf of the International Centre for Heat and Mass Transfer, it was held on April 26- 1st May, In Hammamet, Tunisia. The objective of this conference is to bring together researchers in a forum to exchange innovative ideas, methods and results, and visions of the future related to the general theme of convective heat and mass transfer
Models for fluid flows with heat transfer in mixed convection
International Nuclear Information System (INIS)
Second order models were studied in order to predict turbulent flows with heat transfer. The equations used correspond to the characteristic scale of turbulent flows. The order of magnitude of the terms of the equation is analyzed by using Reynolds and Peclet numbers. The two-equation model (K-?) is applied in the hydrodynamic study. Two models are developed for the heat transfer analysis: the Prt + teta 2 and the complete model. In the first model, the turbulent thermal diffusivity is calculated by using the Prandtl number for turbulent flow and an equation for the variance of the temperature fluctuation. The second model consists of three equations concerning: the turbulent heat flow, the variance of the temperature fluctuation and its dissipation ratio. The equations were validated by four experiments, which were characterized by the analysis of: the air flow after passing through a grid of constant average temperature and with temperature gradient, an axysymmetric air jet submitted to high and low heating temperature, the mixing (cold-hot) of two coaxial jets of sodium at high Peclet number. The complete model is shown to be the most suitable for the investigations presented
Natural convective performance of perforated heat sinks with circular pin fins
Wen, Mao-Yu; Yeh, Cheng-Hsiung
2015-10-01
This study examines heat transfer performance under natural convection for two different types (Type A and Type B) of pin-fin heat sinks with/without a hollow in the heated base. The effects of the rate of heat transfer, the height of fin and base plate, the heat sink porosity and the perforated base plate on the heat-transfer coefficient and enhancement factor were investigated and evaluated. In order to obtain insight into the fluid flow phenomena, flow visualization was also made to observe the detailed fluid flow characteristics of the present pin-fin heat sinks. Experimental results show that heat transfer, using pin-fin heat sinks, is significantly different for Type A and Type B. Comparisons of the heat transfer coefficients, with respect to the unfinned base plate under the same experimental conditions, for the pin-fin heat sinks were about 1.48-fold-1.64-fold (Type A) and 1.81-fold-1.94-fold (Type B), respectively. In addition, results also show that a Type B-heat sink has a higher enhancement factor than a Type A-heat sink.
DEFF Research Database (Denmark)
Steskens, Paul W. M. H.; Janssen, Hans
2013-01-01
Currently, researchers are striving to advance the possibilities to calculate the integrated phenomena of heat, air and moisture flows in buildings, with specific focus on the interactions between the building zones and building components. This paper presents an investigation of the capability and applicability of the sub-zonal airflow model to predict the local indoor environmental conditions, as well as the local surface transfer coefficients near building components. Two test cases were analyzed for, respectively, natural and forced convection in a room. The simulation results predicted from the sub-zonal airflow models are compared to experimental data and numerical computational fluid dynamics (CFD) results. The study shows that sub-zonal models combined with an appropriate surface transfer coefficient model are able to give reliable predictions of the local indoor environmental conditions and surface transfer coefficients near the building component for the analyzed cases. The relatively short computation time and flexibility of the sub-zonal model makes the application attractive for transient simulation of heat, air and moisture transport in buildings. However, the availability of appropriate reference conditions, for example experimental or numerical results, is a prerequisite for the development of a reliable sub-zonal model.
Antonini Alves, Thiago; Santos, Paulo H. D.; Barbur, Murilo A.
2015-09-01
In this research, the temperatures of threedimensional (3D) protruding heaters mounted on a conductive substrate in a horizontal rectangular channel with laminar airflow are related to the independent power dissipation in each heater by using a matrix G + with invariant coefficients, which are dimensionless. These coefficients are defined in this study as the conjugate influence coefficients (g +) caused by the forced convection- conduction nature of the heaters' cooling process. The temperature increase of each heater in the channel is quantified to clearly identify the contributions attributed to the self-heating and power dissipation in the other heaters (both upstream and downstream). The conjugate coefficients are invariant with the heat generation rate in the array of heaters when assuming a defined geometry, invariable fluid and flow rate, and constant substrate and heater conductivities. The results are numerically obtained by considering three 3D protruding heaters on a twodimensional (2D) array by ANSYS/Fluent™ 15.0 software. The conservation equations are solved by a coupled procedure within a single calculation domain comprising of solid and fluid regions and by considering a steady state laminar airflow with constant properties. Some examples are shown, indicating the effects of substrate thermal conductivity and Reynolds number on conjugate influence coefficients.
Transition from forced to natural convective flow in an LMFBR under adverse thermal conditions
International Nuclear Information System (INIS)
One of the most important aspects of the demonstration of the safety of liquid metal cooled fast reactors (LMFBR) is the assurance of adequate cooling of the reactor and critical structures during all potential or hypothetical events. To this end, a design philosophy has developed in which alternate heat removal systems are included in a plant in order to provide independent, diverse, and redundant cooling from the normal heat transport system. Recent studies have examined various aspects of natural convective flow in such systems and preliminary conclusions from these efforts have indicated that the basic phenomena are reasonably well understood and predictable in most cases. However, in certain situations where significant thermal stratification occurs, the resulting buoyancy-driven flow patterns can become quite complex and as a result, the confidence in computer simulations diminishes. The purpose of this paper is to present the results of an experimental and analytical study of one class of such problems in which the development of natural convective flow requires a transition through an unstably stratified condition. The experiments were conducted in the Experimental Breeder Reactor II (EBR-II) and the analytical tool used was the NATDEMO system simulation code
Avallone, F.; Greco, C. S.; Schrijer, F. F. J.; Cardone, G.
2015-04-01
The measurement of the convective wall heat flux in hypersonic flows may be particularly challenging in the presence of high-temperature gradients and when using high-thermal-conductivity materials. In this case, the solution of multidimensional problems is necessary, but it considerably increases the computational cost. In this paper, a low-computational-cost inverse data reduction technique is presented. It uses a recursive least-squares approach in combination with the trust-region-reflective algorithm as optimization procedure. The computational cost is reduced by performing the discrete Fourier transform on the discrete convective heat flux function and by identifying the most relevant coefficients as objects of the optimization algorithm. In the paper, the technique is validated by means of both synthetic data, built in order to reproduce physical conditions, and experimental data, carried out in the Hypersonic Test Facility Delft at Mach 7.5 on two wind tunnel models having different thermal properties.
Atashafrooz, M.; Gandjalikhan Nassab, S. A.; Lari, K.
2015-04-01
In the present work, the interaction between non-gray radiation and forced convection in a laminar radiating gas flow over a recess including two backward and forward facing steps in a duct is investigated numerically. Distributions of absorption coefficients across the spectrum (50 cm-1 effects of radiation-conduction parameter, wall emissivity, scattering coefficient and recess length on heat transfer behaviors of the convection-radiation system are investigated for both gray and non-gray mediums. In addition, the results of gray medium are compared with non-gray results in order to judge if the differences between these two approaches are significant enough to justify the usage of non-gray models. Results show that for air mixture with 10 % CO2 and 20 % H2O, use of gray model for the radiative properties may cause significant errors and should be avoided.
Convective heat transfer on an inlet guide vane.
Holmer, M L; Eriksson, L E; Sunden, B
2001-05-01
The flow and temperature fields around an inlet guide vane are determined numerically by a CFD method. Outer surface temperatures, heat transfer coefficient distributions, and static pressure distributions are presented. Three different thermal boundary conditions on the vane are analysed. The computed results are compared with experimental data. The governing equations are solved by a finite-volume method with the low Reynolds number version of the k-omega turbulence model by Wilcox implemented. It is found that the calculated results agree best with measurements if a conjugate heat transfer approach is applied and thus this wall condition is recommended for future investigations of film cooling of guide vanes and turbine blades. PMID:11460632
Asymptotic solution for heat convection-radiation equation
Energy Technology Data Exchange (ETDEWEB)
Mabood, Fazle; Ismail, Ahmad Izani Md [School of Mathematical Sciences, Universiti Sains Malaysia, 11800 USM, Penang (Malaysia); Khan, Waqar A. [Department of Engineering Sciences, National University of Sciences and Technology, PN Engineering College, Karachi, 75350 (Pakistan)
2014-07-10
In this paper, we employ a new approximate analytical method called the optimal homotopy asymptotic method (OHAM) to solve steady state heat transfer problem in slabs. The heat transfer problem is modeled using nonlinear two-point boundary value problem. Using OHAM, we obtained the approximate analytical solution for dimensionless temperature with different values of a parameter ?. Further, the OHAM results for dimensionless temperature have been presented graphically and in tabular form. Comparison has been provided with existing results from the use of homotopy perturbation method, perturbation method and numerical method. For numerical results, we used Runge-Kutta Fehlberg fourth-fifth order method. It was found that OHAM produces better approximate analytical solutions than those which are obtained by homotopy perturbation and perturbation methods, in the sense of closer agreement with results obtained from the use of Runge-Kutta Fehlberg fourth-fifth order method.
