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1

Forced Convection Heat Transfer in Circular Pipes

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…

Tosun, Ismail

2007-01-01

2

Forced convection heat transfer in He II

International Nuclear Information System (INIS)

An investigation of forced convection heat transfer in He II is conducted. The study includes both experimental and theoretical treatments of the problem. The experiment consists of a hydraulic pump and a copper flow tube, 3 mm in ID and 2m long. The system allows measurements of one-dimensional heat and mass transfer in He II. The heat transfer experiments are performed by applying heat at the midpoint along the length of the flow tube. Two modes of heat input are employed, i.e., step function heat input and square pulse heat input. The heat transfer results are discussed in terms of temperature distribution in the tube. The experimental temperature profiles are compared with numerical solutions of an analytical model developed from the He II energy equation. The bath temperature is set at three different values of 1.65, 1.80, and 1.95 K. The He II flow velocity is varied up to 90 cm/s. Pressure is monitored at each end of the flow tube, and the He II pressure drop is obtained for different flow velocities. Results indicate that He II heat transfer by forced convention is considerably higher than that by internal convection. The theoretical model is in close agreement with the experiment. He II pressure drop and friction factor are very similar to those of an ordinary fluid

1986-01-01

3

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)

1960-01-01

4

Pressure drop, forced convective, free convective and radiant heat transfer of pebble beds

International Nuclear Information System (INIS)

For safety studies low Reynold numbers, free convective heat transfer and high temperature radiation are considered when assuming failure of the total forced convective cooling system of the gas-cooled high temperature reactor with pebble bed core. A computer programme (THERMIX) has been developed to calculate the fluid dynamics and heat transfer of a pebble bed. The Nusselt number for natural convective heat transfer was found to be independent of the void fraction. (DG)

1983-01-01

5

Boiling heat transfer under forced convection of sodium

International Nuclear Information System (INIS)

As the basic research on the heat transfer concerning the safety of sodium-cooled fast neutron reactors, the authors have carried out the research on the boiling heat transfer under the natural convection of sodium. Moreover, the basic research on the non-boiling and boiling heat transfer under forced convection in steady and unsteady states is planned. As the unsteady state, the transient phenomena accompanying the rapid increase of heat generation of a heater and the rapid decrease of sodium flow rate are considered. First, as the research on the boiling heat transfer under the forced convection of sodium, the non-boiling heat transfer in the heat flux below the point of initial bubble generation was examined. It is the interesting problem to examine experimentally the heat transfer characteristics of sodium over the whole range of laminar flow, transition and turbulent flow regions. The experimental apparatuses and the experimental method are explained. The result of experiment is shown in relation to Nusselt number and P'eclet number. In the experiment by the authors, the effect of heat conduction in axial direction was negligibly small. In liquid sodium also, in the transition region from laminar flow to turbulent flow, the tendency similar to the forced convection heat transfer obtained with mercury by Petukhov was shown. (Kako, I

1978-01-01

6

Forced convection heat transfer in integrated microchannel heat sinks

A microchannel heat sink, integrated with pressure and temperature microsensors, was fabricated to study convective boiling under uniform heat flux boundary condition. Utilizing a wafer bond and etch back technology, the heat source, temperature and pressure sensors were separated from the fluid flow by a membrane only 1.5mum in thickness; thus allowing good control of the thermal boundary conditions. Temperature and pressure distributions for various power levels and flow rates were measured. Single-phase liquid flow results, compared with numerical simulations, confirm that the heat flux boundary condition is indeed nearly uniform. The sensor arrays, particularly for two-phase flow, provide the spatial and temporal dependence of both the temperature and pressures fields. During two-phase flow, a pressure peak appears at the location of the liquid-vapor interface region. Simultaneously, qualitative visualizations of the evolving flow patterns have been correlated with quantitative temperature and pressure measurements. Based on the temperature and pressure measurements inside the microchannels, the empirical correlations of local pressure fluctuation frequency and pressure fluctuation amplitude are found to increase with increasing input power and Suratman number, but with decreasing Reynolds number. A flow regime map is plotted to distinguish the different kinds of flow pattern in microchannels. Moreover, the activity of nucleation sites as well as the ensuing bubble dynamics, from incipience to departure, was found to depend on the channel height. The critical size for active nucleation site increases with increasing microchannel height. Furthermore, size and shape effects on two-phase flow patterns in forced convection boiling were investigated in near rectangular microchannels with silicon substrate. Although detected, in contrast with triangular microchannels, annular flow was observed to be unstable. Instead, the dominant flow pattern was an unsteady transition region connecting an upstream vapor zone to a downstream liquid zone with an average location depending on the input power. A physical mechanism based on the force balance across the vapor-liquid interface, and the development of a restoring force, is proposed to explain the flow visualization results.

Lee, Man

7

Forced convective heat transfer in curved diffusers

Energy Technology Data Exchange (ETDEWEB)

Measurements of the velocity characteristics of the flows in two curved diffusers of rectangular cross section with C and S-shaped centerlines are presented and related to measurements of wall heat transfer coefficients along the heated flat walls of the ducts. The velocity results were obtained by laser-Doppler anemometry in a water tunnel and the heat transfer coefficients along flat walls of arbitrary boundary shapes with an accuracy of about 5%. The results show that an increase in secondary flow velocities near the heated wall causes an increase in the local wall heat transfer coefficient, and quantify the variation for the maximum secondary-flow velocities in a range from 1.5 to 17% of the bulk flow velocity.

Rojas, J.; Whitelaw, J.H. (Imperial Coll. of Science and Technology, London (England)); Yianneskis, M. (King' s Coll. London (England))

1987-11-01

8

Forced convective post CHF heat transfer and quenching. [PWR

Energy Technology Data Exchange (ETDEWEB)

This paper discusses mechanisms in the post-CHF region which provide understanding and qualitative prediction capability for several current forced convective heat transfer problems. In the area of nuclear reactor safety, the mechanisms are important in the prediction of fuel rod quenches for the reflood phase, blowdown phase, and possibly some operational transients with dryout. Results using the mechanisms to investigate forced convective quenching are presented. Data reduction of quenching experiments is discussed, and the way in which the quenching transient may affect the results of different types of quenching experiments is investigated. This investigation provides an explanation of how minimum wall superheats greater than the homogeneous nucleation temperature result, as well as how these may appear to be either hydrodynamically or thermodynamically controlled. Finally, the results of a parametric study of the effects of the mechanisms upon the LOFT L2-3 hotpin calculation are presented.

Nelson, R.A.

1980-01-01

9

Calculation of critical heat flux in forced convection boiling

International Nuclear Information System (INIS)

A method of calculating critical heat flux in forced convective boiling in vertical up-flow in round tubes is presented. The heat flux is assumed to be uniform. Results of the calculation are compared with experimental results for a variety of fluids including water at various pressures, and fluorinated hydrocarbons. Very encouraging agreement with experiment is found. The calculation method assumes that the two-phase mixture of liquid and vapour form an annular type of flow. The burnout point is calculated as that at which the liquid film flow rate becomes equal to zero. (U.S.)

1974-09-03

10

Reassessment of forced convection heat transfer correlations for refrigerant-12

International Nuclear Information System (INIS)

In the frame of a Refrigerant-12 experiment on postulated accidental transients in Pressurized Water Reactors under way at Heat Transfer Laboratory (ENEA Casaccia Research Center), an assessment of the main correlation available in scientific literature, for the different heat transfer regions encountered when a liquid is boiled in a confined heated channel, has been performed. Considering a vertical tube uniformly heated over its length with CHF at the exit, the following heat transfer regimes may be individuated: convective heat transfer to liquid, subcooled boiling, saturated nucleate boiling, forced convective heat transfer through liquid film (annular flow regime) and thermal crisis. From the comparison of computed values with an original ENEA dataset, the best correlations in predicting Refrigerant-12 data have been individuated. In a few cases, though preserving the original structure of the correlations, mainly developed for water, it was necessary to adjust some coefficients by means of best-fit procedures through our experimental data. The work has been performed in the frame of the ENEA Thermal Reactor Department Safety Research Project

1986-01-01

11

Steady state forced convection heat transfer in He II

International Nuclear Information System (INIS)

A study of forced convection heat transfer in superfluid helium (He II) is initiated to better understand the physical behavior of this process and to compare it with the more familiar He II heat transfer mechanism of internal convection. An experimental assembly is designed to achieve fluid flow by a motor-driven hydraulic pump which utilizes two stainless steel bellows. Each bellows is connected to one end of a copper tube, 3 mm in diameter and 2 m long. The system allows measurements of one dimensional heat and mass transfer where the measured quantities include: temperature profile and pressure drop. The variable quantities are the helium bath temperature, flow velocity and heat input. The helium bath is held at 1.8 K and under saturation pressure. The flow tube is heated at the middle and the flow velocity is varied up to 97 cm/s. The helium pressure is monitored at both ends of the tube and a friction factor is estimated for He II. Temperature measurements are made at seven evenly spaced locations along the tube. The experimental temperature profile is compared with a numerical solution of an analytical model developed for the problem under study

1986-01-01

12

Transient forced convection heat transfer in He II

International Nuclear Information System (INIS)

Transient heat transfer measurements done on a copper tube, 3 mm in diameter and 2 m long, containing forced convection He II are reported. The present study is an extension of previous work in which steady state heat transfer was studied. A step heat input is applied to the middle of the tube once He II flow is established. The temperature is monitored at eight locations along the tube. Experimental temperature profiles are then compared with a numerical solution to the He II energy equation. The pressure is monitored at each end of the tube and a friction factor for He II is estimated. In the experiment the He II flow velocity is varied up to 90 cm/s. The He II bath is maintained at saturated vapor pressure. Data are reported at three bath temperatures: 1.65 K, 1.8 K and 1.95 K

1986-01-01

13

Comparative analysis of heat transfer correlations for forced convection boiling

International Nuclear Information System (INIS)

A critical survey was conducted of the most relevant correlations of boiling heat transfer in forced convection flow. Most of the investigations carried out on partial nucleate boiling and fully developed nucleate boiling have led to the formulation of correlations that are not able to cover a wide range of operating conditions, due to the empirical approach of the problem. A comparative analysis is therefore required in order to delineate the relative accuracy of the proposed correlations, on the basis of the experimental data presently available. The survey performed allows the evaluation of the accuracy of the different calculating procedure; the results obtained, moreover, indicate the most reliable heat transfer correlations for the different operating conditions investigated. This survey was developed for five pressure range (up to 180bar) and for both saturation and subcooled boiling condition

1978-01-01

14

International Nuclear Information System (INIS)

For predicting the fully developed upward flow in a uniformly heated vertical pipe by taking account of the buoyancy force, the k-? models of turbulence for low Reynolds number flows were adopted. The regime map for forced, mixed and natural convections as well as for laminar and turbulent flows was plotted from the numerical calculations. At the same time, an experiment was carried out at Reynolds numbers of 3000 and 5000 with the Grashof number varied over a wide range by using pressurized nitrogen gas as a test fluid. In agreement with the prediction, buoyancy-induced impairment of heat transfer was measured right in the mixed convection region. Further, from hot-wire measurement, complete laminarization was demonstrated in the mixed convection region at a Reynolds number of 3000. (author)

1986-01-01

15

Analytical Treatment of Heat Sinks Cooled by Forced Convection

Energy Technology Data Exchange (ETDEWEB)

Understanding heat transfer is vital in numerous applications in the field of power electronics. This thesis introduces some new reliable and efficient calculation methods for plate-fin heat sinks. There may be any number of electronic components attached to the base plate. The components may have arbitrary prescribed heat flux distribution. The state-of-the-art calculation methods found in the literature are based on conduction analysis, while the convective heat transfer is only treated as a boundary condition. This may lead to unphysical solutions. In this thesis, conjugated conduction and convection heat transfer problem is solved in the fins. However, the tedious solution of the Navier-Stokes equations is avoided by applying well-known analytical and experimental results for convective heat transfer. The conjugated heat transfer solution for the fins is used to determine the temperature field of the base plate. Some numerical examples are given to illustrate the fact that the present calculation methods give physically more realistic results than the methods found in the literature. The analysis in this thesis has been carried out assuming steady-state conditions. However, it is pointed out that the methods presented in the thesis can easily be generalised for the transient operation of the electronic components. (orig.)

Lehtinen, A.

2005-07-01

16

Transient forces convection heat transfer to helium during a step in heat flux

International Nuclear Information System (INIS)

Transient forced convection heat transfer coefficients for both subcritical and supercritical helium in a rectangular flow channel heated on one side were measured during the application of a step in heat flow. Zero flow data were also obtained. The heater surface which served simultaneously as a thermometer was a fast response carbon film. Operating conditions covered the following range: Pressure, 1.0 x 105 PAia (1 bar) to 1.0 x 106 Pa (10 bar); Temperature, 4 K-10 K; Heat flux, 0.1 W/cm2-10 W/cm2; Reynolds number, 0-8 x 105. The experimental data and a predictive correlation are presented

1983-01-01

17

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

1965-01-01

18

International Nuclear Information System (INIS)

An experimental study on critical heat flux of forced convection boiling in uniformly heated vertical tube was carried out, using Freon-12 as the working fluid with critical heat flux qc measured in a 16 mm diameter circular tube. The effect of steam quality and mass velocity on CHF has been investigated. The results obtained were found to be essentially consistent with the CHF data measured by other authors. However, the influence of pressure on CHF was complex and coupled with mass velocity and steam quality. In the lower steam quality region, CHF decreased with increasing pressure, while in the higher steam quality region, CHF increased with increasing pressure

1998-03-01

19

CFD simulation of forced convective boiling in heated channels

International Nuclear Information System (INIS)

In this paper a forced convective boiling of Refrigerant R-113 in a vertical annular channel has been simulated by a custom version of the CFX-5 code. The employed subcooled boiling model uses a special treatment of the wall boiling boundary, which assures the grid invariant solution. The simulation results have been validated against the published experimental data [1]. In general a good agreement with the experimental data has been achieved, which shows that the current model may be applied for the Refrigerant R-113 without significantly changing the model parameters. The influence of non-drag forces, bubble diameter size and interfacial drag model on the numerical results has been investigated as well. (authors)

2006-09-05

20

NUMERICAL ANALYSIS OF FORCED CONVECTIVE HEAT TRANSFER THROUGH HELICAL CHANNELS

Digital Repository Infrastructure Vision for European Research (DRIVER)

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...