Influence of Chimney Width in Natural Convection Heat Transfer on a Vertical Finned Plate
Energy Technology Data Exchange (ETDEWEB)
Moon, Jeyoung; Heo, Jeonghwan; Chung, Bumjin [Kyung Hee Univ., Yongin (Korea, Republic of)
2014-05-15
The RCCS adopts the chimney system to increase the flow rate. Also the fins installed in the inner walls of the chimney will provide an additional cooling performance by increasing the heat transfer area. On the other hand, the fins also increase the friction loss i. e. the pressure drop. Thus, in order to improve the heat transfer performance of the RCCS, an optimization among the fin parameters is necessary. Many experimental and numerical studies regarding the fin and the chimney are available. In this study, the natural convection heat transfer of the fin system located inside the chimney was measured. Based on the analogy concept, heat transfer experiments were replaced by mass transfer experiments using a sulfuric acid-copper sulfate (H{sub 2}SO{sub 4}-CuSO{sub 4}) electroplating system. The experiments were conducted by varying fin spacing, fin height, chimney width, and chimney height. This study experimentally investigated the natural convection heat transfer of the vertical finned plate in the chimney. Using an analogy, the heat transfer systems were replaced by mass transfer systems. The measured mass transfer coefficients was the difference with the existing heat transfer correlations due to the large value of the Pr, but exhibited similar trends with the existing heat transfer correlations. The heat transfer rate is increased by the decrease of the fin spacing and the increased fin height due to increased heat transfer area and chimney flow pattern. The chimney effect enhances heat transfer rate of vertical finned plate and the chimney effect on the Nu{sub s} In this study, the heat transfer rate on vertical finned plate in the chimney was confirmed by experiment for high values of Ra was stronger for the narrow chimney width, and became stronger by the higher chimney. The chimney effect was not observed when the chimney width becomes larger than a certain value depending on the Ra{sub s}.
Enhanced convective heat transfer using graphene dispersed nanofluids
Baby Tessy; Ramaprabhu Sundara
2011-01-01
Abstract Nanofluids are having wide area of application in electronic and cooling industry. In the present work, hydrogen exfoliated graphene (HEG) dispersed deionized (DI) water, and ethylene glycol (EG) based nanofluids were developed. Further, thermal conductivity and heat transfer properties of these nanofluids were systematically investigated. HEG was synthesized by exfoliating graphite oxide in H2 atmosphere at 200°C. The nanofluids were prepared by dispersing functionalized HEG (f...
Marangoni Convection and Fragmentation in Laser Heat Treatment
Drezet, Jean-Marie; Mokadem, Sélim
2006-01-01
Epitaxial Laser Metal Forming (E-LMF) consists in impinging a jet of metallic powder onto a molten pool formed by controlled laser heating and thereby, generating epitaxially a single crystal deposit onto a single crystal substrate. It is a near net-shape process for rapid prototyping or repair engineering of single crystal high pressure/high temperature gas turbines blades. Single crystal repair using E-LMF requires controlled solidification conditions in order to prevent the nucleation and ...
To the problem of convective heat transfer intensification
International Nuclear Information System (INIS)
Intensifiers of heat exchange of a surface with a geseous current are experimentally investigated. The intensifier is a corrugated perforated packing which creates the system of rectangular zigzag channels of constant cross sections jointly with the main plates. Rectangular system sections are located at a certain angle phi to the initial direction of the current motion and are separated from each other by the distance S. This surface is called the angular corrugated packing (ACP). The ACP effectiveness (phi=16 deg, S=2 mm, section length x=2.6 mm, perforation diameter d=1.2 mm) using aerial models with Re=(0.5-6)x103 is compared with other types of modern intensifying packing (corrugated, louver, notching, wave rod). It is shown that ACP at the given stage of investigation is, at least, on the same level with most effective. Modern packing (e.g. a rod packing which is practically not used in technique due to difficulties connected with its production) or surpasses them (e.g. a notching packing which is widely used when designing compact heat exchange apparatus). ACP caa be recommended for designing compact heat exchange apparatus
Daverat, Christophe; Pabiou, Hervé; Menezo, Christophe; Bouia, Hassan; Xin, Shihe
2013-01-01
The operating temperature of building integrated photovoltaic cells is a key factor in terms of energy efficiency. In order to cool them by natural convection, they are mounted in a double-skin façade configuration, meaning that they are separated from the building wall by a ventilated air channel. Identifying and analysing the physical phenomena governing the air flow and heat transfer in this vertical channel are therefore essential to improve cell-cooling. This analysis can only be perform...
International Nuclear Information System (INIS)
In heated tunnels such as those designated for emplacement of radioactive waste at Yucca Mountain, axial temperature gradients may cause natural convection processes that can significantly influence the moisture conditions in the tunnels and in the surrounding fractured rock. Large-scale convection cells would provide an effective mechanism for axial vapor transport, driving moisture out of the formation away from the heated tunnel section into cool end sections (where no waste is emplaced). To study such processes, we have developed and applied an enhanced version of TOUGH2 (Pruess et al., 1999) adding a new module that solves for natural convection in open cavities. The new TOUGH2 simulator simultaneously handles (1) the flow and energy transport processes in the fractured rock; (2) the flow and energy transport processes in the cavity; and (3) the heat and mass exchange at the rock-cavity interface. The new module is applied to simulate the future thermal-hydrological (TH) conditions within and near a representative waste emplacement tunnel at Yucca Mountain. Particular focus is on the potential for condensation along the emplacement section, a possible result of heat output differences between individual waste packages
Mixed convection heat transfer from confined tandem square cylinders in a horizontal channel
Huang, Zhu
2013-11-01
This paper presents a numerical study on the two-dimensional laminar mixed convective flow and heat transfer around two identical isothermal square cylinders arranged in tandem and confined in a channel. The spacing between the cylinders is fixed with four widths of the cylinder and the blockage ratio and the Prandtl number are fixed at 0.1 and 0.7 respectively. The mixed convective flow and heat transfer is simulated by high accuracy multidomain pseudospectral method. The Reynolds number (Re) is studied in the range 80 ? Re ? 150, the Richardson number (Ri) demonstrating the influence of thermal buoyancy ranges from 0 to 1. Numerical results reveal that, with the thermal buoyancy effect, the mixed convective flow sheds vortex behind the cylinders and keeps periodic oscillating. The variations of characteristic quantities related to flow and heat transfer processes, such as the overall drag and lift coefficients and the Nusselt numbers, are presented and discussed. Furthermore, the influence of thermal buoyancy on the fluid flow and heat transfer are discussed and analysed. © 2013 Elsevier Ltd. All rights reserved.
Energy Technology Data Exchange (ETDEWEB)
Fox, E.; Visser, A.; Bridges, N.
2011-07-18
This paper presents an experimental study of natural convection heat transfer for an Ionic Liquid. The experiments were performed for 1-butyl-2, 3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide, ([C{sub 4}mmim][NTf{sub 2}]) at a Raleigh number range of 1.26 x 10{sup 7} to 8.3 x 10{sup 7}. In addition to determining the convective heat transfer coefficients, this study also included experimental determination of thermophysical properties of [C{sub 4}mmim][NTf{sub 2}] such as, density, viscosity, heat capacity, and thermal conductivity. The results show that the density of [C{sub 4}mmim][NTf{sub 2}] varies from 1.437-1.396 g/cm{sup 3} within the temperature range of 10-50 C, the thermal conductivity varies from 0.105-0.116 W/m.K between a temperature of 10 to 60 C, the heat capacity varies from 1.015 J/g.K - 1.760 J/g.K within temperature range of 25-340 C and the viscosity varies from 18cp-243cp within temperature range 10-75 C. The results for density, thermal conductivity, heat capacity, and viscosity were in close agreement with the values in the literature. Measured dimensionless Nusselt number was observed to be higher for the ionic liquid than that of DI water. This is expected as Nusselt number is the ratio of heat transfer by convection to conduction and the ionic liquid has lower thermal conductivity (approximately 18%) than DI water.
Rehena Nasrin; Abdul Alim
2014-01-01
This work compares heat loss characteristics across a riser pipe of a flat plate solar collector filled water based nanofluid of double nanoparticles (alumina and copper) with single nanoparticle (alumina). Also this study compares heat transfer phenomena among four nanofluids namely water-copper oxide, water-alumina, water-copper and water-silver nanofluids. Comparisons are obtained by numerically solving assisted convective heat transfer problem of a cross section of flat plate solar collec...
Directory of Open Access Journals (Sweden)
Bianco Vincenzo
2011-01-01
Full Text Available Abstract In this article, developing turbulent forced convection flow of a water-Al2O3 nanofluid in a square tube, subjected to constant and uniform wall heat flux, is numerically investigated. The mixture model is employed to simulate the nanofluid flow and the investigation is accomplished for particles size equal to 38 nm. An entropy generation analysis is also proposed in order to find the optimal working condition for the given geometry under given boundary conditions. A simple analytical procedure is proposed to evaluate the entropy generation and its results are compared with the numerical calculations, showing a very good agreement. A comparison of the resulting Nusselt numbers with experimental correlations available in literature is accomplished. To minimize entropy generation, the optimal Reynolds number is determined.
Directory of Open Access Journals (Sweden)
Mehran Khaki JAMEI
2014-04-01
Full Text Available This work presents a boundary-layer analysis of an incompressible viscous steady flow and forced convection over a horizontal flat plate. The solution for velocity and temperature are calculated by applying the Homotopy perturbation method (HPM. A special technique is attempted by which one is able to obtain solutions that are close to the exact solution of the equation. The obtained results are compared to the exact solution and another results provided by previous works so that the high accuracy of the obtained results is clear. Also, the results reveal that this method is effective, simple, and can be applied for other nonlinear problems in different fields of science and engineering, especially some fluid mechanics and heat transfer equations.doi:10.14456/WJST.2014.47
Rainieri, S.; Bozzoli, F.; Cattani, L.; Pagliarini, G.
2012-11-01
In the present analysis, the forced convective heat transfer in smooth and corrugated helical coiled tubes was experimentally studied in the Reynolds and Dean number ranges 50÷1200 and 12÷295 respectively, by adopting Ethylene Glycol as working fluid. The primary aim of the investigation is to study the combined effect of the wall curvature and of the wall corrugation in the thermal entrance region for highly viscous fluids. Two coiled tubes with a curvature ratio of about 0.06, one with smooth wall and the other with spirally corrugated wall, were investigated under the uniform heat flux boundary condition. The main conclusion is that in the Reynolds number range analyzed, both curvature and corrugation enhance the heat transfer. For Dean number values lower than about 120 the wall curvature effect prevails, and the heat transfer enhancement reflects Nusselt numbers that are approximately 2-3 times higher than the straight smooth section. For greater Dean number values, the wall corrugation instead prevails. In fact the corrugated coiled tube reaches Nusselt number values which are up to 8 times higher than the ones expected for the smooth straight tube. The smooth coiled tube shows instead thermal performances at maximum 3.6 times over the straight section.
International Nuclear Information System (INIS)
In the present analysis, the forced convective heat transfer in smooth and corrugated helical coiled tubes was experimentally studied in the Reynolds and Dean number ranges 50÷1200 and 12÷295 respectively, by adopting Ethylene Glycol as working fluid. The primary aim of the investigation is to study the combined effect of the wall curvature and of the wall corrugation in the thermal entrance region for highly viscous fluids. Two coiled tubes with a curvature ratio of about 0.06, one with smooth wall and the other with spirally corrugated wall, were investigated under the uniform heat flux boundary condition. The main conclusion is that in the Reynolds number range analyzed, both curvature and corrugation enhance the heat transfer. For Dean number values lower than about 120 the wall curvature effect prevails, and the heat transfer enhancement reflects Nusselt numbers that are approximately 2–3 times higher than the straight smooth section. For greater Dean number values, the wall corrugation instead prevails. In fact the corrugated coiled tube reaches Nusselt number values which are up to 8 times higher than the ones expected for the smooth straight tube. The smooth coiled tube shows instead thermal performances at maximum 3.6 times over the straight section.
Wei, J. J.; Yu, B.; Wang, H. S.; Tao, W. Q.
Numerical studies were conducted to investigate the natural convection heat transfer around a uniformly heated thin plate with arbitrary inclination in an infinite space. The numerical approach was based on the finite volume technique with a nonstaggered grid arrangement. For handling the pressure-velocity coupling the SIMPLE-algorithm was used. QUICK scheme and first order upwind scheme were employed for discretization of the momentum and energy convective terms respectively. Plate width and heating rate were used to vary the modified Rayleigh number over the range of 4.8×106 to 1.87×108. Local and average heat transfer characteristics were compared with regarding to the inclination angle. The empirical expressions for local and average Nusselt number were correlated. It has been found that for inclination angle less than 10?, the flow and heat transfer characteristics are complicated and the average Nusselt number can not be correlated by one equation while for inclination angle larger than 10?, the average Nusselt number can be correlated into an elegant correlation.
Study on radiation heat transfer and natural convection outside passive containment cooling system
International Nuclear Information System (INIS)
A 1 : 10 2D model of radiation heat transfer and natural convection outside the 2D steel containment was established in this paper. The advanced computational fluid dynamic analysis software Fluent was applied to complete the calculation. The velocity nephogram and the velocity vector diagram on the head of steel containment were obtained, and the results show that there is an air stagnant zone on the head of steel containment. The influence of channel width, airflow speed and emissivity of wall surface on heat transfer was studied as well. The appropriate channel width and airflow speed can enhance the heat transfer and the efficiency of heat exchange. The increase of the emissivity of wall surface can obviously improve the radiation heat transfer on the head of steel containment. (authors)
Nonlinear thermal convection in a layer of nanofluid under G-jitter and internal heating effects
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Bhadauria B. S.
2014-01-01
Full Text Available This paper deals with a mathematical model of controlling heat transfer in nanofluids. The time-periodic vertical vibrations of the system are considered to effect an external control of heat transport along with internal heating effects. A weakly non-linear stability analysis is based on the five-mode Lorenz model using which the Nusselt number is obtained as a function of the thermal Rayleigh number, nano-particle concentration based Rayleigh number, Prandtl number, Lewis number, modified diffusivity ratio, amplitude and frequency of modulation. It is shown that modulation can be effectively used to control convection and thereby heat transport. Further, it is found that the effect of internal Rayleigh number is to enhance the heat and nano-particles transport.
Energy Technology Data Exchange (ETDEWEB)
Basit, M. Abdul [Department of Chemical and Material Engineering, Pakistan Institute of Engineering and Applied Science Nilore, Islamabad 45650 (Pakistan); Rafique, Muhammad [Department of Physics, University of Azad Jammu and Kashmir Muzaffarabad 13100, Azad Kashmir (Pakistan)]. E-mail: rafi_722002@yahoo.com; Chughtai, Imran R. [Department of Chemical and Material Engineering, Pakistan Institute of Engineering and Applied Science Nilore, Islamabad 45650 (Pakistan); Inayat, Mansoor H. [Department of Chemical and Material Engineering, Pakistan Institute of Engineering and Applied Science Nilore, Islamabad 45650 (Pakistan)
2007-01-15
In this paper a Computer Code COSINAC (Computer Simulation of Natural Convection from Assembly of vertical Cylinders) has been developed to simulate the natural convection heat transfer from an assembly of vertical cylinders of Pakistan Research Reactor-2 (PARR-2), under the steady state reactor operation. The momentum and energy equations in cylindrical co-ordinates, representing the thermal hydraulic behavior of a typical fuel pin in Pakistan Research Reactor-2, have been solved numerically for a two dimensional axisymmetric domain. The temperature and velocity profiles and Nusselt number variations have been studied and results have been presented. The computer code COSINAC has been validated against experimental results carried out in previous studies at different occasions. Average outlet coolant temperature simulated by computer code, at different wall heat fluxes, has been found in good agreement with experimental results.
International Nuclear Information System (INIS)
In this paper a Computer Code COSINAC (Computer Simulation of Natural Convection from Assembly of vertical Cylinders) has been developed to simulate the natural convection heat transfer from an assembly of vertical cylinders of Pakistan Research Reactor-2 (PARR-2), under the steady state reactor operation. The momentum and energy equations in cylindrical co-ordinates, representing the thermal hydraulic behavior of a typical fuel pin in Pakistan Research Reactor-2, have been solved numerically for a two dimensional axisymmetric domain. The temperature and velocity profiles and Nusselt number variations have been studied and results have been presented. The computer code COSINAC has been validated against experimental results carried out in previous studies at different occasions. Average outlet coolant temperature simulated by computer code, at different wall heat fluxes, has been found in good agreement with experimental results
Review of fluid flow and convective heat transfer within rotating disk cavities with impinging jet
Harmand, Souad; Poncet, Sébastien; Shevchuk, Igor V; 10.1016/j.ijthermalsci.2012.11.009
2013-01-01
Fluid flow and convective heat transfer in rotor-stator configurations, which are of great importance in different engineering applications, are treated in details in this review. The review focuses on convective heat transfer in predominantly outward air flow in the rotor-stator geometries with and without impinging jets and incorporates two main parts, namely, experimental/theoretical methodologies and geometries/results. Experimental methodologies include naphthalene sublimation techniques, steady state (thin layer) and transient (thermochromic liquid crystals) thermal measurements, thermocouples and infra-red cameras, hot-wire anemometry, laser Doppler and particle image velocimetry, laser plane and smoke generator. Theoretical approaches incorporate modern CFD computational tools (DNS, LES, RANS etc). Geometries and results part being mentioned starting from simple to complex elucidates cases of a free rotating disk, a single disk in the crossflow, single jets impinging onto stationary and rotating disk,...
Natural convection and boiling heat transfer of a liquid metal in a magnetic field
International Nuclear Information System (INIS)
A liquid metal is often assumed as a coolant and a breeding material of a Tokamak fusion reactor. However, many problems on the thermo-hydraulics of a liquid metal in a magnetic field are still remained to be studied. In the present report, natural convection and boiling of a liquid metal in a strong magnetic field are studied to examine a fundamental feasibility of a fusion reactor cooled by a liquid metal. In the experimental study of the natural convection, the circulation of a liquid metal was found to be surpressed even by a magnetic field parallel to the gravity. A numerical study has confirmed the conclusion drawn by the experiment. In the study of boiling heat transfer, stable boiling of a liquid metal has been found also in a strong magnetic field. The burnout heat flux hardly affected by the magnetic field. These indicate a fundamental feasibility of the liquid-metal cooling for a Tokamak fusion reactor. (author)
International Nuclear Information System (INIS)
Highlights: • Analytical model for thermal analysis of moving porous fins. • Heat transfer from the fin surface due to convection and radiation. • For practical design aspects, optimization analysis was carried out. • Comparative study was made between the solid and porous moving fins. • Porous moving fin has more heat transfer ability than the stationary fin. - Abstract: In the present article, an exercise has been devoted to establish an analytical model for the determination of temperature distribution, fin efficiency and optimum design parameters of a porous moving fin which is losing heat by simultaneous convection and radiation to its surroundings. For the adaptation of this consideration, the governing equation becomes highly nonlinear. An analytical technique called Adomian decomposition method (ADM) is proposed for the solution methodology. The accuracy of the analytic solution is validated by using a numeric scheme called finite difference method. The results indicate that the numerical data and analytical approach are in agreement with each other. As the present study is an analytic, it is extended to the analysis for determination of optimum dimensions of said fin by satisfying either the maximization of rate of heat transfer for a given fin volume or by the minimization of fin volume for a desired heat transfer rate. The study is further extended to the porous fin in stationary condition and it is found that porous fin in moving condition transfers more heat than stationary condition. Investigation has also been made on solid moving fin to compare the outcomes of these parameters
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
A Review on Natural Convection in Nanofluid Flow
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Mohadeseh Miri
2015-09-01
Full Text Available The Heat transfer is performed in three forms including conduction, convection and radiation. Convection is one of the major modes which classifies into three categories: natural, mixed or forced convection. In recent years, much attention was given to the use of natural convection with nanofluid in engineering application, satellite, cooling system, solar collector etc. and many studies were conducted in this area. This paper reviews some results of numerical, experimental and theoretical studies in the field of natural convection heat transfer with nanofluid and application of natural convection. Finally, some suggestions are given for future studies in this area.