2012-01-01

21

Analysis of Rectangular Microchannel under Forced convection heat transfer condition

Directory of Open Access Journals (Sweden)

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.

Dr. B.S.Gawali,

2011-03-01

22

International Nuclear Information System (INIS)

Combined forced and free convective heat transfer characteristics were experimentally investigated for water flowing under about 1 atm in a narrow vertical rectangular (750 mm long, 50 mm wide and 18 mm in gap) channel heated from both sides. Experiments were carried out for both downward and upward forced convective flows for Reynolds number Re of 4 x 10"1 ? 5 x 10"5 and Grashof number Gr of 5 x 10"4 ? 4 x 10"1"1, where the distance x from the inlet of the channel is adopted as the characteristic length in Re and Gr. As the results, the following were revealed : (1) Heat transfer coefficients for the combined convective flow region can be expressed in simple forms with a dimensionless parameter Gr/(Re"2"1"/"8 Pr"1"/"2) which is constituted by Gr, Re and Prandtl number Pr. (2) From the above-mentioned simple expressions it can be identified that the combined convective heat transfer is significant for the condition of 2.5 x 10"-"4 < Gr/(Re"2"1"/"8 Pr"2"/"1) < 1.8 x 10"-"3. (3) In the present experiment, little significant differences were observed in heat transfer characteristics between the combined convective flow region with upward forced convective flow and that with downward forced convective flow. (author)

1987-01-01

23

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

1991-03-17

24

Prediction of forced convection heat transfer to Lead-Bismuth-Eutectic

Digital Repository Infrastructure Vision for European Research (DRIVER)

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...

Thiele, Roman

2013-01-01

25

Forced-convection heat transfer in a spherical annulus heat exchanger

International Nuclear Information System (INIS)

Results are presented of a combined numerical and experimental study of steady, forced-convection heat transfer in a spherical annulus heat exchanger with 53 0C water flowing in an annulus formed by an insulated outer sphere and a 0 0C inner sphere. The inner sphere radius is 139.7 mm, the outer sphere radius is 168.3 mm. The transient laminar incompressible axisymmetric Navier-Stokes equations and energy equation in spherical coordinates are solved by an explicit finite-difference solution technique. Turbulence and buoyancy are neglected in the numerical analysis. Steady solutions are obtained by allowing the transient solution to achieve steady state. Numerically obtained temperature and heat-flux rate distributions are presented for gap Reynolds numbers from 41 to 465. Measurements of inner sphere heat-flux rate distribution, flow separation angle, annulus fluid temperatures, and total heat transfer are made for Reynolds numbers from 41 to 1086. The angle of separation along the inner sphere is found to vary as a function of Reynolds number. Measured total Nusselt numbers agree with results reported in the literature to within 2.0 percent at a Reynolds number of 974, and 26.0 percent at a Reynolds number of 66

1982-01-01

26

International Nuclear Information System (INIS)

A numerical study has been made of heat transfer across a shallow cavity containing liquid sodium. The cavity is heated at the roof and open at the base to a turbulent forced flow. The mixed convection problem is important in nuclear fast reactors. The Nusselt number of the cavity was calculated as a function of the forced flow velocity and the thermal resistance of the roof. A criterion was obtained for the switch from a forced flow to a stratified regime. The results were shown to be consistent with published observations. (author)

1984-01-01

27

Unsteady forced convection heat transfer in a channel

International Nuclear Information System (INIS)

An exact analytical solution is found for unsteady heat transfer, in the first time domain, for a fluid flowing in a laminar, fully developed manner in a duct when the wall temperature is suddenly changed to ?w = bn?. The solution to the governing partial differential equation is effected by the use of the Laplace transform and yields the transient surface heat flux as a function of time and position down the duct for n = 1-6. For comparison purposes, solutions are also obtained for a number of approximate models, namely, pure conduction, slug flow, quasi-steady, and a new model proposed by Sucec. (author)

1990-01-01

28

Unsteady forced convection heat transfer in a channel

An exact analytical solution is found for unsteady heat transfer, in the first time domain, for a fluid flowing in a laminar fully developed manner in a duct when the wall temperature is suddenly changed. The solution to the governing partial differential equation is effected by the use of the Laplace transform and yields the transient surface heat flux as a function of time and position down the duct for n = 1.6. For comparison purposes, solutions are also obtained for a number of approximate models; namely, pure conduction, slug flow, quasi-steady flow, and a model proposed by Sucec (1981).

Sucec, James; Radley, David

1990-04-01

29

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

2010-09-01

30

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

1979-01-01

31

Analysis of forced convection heat flow effects in horizontal ribbon growth from the melt

A heat transport analysis which considers forced convective fluid flow induced by the motion of a continuous solid ribbon over a melt has been done for horizontal ribbon growth. A model has been developed which treats both 'active' and 'passive' cooling at the ribbon surface. The results show that heat flow from the melt requires active cooling in the region of the leading growth edge or growth tip. Steady-state liquid-solid interface shape is analyzed and numerical results are given for steady-state pulling of silicon ribbon.

Zoutendyk, J. A.

1980-01-01

32

The effects of buoyancy on the critical heat flux in forced convection

The critical heat flux (CHF) in forced convection over a flat surface at relatively low flow velocities has been found, not unexpectedly, to depend upon the orientation of the buoyancy. The CHF for R-113 was measured at various heating surface orientations for test section Reynolds numbers ranging between 3000 and 6500. In this flow range, the buoyancy force acting on the vapor generally dominates over the flow inertia, yet the inertia would still be substantial were gravity to be reduced. In the experiments of this study, the CHF is determined for heating surface orientations ranging from 0 deg to 360 deg, for flow velocities between 4 cm/s and 35 cm/s, and for subcoolings between 2.8 C and 22.2 C. The results presented here demonstrate the strong influence of buoyancy at low flow velocities, which diminishes as the flow velocity and subcooling are increased.

Brusstar, Matthew J.; Merte, Herman, Jr.

1993-01-01

33

Performance of RNG Turbulence Modelling for Turbulent Forced Convective Heat Transfer in Ducts

DEFF Research Database (Denmark)

This investigation concerns numerical calculation of turbulent forced convective heat transfer and fluid flow in straight ducts using the RNG (Re-Normalized Group) turbulence method. A computational method has been developed to predict the turbulent Reynolds stresses and turbulent heat fluxes in ducts with different turbulence models. The turbulent Reynolds stresses and other turbulent flow quantities are predicted with the RNG k-e model and the RNG non-linear k-e model of Speziale. The turbulent heat fluxes are modeled by the simple eddy diffusivity (SED) concept, GGDH and WET methods. Two wall functions are used, one for the velocity field and one for the temperature field. All the models arc implemented for an arbitrary three dimensional duct. Fully developed condition is achieved by imposing cyclic boundary conditions in the main flow direction. The numerical approach is based on the finite volume technique with a non-staggered grid arrangement. The pressure-velocity coupling is handled by using the SIMPLEC-algorithm. The convective terms are treated by the QUICK, scheme while the diffusive terms are handled by the central-difference scheme. The hybrid scheme is used for solving the k and e equations. The overall comparison between the models is presented in terms of friction factor and Nusselt number. The secondary flow generation is also of major concern. Keyword: RNG; turbulent flow; secondary motions; convective; heal transfer; ducts; GGDH; WET

Rokni, Masoud; SundĂ©n, Bengt

1998-01-01

34

Convective heat transfer in an annular porous layer with centrifugal force field

Energy Technology Data Exchange (ETDEWEB)

The present study deals with natural convection in an annular porous layer under the influence of a centrifugal force field. It is assumed that the outer boundary is heated by a constant heat flux, while the inner boundary is perfectly insulated. The problem is formulated in terms of Darcy-Boussinesq equations and solved using analytical and numerical techniques. An analytical solution for the flow and heat transfer variables, based on a concentric flow assumption, is obtained in terms of the Rayleigh number and the radius ratio. Finite amplitude results are verified by a numerical approach. Predicted thresholds in terms of critical Rayleigh numbers are verified by a linear stability analysis. Results obtained from the numerical approach indicate the existence of multiple solutions differing by the number of cells involved.

Aboubi, K.; Robillard, L.; Bilgen, E. [Univ. of Montreal, Quebec (Canada). Dept. of Mechanical Engineering

1995-09-01

35

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)

1967-01-01

36

Forced convection boiling heat transfer of CO{sub 2} in horizontal tubes

Energy Technology Data Exchange (ETDEWEB)

From ozone depletion and global warming (ODP and GWP) viewpoints, natural refrigerants offer a clear advantage over CFCs and their substitutes. Here, an experimental investigation is performed to study the forced convection boiling heat transfer of CO{sub 2} in a smooth tube. Tests are conducted in a horizontal stainless steel tube. The tube has an inner diameter of 6.00 mm, wall thickness of 1.00 mm, and length of 1.78 m. Heat to the test section is provided by a heating wire wrapped around the tube. Experiments are conducted with lubricant-free CO{sub 2} at saturation temperature of 283 K, mass fluxes of 160 to 320 kg/m{sup 2}s, heat fluxes of 10 to 20 kW/m{sup 2}, and inlet quality of 5% to 60%. The heat transfer coefficients of CO{sub 2} increase with increasing the heat and/or mass flux. For the test conditions considered, the vapor quality had an adverse effect on the heat transfer coefficients. The present experimental data are found to be close to the results predicted by Gungor and Winterton (1987) with the mean deviation of less than 10%. The experimental data are correlated and the results are discussed.

Zhao, Y.; Ohadi, M.M.; Dessiatoun, S.V.; Molki, M.; Darabi, J.

1999-07-01

37

Non-Newtonian fluid laminar flow and forced convection heat transfer in rectangular ducts

Energy Technology Data Exchange (ETDEWEB)

Numerical solutions for fully developed laminar flow forced convection heat transfer of a power law non-Newtonian fluid in rectangular ducts are presented in this paper. Finite difference methods are developed for the governing equations to obtain the velocity and temperature distributions. Friction factor results are given for flow through rectangular ducts of aspect ratios of 0.2, 0.5 and 1.0 with power law index n values of 0.5 to 1.0. For the same flow conditions the Nusselt values, maximum wall temperatures, and minimum wall temperatures for the H2 thermal boundary condition for different combinations of heated and adiabatic walls are obtained. Also the Nusselt values for slug flow (n = 0) are presented for the H2 boundary condition.

Gao, S.X.; Hartnett, J.P. (Illinois Univ., Chicago, IL (United States). Energy Resources Center)

1992-09-01

38

Non-Newtonian fluid laminar flow and forced convection heat transfer in rectangular ducts

International Nuclear Information System (INIS)

Numerical solutions for fully developed laminar flow forced convection heat transfer of a power law non-Newtonian fluid in rectangular ducts are presented in this paper. Finite difference methods are developed for the governing equations to obtain the velocity and temperature distributions. Friction factor results are given for flow through rectangular ducts of aspect ratios of 0.2, 0.5 and 1.0 with power law index n values of 0.5 to 1.0. For the same flow conditions the Nusselt values, maximum wall temperatures, and minimum wall temperatures for the H2 thermal boundary condition for different combinations of heated and adiabatic walls are obtained. Also the Nusselt values for slug flow (n = 0) are presented for the H2 boundary condition

1992-09-01

39

Critical heat flux (CHF) has been measured in saturated forced convective boiling with a wall jet on a rectangular heated surface of 40 and 80mm in length and 20mm in width. The jet velocity is varied from 3 to 15 m/s, and the system pressure is 0.1, 0.2, and 0.4MPa for R113. It is found that the existing correlation for saturation condition can be applied to the CHF at high and low ?l/?g values (e.g. water and R22), but hardly to the CHF at medium ?l/?g values (e.g. R113 at 0.2 and 0.4MPa). A revised correlation is proposed to predict most of the CHF data within an accuracy of +/-25%.

Monde, M.; Wang, X.

40

Laminar forced convective heat transfer in a two-dimensional branching tee junction

International Nuclear Information System (INIS)

Laminar forced convective heat transfer in a two-dimensional ninety degree branching tee junction is studied numerically. The governing elliptic equations are solved by a finite-difference numerical scheme utilizing primitive dependent variables. A wide range of Reynolds numbers and dividing flowrates is studied while the working fluid is air with constant properties which is heated via the constant temperature walls of the bifurcation. The location of the separation and reattachment points corresponding to the two recirculation zones which form near the bifurcation are quantified as a function of the Reynolds number and dividing flowrate. The variation of the local Nusselt number along the walls of the bifurcation is discussed in light of the direct effects of the highly perturbed flowfield

1985-01-01

41

LAMINAR FORCED CONVECTION HEAT TRANSFER OVER STAGGERED CIRCULAR TUBE BANKS: A CFD APPROACH

Directory of Open Access Journals (Sweden)

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.

Tahseen A. Tahseen

2013-06-01

42

Conceptual Design of Forced Convection Molten Salt Heat Transfer Testing Loop

International Nuclear Information System (INIS)

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: (1) Molten Salts: The candidate molten salts for investigation will be selected. (2) 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. (3) Scaling Analysis: Scaling analysis to design the test loop will be performed. (4) 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. (5) 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. (6) Fabricate the Test Loop. (7) Perform the Tests. (8) 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.

2010-01-01

43

Conceptual Design of Forced Convection Molten Salt Heat Transfer Testing Loop

Energy Technology Data Exchange (ETDEWEB)

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.

Manohar S. Sohal; Piyush Sabharwall; Pattrick Calderoni; Alan K. Wertsching; S. Brandon Grover

2010-09-01

44

The major focus of this article is to analyze the forced convective heat transfer in a steady boundary layer flow of Sisko fluid over a nonlinear stretching sheet. Two cases are studied, namely (i) the sheet with variable temperature (PST case) and (ii) the sheet with variable heat flux (PHF case). The heat transfer aspects are investigated for both integer and non-integer values of the power-law index. The governing partial differential equations are reduced to a system of nonlinear ordinary differential equations using appropriate similarity variables and solved numerically. The numerical results are obtained by the shooting method using adaptive Runge Kutta method with Broyden’s method in the domain. The numerical results for the temperature field are found to be strongly dependent upon the power-law index, stretching parameter, wall temperature parameter, material parameter of the Sisko fluid and Prandtl number. In addition, the local Nusselt number versus wall temperature parameter is also graphed and tabulated for different values of pertaining parameters. Further, numerical results are validated by comparison with exact solutions as well as previously published results in the literature.