A p-version finite element method for steady incompressible fluid flow and convective heat transfer
Winterscheidt, Daniel L.
1993-01-01
A new p-version finite element formulation for steady, incompressible fluid flow and convective heat transfer problems is presented. The steady-state residual equations are obtained by considering a limiting case of the least-squares formulation for the transient problem. The method circumvents the Babuska-Brezzi condition, permitting the use of equal-order interpolation for velocity and pressure, without requiring the use of arbitrary parameters. Numerical results are presented to demonstrate the accuracy and generality of the method.
Calculation of Post-Closure Natural Convection Heat and Mass Transfer in Yucca Mountain Drifts
International Nuclear Information System (INIS)
Natural convection heat and mass transfer under post-closure conditions has been calculated for Yucca Mountain drifts using the computational fluid dynamics (CFD) code FLUENT. Calculations have been performed for 300, 1000, 3000, and 10,000 years after repository closure. Effective dispersion coefficients that can be used to calculate mass transfer in the drift have been evaluated as a function of time and boundary temperature tilt
Mariana-Atena Poiana
2012-01-01
This study was performed to investigate the effectiveness of grape seed extract (GSE) compared to butylated hydroxytoluene (BHT) on retarding lipid oxidation of sunflower oil subjected to convection and microwave heating up to 240 min under simulated frying conditions. The progress of lipid oxidation was assessed in terms of peroxide value (PV), p-anisidine value (p-AV), conjugated dienes and trienes (CD, CT), inhibition of oil oxidation (IO) and TOTOX value. In addition, total phenolic conte...
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 models for convection. In a full-scale test room, the heat transfer was investigated during 12 h of discharge by night-time ventilation. A total of 34 experiments have been performed, with different vent...
Drying Maize Using Biomass-Heated Natural Convection Dryer Improves Grain Quality During Storage
W. Kyamuhangire; A.N. Kaaya
2010-01-01
The objective of this study was establish the effect of the biomass-heated natural convection dryer on insect damage, mould infection, aflatoxin contamination and the germination potential of maize grain during storage. Maize grains were dried using two methods; on bare ground simulating farmer practices and using the biomass dryer. The maize dried on bare ground took five days to dry to safe storage moisture content of 14% while that of the biomass dryer took only 6 h. Drying maize grain usi...
A wavelet regularization method for an inverse heat conduction problem with convection term
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Wei Cheng
2013-05-01
Full Text Available In this article, we consider an inverse heat conduction problem with convection, which is ill-posed; i.e., the solution does not depend continuously on the given data. A special projection dual least squares method generated by the family of Shannon wavelets is applied to formulate an approximate solution. Also an optimal-order estimate for the error between the approximate solution and exact solution is obtained.
A wavelet regularization method for an inverse heat conduction problem with convection term
Wei Cheng; Ying-Qi Zhang; Chu-Li Fu
2013-01-01
In this article, we consider an inverse heat conduction problem with convection, which is ill-posed; i.e., the solution does not depend continuously on the given data. A special projection dual least squares method generated by the family of Shannon wavelets is applied to formulate an approximate solution. Also an optimal-order estimate for the error between the approximate solution and exact solution is obtained.
DEFF Research Database (Denmark)
Hosseini, R.; Kolaei, Alireza Rezania
2012-01-01
In this work, the natural convection heat transfer from a long vertical electrically heated cylinder to an adjacent air gap is experimentally studied. The aspect and diameter ratios of the cylinder are 55.56 and 6.33, respectively. The experimental measurements were obtained for a concentric condition and six eccentricities from 0.1 to 0.92 at five different heat fluxes. The surface temperature of the heated rod is measured at different heights, and the Nusselt number is calculated at the temperature measurement locations. A correlation is suggested to determine the Nusselt number based on the variation of the eccentric ratio values. The experimental results show a good agreement with other studies.
Heat and mass transfer bibliography - Soviet works
International Nuclear Information System (INIS)
A bibliography of over 350 references of Soviet works in heat and mass transfer is presented. The topics covered are; thermodynamics, thermophysical (transport) properties, heat conduction, hydromechanics, natural convection, forced convection, phase changes, radiation, combined heat and mass transfer, high temperature thermophysics, rheophysics and heat and mass transfer in technological process (drying, dispersed systems, and heat and mass exchangers). (U.K.)
Integral transform solution of natural convection in a square cavity with volumetric heat generation
Scientific Electronic Library Online (English)
C., An; C. B., Vieira; J., Su.
2013-12-01
Full Text Available The generalized integral transform technique (GITT) is employed to obtain a hybrid numerical-analytical solution of natural convection in a cavity with volumetric heat generation. The hybrid nature of this approach allows for the establishment of benchmark results in the solution of non-linear parti [...] al differential equation systems, including the coupled set of heat and fluid flow equations that govern the steady natural convection problem under consideration. Through performing the GITT, the resulting transformed ODE system is then numerically solved by making use of the subroutine DBVPFD from the IMSL Library. Therefore, numerical results under user prescribed accuracy are obtained for different values of Rayleigh numbers, and the convergence behavior of the proposed eigenfunction expansions is illustrated. Critical comparisons against solutions produced by ANSYS CFX 12.0 are then conducted, which demonstrate excellent agreement. Several sets of reference results for natural convection with volumetric heat generation in a bi-dimensional square cavity are also provided for future verification of numerical results obtained by other researchers.
Integral transform solution of natural convection in a square cavity with volumetric heat generation
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C. An
2013-12-01
Full Text Available The generalized integral transform technique (GITT is employed to obtain a hybrid numerical-analytical solution of natural convection in a cavity with volumetric heat generation. The hybrid nature of this approach allows for the establishment of benchmark results in the solution of non-linear partial differential equation systems, including the coupled set of heat and fluid flow equations that govern the steady natural convection problem under consideration. Through performing the GITT, the resulting transformed ODE system is then numerically solved by making use of the subroutine DBVPFD from the IMSL Library. Therefore, numerical results under user prescribed accuracy are obtained for different values of Rayleigh numbers, and the convergence behavior of the proposed eigenfunction expansions is illustrated. Critical comparisons against solutions produced by ANSYS CFX 12.0 are then conducted, which demonstrate excellent agreement. Several sets of reference results for natural convection with volumetric heat generation in a bi-dimensional square cavity are also provided for future verification of numerical results obtained by other researchers.
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)
A block-iterative nodal integral method for forced convection problems
International Nuclear Information System (INIS)
A new efficient iterative nodal integral method for the time-dependent two- and three-dimensional incompressible Navier-Stokes equations has been developed. Using the approach introduced by Azmy and Droning to develop nodal mehtods with high accuracy on coarse spatial grids for two-dimensional steady-state problems and extended to coarse two-dimensional space-time grids by Wilson et al. for thermal convection problems, we have developed a new iterative nodal integral method for the time-dependent Navier-Stokes equations for mechanically forced convection. A new, extremely efficient block iterative scheme is employed to invert the Jacobian within each of the Newton-Raphson iterations used to solve the final nonlinear discrete-variable equations. By taking advantage of the special structure of the Jacobian, this scheme greatly reduces memory requirements. The accuracy of the overall method is illustrated by appliying it to the time-dependent version of the classic two-dimensional driven cavity problem of computational fluid dynamics
Natural convection in an adiabatic vertical channel due to a dissipated heat element
International Nuclear Information System (INIS)
An experimental study was perfomed on natural convection heat transfer to air in a vertical channel due to an isothermal heated element attached in one of the walls of the channel. The heated element dissipates heat due to the Joule effect. To determine the heat transfer coefficient, it is necessary to evaluate the heat transferred to air by natural convenction alone. Hence, the heat lost by the element due to conduction and radiation is evaluated in order to correct the measured heat transfer. The natural-convenction heat transfer coefficient is a function of the following parameters: the temperature difference between the element and the ambient air, the position of the element in the channel, and the channel spacing. An optimal value of the channel spacing, when the heat transfer coefficient attains its maximum value, was observed for each of the temperature difference investigated. These maximum values may be up to 25% higher than the value for the case of infinite spacing. Comparisons are made with results available in the literature for similar configurations, and the values found in this work are higher. (author)
A Magneto-convection Over a Semi -infinite Porous Plate with Heat Generation
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T. Raja
2013-01-01
Full Text Available Convective flow through porous media is a branch of research undergoing rapid growth in fluid mechanics and heat transfer. This is quite natural because of its important applications in environmental, geophysical and energy related engineering problems. Prominent applications are the utilization of geothermal energy, the control of pollutant spread in ground water, the design of nuclear reactors, solar power collectors and the heat transfer associated with the deep storage of nuclear waste. The study of heat generation in moving fluids is important in problems dealing with chemical reactions and those concerned with dissociating ?uids. Heat generation effects may alter the temperature distribution and this in turn can affect the particle deposition rate in nuclear reactors, electronic chips and semi conductor wafers. Although exact modeling of internal heat generation is quite difficult, some simple mathematical models can be used to express its general behaviour for most physical situations. The objective of this work is to investigate the effects of internal heat generation on an unsteady two-dimensional magnetohydrodynamic free convection flow of a viscous, incompressible fluid free convection flow past a semi-infinite vertical porous plate embedded in a porous medium, in the presence of variable suction. The equations of continuity, linear momentum and energy, which govern the flow field, are transformed to a system of ordinary differential equations by perturbation technique. The resulting equations are solved analytically to obtain the solutions for the velocity and temperature fields. The behavior of the velocity, temperature, skin-friction and Nusselt number have been discussed for variations in the physical parameters.