Munir, Asif; Shahzad, Azeem; Khan, Masood

2014-01-01

45

The major focus of this article is to analyze the forced convective heat transfer in a steady boundary layer flow of Sisko fluid over a nonlinear stretching sheet. Two cases are studied, namely (i) the sheet with variable temperature (PST case) and (ii) the sheet with variable heat flux (PHF case). The heat transfer aspects are investigated for both integer and non-integer values of the power-law index. The governing partial differential equations are reduced to a system of nonlinear ordinary differential equations using appropriate similarity variables and solved numerically. The numerical results are obtained by the shooting method using adaptive Runge Kutta method with Broyden's method in the domain[Formula: see text]. The numerical results for the temperature field are found to be strongly dependent upon the power-law index, stretching parameter, wall temperature parameter, material parameter of the Sisko fluid and Prandtl number. In addition, the local Nusselt number versus wall temperature parameter is also graphed and tabulated for different values of pertaining parameters. Further, numerical results are validated by comparison with exact solutions as well as previously published results in the literature. PMID:24949738

Munir, Asif; Shahzad, Azeem; Khan, Masood

2014-01-01

46

International Nuclear Information System (INIS)

Highlights: ? Superposition of forced and thermal convection is studied in a rectangular cavity. ? For pure forced convection the mean wind exhibits a solid body rotation. ? Four buoyancy induced convection rolls are formed for mixed convection at Ar ? 3.3. ? The enthalpy flux difference between out- and inflowing air has a maximum at Ar ? 0.6. - Abstract: Results of an experimental study of flow structure formation and heat transport in turbulent forced and mixed convection are presented. The experiments were conducted in a rectangular cavity with a square cross section, which has an aspect ratio between length and height of ?xz = 5. Air at atmospheric pressure was used as working fluid. The air inflow was supplied through a slot below the ceiling, while exhausting was provided by another slot, which is located directly above the floor. Both vents extend over the whole length of the cell. In order to induce thermal convection the bottom of the cell is heated while the ceiling is maintained at a constant temperature. This configuration allows to generate and study mixed convection under well defined conditions. Results of forced convection at Re = 1.07 x 104 as well as mixed convection at 1.01 x 104 ? Re ? 3.4 x 104 and Ra = 2.4 x 108 (3.3 ? Ar ? 0.3), which were obtained by means of Particle Image Velocimetry and local temperature measurements, are presented. For purely forced convection a 2D mean wind, which can be approximated by a solid body rotation, is found. With increasing Archimedes number this structure becomes unstable, leading to a transition of the solid body rotation into additional smaller convection rolls. Proper orthogonal decomposition of the instantaneous velocity fields has been performed for further analysis of these coherent large-scale structures. Their fingerprint is found in the spatial temperature distribution of the out flowing air at the end of the outlet channel, which reveals a temporally stable profile with two maxima over the length of the outlet. Moreover a maximum in the global enthalpy transport by the fluid is found at Ar ? 0.6.

2011-10-01

47

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.

2012-01-01

48

Energy Technology Data Exchange (ETDEWEB)

In this study, numerical investigations are conducted for forced convective heat transfer in an annular helicoidal tube under uniform wall temperature condition for laminar flow including developing region. The numerical computations reveal the developments and distributions of heat transfer and flow fields in the annular helicoidal tube when the outer tube wall is heated and the inner tube wall is insulated. The effects of Reynolds number, curvature ratio, and coil pitch on the circumferential average friction factor and Nusselt number at different axial locations, and the non-dimensional entropy generation number of laminar convection in an annular helicoidal tube are investigated. In addition, the differences of flow and heat transfer characteristics between the annular helicoidal tube and circular helicoidal tube are also described.

Wu, Shuang Ying; Chen, Su Jun; Xiao, Lan; Li, You Rong [Chongqing University, Chongqing (China)

2011-06-15

49

International Nuclear Information System (INIS)

In this study, numerical investigations are conducted for forced convective heat transfer in an annular helicoidal tube under uniform wall temperature condition for laminar flow including developing region. The numerical computations reveal the developments and distributions of heat transfer and flow fields in the annular helicoidal tube when the outer tube wall is heated and the inner tube wall is insulated. The effects of Reynolds number, curvature ratio, and coil pitch on the circumferential average friction factor and Nusselt number at different axial locations, and the non-dimensional entropy generation number of laminar convection in an annular helicoidal tube are investigated. In addition, the differences of flow and heat transfer characteristics between the annular helicoidal tube and circular helicoidal tube are also described

2011-06-01

50

Directory of Open Access Journals (Sweden)

Full Text Available An indirect forced convection solar drier integrated with different sensible heat storage maternal has been developed and tested its performance for drying chili under the metrological conditions of Pollachi, India. The system consists of a flat plate solar air heater with heat storage unit, a drying chamber and a centrifugal blower. Drying experiments have been performed at an air flow rate of 0.25 kg/s. Drying of chili in a forced convection solar drier reduces the moisture content from around 72.8% (wet basis to the final moisture content about 9.1% in 24 h. Average drier efficiency was estimated to be about 21%. The specific moisture extraction rate was estimated to be about 0.87 kg/kWh.

M. MOHANRAJ

2009-09-01

51

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

1977-01-01

52

Low Reynolds number forced convection steam cooling heat transfer in rod bundles

International Nuclear Information System (INIS)

A series of forced convection steam cooling tests at low Reynolds numbers were conducted in the rod bundle test facility of the FLECHT-SEASET program. The data was reduced using a rod-centered subchannel energy balance to obtain the vapor temperature and by modeling the bundle with the COBRA-IV-I computer code. The comparisons between the COBRA-IV-I vapor temperatures and subchannel energy balance vapor temperatures were quite good. 5 refs

1980-01-01

53

Design of Test Loops for Forced Convection Heat Transfer Studies at Supercritical State

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).

Balouch, Masih N.

54

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 vari...

2012-01-01

55

Transient convective heat transfer

Directory of Open Access Journals (Sweden)

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 calculation 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.

J. Padet

2005-03-01

56

Transient convective heat transfer

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english 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.

J., Padet.

57

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

1977-01-01

58

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....

Kakac, Sadik; Pramuanjaroenkij, Anchasa

2014-01-01

59

International Nuclear Information System (INIS)

Experiments have been made of unsteady forced convection heat transfer from a heated surface of 52 mm in length on an inner cylinder of 7.6 mm in diameter to liquid sodium flowing in a vertically orientated concentric annulus with 14.3 mm inside diameter. With constant heat flux of 1.0 x 106 W/m2, liquid sodium flow rate was reduced ramp-wise within about 25 seconds from an initial equilibrium state, Pe=72, to Pe=6.4 and from Pe=141 to 11.5 where Pe denotes the Peclet number. A theoretical model capable of describing the effect of heat conduction in the inner and outer walls of an annulus as well as in liquid sodium was developed. The numerical results obtained by the model agree with the experimental results for the two different flow transient conditions. (author)

1997-05-01

60

Application of Full Reynolds Stress Modeling to Turbulent Forced Convective Heat Transfer in Ducts

DEFF Research Database (Denmark)

A computational method has been developed to predict the turbulent Reynolds stresses and turbulent heat fluxes in ducts by different turbulence models. The turbulent Reynolds stresses and other turbulent flow quantities are predicted with a full Reynolds Stress Model (RSM). The turbulent heat fluxes are modeled by a SED concept, the GGDH and the WET methods. Two wall functions are used, one for the velocity field and one for the temperature field. All the models are implemented for an arbitrary three dimensional channel. Fully developed condition is achieved by imposing cyclic boundary conditions in the main flow direction. The numerical approach is based on the finite volume technique with a non-staggered grid arrangement. The pressure-velocity coupling is handled by using the SIMPLEC-algorithm. The convective terms are treated by the van Leer scheme while the diffusive terms are handled by the central-difference scheme. The hybrid scheme is used for solving the ï¥ equation. The secondary flow generation using the RSM model is compared with a non-linear k-ï¥ model. The overall comparison between the models is presented in terms of the friction factor and Nusselt number Keyword: RSM, Reynolds Stress, Turbulent, Convective, Fluid Flow

Rokni, Masoud; SundĂ©n, Bengt

1997-01-01

61

International Nuclear Information System (INIS)

Experiments have been made of unsteady forced convection heat transfer from a heated surface of 52 mm in length on an inner cylinder of 7.6 mm in diameter to liquid sodium flowing in a vertically orientated concentric annulus with 14.3 mm inside diameter. With constant heat flux of 1.0 x 106 W/m2, liquid sodium flow rate was reduced ramp-wise within about 25 seconds from an initial equilibrium state, Pe=72, to Pe=6.4 and from Pe=141 to 11.5 where Pe denotes the Peclet number. A numerical model capable of describing the effect of heat conduction in the inner and outer walls of an annulus as well as in liquid sodium was developed. The numerical results obtained by the model agree with the experimental results for the two different flow transient conditions. The numerical model proved to be valid for expressing the unsteady laminar forced convection heat transfer caused by a decrease in sodium flow rate in a concentric annular passage. The theoretical results for various flow reduction periods revealed that the time delays in heated wall temperature rise from that predicted by a model for constant flow rates were little influenced by the flow reduction periods ranging from about 25 down to 2 sec. As the surface temperatures calculated by the present model became always equal to or lower than those by the constant flow rate model over the investigated flow reduction range and periods, the constant flow rate model is a simple but effective method for safety evaluation of transient heated wall temperature rise caused by rapid decrease in flow rate. (author)

1997-10-04

62

Directory of Open Access Journals (Sweden)

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.

M.M. Rahman

2012-12-01

63

Forced convection heat transfer from rib-roughened pins tested singly and in a cluster of seven

International Nuclear Information System (INIS)

Rib roughened surfaces are used to improve forced convection heat transfer. The basic data on roughened surface performance is usually obtained from tests using a roughened cylinder in a smooth cylindrical flow channel. The practical applications, in particular nuclear fuel elements, usually involve a cluster of 'pins' with roughened surfaces. Methods have been developed to apply the basic data to clusters and this report presents experimental results which can be used to check the accuracy of these methods. A single roughened pin has been tested in two smooth-walled channels of different diameters. An identical pin surrounded by a ring of six pins in a smooth channel has been tested with all the pins uniformly heated and only the centre pin heated. All the tests were repeated for a range of coolant flow rates. (author)

1980-01-01

64

International Nuclear Information System (INIS)

Emphasizing the integration of mathematical expressions with clear physical associations, this textbook on convective heat and mass transfer reviews the laws of thermodynamics and fluid motions, behavior of laminar and turbulent flows in a variety of conditions, natural free convection in space, and flows through porous media

1984-01-01

65

International Nuclear Information System (INIS)

Forced convective boiling heat transfer coefficients were predicted for an annular flow inside a horizontal tube for pure refrigerants and nonazeotropic binary refrigerant mixtures. The heat transfer coefficients were calculated based on the turbulent temperature profile in liquid film and vapor core considering the composition difference in vapor and liquid phases, and the nonlinearity in mixing rules for the calculation of mixture properties. The heat transfer coefficients of pure refrigerants were estimated within a standard deviation of 14% compared with available experimental data. For nonazeotropic binary refrigerant mixtures, prediction of the heat transfer coefficients was made with a standard deviation of 18%. The heat transfer coefficients of refrigerant mixtures were lower than linearly interpolated values calculated from the heat transfer coefficients of pure refrigerants. This degradation was represented by several factors such as the difference between the liquid and the overall compositions, the conductivity ratio and the viscosity ratio of both components in refrigerant mixtures. The temperature change due to the concentration gradient was a major factor for the heat transfer degradation and the mass flux itself at the interface had a minor effect

2003-06-01

66

Energy Technology Data Exchange (ETDEWEB)

The performance of various turbulent flow prediction models were evaluated. In order to improve the accuracy of two-equation turbulence models, modifications have been made to account for streamline curvature, reduction of the eddy viscosity in reattachment regions, and better dynamical response of the equation governing the rate of dissipation of turbulent kinetic energy to sudden alterations of the velocity field. This paper reported on the computation of forced convection heat transfer in flow over a backward-facing step, using different eddy-viscosity low-Reynolds number turbulence models. Although the models showed different abilities, it was proposed that applying a simple damping function to the LSY (Launder and Sharma with Yap correction) model would yield accurate predictions of the turbulent heat transfer. 15 refs., 2 tabs., 7 figs.

Vasic, S. [Centre de Recherche en Calcul Applique, Montreal, PQ (Canada); Baliga, B.R. [McGill Univ., Montreal, PQ (Canada). Dept. of Mechanical Engineering

1996-09-01

67

Energy Technology Data Exchange (ETDEWEB)

Highlights: Black-Right-Pointing-Pointer Three different eddy viscosity turbulence models are validated. Black-Right-Pointing-Pointer Two data sets and an extensive sensitivity study are employed for validation. Black-Right-Pointing-Pointer Prediction errors for the velocity and temperature fields are analyzed. Black-Right-Pointing-Pointer Turbulence Prandtl number has to be chosen in dependence of the turbulence model. - Abstract: This paper provides temperature and velocity distribution computations in heated annuli using RANS approach and employing three different turbulent viscosity models. In addition to comparison calculations an extensive sensitivity study was performed. The results show that the RANS approach and the turbulent viscosity models can be used for prediction of forced convection heat transfer to lead-bismuth-eutectic. However, the turbulent Prandtl number has to be carefully selected depending on the respective turbulence model.

Thiele, Roman, E-mail: romant@kth.se [Nuclear Reactor Technology, School of Engineering Sciences, Royal Institute of Technology, SE-10691 Stockholm (Sweden); Anglart, Henryk, E-mail: henryk@kth.se [Nuclear Reactor Technology, School of Engineering Sciences, Royal Institute of Technology, SE-10691 Stockholm (Sweden)

2013-01-15

68

Energy Technology Data Exchange (ETDEWEB)

Two-dimensional forced convective flow over a circular cylinder embedded in a saturated porous medium has been numerically studied. The axis of a cylinder is arranged at right angles to the oncoming forced flow. The cylinder is heated to a uniform surface temperature which is different from than that of the fluid in the forced flow ahead of the cylinder. There is an impermeable plane surface above the cylinder, the surface being parallel to the forced flow. This surface is either adiabatic or at a uniform specified temperature that is between the temperature of the liquid flowing over the cylinder and the temperature of the cylinder surface. The study is based on the Darcy type assumptions. The governing equations, written in dimensionless form, have been solved using the finite element method. The solution has as parameters the dimensionless depth of the cylinder from the impermeable surface, the Peclet number and the dimensionless thermal boundary condition at the impermeable surface. Solutions have been obtained for a range of values of all these parameters. The results indicate that the dimensionless impermeable surface temperature has a strong influence of the heat transfer rate from the cylinder at smaller values of the dimensionless cylinder-to-surface distance.