Enhanced convective heat transfer using graphene dispersed nanofluids
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Baby Tessy
2011-01-01
Full Text Available Abstract Nanofluids are having wide area of application in electronic and cooling industry. In the present work, hydrogen exfoliated graphene (HEG dispersed deionized (DI water, and ethylene glycol (EG based nanofluids were developed. Further, thermal conductivity and heat transfer properties of these nanofluids were systematically investigated. HEG was synthesized by exfoliating graphite oxide in H2 atmosphere at 200°C. The nanofluids were prepared by dispersing functionalized HEG (f-HEG in DI water and EG without the use of any surfactant. HEG and f-HEG were characterized by powder X-ray diffractometry, electron microscopy, Raman and FTIR spectroscopy. Thermal and electrical conductivities of f-HEG dispersed DI water and EG based nanofluids were measured for different volume fractions and at different temperatures. A 0.05% volume fraction of f-HEG dispersed DI water based nanofluid shows an enhancement in thermal conductivity of about 16% at 25°C and 75% at 50°C. The enhancement in Nusselts number for these nanofluids is more than that of thermal conductivity.
Aspect ratio dependence of heat transfer and large-scale flow in turbulent convection
Bailon-Cuba, Jorge; Emran, Mohammad S.; Schumacher, Joerg
2010-01-01
The heat transport and corresponding changes in the large-scale circulation (LSC) in turbulent Rayleigh-B\\'{e}nard convection are studied by means of three-dimensional direct numerical simulations as a function of the aspect ratio $\\Gamma$ of a closed cylindrical cell and the Rayleigh number $Ra$. For small and moderate aspect ratios, the global heat transfer law $Nu=A\\times Ra^{\\beta}$ shows a power law dependence of both fit coefficients $A$ and $\\beta$ on the aspect ratio...
Modeling free convection flow of liquid hydrogen within a cylindrical heat exchanger cooled to 14 K
Yang, SQ; Green, MA; Lau, W
2005-01-01
A liquid hydrogen in a absorber for muon cooling requires that up to 300 W be removed from 20 liters of liquid hydrogen. The wall of the container is a heat exchanger between the hydrogen and 14 K helium gas in channels within the wall. The warm liquid hydrogen is circulated down the cylindrical walls of the absorber by free convection. The flow of the hydrogen is studied using FEA methods for two cases and the heat transfer coefficient to the wall is calculated. The first case is when the wa...
International Nuclear Information System (INIS)
The Letter presents a numerical study of convection in a finite fluid layer heated from below and with homogeneous internal heat generation. Transitions between the conducting state, hexagonal, and roll flows are investigated for the Prandtl number in the range [0.1,100] and the Rayleigh number from subcritical values up to 1.5Racr. The calculations reveal different directions of circulation in the stable hexagons above and below a critical Prandtl number value Prcr. Close to the Prcr, stable overcritical rolls are detected.
Virtual Study of Natural Convection Heat Transfer in an Inclined Square Cavity
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C.S. Nor Azwadi
2010-01-01
Full Text Available This study presented numerical prediction of natural convection heat transfer inside an inclined square cavity with perfectly conducting boundary conditions for the top and bottom walls. The modified Navier Stokes equations were solved using finite difference approach with uniform mesh resolution. The inclination angels were varied from 0° to 90° with 20° intervals. The results were presented in terms of streamlines and isotherms plots. The detailed heat transfer mechanism based on the average Nusselt number and inclination angles are presented. The effects of the boundary conditions on the sidewalls on the flow behavior are also demonstrated numerically.
Sahoo, Niranjan; Kumar, Rakesh
2015-10-01
The determination of convective surface heating is a very crucial parameter in high speed flow environment. Most of the ground based facilities in this domain have short duration experimental time scale (~milliseconds) of measurements. In these facilities, the calorimetric heat transfer sensors such as thin film gauges (TFGs) and coaxial surface junction thermocouple (CSJT) are quite effective temperature detectors. They have thickness in the range of few microns and have capability of responding in microsecond time scale. The temperature coefficient of resistance (TCR) and the sensitivity are calibration parameter indicators that show the linear change in the resistance of the gauge as a function of temperature. In the present investigation, three of types of heat transfer gauges are fabricated in the laboratory namely, TFG made out of platinum, TFG made out of platinum mixed with CNT and chromel-alumel surface junction coaxial thermocouple (K-type). The calibration parameters of the gauges are determined though oil-bath experiments. The average value TCR and sensitivity of platinum TFG is found to be 0.0024 K-1 and 465 ?V/K, while similar values of CSJT are obtained as, 0.064 K-1 and 40.5 ?V/K, respectively. The TFG made out of platinum mixed with CNT (5 % by mass) shows the enhancement of TCR as well as sensitivity and the corresponding values are 0.0034 K-1 and 735 ?V/K, respectively. The relative performances of heat transfer gauges are compared in a simple laboratory scale experiment in which the gauges are exposed to a sudden step heat load in convection mode for the time duration of 200 ms. The surface heat fluxes are predicted from the temperature history through one dimensional heat conduction modeling. While comparing the experimental results, it is seen that prediction of surface heat flux from all the heat transfer gauges are within the range of ±4 %.
David, Damien; Kuznik, F.; Roux, J.-J.
2011-01-01
In construction, the use of Phase Change Materials (PCM) allows the storage/release of energy from solar radiation and internal loads. The ap- plication of such materials for lightweight construction (e.g., a wood house) makes it possible to improve thermal comfort and reduce energy consump- tion. The heat transfer process between the wall and the indoor air is con-vection. In this paper, we have developed a numerical model to evaluate several convective heat transfer correlations from the li...
HEAT TRANSFER BY NATURAL CONVECTION IN TWO VERTICAL AND ONE HORIZONTAL PLATE – AN OVERVIEW
Directory of Open Access Journals (Sweden)
MAHENDRA P NIMKAR
2011-02-01
Full Text Available Natural Convection flow in a vertical channel with internal objects is encountered in several technological applications of particular interest of heat dissipation from electronic circuits, refrigerators, heat exchangers, nuclear reactors fuel elements, dry cooling towers, and home ventilation etc.This study deals with the study of natural convection in horizontal plate with vertical channels. The parameters varied during the experimentation are heat input, aspect ratio (the ratio of gap of horizontal plate with respect to vertical plate and gap of horizontal plate from bottom to top, and horizontal plate with and without Vslot. The present study aims to determine the heat transfer characteristics, temperature distribution along the plate to develop a correlation in the form of Nu = c (Ra n for different values of aspect ratio, for the selection of optimum dimension for design purpose. Further, the influences of aspect ratio on theperformance characteristics of heat transfer will be studied and also, the experimental results will be validated with CFD simulation (FLUENT SOFTWARE.
International Nuclear Information System (INIS)
The steam-gas pressurizer in integrated small reactors experiences very complicated thermal-hydraulic phenomena. Especially, the condensation heat transfer with noncondensable gas under natural convection is an important factor to evaluate the pressurizer behavior. However, few studies have investigated the condensation in the presence of noncondensable gas at high pressure. In this study, therefore, a theoretical model is proposed to estimate the condensation heat transfer at high pressure using the heat and mass transfer analogy. For the high pressure effect, the steam and nitrogen gas tables are used directly to determine the density of the gas mixture and the heat and mass transfer analogy based on mass approach is applied instead of that based on the ideal gas law. A comparison of the results from the proposed model with experimental data obtained from Seoul National University indicates that the condensation heat transfer coefficients increase with increasing system pressure and with decreasing mass fraction of the nitrogen gas. The proposed model is also compared with other conventional correlations proposed in the literature. The proposed model demonstrates the capability to predict the condensation heat transfer coefficients at high pressure better than any other correlation. Finally, the condensate rate is compared to verify the application of the heat and mass transfer analogy at high pressure. The comparison results confirm that the heat and mass transfer analogy can be applied to evaluate the condensation heat and mass transfer at high pressure
Convective heat transfer from rotating disks subjected to streams of air
aus der Wiesche, Stefan
2016-01-01
This Brief describes systematically results of research studies on a series of convective heat transfer phenomena from rotating disks in air crossflow. Phenomena described in this volume were investigated experimentally using an electrically heated disk placed in the test section of a wind tunnel. The authors describe findings in which transitions between different heat transfer regimes can occur in dependency on the involved Reynolds numbers and the angle of incidence, and that these transitions could be related to phenomenological Landau and Landau-de Gennes models. The concise volume closes a substantial gap in the scientific literature with respect to flow and heat transfer in rotating disk systems and provides a comprehensive presentation of new and recent results not previously published in book form.
Optimal Prandtl number for heat transfer in rotating Rayleigh-Benard convection
International Nuclear Information System (INIS)
Numerical data for the heat transfer as a function of the Prandtl (Pr) and Rossby (Ro) numbers in turbulent rotating Rayleigh-Benard convection are presented for Rayleigh number Ra=108. When Ro is fixed, the heat transfer enhancement with respect to the non-rotating value shows a maximum as a function of Pr. This maximum is due to the reduced effect of Ekman pumping when Pr becomes too small or too large. When Pr becomes small, i.e. for large thermal diffusivity, the heat that is carried by the vertical vortices spreads out in the middle of the cell and Ekman pumping thus becomes less effective. For higher Pr the thermal boundary layers (BLs) are thinner than the kinetic BLs and therefore the Ekman vortices do not reach the thermal BL. This means that the fluid that is sucked into the vertical vortices is colder than that for lower Pr, which limits the upwards heat transfer.
Study of natural convection heat transfer characteristics. (2) Verification for numerical simulation
International Nuclear Information System (INIS)
In the natural cooling system for waste storage, it is important to evaluate the flow by natural draft enough to remove the decay heat from the waste. In this study, we carried out the fundamental study of natural convection on vertical cylindrical heater by experiment and numerical simulation. The dimension of test facility is about 4m heights with single heater. Heating power is varied in the range of 33-110W, where Rayleigh number is over 1010. We surveyed the velocity distribution around heater by some turbulent models, mesh sizes around heated wall and turbulent Prandtl numbers. Results of numerical simulation of the velocity distribution and averaged heat transfer coefficient agreed well with experimental data and references. (author)
Directory of Open Access Journals (Sweden)
Ahmed T. Ahmed
2013-05-01
Full Text Available An experimental study on natural convection heat transfer from two parallel horizontal cylinders in horizontal cylindrical enclosure was carried out under condition of constant surfaces temperature for two cylinders and cylindrical enclosure. The study included the effect of Rayleigh number, rotation angle that represent the confined angle between the passing horizontal plane in cylindrical enclosure center and passing line in two cylinders centers, and the spaces between two cylinders on their heat loss ability.39An experimental set-up was used for this purpose which consist watercontainer, test section which is formed of plastic cylinder that represent the cylindrical enclosure, and two heating elements which are formed of two copper cylinders with (19 mm in diameters heated internally by electrical sources that represents transfer and heat loss elements through this set-up. The experiments were done at the range of Rayleigh number between ( , cylinders rotation angle at ( , and spacing ratio at ( . The study showed that the ability of heat loss from two cylinders is a function of Rayleigh number, cylinders rotation angle, and the spaces between them. This ability is increased by increasing of Rayleigh number and it was showed that this ability reaches maximum value at the first cylinder ( and minimum value at the second cylinder ( at spacing ratio (S/D=3 and rotation angle ( for the first and ( for the second cylinder respectively. The effective variables on natural convection heat transfer from the above two cylinders are related by two correlating equations, each one explains dimensionless relation of heat transfer from each cylinder that represented by Nusselt number against Rayleigh number, rotation angle, and the spacing ratio between two cylinders.
Convective and Stratiform Precipitation Processes and their Relationship to Latent Heating
Tao, Wei-Kuo; Lang, Steve; Zeng, Xiping; Shige, Shoichi; Takayabu, Yukari
2009-01-01
The global hydrological cycle is central to the Earth's climate system, with rainfall and the physics of its formation acting as the key links in the cycle. Two-thirds of global rainfall occurs in the Tropics. Associated with this rainfall is a vast amount of heat, which is known as latent heat. It arises mainly due to the phase change of water vapor condensing into liquid droplets; three-fourths of the total heat energy available to the Earth's atmosphere comes from tropical rainfall. In addition, fresh water provided by tropical rainfall and its variability exerts a large impact upon the structure and motions of the upper ocean layer. An improved convective -stratiform heating (CSH) algorithm has been developed to obtain the 3D structure of cloud heating over the Tropics based on two sources of information: 1) rainfall information, namely its amount and the fraction due to light rain intensity, observed directly from the Precipitation Radar (PR) on board the TRMM satellite and 2) synthetic cloud physics information obtained from cloud-resolving model (CRM) simulations of cloud systems. The cloud simulations provide details on cloud processes, specifically latent heating, eddy heat flux convergence and radiative heating/cooling, that. are not directly observable by satellite. The new CSH algorithm-derived heating has a noticeably different heating structure over both ocean and land regions compared to the previous CSH algorithm. One of the major differences between new and old algorithms is that the level of maximum cloud heating occurs 1 to 1.5 km lower in the atmosphere in the new algorithm. This can effect the structure of the implied air currents associated with the general circulation of the atmosphere in the Tropics. The new CSH algorithm will be used provide retrieved heating data to other heating algorithms to supplement their performance.
Directory of Open Access Journals (Sweden)
Sarvari Seyed
2011-01-01
Full Text Available Abstract Laminar mixed convection of a nanofluid consisting of water and Al2O3 in an inclined tube with heating at the top half surface of a copper tube has been studied numerically. The bottom half of the tube wall is assumed to be adiabatic (presenting a tube of a solar collector. Heat conduction mechanism through the tube wall is considered. Three-dimensional governing equations with using two-phase mixture model have been solved to investigate hydrodynamic and thermal behaviours of the nanofluid over wide range of nanoparticle volume fractions. For a given nanoparticle mean diameter the effects of nanoparticle volume fractions on the hydrodynamics and thermal parameters are presented and discussed at different Richardson numbers and different tube inclinations. Significant augmentation on the heat transfer coefficient as well as on the wall shear stress is seen.
Cockrell, Allison L; Fitzgerald, Lisa A; Cusick, Kathleen D; Barlow, Daniel E; Tsoi, Stanislav D; Soto, Carissa M; Baldwin, Jeffrey W; Dale, Jason R; Morris, Robert E; Little, Brenda J; Biffinger, Justin C
2015-09-15
A thermophile, Thermus scotoductus SA-01, was cultured within a constant-temperature (65°C) microwave (MW) digester to determine if MW-specific effects influenced the growth and physiology of the organism. As a control, T. scotoductus cells were also cultured using convection heating at the same temperature as the MW studies. Cell growth was analyzed by optical density (OD) measurements, and cell morphologies were characterized using electron microscopy imaging (scanning electron microscopy [SEM] and transmission electron microscopy [TEM]), dynamic light scattering (DLS), and atomic force microscopy (AFM). Biophysical properties (i.e., turgor pressure) were also calculated with AFM, and biochemical compositions (i.e., proteins, nucleic acids, fatty acids) were analyzed by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. Gas chromatography-mass spectrometry (GC-MS) was used to analyze the fatty acid methyl esters extracted from cell membranes. Here we report successful cultivation of a thermophile with only dielectric heating. Under the MW conditions for growth, cell walls remained intact and there were no indications of membrane damage or cell leakage. Results from these studies also demonstrated that T. scotoductus cells grown with MW heating exhibited accelerated growth rates in addition to altered cell morphologies and biochemical compositions compared with oven-grown cells. PMID:26150459
International Nuclear Information System (INIS)
Forced convection boiling is the process used widely in a lot of industry branches including NPP. Heat transfer intensity under forced convection boiling is considered in different way in dependence on conditions. One of main problems for the process considered is an influence of interaction between forced flow and boiling on heat transfer character. For saturated water case a transition from ''pure'' forced convection to nucleate boiling can be realized in smooth form. (author)
Forced flow dryout heat flux in a stratified debris bed
International Nuclear Information System (INIS)
The purpose of this study is to obtain qualitative information through a series of tests for forced flow dryout heat flux in a heat-generating stratified debris bed that simulates a degraded nuclear reactor core. An experimental investigation has been conducted of dryout heat flux in an inductively heated stratified debris bed with upward forced flow under atmospheric pressure. The present observations focused mainly on the effect of coolant mass flux on the dryout heat flux in the stratified debris bed, which consists of various layers with different particle size under constant bed depth and inlet temperature of coolant conditions. The data were obtained when carbon steel particle layers were placed in a 55-mm-i.d. Pyrex glass column and inductively heated by passing radio-frequency current through a multiturn work coil encircling the column. The paper shows the comparison of experimental data and Lipinski's one-dimensional model based on surface area mean diameter of the particles. In the region of mass flux between 0 and 1 kg/m2 x s, Lipinski's one-dimensional model predicts the data trend and magnitude well. Beyond 1 kg/m2 x s mass flux, however, the trend of an increasing rate of dryout heat flux deviates extremely from both Lipinski's curve and the total evaporation energy curve
International Nuclear Information System (INIS)
Graphical abstract: The core of the water-cooled pebble bed reactor is the porous channels which stacked with spherical fuel elements. The gaps between the adjacent fuel elements are complex because they are stochastic and often shift. We adopt electromagnetic induction heating method to overall heat the pebble bed. By comparing and analyzing the experimental data, we get the rule of power distribution and the rule of heat transfer coefficient with particle diameter, heat flux density, inlet temperature and working fluid's Re number. Highlights: ? We adopt electromagnetic induction heating method to overall heat the pebble bed to be the internal heat source. ? The ball diameter is smaller, the effect of the heat transfer is better. ? With Re number increasing, heat transfer coefficient is also increasing and eventually tends to stabilize. ? The changing of heat power makes little effect on the heat transfer coefficient of pebble bed channels. - Abstract: The reactor core of a water-cooled pebble bed reactor includes porous channels that are formed by spherical fuel elements. This structure has notably improved heat transfer. Due to the variability and randomness of the interstices in pebble bed channels, heat transfer is complex, and there are few studies regarding this topic. To study the heat transfer characters of pebble bed channels with internal heat sources, oxidized stainless steel spheres with diameters of 3 and 8 mm and carbon steel spheres with 8 mm diameters are used in a stacked pebble bed. Distilled water is used as a refrigerant for the experiments, and the electromagnetic induction heating method is used to heat the pebble bed. By comparing and analyzing the experimental results, we obtain the governing rules for the power distribution and the heat transfer coefficient with respect to particle diameter, heat flux density, inlet temperature and working fluid Re number. From fitting of the experimental data, we obtain the dimensionless average heat transfer coefficient correlation criteria and find that the deviation between the fitted results and the experimental results is 12% or less.