Oosthuizen, P.H.

1999-07-01

69

International Nuclear Information System (INIS)

A forced convective heat transfer correlation is proposed by determine the fractal dimension based on the self-avoiding random walk statistics. Nusselt number measuring the convective heat transfer area is correlated with Reynolds number measuring the line generated by the turbulent eddies. The fractal dimension , ?F = 3 over d+2 is derived from the self-avoiding random walking model. The proposed heat transfer correlation here is Nu = C Re?FPr1/3. The present model is well fitted with the Reynolds analogy between the friction factor and the heat transfer correlation

1999-10-01

70

Directory of Open Access Journals (Sweden)

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.

Thamir K. Jassem

2013-05-01

71

International Nuclear Information System (INIS)

An analytical study on laminar and fully developed forced convection heat transfer in a parallel-plate horizontal channel filled with an anisotropic permeability porous medium is performed. The principal axis of the anisotropic porous medium is oriented from 0 to 90 degrees. A constant heat flux is applied on the outer wall of the channel. Both clear (Newtonian) fluid and Darcy viscous dissipations are considered in the energy equation. Directional permeability ratio parameter A* is defined to combine both the effect of the dimensionless permeability ratio parameter K*=(K1/K2) and orientation angle ? into one parameter. The effects of the parameter A*, the Darcy number Da and the modified Brinkman number Br* on the heat transfer and fluid flow characteristics in the channels are investigated and presented in graphs. The obtained results show that the parameters A*, Da and Br* have strong effects on the dimensionless normalized velocity and temperature profiles as well as on the Nusselt number. It is found that for a particular value of A*, called as critical value Acr*, the external heat applied to the surface of the channel is balanced by the internal heat generation due to viscous dissipation and the bulk mean temperature approaches the wall temperature. Hence, the Nusselt number approaches infinity for the critical values Acr*. (authors)

2010-10-01

72

Energy Technology Data Exchange (ETDEWEB)

An analytical study on laminar and fully developed forced convection heat transfer in a parallel-plate horizontal channel filled with an anisotropic permeability porous medium is performed. The principal axis of the anisotropic porous medium is oriented from 0 to 90 degrees. A constant heat flux is applied on the outer wall of the channel. Both clear (Newtonian) fluid and Darcy viscous dissipations are considered in the energy equation. Directional permeability ratio parameter A{sup *} is defined to combine both the effect of the dimensionless permeability ratio parameter K{sup *}=(K{sub 1}/K{sub 2}) and orientation angle {phi} into one parameter. The effects of the parameter A{sup *}, the Darcy number Da and the modified Brinkman number Br{sup *} on the heat transfer and fluid flow characteristics in the channels are investigated and presented in graphs. The obtained results show that the parameters A{sup *}, Da and Br{sup *} have strong effects on the dimensionless normalized velocity and temperature profiles as well as on the Nusselt number. It is found that for a particular value of A{sup *}, called as critical value A{sub cr}{sup *}, the external heat applied to the surface of the channel is balanced by the internal heat generation due to viscous dissipation and the bulk mean temperature approaches the wall temperature. Hence, the Nusselt number approaches infinity for the critical values A{sub cr}{sup *}. (authors)

Moghtada, Mobedi [Mechanical Engineering Department, Izmir Institute of Technology, Urla, Izmir (Turkey); Ozgur, Cekmer [Energy Engineering Program, Izmir Institute of Technology, Urla, Izmir (Turkey); Pop, I. [Faculty of Mathematics, University of Cluj, CP, Cluj (Romania)

2010-10-15

73

International Nuclear Information System (INIS)

An experimental model was developed for a nuclear waste repository storage room. Data were taken over a Reynolds number range of 6,000 to 180,000, covering both the forced and mixed (combined natural and forced) regimes of convection. Data are presented for several circumferential boundary conditions. Results indicate that the natural convection component is significant. Also presented is a plot of the tunnel air isotherms for Re = 10,000

1986-01-01

74

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

75

Critical heat flux (CHF) during forced convective subcooled boiling with a plane jet has been yet made insufficient except for saturation condition when comparing CHF with impinging jet system including multiple jets. The present experiment has measured the CHF with plane jet on a rectangular heated surface of 40, 60, and 80mm in length and 10 and 20mm in width. Subcooled liquid being supplied through the plane jet with a different thickness of 1 and 2mm, covers the heated surface where rigorous boiling takes place. The experiment varies a jet velocity from 3 to 15m/s, a subcooling from 0 to 60K, and system pressure at 0.1MPa for water and at 1.5 to 3.0MPa for R22. It is found that the existing correlation for saturation can be applied to the CHF of water, but hardly to the CHF of R22 in spite of saturation condition. After the effects of jet velocity and subcooling on the CHF can be elucidated, a new correlation including the effect of subcooling is proposed to predict most of the CHF data within an accuracy of +/-20 percent. This correlation for saturated condition is found to interestingly agree with that theoretically derived by applying the Katto and Haramura criterion to this system.

Wang, X.; Monde, M.

76

International Nuclear Information System (INIS)

The Forced-Convective film-boiling heat transfer was investigated experimentally focusing on turbulence on liquid phase in high Reynolds number region and the mechanism of the critical condition on the heat transfer characteristics. Quenching tests of a vertical-long-thick cylindrical pipe were conduced for subcooled R-113 near 0.1 Mpa. A range of bulk-liquid velocity was 0.1 to 11.4 m/s. The characteristics of the heat-transfer coefficients for the liquid velocity above 2.4 m/s were qualitatively close to the Dittus-Boelter type correlation based upon the turbulent flow. The critical condition was about Re1=3.4 x 104 for R-113. Furthermore, it was noticeable that thickness of vapor film in the film boiling at the critical condition was just as that of the viscous sublayer in turbulent boundary layer of the bulk liquid. (author)

2000-05-01

77

Optimal discrete distribution of heat flux elements for in-tube laminar forced convection

International Nuclear Information System (INIS)

Highlights: ? Effect of an insulated segment on the peak temperature of a heated pipe is studied. ? Peak temperatures in a pipe wall heated by step-wise heat flux are minimized. ? Effects of Graetz number, fraction and number of insulated segments are studied. ? Use of upper unheated section(s) enhance(s) the thermal performance. -- Abstract: A new technique is proposed to enhance the heat transfer from a discretely heated pipe to a developing laminar fluid flow. Unlike the common heating situation where the fluid is continuously heated along the pipe wall with uniform heat flux, the proposed technique consists in heating the fluid with stepwise distributed heat flux, namely by placing insulated segments between the heated segments. Applying this technique, the effective length of the thermal entrance region is enlarged and as a result, the average heat transfer is invigorated. In order to maximize the heating performance, an optimal placement of the insulated segments between the heated segments is calculated according to constructal design. This serves to describe the optimal stepwise distribution of the heat flux. Owing that the total heat load is considered fixed, the maximization of the heating performance translates into the minimization of the peak temperature (‘hot spot’) of the pipe wall. The analytical results demonstrate that the optimal location of the insulated segments along with the reduction of the peak temperature strongly depend on the Graetz number. It is also shown that for intermediate values of the Graetz number, the peak temperatures are remarkably reduced in response to the optimal placement of the insulated/heated segments

2013-04-01

78

Critical heat flux and associated phenomena in forced convective boiling in nuclear systems

Digital Repository Infrastructure Vision for European Research (DRIVER)

In evaporation of a liquid flowing in a tube or nuclear fuel element, there exists a transition (known as "dryout", "burnout", "boiling crisis" or "critical heat flux", CHF) from a high heat transfer coefficient regime to one of greatly reduced heat transfer coefficient. The conditions leading to dryout or CHF and the behaviour of wall temperatures in the ("post dryout or post CHF") region beyond it are of immense importance in nuclear reactor safety. I...

Ahmad, Masroor

2012-01-01

79

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)

1994-01-01

80

Energy Technology Data Exchange (ETDEWEB)

A model has been developed for the simulation of laminar hydrodynamic and heat transfer characteristics in a horizontal circular tube for a suspension flow with micro-size phase-change material (PCM) particles. The energy equation is formulated by taking into consideration the heat absorption (or release) due to the phase-change process. The heat source or heat generation term in the energy equation is derived from solutions for freezing or melting inside the particles. The supercooling breakdown induced by the stochastic character of the crystallization is taken into account in the modeling with nucleation laws. The solutions were compared to experimental results obtained by different authors. (author)

Kousksou, T.; El Rhafiki, T.; El Omari, K.; Zeraouli, Y.; Le Guer, Y. [Laboratoire de Thermique Energetique et Procedes, Universite de Pau et des Pays de l' Adour (UPPA), Campus Universitaire, 64000 Pau (France)

2010-12-15

81

A review is made of some of the experimental data and analyses applicable to convective heat transfer in fully turbulent flow in smooth tubes with liquid metals and viscous Newtonian fluids. An empirical equation is evolved that closely approximates heat-transfer values obtained from selected analyses and experimental data for Prandtl numbers from 0.001 to 1000. The terms included in the equation are Reynolds number, Prandtl number, and an empirical diffusivity ratio between heat and momentum.

vonGlahn, Uwe H.

1960-01-01

82

Energy Technology Data Exchange (ETDEWEB)

Highlights: {yields} The effect of holes placed on perforated finned heat exchangers on convective heat transfer experimentally investigated. {yields} Six millimeter-diameter holes were opened on each circular fin on a heating tube in order to increase convective heat transfer. {yields} These holes were placed on the circular fins in such a way as to follow each other at the same chosen angle. {yields} The holes created turbulence in a region near the heating tube surface on the bottom of the fin. - Abstract: In this study, the effect of holes placed on perforated finned heat exchangers on convective heat transfer experimentally investigated. Six millimeter-diameter holes were opened on each circular fin on a heating tube in order to increase convective heat transfer. These holes were placed on the circular fins in such a way as to follow each other at the same chosen angle. The holes created turbulence in a region near the heating tube surface on the bottom of the fin. Some experiments were then performed to analyze the effect of this turbulence on heat transfer and pressure drop. These experiments were carried out at six different angular locations in order to determine the best angular location. In addition, a perforated finned heater was compared with an imperforate finned heater to observe the differences. In the cases of the Re above the critical value, Nusselt numbers for the perforated finned positions are 12% higher than the Nusselt numbers for the imperforate state. Moreover, a correlation has been obtained between the Re and Nu in the Re number above the critical value and the Re below the critical value. Meanwhile, correlations regarding pressure drops in the flow areas have been obtained.

Karabacak, Rasim [Department of Mechanical Engineering, Pamukkale University, 20070 Kinikli, Denizli (Turkey); Yakar, Guelay, E-mail: gyakar@pau.edu.tr [Department of Mechanical Engineering, Pamukkale University, 20070 Kinikli, Denizli (Turkey)

2011-08-15

83

International Nuclear Information System (INIS)

Highlights: ? The effect of holes placed on perforated finned heat exchangers on convective heat transfer experimentally investigated. ? Six millimeter-diameter holes were opened on each circular fin on a heating tube in order to increase convective heat transfer. ? These holes were placed on the circular fins in such a way as to follow each other at the same chosen angle. ? The holes created turbulence in a region near the heating tube surface on the bottom of the fin. - Abstract: In this study, the effect of holes placed on perforated finned heat exchangers on convective heat transfer experimentally investigated. Six millimeter-diameter holes were opened on each circular fin on a heating tube in order to increase convective heat transfer. These holes were placed on the circular fins in such a way as to follow each other at the same chosen angle. The holes created turbulence in a region near the heating tube surface on the bottom of the fin. Some experiments were then performed to analyze the effect of this turbulence on heat transfer and pressure drop. These experiments were carried out at six different angular locations in order to determine the best angular location. In addition, a perforated finned heater was compared with an imperforate finned heater to observe the differences. In the cases of the Re above the critical value, Nusselt numbers for the perforated finned positions are 12% higher than the Nusselt numbers for the imperforate state. Moreover, a correlation has been obtained between the Re and Nu in the Re number above the critical value and the Re below the critical value. Meanwhile, correlations regarding pressure drops in the flow areas have been obtained.

2011-08-01

84

Directory of Open Access Journals (Sweden)

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.

Sundus Hussein Abd

2012-01-01

85

International Nuclear Information System (INIS)

It has been pointed out that the film-boiling heat transfer is affected by the structures of the vapor flow in vapor film, the liquid-vapor interface and the liquid boundary layer adjacent to the interface. In this study the forced-convection film-boiling heat transfer was investigated experimentally focusing on the heat transfer characteristics in high Reynolds number region and the critical condition on film-boiling heat transfer characteristics. The characteristics of the measured heat-transfer coefficients for the liquid velocity above 1.5 m/s were qualitatively close to the Dittus-Boelter type correlation based upon the turbulent flow. It was found that the critical condition, which was defined as the heat-transfer coefficient increased with the liquid velocity, was about Rel0.84Prl = Pel* = 6 x 104 in the present experimental range. Furthermore, it was noticeable that thickness of vapor film in the film boiling at the critical condition was just as that of the viscous sublayer in turbulent boundary layer of the bulk liquid. (author)

1999-04-19

86

Forced convection heat transfer of steam in a square ribbed channel

Energy Technology Data Exchange (ETDEWEB)

An experimental study of heat transfer characteristics of steam in a square channel (simulating a gas turbine blade cooling passage) with two opposite surfaces roughened by 60 deg parallel ribs was performed. The ranges of key governing parameters were: Reynolds numbers (Re) based on the channel hydraulic diameter (30000-140000), entry gauge pressure (0.2Mpa-0.5Mpa), heat flux of heat transfer surface area (5kWm{sup -2}-20kWm{sup -2}), and steam superheat (13 .deg. C-51 .deg. C). The test channel length was 1000mm, while the rib spacing (p/e) was 10, and the ratio of rib height (e) to hydraulic diameter (D) was 0.048. The test channel was heated by passing current through stainless steel walls instrumented with thermocouples. The local heat transfer coefficients on the ribbed wall from the channel entrance to the fully developed regions were measured. The semi-empirical correlation was fitted out by using the average Nusselt numbers in the fully developed region to cover the range of Reynolds number. The correlation can be used in the design of new generation of gas turbine blade cooled by steam.

Liu, Jiazeng; Gao, Jianmin; Gao, Tieyu [Xi' an Jiaotong University, Shaanxi (China)

2012-04-15

87

Forced convection heat transfer of steam in a square ribbed channel

International Nuclear Information System (INIS)

An experimental study of heat transfer characteristics of steam in a square channel (simulating a gas turbine blade cooling passage) with two opposite surfaces roughened by 60 deg parallel ribs was performed. The ranges of key governing parameters were: Reynolds numbers (Re) based on the channel hydraulic diameter (30000-140000), entry gauge pressure (0.2Mpa-0.5Mpa), heat flux of heat transfer surface area (5kWm-2-20kWm-2), and steam superheat (13 .deg. C-51 .deg. C). The test channel length was 1000mm, while the rib spacing (p/e) was 10, and the ratio of rib height (e) to hydraulic diameter (D) was 0.048. The test channel was heated by passing current through stainless steel walls instrumented with thermocouples. The local heat transfer coefficients on the ribbed wall from the channel entrance to the fully developed regions were measured. The semi-empirical correlation was fitted out by using the average Nusselt numbers in the fully developed region to cover the range of Reynolds number. The correlation can be used in the design of new generation of gas turbine blade cooled by steam

2012-04-01

88

Experimental investigation of forced convective heat transfer in rectangular micro-channels

Directory of Open Access Journals (Sweden)

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.

R. Kalaivanan

2010-05-01

89

Application of Rayleigh-Ritz method to forced convection turbulent heat transfer

Energy Technology Data Exchange (ETDEWEB)

An analytical model to predict heat transfer rates to an incompressible fluid in turbulent flow, with fully developed velocity profile, between a heated plate and a parallel, insulated plate is developed. The model employs van Driest's mixing length expression near the wall, a constant eddy diffusivity in the core and a constant turbulent Prandtl number. An approximate solution obtained by employing Rayleigh-Ritz method is shown to compare well with the 'exact' solution obtained by numerical integration of the differential equations. The results are compared with the available experimental data and analytical solutions.

Diaz, L.A.; Suryanarayana, N.V.

1981-06-16

90

Heterogeneous nanofluids: natural convection heat transfer enhancement

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 ...

Oueslati Fakhreddine; Bennacer Rachid

2011-01-01

91

A new mechanistic model of critical heat flux in forced-convection subcooled boiling

Energy Technology Data Exchange (ETDEWEB)

Because of its practical importance and various industrial applications, the process of subcooled flow boiling has attracted a lot of attention in the research community in the past. However, the existing models are primarily phenomenological and are based on correlating experimental data rather than on a first-principle analysis of the governing physical phenomena. Even though the mechanisms leading to critical heat flux (CHF) are very complex, the recent progress in the understanding of local phenomena of multiphase flow and heat transfer, combined with the development of mathematical models and advanced Computational Fluid Dynamics (CFD) methods, makes analytical predictions of CHF quite feasible. Various mechanisms leading to CHF in subcooled boiling have been investigated. A new model for the predictions of the onset of CHF has been developed. This new model has been coupled with the overall boiling channel model, numerically implemented in the CFX 4 computer code, tested and validated against the experimental data of Hino and Ueda. The predicted critical heat flux for various channel operating conditions shows good agreement with the measurements using the aforementioned closure laws for the various local phenomena governing nucleation and bubble departure from the wall. The observed differences are consistent with typical uncertainties associated with CHF data.

Alajbegovic, A.; Kurul, N.; Podowski, M.Z.; Drew, D.A.; Lahey, R.T. Jr. [Rensselaer Polytechnic Inst., Troy, NY (United States). Center for Multiphase Research

1997-10-01

92

A new mechanistic model of critical heat flux in forced-convection subcooled boiling

International Nuclear Information System (INIS)

Because of its practical importance and various industrial applications, the process of subcooled flow boiling has attracted a lot of attention in the research community in the past. However, the existing models are primarily phenomenological and are based on correlating experimental data rather than on a first-principle analysis of the governing physical phenomena. Even though the mechanisms leading to critical heat flux (CHF) are very complex, the recent progress in the understanding of local phenomena of multiphase flow and heat transfer, combined with the development of mathematical models and advanced Computational Fluid Dynamics (CFD) methods, makes analytical predictions of CHF quite feasible. Various mechanisms leading to CHF in subcooled boiling have been investigated. A new model for the predictions of the onset of CHF has been developed. This new model has been coupled with the overall boiling channel model, numerically implemented in the CFX 4 computer code, tested and validated against the experimental data of Hino and Ueda. The predicted critical heat flux for various channel operating conditions shows good agreement with the measurements using the aforementioned closure laws for the various local phenomena governing nucleation and bubble departure from the wall. The observed differences are consistent with typical uncertainties associated with CHF data

1997-10-04

93

Forced-convection peak heat flux on cylindrical heaters in water and refrigerant 113

An investigation was conducted of the peak heat flux on cylindrical heaters in a fluid flowing perpendicular to the major axis of the heater. The test fluids were water and Refrigerant 113. Heaters of 0.049 to 0.181 cm diameter were tested over a fluid velocity range of 10.1 to 81.1 cm/sec. The experimental results were observed to fall within two regions based on the vapor removal geometry: jets or sheets. Mathematical models for each region successfully correlated the data for both fluids.

Cochran, T. H.; Andracchio, C. R.

1974-01-01

94

Two-phase pressure drop and heat transfer of sodium at forced convection

International Nuclear Information System (INIS)

Experiments with sodium for the two-phase pressure drop in vertical tubes with upward flow (internal diameters 6 and 9 mm) performed at the Joint Research Centre (JRC) of the European Communities in Ispra, Italy, and at the Nuclear Research Centre in Karlsruhe (KfK) were evaluated and analysed. Furthermore, experiments for the single-phase and two-phase heat transfer in the grid spaced twelve-rod bundle (pd/d =1.3, rod diameter 8 mm) with flow in axial direction performed at the JRC were evaluated and analysed. The pressure drop measurements were carried out at moderate to high mass flow rates (30 to 4500 kg/(m2s)) and at moderate pressures (50 to 300 kPa, density ratio ?f/?g = 950 to 5400). The measurements for the single-phase heat transfer at high heat fluxes (0.16 to 1.6 MW/m2) were carried out in the Reynolds number region (3100 2s)) and at high heat fluxes (0.46 to 1.6 MW/m2) within the temperature range from 870 to 9700C. For the subsequent calculation of the experiments relating to the two-phase pressure drop a computer program was developed, which is based on the so-called slip model. It requires a friction pressure loss correlation and a slip correlation. The tested correlations were not suitable for describing the experimental measurements. Accordingly, simplified equations of momentum were used to develop a new slip correlation for the case of annular flow together with the annular-mist flow, the most important two-phase flow regimes for sodium in the measurement range. After the inception of the entrainment - transition from the annular flow to the annular-mist flow - an even larger fraction of liquid enters the vapour core in the form of droplets, as the vapour quality increases. An equation was formulated for the slip in this region and adapted to the experiments via coefficients. (orig./GL)

1989-01-01

95

Convective Flow under Rotating Force.

Directory of Open Access Journals (Sweden)

Full Text Available A finite difference code using the primitive variables is used to simulate the mixed convection in air (Pr=0.7 and liquid metals (Pr=0.015. The present study involves numerical simulation of momentum and energy equations in order to analyze two dimensional mixed convection in air and liquid metals in a differentially heated square cavity subjected to rotation for a broad range of operating parameters i.e. Rayleigh number (Ra , Taylor number (Ta and rotational Raleigh number (Raw.

Meenu Hooda

2013-09-01

96

Experimental study of micro-particle fouling under forced convective heat transfer

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english 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.

Peyghambarzadeh, S. M.; Vatani, A.; Jamialahmadi, M..

97

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 ...

Sundus Hussein Abd

2012-01-01

98

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)

1965-01-01

99

International Nuclear Information System (INIS)

This paper explores the bearing that a non-uniform distribution of heat flux used as a wall boundary condition exerts on the heat transfer improvement in a round pipe. Because the overall heat load is considered fixed, the heat transfer improvement is viewed through a reduction in the maximum temperature (‘hot spot’) by imposing optimal distribution of heat flux. Two cases are studied in detail 1) fully developed and 2) developing flow. Peak temperatures in the heated pipe wall are calculated via an analytical approach for the fully developed case, while a numerical simulation based on CFD is employed for the developing case. By relaxing the heat flux distribution on the pipe wall, the numerical results imply that the optimum distribution of heat flux, which minimizes the peak temperatures corresponds with the ‘descending’ distribution. Given that the foregoing approach is quite different from the ‘ascending’ heat flux distribution recommended in the literature by means of the entropy generation minimization (EGM) method, it is inferred that the optimization of heat transfer and fluid flow, in comparison with the thermodynamic optimization, may bring forth quite different guidelines for the designs of thermal systems under the same constraints and circumstances. -- Highlights: • Considered the bearing of non-uniform distribution of heat flux on the hot spots. • Determined the optimal distribution of heat flux that minimizes the hot spots. • Results are compared with those obtained by EGM method

2013-11-03

100

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)

1994-01-01

101

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

1996-08-19

102

Digital Repository Infrastructure Vision for European Research (DRIVER)

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...

2009-01-01

103

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 m...

2011-01-01

104

Thermal instability of forced convection boundary layers

International Nuclear Information System (INIS)

Thermal instability of forced convection boundary layers with non-zero steamwise pressure gradient is examined. Such an instability increases as the third power of the layer thickness, hence could be significant in the low Peclet number flows encountered in solar and microelectronic heat transfer as well as in reactor safety considerations associated with pump shutdowns. The analysis is carried out for the family of Falkner-Skan flows, here viewed as the lowest order local similarity approximation of general forced convection boundary layers. Only the streamwise vortex mode, which had previously been shown to be the dominant mode for buoyancy generated instability for fluid layers with shear at low Reynolds numbers are considered. 19 refs

1981-01-01

105

International Nuclear Information System (INIS)

Steady-state numerical results for the solution to the non-linear thermal problem of combined free and forced laminar convection in inclined rectangular channels with constant but unequal surface temperature are presented for an incompressible, viscous fluid whose Prandtl number, Pr = 0.73. Fluid properties are assumed constant, except for density variations with temperature. Maximum values exist for the mean friction factor, Nusselt and Stanton numbers when the inclination to the horizontal lies between 30"0 and 60"0 for a given Archimedes number, Ar. Also, for any given inclination a unique solution exists when Ar = 0,50. (Author)

1981-12-18

106

Turbulent Forced Convection in Ducts

DEFF Research Database (Denmark)

The need for a reliable and reasonably accurate turbulence model without specific convergence problems for calculating duct flows in industrial applications has become more evident. In this chapter a general computational method is presented for calculating turbulent quantities in any arbitrary three-dimensional duct. Four different turbulence models for predicting the turbulent Reynolds stresses, namely the standard k-ï¥ model, the non-linear k-ï¥ model of Speziale, an Explicit Algebraic Stress Model (EASM) and a full Reynolds Stress Model (RSM) are compared with each other. The turbulent heat fluxes are modeled by the SED concept, the GGDH and the WET methods. The two-equation model of temperature invariance and its dissipation rate for calculating turbulent heat fluxes is also discussed. The numerical method is based on the finite volume technique with non-staggered grid arrangement. The SIMPLEC algorithm is used for pressure~ velocity coupling. A modified SIP solving method is implemented for solving the equations. The van Leer, QUICK and hybrid schemes are applied for treating the convective terms. Periodic boundary conditions are imposed in the main flow direction for decreasing the number of grid points in this direction. In practical applications, periodic conditions in the main flow direction are commonly justified because in wavy or corrugated ducts such conditions occur naturally. Keyword: Turbulent, Heat Transfer, Convection, Turbulence Modelling

Rokni, Masoud

2000-01-01

107

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.

2012-11-26

108

Turbulent mixed convection in asymmetrically heated vertical channel

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 widt...

Mokni Ameni; Mhiri Hatem; Le Palec Georges; Bournot Philippe

2012-01-01

109

Transient convective heat transfer

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 calculation 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 ...

Padet, J.

2005-01-01

110

International Nuclear Information System (INIS)

Highlights: ? ANN was trained to predict the CHF with a better accuracy than GA. ? CHF increases with jet velocity. ? CHF decreases with an increase in L/d and the number of jets. ? CHF increases at first and then decreases with an increase of pressure. - Abstract: In this paper, a three-layer Back Propagation (BP) algorithm artificial neural network (ANN) for predicting critical heat flux (CHF) in saturated forced convective boiling on a heated surface with impinging jets was trained successfully with a root mean square (RMS) error of 17.39%. The input parameters of the ANN are liquid-to-vapor density ratio, ?l/?v, the ratio of characteristic dimension of the heated surface to the diameter of the impinging jet, L/d, reciprocal of the Weber number, 2?/?lu2(L - d), and the number of impinging jets, Nj. The output is dimensionless heat flux, qco/?vHfgu. Based on the trained ANN, the influence of principal parameters on CHF has been analyzed as follows. CHF increases with an increase in jet velocity and decreases with an increase in L/d and Nj. CHF increases with an increase in pressure at first and then decreases. Besides, a new correlation was generalized using genetic algorithm (GA) as a comparison with ANN to confirm the advantage of ANN.

2011-09-01

111

Convection driven by internal heating

Two-dimensional direct numerical simulations are conducted for convection sustained by uniform internal heating in a horizontal fluid layer. Top and bottom boundary temperatures are fixed and equal. Prandtl numbers range from 0.01 to 100, and Rayleigh numbers (R) are up to 5x10^5 times the critical R at the onset of convection. The asymmetry between upward and downward heat fluxes is non-monotonic in R. In a broad high-R regime, dimensionless mean temperature scales as R^{-1/5}. We discuss the scaling of mean temperature and heat-flux-asymmetry, which we argue are better diagnostic quantities than the conventionally used top and bottom Nusselt numbers.

Goluskin, David

2012-01-01

112

Scientific Electronic Library Online (English)

Full Text Available SciELO South Africa | Language: English Abstract in english 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.

Philip O., Olanrewaju; Jacob A., Gbadeyan; Tasawar, Hayat; Awatif A., Hendi.

113

Directory of Open Access Journals (Sweden)

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.