Energy Technology Data Exchange (ETDEWEB)
Kaya, O. [Pamukkale University, Department of Mechanical Engineering, Camlik/Denizli (Turkey); Teke, I. [Yildiz Technical University, Department of Mechanical Engineering, Besiktas/Istanbul (Turkey)
2005-12-01
In this study, effects of geometrical parameters on the average convection heat transfer characteristics in helical square ducts were investigated both experimentally and numerically. The inner wall of the helical square duct was uniformly temperatured, and the top, bottom, and outer walls were adiabatic. The Renormalization Group (RNG) k-{epsilon} turbulence model was used to simulate turbulent flow and heat transfer. The governing equations were solved by a finite volume method. Numerical results were found to be in good agreement with the presented experimental data. The new correlation was proposed for the average heat transfer coefficient on the inner wall of the helical square duct. The results showed that the ratio of pitch to coil radius b/R has no obvious effect on the inner wall convective heat transfer coefficient but the ratio of hydraulic radius to coil radius a/R has considerable effect. (orig.)
Convective heat transfer from circular cylinders located within perforated cylindrical shrouds
Daryabeigi, K.; Ash, R. L.
1986-01-01
The influence of perforated cylindrical shrouds on the convective heat transfer to circular cylinders in transverse flow has been studied experimentally. Geometries studied were similar to those used in industrial platinum resistance thermometers. The influence of Reynolds number, ventilation factor (ratio of the open area to the total surface area of shroud), radius ratio (ratio of shroud's inside radius to bare cylinder's radius), and shroud orientation with respect to flow were studied. The experiments showed that perforated shrouds with ventilation factors in the range 0.1 to 0.4 and radius ratios in the range 1.1 to 2.1 could enhance the convective heat transfer to bare cylinders up to 50%. The maximum enhancement occurred for a radius ratio of 1.4 and ventilation factors between 0.2 and 0.3. It was found that shroud orientation influenced the heat transfer, with maximum heat transfer generally occurring when the shroud's holes were centered on either side of the stagnation line. However, the hole orientation effect is of second order compared to the influence of ventilation factor and radius ratio.
Energy Technology Data Exchange (ETDEWEB)
Strongrich, Andrew D.; Alexeenko, Alina A. [School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN 47907 (United States)
2014-12-09
We present experimental measurements and numerical simulations of convective heat transfer performance in the transitional rarefied regime for an isolated rectangular beam geometry. Experiments were performed using single crystalline silicon beam elements having width-to-thickness aspect ratios of 8.5 and 17.4. Devices were enclosed in a vacuum chamber and heated resistively using a DC power supply. A range of pressures corresponding to Knudsen numbers between 0.096 and 43.2 in terms of device thickness were swept, adjusting applied power to maintain a constant temperature of 50 K above the ambient temperature. Both parasitic electrical resistance associated with the hardware and radiative exchange with the environment were removed from measured data, allowing purely convective heat flux to be extracted. Numerical simulations were carried out deterministically through solution of the Ellipsoidal Statistical Bhatnagar-Gross-Krook collision model of the Boltzmann equation. Results agree with experimental data, revealing a strong coupling between dissipated heat flux and thermal stresses within the flowfield as well as a nonlinear transition between the free-molecule and continuum regimes.
International Nuclear Information System (INIS)
An analytical study of the coolability of the control rods in the Savannah River Site (SRS) K-Production Reactor under conditions of loss of normal forced convection cooling has been performed. The study was performed as part of the overall safety analysis of the reactor supporting its restart. The analysis addresses the buoyancy-driven flow over the control rods that occurs when forced cooling is lost, and the limit of critical heat flux that sets the acceptance criteria for the study. The objective of the study is to demonstrate that the control rods will remain cooled at powers representative of those anticipated for restart of the reactor. The study accomplishes this objective with a very tractable simplified analysis for the modest restart power. In addition, a best-estimate calculation is performed, and the results are compared to results from sub-scale scoping experiments. 5 refs
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.
Experimental Investigation of Solar Drying for Orange Peels by Forced convection
International Nuclear Information System (INIS)
Solar drier does not degrade any more the dried products with the manner of the products dried at the natural sun. The drying unit is composed mainly of a solar air collector and an enclosure of drying. The transformation of the solar radiation into heat is done thanks to the solar collector whose effectiveness is increased by the addition of suitable baffles in the mobile air vein. The efficiency of the collector reaches then 80. The hot air on the outlet side of the collector arrives in the enclosure of drying where the heat transfer with the product to be dried is done by convection. The kinetics drying study shows that in addition to the dependence of the temperature and air velocity of drying, the speed of drying also depends on fragmentation on the product to dry, and mainly, of the product surface in contact with the drying air. Thus, the hygrometry is reduced from 76 to 13 pour cent in one day.. The total efficiency of the drier reached 28 pour cent
International Nuclear Information System (INIS)
A holographic interferometer is used to measure the local heat transfer by natural convection in an inclined air-filled enclosure with isotherm hot and cold walls and adiabatic side walls. The air layer is heated from below. It is shown that at an angle of inclination greater than 40 deg the local Nusselt number is maximum at the lower side and minimum at the upper side of the hot plate. At an angle of inclination less than 30 deg, the local Nusselt number has several maxima and minima at different locations along the hot plate. With decreasing angle of inclination, the local heat transfer decreases at the lower side while it increases at the upper side. Mean Nusselt number increases with decreasing angle of inclination for all Rayleigh numbers considered
Laminar free convection over inclined plates with non-uniform surface temperature or heat flux
International Nuclear Information System (INIS)
An analytical solution is obtained for the problem of natural-convection heat transfer by laminar flow along inclined and vertical flat plates whose surface temperature or heat flux varies as a power function of the axial coordinate along the plate surface. The integral energy equation is solved simultaneously with the momentum equation. To simplify the analytical solution, the pressure gradient along the plate length is neglected in the momentum equation. This approximation is found to be very good if the angle of inclination from the horizontal is greater than 30deg. For the special cases of uniform surface temperature or heat flux, the results agree very well with published numerical experimental data. (orig.)
3D modelling of coupled mass and heat transfer of a convection-oven roasting process
DEFF Research Database (Denmark)
Feyissa, Aberham Hailu; Adler-Nissen, Jens
2013-01-01
A 3D mathematical model of coupled heat and mass transfer describing oven roasting of meat has been developed from first principles. The proposed mechanism for the mass transfer of water is modified and based on a critical literature review of the effect of heat on meat. The model equations are based on a conservation of mass and energy, coupled through Darcy's equations of porous media - the water flow is mainly pressure-driven. The developed model together with theoretical and experimental assessments were used to explain the heat and water transport and the effect of the change in microstructure (permeability, water binding capacity and elastic modulus) that occur during the meat roasting process. The developed coupled partial differential equations were solved by using COMSOL Multiphysics®3.5 and state variables are predicted as functions of both position and time. The proposed mechanism was partially validated by experiments in a convection oven where temperatures were measured online. © 2012 Elsevier Ltd.
On the design of heater sections of natural-convection, liquid metals heat-pipes
International Nuclear Information System (INIS)
In this article analytic relations for the design of heater sections of natural-convection, liquid-metals heat-pipes are considered, that account for the internal thermal resistance between the walls and the boiling liquid and its variations along the heater section. The comparison between the results obtained by this way in the calculation of the heat exchanger surfaces, and those deriving from simplified hypotheses (for example, assuming as null the thermal resistance between the wall and the boiling liquid) shows a significant difference, in particular as concerns heat exchangers in nuclear power plants using liquid metals as primary coolants. The present approach permits then a more realistic calculation of the exchange surfaces
Directory of Open Access Journals (Sweden)
C.S.N. Azwadi
2010-01-01
Full Text Available In this study, we present the behavior of free convective heat transfer in a rectangular cavity by two entirely different scales of numerical methods, namely a mesoscale and macroscale methods. The cavity is bounded by two vertical isothermal walls kept at different temperatures and by two horizontal perfectly conducting walls. The heat flow simulations were carried out across the two isothermal walls by varying the aspect ratio of cavity with six different values -0.2, 0.5, 1.5, 2.0, 2.5 and 3.0. The macroscale simulations were performed by solving the Navier-Stokes equation using finite difference scheme while the mesoscale simulations were done using lattice Boltzmann method. This study found that the heat transfer mechanism, fluid flow behavior in terms of formation, size and strength of vortex are critically depending on the aspect ratio of the geometry. Numerical results also show excellent agreement between these two scales of simulations.