Tasawar Hayat

2011-09-01

114

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

1985-03-01

115

Convection coefficient equations for forced air flow over flat surfaces

Energy Technology Data Exchange (ETDEWEB)

In this paper, various comparisons among well-known equations of the convection heat transfer coefficient for forced air flow over flat surfaces and particularly over flat plate solar collectors, with the aim at arriving at a consensus on which of such equations is more accurate are carried out. Through the application of basic principles, various accuracies, inaccuracies and validations of the considered equations have been found and shown, and a consensus reached. Such consensual equation, which comes from the boundary layer theory and takes into account the determining laminar and turbulent flows as well as the wind direction and the decay of the convection coefficient over the surface, also showed close agreement with different experimental works and tends to represent more accurately the actual heat transfer from/to any flat surface submitted to forced convection. (author)

Sartori, Ernani [Universidade Federal da Paraiba, Av. Silvino Lopes 425/404, 58039-190 Joao Pessoa PB (Brazil)

2006-09-15

116

International Nuclear Information System (INIS)

To clarify thermohydrodynamic processes in a heat exchanger with a low relative length of heated sections (l/d < 50), convective heat transfer in a heated tube is analytically investigated. Equations of heat transfer in the wall and in liquid are considered along with motion equations, i.e. a conjugate problem is solved. Iterations over the joint of temperature fields of wall inner surface and the external current surface is carried out for boundary conditions of the 1st kind from the wall and the second kind from the current. The model adequacy to real physical processes is experimentally established. The information obtained permits to calculate practically any termohydrodynamic characteristic of the model considered. Results of investigations permit to recommend the above mathematical model to investigate analogous problems connected with the calculation of convection heat exchange in porced liquid currents with variable properties under conditions of considerable nonisothermality characteristic of small sections of active heating

1980-09-01

117

Modeling of laminar forced convection in spherical- pebble packed beds

Energy Technology Data Exchange (ETDEWEB)

There are many parameters that have significant effects on forced convection heat transfer in packed beds, including Reynolds and Prandtl numbers of flow, porosity, pebble geometry, local flow conditions, wall and end effects. In addition, there have been many experimental investigations on forced convection heat transfer in packed beds and each have studied the effect of some of these parameters. Yet, there is not a reliable correlation that includes the effect of main parameters: at the same time, the prediction of precise correct limits for very low and high Reynolds numbers is off hand. In this article a general well-known model of convection heat transfer from isothermal bodies, next to some previous reliable experimental data has been used as a basis for a more comprehensive and accurate correlation to calculate the laminar constant temperature pebble-fluid forced convection heat transfer in a homogeneous saturated bed with spherical pebbles. Finally, for corroboration, the present results are compared with previous works and show a very good agreement for laminar flows at any Prandtl number and all porosities.

Hadad, Yaser; Jafarpur, Khosrow [Shiraz University, Shiraz (Iran, Islamic Republic of)

2012-02-15

118

Directory of Open Access Journals (Sweden)

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.

Rajesh Khatri

2012-03-01

119

The optimal spacing for cylinders in crossflow forced convection

International Nuclear Information System (INIS)

In this note I draw attention to a new fundamental aspect of the heat transfer performance of a bundle of parallel cylinders with crossflow forced convection, namely, the maximization of the thermal contact between the bundle and the fluid, when the volume occupied by the bundle is fixed. In the experiments described by Jubran et al. we have seen empirical evidence that the total heat transfer rate is maximum when the cylinder-to-cylinder spacing S has a certain value. This finding is important because it has been overlooked for decades, while forced convection from cylinders in crossflow grew into one of the most researched topics in heat transfer. 8 refs., 3 figs

1995-08-01

120

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

1997-05-18

121

Mechanistic modeling of CHF in forced-convection subcooled boiling

Energy Technology Data Exchange (ETDEWEB)

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.

Podowski, M.Z.; Alajbegovic, A.; Kurul, N.; Drew, D.A.; Lahey, R.T. Jr. [Rensselaer Polytechnic Inst., Troy, NY (United States). Center for Multiphase Research

1997-05-01

122

Natural, forced and mixed convection in fibrous insulation

International Nuclear Information System (INIS)

A numerical solution of flow and temperature distribution in fibrous insulation has been obtained. Rectangular and cylindrical geometries have been modeled. Boundary conditions included permeable hot wall and convective heat transfer at the walls. Good agreement has been obtained with published experimental and numerical values on rectangular cavities. The computed velocity and temperature profiles gave a better understanding of flow and heat transfer phenomena in fibrous insulation. Local cold wall and average Nusselt numbers presented, provide useful information in the design of the fibrous insulation for concrete reactor vessel and primary coolant piping of the gas cooled nuclear power plants. Average Nusselt number has been correlated with Rayleigh number when only natural convection is present, and with Rayleigh and Reynolds numbers when a combination of natural and forced convection is present

1978-01-01

123

Scientific Electronic Library Online (English)

Full Text Available SciELO Colombia | Language: Spanish Abstract in spanish 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.

Néstor Enrique, Cerquera Peña; Yaneth Liliana, Ruiz Osorio; Eduardo, Pastrana Bonilla.

2010-04-01

124

Directory of Open Access Journals (Sweden)

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 â??V-shaped orifice tubulators, VOTâ?. The effects of Blockage Ratios (BR = 0.05-0.20, flow attack angles (Î± = 20Â°, 30Â° and 45Â° 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, Î± = 30Â° at the highest Reynolds number of V-Downstream and V-Upstream, respectively.

Amnart Boonloi

2014-01-01

125

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)

2005-10-02

126

Convective cooling of three discrete heat sources in channel flow

Digital Repository Infrastructure Vision for European Research (DRIVER)

A numerical investigation was performed to evaluate distinct convective heat transfer coefficients for three discrete strip heat sources flush mounted to a wall of a parallel plates channel. Uniform heat flux was considered along each heat source, but the remaining channel surfaces were assumed adiabatic. A laminar airflow with constant properties was forced into the channel considering either developed flow or a uniform velocity at the channel entrance. The conservation equations were solved...

Thiago Antonini Alves; Altemani, Carlos A. C.

2008-01-01

127

Optimization principles for convective heat transfer

Energy Technology Data Exchange (ETDEWEB)

Optimization for convective heat transfer plays a significant role in energy saving and high-efficiency utilizing. We compared two optimization principles for convective heat transfer, the minimum entropy generation principle and the entransy dissipation extremum principle, and analyzed their physical implications and applicability. We derived the optimization equation for each optimization principle. The theoretical analysis indicates that both principles can be used to optimize convective heat-transfer process, subject to different objectives of optimization. The minimum entropy generation principle, originally derived from the heat engine cycle process, optimizes the convective heat-transfer process with minimum usable energy dissipation focusing on the heat-work conversion. The entransy dissipation extremum principle however, originally for pure heat conduction process, optimizes the heat-transfer process with minimum heat-transfer ability dissipation, and therefore is more suitable for optimization of the processes not involving heat-work conversion. To validate the theoretical results, we simulated the convective heat-transfer process in a two-dimensional foursquare cavity with a uniform heat source and different temperature boundaries. Under the same constraints, the results indicate that the minimum entropy production principle leads to the highest heat-work conversion while the entransy dissipation extremum principle yields the maximum convective heat-transfer efficiency. (author)

Chen, Qun; Guo, Zeng-Yuan [Department of Engineering Mechanics, Tsinghua University, Beijing 100084 (China); Wang, Moran [Division of Earth and Environmental Science, Los Alamos National Laboratory, P.O. Box 1663, Mail Stop T003, Los Alamos, NM 87545 (United States); Pan, Ning [Department of Biological System Engineering, University of California, Davis, CA 95616 (United States)

2009-09-15

128

Convection in a horizontally heated sphere

Natural convection in horizontally heated spherical fluid-filled cavities is considered in the low Grashof number limit. The equations governing the asymptotic expansion are derived for all orders. At each order a Stokes problem must be solved for the momentum correction. The general solution of the Stokes problem in a sphere with arbitrary smooth body force is derived and used to obtain the zeroth-order (creeping) flow and the first-order corrections due to inertia and buoyancy. The solutions illustrate the two mechanisms adduced by Mallinson & de Vahl Davis (1973, 1977) for spanwise flow in horizontally heated cuboids. Further, the analytical derivations and expressions clarify these mechanisms and the conditions under which they do not operate. The inertia and buoyancy effects vanish with the Grashof and Rayleigh numbers, respectively, and both vanish if the flow is purely vertical, as in a very tall and narrow cuboid.

McBain, G. D.

2001-07-01

129

Two regimes of forced turbulent convection

We study experimentally a forced turbulent convection in the Rayleigh-B\\'{e}nard apparatus with an additional source of turbulence produced by the two oscillating grids located nearby the side walls of the chamber. Two different regimes have been observed in the forced turbulent convection. When the frequency of the grid oscillations f>2 Hz, the large-scale circulation (LSC) is totally destroyed, and the destruction of the LSC is accompanied by a strong change of the mean temperature distribution. For the very low frequency the thermal structure inside the LSC is inhomogeneous and anisotropic. The hot thermal plumes accumulate at one side of LSC, and cold plumes concentrate at the opposite side of LSC. The mean temperature gradient in the horizontal direction inside the LSC is significantly larger than in the vertical direction. For the high frequency (f > 10 Hz), LSC has not been observed and the mean temperature gradient in the central flow region in the vertical direction, \

Eidelman, A; Kleeorin, N; Rogachevskii, I; Sapir-Katiraie, I

2009-01-01

130

Measurement of thermoacoustic convection heat transfer phenomenon

In this paper the results of an experimental investigation of thermoacoustic convection (TAC) heat transfer phenomenon in both zero-gravity and gravity environment are presented and compared with pure conduction heat transfer. The numerical solutions of the governing equations obtained by others for TAC heat transfer phenomenon are also discussed. The experimental results show that for rapid heating rate at a boundary, the contribution of TAC heat transfer to a gas could be significantly (one order of magnitude) higher than heat transfer rate from pure conduction. The results also show significantly reduced transient time in heat transfer processes involving thermoacoustic convective heat transfer mode in both space and gravity environment.

Parang, M.; Salah-Eddine, A.

1983-01-01

131

Forced-convection boiling tests performed in parallel simulated LMR fuel assemblies

International Nuclear Information System (INIS)

Forced-convection tests have been carried out using parallel simulated Liquid Metal Reactor fuel assemblies in an engineering-scale sodium loop, the Thermal-Hydraulic Out-of-Reactor Safety facility. The tests, performed under single- and two-phase conditions, have shown that for low forced-convection flow there is significant flow augmentation by thermal convection, an important phenomenon under degraded shutdown heat removal conditions in an LMR. The power and flows required for boiling and dryout to occur are much higher than decay heat levels. The experimental evidence supports analytical results that heat removal from an LMR is possible with a degraded shutdown heat removal system

1985-04-21

132

Forced-convection boiling tests performed in parallel simulated LMR fuel assemblies

Energy Technology Data Exchange (ETDEWEB)

Forced-convection tests have been carried out using parallel simulated Liquid Metal Reactor fuel assemblies in an engineering-scale sodium loop, the Thermal-Hydraulic Out-of-Reactor Safety facility. The tests, performed under single- and two-phase conditions, have shown that for low forced-convection flow there is significant flow augmentation by thermal convection, an important phenomenon under degraded shutdown heat removal conditions in an LMR. The power and flows required for boiling and dryout to occur are much higher than decay heat levels. The experimental evidence supports analytical results that heat removal from an LMR is possible with a degraded shutdown heat removal system.

Rose, S.D.; Carbajo, J.J.; Levin, A.E.; Lloyd, D.B.; Montgomery, B.H.; Wantland, J.L.

1985-04-21

133

Photographic study of bubble behaviors in forced convection subcooled boiling

Energy Technology Data Exchange (ETDEWEB)

Forced convection subcooled water boiling experiments were conducted in a vertical annular channel. A high-speed digital video camera was applied to record the dynamics of the subcooled boiling process. The flow visualization results show that the bubble departure frequency generally increases as the heat flux increases. For some cases, the departure frequency may reach a limit around 1000 bubbles/s. In addition, bubble lift-off diameter, bubble growth rate and bubble velocity after bubble lift-off were determined by analyzing the images. The experimental data obtained from this study can be used in modeling the bubble departure frequency, bubble lift-off diameter, and bubble dynamics in forced convection subcooled boiling. (Author)

Rong Situ; Ye Mi; Ishii, Mamoru [Purdue Univ., School of Nuclear Engineering, West Lafayette, IN (United States); Mori, Michitsugu [Tokyo Electric Power Co., Yokohama (Japan)

2004-08-01

134

Turbulent mixed convection in asymmetrically heated vertical channel

Directory of Open Access Journals (Sweden)

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.

Mokni Ameni

2012-01-01

135

Transient conjugated forced convection in turbulent pipe flows

International Nuclear Information System (INIS)

This work deals with the problem of transient conjugated forced convection heat transfer in turbulent pipe flows. The external surface of the pipe over a finite heated section is subjected to either uniform heat flux or uniform wall temperature. The governing parameters identified in this work are the Reynolds number Re, the wall-to-fluid conductivity ratio K, the wall-to-fluid diffusivity ratio A, the dimensionless wall thickness ?, and the Prandtl number Pr. A modified low-Re ?-? turbulent model is adopted to solve for the fully developed velocity and eddy viscosity distributions. Predicted results show that effects of wall conduction and wall heat capacity have a significant impact on the unsteady heat transfer, especially in the early transient period

1995-02-01

136

Forced convective boiling of hydrocarbon mixtures

Energy Technology Data Exchange (ETDEWEB)

This paper describes a model for evaporation of hydrocarbon mixtures in vertical pipes, the evaporation taking place mainly in the forced convective (annular flow) region where nucleate boiling is expected to be greatly suppressed. The model takes into account of the detailed processes of droplet entrainment and deposition and the results are compared with recent data obtained from a large scale high pressure boiling facility. It is shown that taking into account of liquid phase mixing due to entrainment and deposition gives predictions closer to the observations.

Barbosa, J.R.; Hewitt, Jr.; Hewitt, G.F. [Imperial College of Science, Technology and Medicine, London (United Kingdom). Dept. of Chemical Engineering and Chemical Technology

1999-07-01

137

An enhanced adsorption cycle operated by periodic reversal forced convection

Energy Technology Data Exchange (ETDEWEB)

A different cycle configuration is suggested in this work. Interesting improvement in performance is envisaged by applying the fixed bed adsorbers with the forced unsteady-state operation of periodic flow reversal to convective thermal wave cycle, which results in the so-called Periodic Reversal Forced Convective cycle. To describe process behavior for this kind of cycle with seven possible operating modes of flow reversal, a two-phase, one-dimensional model for the zeolite 13X-water pair is developed. Some important phenomena, such as the heat pipe effect, internal and external thermal regeneration etc., are discussed. The results of numerical simulations show that among all possible operating modes of flow reversal, only the mode of CCAA (switching the flow directions each phase duration) could improve the global performances of non-reversed flow operation, but others could not. Under the current conditions of cycle, the proposed CCAA cycle with heat exchanger regenerator could offer significant improvements in coefficient of performance (COP) over both the conventional cycle without flow reversal operation and existing convective thermal wave cycle with inert packed bed regenerator. The results of calculation also show that more than 1.7 of COP{sub h}, 0.9 of COP{sub c} and 125 W/kg adsorbent of specific cooling power (SCP) within this system could be possible. Analysis gives satisfactory explanation of these results. (author)

Hai-Ming Lai [Beijing University of Chemical Technology (China). Research Institute of Chemical Engineering

2000-05-01

138

Effects of Heated Wall on Mixed Convective Flow and Heat Transfer in a Fuel Cell Duct

DEFF Research Database (Denmark)

A general 3-D CFD code is used to simulate the effects of the heated wall on buoyancy-affected secondary flow, mixed convective flow and heat transfer for a horizontal fuel cell duct with rectangular cross section. The constant thermal-properties assumption except for linear density variation with temperature in the body force term is applied, and a constant heat flux is prescribed on either the bottom, top or side wall, while thermal insulation is implemented on the other remaining three walls. The secondary flow forms vortices in the duct and has various effects on friction factor and heat transfer. Calculations have been performed to determine the effects of various placements of the heated wall, Grashof number Gr* and Reynolds number Re. This study may be regarded as an improved modeling procedure for gas flow and convective heat transfer in fuel cell ducts. Keyword: SOFC, Heat transfer, Gas flow, Convection, Heated Wall

Yuan, Jinliang; Rokni, Masoud

2001-01-01

139

Latent Heating Processes within Tropical Deep Convection

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.

van den Heever, S. C.; Mcgee, C. J.

2013-12-01

140

An assessment on forced convection in metal foams

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.

Mancin, S.; Rossetto, L.

2012-11-01

141

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.

2012-11-26

142

We examine the effect of magnetic field on boundary layer flow of an incompressible electrically conducting water-based nanofluids past a convectively heated vertical porous plate with Navier slip boundary condition. A suitable similarity transformation is employed to reduce the governing partial differential equations into nonlinear ordinary differential equations, which are solved numerically by employing fourth-order Runge-Kutta with a shooting technique. Three different water-based nanofluids containing copper (Cu), aluminium oxide (Al2O3), and titanium dioxide (TiO2) are taken into consideration. Graphical results are presented and discussed quantitatively with respect to the influence of pertinent parameters, such as solid volume fraction of nanoparticles (?), magnetic field parameter (Ha), buoyancy effect (Gr), Eckert number (Ec), suction/injection parameter (fw), Biot number (Bi), and slip parameter (?), on the dimensionless velocity, temperature, skin friction coefficient, and heat transfer rate.

2013-01-01

143

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 t...

E. Herrera

1994-01-01

144

Convective heat transfer in buildings: recent research results. Rev

Energy Technology Data Exchange (ETDEWEB)

Recent experimental and numerical studies of convective heat transfer in buildings are described, and important results are presented. The experimental work has been performed on small-scale, water-filled enclosures; the numerical analysis results have been produced by a computer program based on a finite-difference scheme. The convective processes investigated in this research are: (1) natural convective heat transfer between room surfaces and the adjacent air, (2) natural convective heat transfer between adjacent rooms through a doorway or other openings, and (3) forced convection between the building and its external environment (such as wind-driven ventilation through windows, doors, or other openings). Results obtained at Lawrence Berkeley Laboratory (LBL) for surface convection coefficients are compared with existing ASHRAE correlations, and differences can have a significant impact on the accuracy of building energy analysis computer simulations. Interzone coupling correlations obtained from experimental work are in reasonable agreement with recently published experimental results and with earlier published work. Numerical simulations of wind-driven natural ventilation are presented. They exhibit good qualitative agreement with published wind-tunnel data.

Bauman, F.; Gadgil, A.; Kammerud, R.; Altmayer, E.; Nansteel, M.W.

1982-10-01

145

Double tube heat exchanger with novel enhancement: Part II - single phase convective heat transfer

Energy Technology Data Exchange (ETDEWEB)

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.)

Tiruselvam, R.; Chin, W.M.; Raghavan, Vijay R. [OYL Sdn. Bhd., Research and Application Department, Kuala Lumpur (Malaysia)

2012-08-15

146

Energy Technology Data Exchange (ETDEWEB)

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.

Green, M.A.; Ishimoto, S.; Lau, W.; Yang, S.

2003-09-15

147

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

2003-09-22

148

The effect of negative aerodynamic drag in an ideal fluid subject to convective instability is considered. It is shown that a cylinder moving in such a fluid is propelled forward in its motion by the convective forces and that the characteristic acceleration time is comparable to the onset time of convective motions in the fluid. It is suggested that convective propulsion plays an important role in the dynamics of flux tubes extending through the surface of the sun. The suppression of the upward heat flow in a Boussinesq convective cell with free upper and lower boundaries by a downdraft is then analyzed. Application to the solar convection zone indicates that downdrafts of 1 to 2 km/s at depths of 1000 to 4000 km beneath the visible surface of the sun are sufficient to reduce the upward heat flux to a small fraction of the ambient value.

Parker, E. N.

1979-01-01

149

Prandtl Number Dependent Natural Convection with Internal Heat Sources

International Nuclear Information System (INIS)

Natural convection plays an important role in determining the thermal load from debris accumulated in the reactor vessel lower head during a severe accident. Recently, attention is being paid to the feasibility of external vessel flooding as a severe accident management strategy and to the phenomena affecting the success path for retaining the molten core material inside the vessel. The heat transfer inside the molten core material can be characterized by the strong buoyancy-induced flows resulting from internal heating due to decay of fission products. The thermo-fluid dynamic characteristics of such flow depend strongly on the thermal boundary conditions. The spatial and temporal variation of heat flux on the pool wall boundaries and the pool superheat are mainly characterized by the natural convection flow inside the molten pool. In general, the natural convection heat transfer phenomena involving the internal heat generation are represented by the modified Rayleigh number (Ra'), which quantifies the internal heat source and hence the strength of the buoyancy force. In this study, tests were conducted in a rectangular section 250 mm high, 500 mm long and 160 mm wide. Twenty-four T-type thermocouples were installed in the test section to measure temperatures. Four T-type thermocouples were used to measure the boundary temperatures. The thermocouples were placed in designated locations after calibration. A direct heating method was adopted in this test to simulate the uniform heat generation. The experiments covered a range of Ra' between 1.5x106 and 7.42x1015 and the Prandtl number (Pr) between 0.7 and 6.5. Tests were conducted with water and air as simulant. The upper and lower boundary conditions were maintained uniform. The results demonstrated feasibility of the direct heating method to simulate uniform volumetric heat generation. Particular attentions were paid to the effect of Pr on natural convection heat transfer within the rectangular pool

2004-06-13

150

Notions of convection heat transfer; Notions de transfert thermique par convection

Energy Technology Data Exchange (ETDEWEB)

The concept of convection covers the description of the velocity and temperature fields of homogenous and isotropic fluids. This concept excludes: the two-phase transfers and flows, the fluidized beds, the transfers in complex fluids, and the transfers inside porous media. Basically, the equations of convection are the equations of fluid mechanics applied to a conduction problem in a deformable environment. The resolution of these equations is in general complex and requires the use of iterative calculus. This article treats of the resolution of these equations in different flow conditions: 1 - introduction; 2 - general considerations (equations, hypotheses); 3 - stationariness, boundary conditions, Newton's hypothesis and mixing temperature; 4 - implementation of Newton's hypothesis (characteristic temperature, non-dimensioning and characteristic numbers); 5 - similarity limits and Nusselt number (irreducible classes in forced and natural convection, expressions of the Nusselt number); 6 - numerical expressions of the Nusselt number (external and internal forced, natural and mixed convection). Three appendixes about the heat transfer equation in a deformable medium, about the non-dimensioning method and about the particular case of convection inside liquid metals complete this analysis. (J.S.)

Huetz, J. [Centre National de la Recherche Scientifique (CNRS), 75 - Paris (France); Petit, J.P. [Ecole Centrale des Arts et Manufactures, 92 - Chatenay-Malabry (France)

2004-07-15

151

Experimental study on convective heat transfer of TiO2 nanofluids

In this study, nanofluids with different TiO2 nanoparticle concentrations were synthesized and measured in different constant heat fluxes for their heat transfer behavior upon flowing through a vertical pipe. Addition of nanoparticles into the base fluid enhances the forced convective heat transfer coefficient. The results show that the enhancement of the convective heat transfer coefficient in the mixture consisting of ethylene glycol and distilled water is more than distilled water as a base fluid.

Vakili, M.; Mohebbi, A.; Hashemipour, H.

2013-08-01

152

Laminar Boundary Layers in Convective Heat Transport

We study Rayleigh-Bénard convection in the high-Rayleigh-number regime and infinite-Prandtl-number limit, i.e., we consider a fluid in a container that is exposed to strong heating of the bottom and cooling of the top plate in the absence of inertia effects. While the dynamics in the bulk are characterized by a chaotic heat flow, close to the horizontal walls, the fluid is essentially motionless. We derive local bounds on the temperature field in the boundary layers and prove that the temperature profile is essentially linear. The results depend only logarithmically on the system parameters. An important tool in our analysis is a new Hardy-type estimate for the convecting velocity field, which yields control of the fluid motion in the layer. The bounds on the temperature field are derived via local maximal regularity estimates for convection-diffusion equations.

Seis, Christian

2013-12-01

153

Natural and mixed convection heat transfer from a horizontal heated pipe buried in a porous medium

International Nuclear Information System (INIS)

The natural and mixed convection heat transfer from horizontal heated pipe (diam. 0.646'') embedded in a water saturated a porous medium (MIT Coarse Sand, porosity 30%) for three different depth of burial to pipe diameter ratios from 11.11, 9.69 and 4.55, have been studied experimentally. A galvanized steel box, 2.5 ft long x 1.25 ft wide x 0.5 ft deep, containing the coarse sand bed measuring 1.875 ft x 1.25 ft, was used for the experimental model. The pipe was heated by an electric heating element. In the mixed convection studies, water was allowed to flow in the sand bed and past the heated cylinder in a crosswise direction. The experimental results were correlated by using the normalized parameters - Nusselt number, Rayleigh number and Reynolds number, and the aspect ratios. In the case of crossflow past the pipe, the average Reynolds number at which forced convection becomes dominant over the natural convection mode of heat transfer has been experimentally determined. In order to check the accuracy of the experimental set up and instrumentations, natural convection heat transfer from the heated cylinder to water without the porous medium was conducted and the experimental results compared favorably with those of the previous investigators

1986-01-01

154

Experimental Study on Convective Heat Transfer of Water Flow in Tubes under Natural Circulation

This paper reports on an experimental study on convective heat transfer of single phase water in an upflow heated tube, an upflow cooled tube and a downflow cooled tube under natural circulation conditions. In the experiments the average heat transfer coefficients in the three test tubes were obtained. The experimental data were compared with the predictions by a forced flow correlation available in the literature. The comparisons show that free convection caused by buoyancy in the tubes has great influence on heat transfer under natural circulation condition. By using the Grashof number Gr which can represent the influence of free convection on heat transfer in natural circulation to modify the known correlations for the calculation of heat transfer in forced flow, the empirical correlations for the calculation of convective heat transfer in tubes under natural circulation condition have been developed. The empirical correlations are in good agreement with the experimental data.

Yang, R. C.; Zhong, Y.; Liu, R. L.; Liu, T.

2010-03-01

155

Solar Hot Water Heating by Natural Convection.

Presents an undergraduate laboratory experiment in which a solar collector is used to heat water for domestic use. The working fluid is moved by natural convection so no pumps are required. Experimental apparatus is simple in design and operation so that data can be collected quickly and easily. (Author/JN)

Noble, Richard D.

1983-01-01

156

Combined convective heat transfer of liquid sodium flowing across tube banks

International Nuclear Information System (INIS)

In order to clarify the heat transfer characteristics of combined convection of liquid sodium, a numerical analysis is performed for liquid sodium which flows through a single horizontal row of tubes in the direction of gravity. The correlation of heat transfer characteristics between liquid sodium and ordinary fluids is also discussed. The heat transfer characteristics at large Reynolds numbers are improved when the Richardson number is increased, and the improvement rate is enlarged with increase in p/d value, since convection effect is relatively large. However heat transfer coefficients do not differ from those of forced convection at small Reynolds numbers even when the Richardson number reaches a high value because of conduction effect. A good consistence of heat transfer characteristics of combined convection between liquid sodium and air is obtained at the same Peclet number and Richardson number. This means that the fundamental heat transfer characteristics of combined convection of liquid sodium can be investigated with ordinary fluids. (author)

1989-01-01

157

Natural convection in a uniformly heated pool

International Nuclear Information System (INIS)

In the event of a core meltdown accident, to prevent reactor vessel failure from molten corium relocation to the reactor vessel lower head, the establishment of a coolable configuration has been proposed by flooding with water the reactor cavity. In Reference 3, it was shown that for the heavy-water new production reactor (NPW-HWR) design, this strategy, e.g., the rejection of decay heat to a containment decay heat removal system by boiling of water in the reactor cavity, could keep the reactor vessel temperature below failure limits. The analysis of Ref. 3 was performed with the computer code COMMIX-1AR/P, and showed that natural convection in the molten-corium pool was the dominant mechanism of heat transfer from the pool to the wall of the reactor vessel lower head. To determine whether COMMIX adequately predicts natural convection in a pool heated by a uniform heat source, in Ref. 4, the experiments of free convection in a semicircular cavity of Jahn and Reineke were analyzed with COMMIX. It was found that the Nusselt (Nu) number predicted by COMMIX was within the spread of the experimental measurements. In the COMMIX analysis of Ref. 4, the semicircular cavity was treated as symmetric. The objective of the work presented in this paper was to extend the COMMIX validation analysis of Ref. 4 by removing the assumption of symmetry and expanding the analysis up to the highest Rayleigh (Ra) number that leads to a steady state. In conclusion, this work shows that the numerical predictions of natural convection in an internally heated pool bounded by a curved bottom are in reasonably good agreement with experimental measurements

1996-06-16

158

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 v...