Simulation of Free Convection from an Inclined HeatedThin Plate in a Square Enclosure
Directory of Open Access Journals (Sweden)
Nora M. Sahib
2009-01-01
Full Text Available Simulation of free convection heat transfer in a square enclosure induced by heated thin plate is represented numerically. All the enclosure walls have constant temperature lower than the plate?s temperature. The flow is assumed to be two-dimensional. The discretized equations were solved stream function, vorticity, and energy equations by finite difference method using explicit technique and Successive Over- Relaxation method. The study was performed for different values of Rayleigh number ranging from 103 to 105 for different angle position of heated thin plate(0°, 45°, 90°. Air was chosen as a working fluid (Pr = 0.71. Aspect ratio of center of plate to the parallel left wall A2 take a constant and is equal to 0.5. The effect of the angle position of the heated thin plate on heat transfer and flow were addressed. With the increase of Rayleigh number heat transfer rate increased in both vertical and horizontal position of the plate. For the vertical situation (?=90° of thin plate, heat transfer becomes more enhanced than for the horizontal situation (?=0° and the inclined situation (?=45° especially when the value of aspect ratio A1 is equal to 0.25.
Evaluation of heat transfer in acupuncture needles: convection and conduction approaches.
Tzou, Chieh-Han John; Yang, Tzyy-Yih; Chung, Ya-Chien
2015-04-01
Originating in ancient China, acupuncture using needles has been developed for thousands of years and has received attention for its reported medical remedies, such as pain relief and chronic disease treatment. Heat transfer through the needles, which might have effects on the biomechanism of acupuncture, providing a stimulus and regulating homeostasis, has never been studied. This article analyzes the significance of heat transfer through needles via convection and conduction, approached by means of computational analysis. The needle is a cylindrical body, and an axis symmetrical steady-state heat-transfer model that viscosity and static pressure was not applied. This article evaluates heat transfer via acupuncture needles by using five metal materials: silver, copper, brass, iron, and stainless steel. A silver needle of the type extensively applied in acupuncture can dissipate more than seven times as much heat as a stainless steel needle of the same type. Heat transfer through such a needle is significant, compared to natural body-energy consumption over a range of ambient temperatures. The mechanism by which heat flows in or out of the body through the needles may be crucial in the remedial efficacy of acupuncture. PMID:25952124
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 peuvent etre environ 10 fois plus eleves que ceux calcules a l'aide des formules classiques. On presente une correlation des resultats experimentaux avec l'enthalpie comme force motrice pour la transmission de la chaleur. Les pertes de charge peuvent etre calculees a l'aide du coefficient de frottement classique. (auteurs)
Energy Technology Data Exchange (ETDEWEB)
Cho, Jae Seon; Suh, Kune Yull; Chung, Chang Hyun [Seoul National University, Seoul (Korea, Republic of); Paark, Rae Joon; Kim, Sang Baik [Korea Atomic Energy Research Institute, Taejon (Korea, Republic of)
1997-12-31
This paper presents results of experimental studies on the heat transfer and solidification of the molten metal pool with overlying coolant with boiling. The metal pool is heated from the bottom surface and coolant is injected onto the molten metal pool. Ad a result, the crust, which is a solidified layer, may form at the top of the molten metal pool. Heat transfer is accomplished by a conjugate mechanism, which consists of the natural convection of the molten metal pool, the conduction in the crust layer and the convective boiling heat transfer in the coolant. This work examines the crust formation and the heat transfer rate on the molten metal pool with boiling coolant. The simulant molten pool material is tin (Sn) with the melting temperature of 232 deg C. Demineralized water is used as the working coolant. The crust layer thickness was ostensibly varied by the heated bottom surface temperature of the test section, but not much affected by the coolant injection rate. The correlation between the Nusselt number and the Rayleigh number in the molten metal pool region of this study is compared against the crust formation experiment without coolant boiling and the literature correlations. The present experimental results are higher than those from the experiment without coolant boiling, but show general agreement with the Eckert correlation, with some deviations in the high and low ends of the Rayleigh number. This discrepancy is currently attributed to concurrent rapid boiling of the coolant on top of the metal layer. 10 refs., 4 figs., 1 tab. (Author)
Topology Optimisation for Coupled Convection Problems
DEFF Research Database (Denmark)
Alexandersen, Joe; Andreasen, Casper Schousboe; Aage, Niels; Lazarov, Boyan Stefanov; Sigmund, Ole
2013-01-01
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 temperat...
International Nuclear Information System (INIS)
Highlights: ? Turbulent natural convection is studied numerically and experimentally. ? DNS of full conduction–convection–radiation coupling is performed. ? Spectral methods are combined with domain decomposition. ? Considering surface radiation improves strongly numerical results. ? Surface radiation is responsible for the weak stratification. -- Abstract: The present study concerns an air-filled differentially heated cavity of 1 m × 0.32 m × 1 m (width × depth × height) subject to a temperature difference of 15 K and is motivated by the need to understand the persistent discrepancy observed between numerical and experimental results on thermal stratification in the cavity core. An improved experiment with enhanced metrology was set up and experimental data have been obtained along with the characteristics of the surfaces and materials used. Experimental temperature distributions on the passive walls have been introduced in numerical simulations in order to provide a faithful prediction of experimental data. By means of DNS using spectral methods, heat conduction in the insulating material is first coupled with natural convection in the cavity. As heat conduction influences only the temperature distribution on the top and bottom surfaces and in the near wall regions, surface radiation is added to the coupling of natural convection with heat conduction. The temperature distribution in the cavity is strongly affected by the polycarbonate front and rear walls of the cavity, which are almost black surfaces for low temperature radiation, and also other low emissivity walls. The thermal stratification is considerably weakened by surface radiation. Good agreement between numerical simulations and experiments is observed on both time-averaged fields and turbulent statistics. Treating the full conduction–convection–radiation coupling allowed to confirm that experimental wall temperatures resulted from the coupled phenomena and this is another way to predict correctly the experimental results in the cavity
Experimental and analytical study of natural-convection heat transfer of internally heated liquids
International Nuclear Information System (INIS)
Boundary heat transfer from a liquid pool with a uniform internal heat source to a vertical or inclined boundary was investigated. The experiments were performed in an open rectangular liquid pool in which the internal heat source was generated by electrical heating. The local heat flux was measured to a boron nitride test wall which was able to be continuously inclined from vertical. Gold plated microthermocouples of 0.01 inch outside diameter were developed to measure the local surface temperature, both front and back, of the boron nitride. The local heat flux and, thus, the local heat transfer coefficient was measured at nineteen locations along the vertical axis of the test plate. A theoretical analysis of the coupled nonlinear boundary layer equations was performed. The parametric effect of the Prandtl number and the dimensionless wall temperature on the boundary heat transfer were investigated When the analytical model was used to calculate the boundary heat transfer data, agreement was achieved with the experimental data within 3% for the local heat transfer and within 2% for the average heat transfer
Scientific Electronic Library Online (English)
Álvaro J. H., Siqueira; Andréa O. da, Costa; Esly F. da, Costa Junior.
2012-02-01
Full Text Available No processo de resfriamento de frutas por ar forçado, ocorre, além da transferência convectiva de calor, a transferência de massa por evaporação. A energia necessária para a evaporação é retirada da fruta que tem sua temperatura diminuída. Neste trabalho, propõe-se o uso de correlações empíricas par [...] a o cômputo do coeficiente convectivo de transferência de calor em função da temperatura da superfície do morango durante este processo de resfriamento. O objetivo desta variação do coeficiente convectivo é compensar o efeito da evaporação no processo de transferência de calor. São testadas correlações lineares e exponenciais, ambas com dois parâmetros ajustáveis. As simulações são realizadas, utilizando-se de condições experimentais reportadas na literatura para o resfriamento de morango. Os resultados encontrados confirmam a adequação da metodologia proposta. Abstract in english In the forced-air cooling process of fruits occurs, besides the convective heat transfer, the mass transfer by evaporation. The energy need in the evaporation is taken from fruit that has its temperature lowered. In this study it has been proposed the use of empirical correlations for calculating th [...] e convective heat transfer coefficient as a function of surface temperature of the strawberry during the cooling process. The aim of this variation of the convective coefficient is to compensate the effect of evaporation in the heat transfer process. Linear and exponential correlations are tested, both with two adjustable parameters. The simulations are performed using experimental conditions reported in the literature for the cooling of strawberries. The results confirm the suitability of the proposed methodology.
Directory of Open Access Journals (Sweden)
Mariana-Atena Poiana
2012-07-01
Full Text Available This study was performed to investigate the effectiveness of grape seed extract (GSE compared to butylated hydroxytoluene (BHT on retarding lipid oxidation of sunflower oil subjected to convection and microwave heating up to 240 min under simulated frying conditions. The progress of lipid oxidation was assessed in terms of peroxide value (PV, p-anisidine value (p-AV, conjugated dienes and trienes (CD, CT, inhibition of oil oxidation (IO and TOTOX value. In addition, total phenolic content (TP was evaluated in samples before and after heating in order to assess the changes in these compounds relative to the extent of lipid oxidation. The results of this study highlight that GSE showed a significantly inhibitory effect on lipid oxidation during both treatments, although to a different extent. This ability was dose-dependent; therefore, the extent of lipid oxidation was inversely related to GSE level. Convective heating, respective microwave exposure for 240 min of samples supplemented by GSE to a level of 1000 ppm, resulted in significant decreases of investigated indices relative to the control values as follows: PV (48%; 30%, p-AV (29%; 40%, CD (45%; 30%, CT (41%; 36%, TOTOX (35%; 37%. GSE to a level of 600–800 ppm inhibited the lipid oxidation in a similar manner to BHT. These results suggested that GSE can be used as a potential natural extract for improving oxidative stability of sunflower oil during thermal applications.