2006-01-01

159

Natural convection heat transfer in a uniformly heated horizontal pipe

Natural convection heat transfers inside horizontal pipes were measured. The Rayleigh numbers were varied from 6.8 × 108 to 1.5 × 1012, while the Prandtl number was fixed at 2,094. Based on the analogy concept, a copper sulfate electroplating system was adopted to measure mass transfer rates in place of heat transfer rates. Test results using single-piece electrodes were in good agreement with the work of Sarac and Korkut. The angle-dependent mass transfer rates, measured using piecewise electrodes, were compared with the results of studies on natural convection in concentric annuli, and showed similar trends. The experiments were expanded to the turbulent region, and a transition criterion was proposed. Angle-dependent natural convection heat transfer correlations for the laminar and turbulent regions were derived.

Chae, Myeong-Seon; Chung, Bum-Jin

2014-01-01

160

Convective heat transport in geothermal systems

Energy Technology Data Exchange (ETDEWEB)

Most geothermal systems under exploitation for direct use or electrical power production are of the hydrothermal type, where heat is transferred essentially by convection in the reservoir, conduction being secondary. In geothermal systems, buoyancy effects are generally important, but often the fluid and heat flow patterns are largely controlled by geologic features (e.g., faults, fractures, continuity of layers) and location of recharge and discharge zones. During exploitation, these flow patterns can drastically change in response to pressure and temperature declines, and changes in recharge/discharge patterns. Convective circulation models of several geothermal systems, before and after start of fluid production, are described, with emphasis on different characteristics of the systems and the effects of exploitation on their evolution. Convective heat transport in geothermal fields is discussed, taking into consideration (1) major geologic features; (2) temperature-dependent rock and fluid properties; (3) fracture- versus porous-medium characteristics; (4) single- versus two-phase reservoir systems; and (5) the presence of noncondensible gases.

Lippmann, M.J.; Bodvarsson, G.S.

1986-08-01

161

Convective cooling of three discrete heat sources in channel flow

Directory of Open Access Journals (Sweden)

Full Text Available A numerical investigation was performed to evaluate distinct convective heat transfer coefficients for three discrete strip heat sources flush mounted to a wall of a parallel plates channel. Uniform heat flux was considered along each heat source, but the remaining channel surfaces were assumed adiabatic. A laminar airflow with constant properties was forced into the channel considering either developed flow or a uniform velocity at the channel entrance. The conservation equations were solved using the finite volumes method together with the SIMPLE algorithm. The convective coefficients were evaluated considering three possibilities for the reference temperature. The first was the fluid entrance temperature into the channel, the second was the flow mixed mean temperature just upstream any heat source, and the third option employed the adiabatic wall temperature concept. It is shown that the last alternative gives rise to an invariant descriptor, the adiabatic heat transfer coefficient, which depends solely on the flow and the geometry. This is very convenient for the thermal analysis of electronic equipment, where the components' heating is discrete and can be highly non-uniform.

Thiago Antonini Alves

2008-09-01

162

Convective cooling of three discrete heat sources in channel flow

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english A numerical investigation was performed to evaluate distinct convective heat transfer coefficients for three discrete strip heat sources flush mounted to a wall of a parallel plates channel. Uniform heat flux was considered along each heat source, but the remaining channel surfaces were assumed adia [...] batic. A laminar airflow with constant properties was forced into the channel considering either developed flow or a uniform velocity at the channel entrance. The conservation equations were solved using the finite volumes method together with the SIMPLE algorithm. The convective coefficients were evaluated considering three possibilities for the reference temperature. The first was the fluid entrance temperature into the channel, the second was the flow mixed mean temperature just upstream any heat source, and the third option employed the adiabatic wall temperature concept. It is shown that the last alternative gives rise to an invariant descriptor, the adiabatic heat transfer coefficient, which depends solely on the flow and the geometry. This is very convenient for the thermal analysis of electronic equipment, where the components' heating is discrete and can be highly non-uniform.

Thiago Antonini, Alves; Carlos A. C., Altemani.

163

Convective heat conductivity of layered HTSC

Energy Technology Data Exchange (ETDEWEB)

On the basis of the assumption that the Cu-O layers in strongly anisotropic high-temperature superconductors (HTSCs) are characterized by magnetic vortices of variable polarity, it is suggested that the properties of heat transfer in zero-magnetic-field HTSC materials must differ from those of traditional HTSCs. Calculations are presented showing that the convective heat flow in planar or layered HTSCs is determined not only by the energy transfer due to regular mechanisms of excitation, but also by a flow of vortexlike magnetic excitations carrying surplus entropy of electrons localized in the HTSC core. 14 refs.

Genenko, IU.A.; Medvedev, IU.V. (Donetskii Fiziko-Tekhnicheskii Inst., Donetsk (Ukraine))

1992-03-01

164

Natural convection in a uniformly heated pool

International Nuclear Information System (INIS)

To prevent reactor vessel failure from molten corium relocation to the reactor vessel lower head in the event of a core meltdown accident, the establishment of a coolable configuration has been proposed by flooding the reactor cavity with water. In Ref. 3, it was shown that for the heavy-water new production reactor (NPW-HWR) design, this strategy (e.g., the rejection of decay heat to a containment decay heat removal system by boiling of water in the reactor cavity) could keep the reactor vessel temperature below failure limits. The analysis of Ref. 3 was performed with the COMMIX-IAR/P computer code and showed that natural convection in the molten-corium pool was the dominant mechanism of heat transfer from the pool to the wall of the reactor vessel lower head. COMMIX is a general-purpose thermal-hydraulics code based on finite differencing by the first-order upwind scheme. To determine whether COMMIX adequately predicts natural convection in a pool heated by a uniform heat source, in Ref. 4, the experiments of free convection in a semicircular cavity of Jahn and Reineke were analyzed with COMMIX in Ref. 5. It was found that the Nusselt number predicted by COMMIX was within the spread of the experimental measurements. In the COMMIX analysis of Ref. 5, the semicircular cavity was treated as symmetric. The objective of this paper was to extend the COMMIX validation analysis of Ref. 5 by removing the assumption of symmetry and expanding the analysis from the highest Rayleigh number of the experiments of Ref. 4 to the highest Rayleigh number that leads to a steady state

1996-06-16

165

Laminar boundary layers in convective heat transport

We study Rayleigh-Benard convection in the high-Rayleigh-number and high-Prandtl-number regime, i.e., we consider a fluid in a container that is exposed to strong heating of the bottom and cooling of the top plate in the absence of inertia effects. While the dynamics in the bulk are characterized by a chaotic convective heat flow, the boundary layers at the horizontal container plates are essentially conducting and thus the fluid is motionless. Consequently, the average temperature exhibits a linear profile in the boundary layers. In this article, we rigorously investigate the average temperature and oscillations in the boundary layer via local bounds on the temperature field. Moreover, we deduce that the temperature profile is indeed essentially linear close to the horizontal container plates. Our results are uniform in the system parameters (e.g. the Rayleigh number) up to logarithmic correction terms. An important tool in our analysis is a new Hardy-type estimate for the convecting velocity field, which ca...

Seis, Christian

2012-01-01

166

Pattern formation without heating in an evaporative convection experiment

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present an evaporation experiment in a single fluid layer. When latent heat associated to the evaporation is large enough, the heat flow through the free surface of the layer generates temperature gradients that can destabilize the conductive motionless state giving rise to convective cellular structures without any external heating. The sequence of convective patterns obtained here without heating, is similar to that obtained in B\\'enard-Marangoni convection. This work p...

Mancini, Hector; Maza, Diego

2003-01-01

167

Investigation of the transition from forced to natural convection in the research reactor Munich II

International Nuclear Information System (INIS)

The new research reactor Munich II (FRM-II), which is under construction at the Technical University Munich, Germany, makes use of a newly developed compact reactor core consisting of a single fuel element, which is assembled of two concentric pipes. Between the fuel element's inner and outer pipe 113 involutely bent fuel plates are placed rotationally symmetric, forming 113 cooling channels of a constant width of 2.2 mm. After a shut down of the reactor, battery supported cooling pumps are started by the reactor safety system in order to remove the decay heat by a downwards directed forced flow. Three hours after they have been started, the cooling pumps are shut down and so-called 'natural convection flaps' are opened by their own weight. Through a flow path, which is provided by the opening of the natural convection flaps, the decay heat is given off to the water in the reactor pool after the direction of the flow has changed and an upwards directed natural convection flow has developed. At the Department for Nuclear and New Energy Systems of the Ruhr-University Bochum, Germany, a test facility has been built in order to confirm the concept of the decay heat removal in the FRM-II, to acquire data of single and two phase natural convection flows and to detect the dry out in a narrow channel. The thermohydraulics of the FRM-II are simulated by an electrically heated test section, which represents one cooling channel of the fuel element. At first experiments have been performed, which simulated the transition from forced to natural convection in the core of the FRM-II, both at normal operation and at a complete loss of the decay heat removal pumps. In case of normal operation, the transition from forced to natural convection takes place single phased. If a complete loss of the active decay heat removal system occurs, the decay heat removal is ensured by a quasi-steady two phase flow. In a second test series minimum heat flux densities leading to pressure pulsations up to limiting amplitudes of 0.1 bar, 0.2 bar and 0.3 bar at the transition from forced to natural convection have been determined. Further tests have been performed to determine minimum heat flux densities leading to boiling processes in the cooling channel and critical heat flux densities causing dry outs of the cooling channel at downwards directed forced flow. During the tests, flow reversals have been observed because of the buoyancy forces in the coolant causing a mixed convection flow. The last test series, which has been finished in March 1999, has been performed in order to determine critical heat flux densities during the transition from forced to natural convection and to measure the occurring pressure amplitudes. All results prove the possibility to remove the decay heat of the FRM-II by natural convection, even in case of a complete loss of the active decay heat removal system. Above this, large safety margins in the FRM-II concerning pressure pulsations, beginning of boiling and dry out could be verified. (author)

1999-10-26

168

Convection of liquid with internal heat release in a rotating container

The convection of heat-generating fluid in a rotating horizontal cylinder is experimentally investigated. The threshold of convection excitation, the structure of convective flows and the heat transfer in the cylinder depending on the heat release capacity, liquid viscosity and aspect ratio of the cavity are studied. It is found that the average convection is excited by the thermovibrational mechanism —the gravity force, rotating in the cavity frame, produces the oscillations of non-isothermal fluid relative to the wall, which in turn result in excitation of mean convective flows. It is shown that the structure of convective flows depends on the dimensionless velocity of rotation. At relatively low rotation velocity the convection develops in the form of a periodic system of vortices regularly distributed along the cylinder axis. The threshold of excitation (critical value of vibration parameter) of three-dimensional vortex structures grows with rotation velocity. Above some definite rotation velocity the convection develops as two-dimensional rolls parallel to the axis of rotation. The threshold of two-dimensional structures excitation does not depend on the rotation velocity. Besides the structure of thermal convective flows the analysis of the relatively weak currents generated by the inertial waves below the threshold of convection is performed.

Kozlov, V.; Vjatkin, A.; Sabirov, R.

2013-08-01

169

Entropy generation around a solid sphere in the forced convection regime

International Nuclear Information System (INIS)

The entropy generation around an isothermal solid sphere subjected to an air stream is calculated for forced convection heat transfer case. Local and average entropy generation terms are calculated. The entropy generated due to heat transfer and due to fluid friction are calculated separately and the relative magnitude for each one is evaluated in the total entropy generation expression. The effect of the controlling parameters on the entropy generation, namely Reynolds number and Eckert number is investigated and discussed. (authors)

2005-05-17

170

International Nuclear Information System (INIS)

An experimental model was developed for a nuclear waste repository storage room. Data were taken over a Reynolds number range of 6,000 to 180,000, covering both the forced and mixed (combined natural and forced) regimes of convection. Data are presented for several circumferential boundary conditions. Results indicate that the natural convection component is significant. The finite difference heat conduction code HEATING5 was used to plot temperatures around the repository room for the different heat transfer coefficients. The plots show that the use of standard forced convection relations can result in over estimating the room temperature by as much as 70"0C

1986-01-01

171

International Nuclear Information System (INIS)

The numerical solution of flow and heat transfer for steady and transient laminar mixed convection near a vertical uniformly heated surface exposed to a horizontal cross-flow is presented. The transients considered include the simultaneous initiation of flow and heating and the time-varying flow generated by starting or stopping forced convection with the surface heating condition unchanged. The partial differential equations describing the conservation of mass, momentum, and energy were solved in their time-dependent forms by an explicit finite-difference technique. Calculations were performed for fluids with Prandtl numbers of 0.733 and 6.7, nominally air and water. During both steady and transient circumstances, the effects of the horizontal forced flow were dominant near the vertical leading edge, whereas natural convection dictated the flow at large values of the horizontal coordinate. The heat transfer coefficient during mixed convection was significantly higher than that with either forced or free convection alone. During simultaneous starting of flow and heating, the time to reach steady state decreased with increase in cross-flow velocity. For the transients initiated by suddenly imposing a horizontal forced flow on an existing natural convection flow, the local temperature and vertical velocity were found to undershoot before reaching their respective steady-state values. The overshoot in heat transfer coefficient in such situations was significant for fluids with smaller Prandtl number. The transient initiated by stopping the horizontal flow during mixed convection was associated with overshoot in both vertical component of velocity and local fluid temperature

1986-01-01

172

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)

1991-01-01

173

Energy Technology Data Exchange (ETDEWEB)

The Thermal-Hydraulic Out of Reactor Safety (THORS) Program at Oak Ridge National Laboratory (ORNL) had as its objective the testing of simulated, electrically heated liquid metal reactor (LMR) fuel assemblies in an engineering-scale, sodium loop. Between 1971 and 1985, the THORS Program operated 11 simulated fuel bundles in conditions covering a wide range of normal and off-normal conditions. The last test series in the Program, THORS-SHRS Assembly 1, employed two parallel, 19-pin, full-length, simulated fuel assemblies of a design consistent with the large LMR (Large Scale Prototype Breeder -- LSPB) under development at that time. These bundles were installed in the THORS Facility, allowing single- and parallel-bundle testing in thermal-hydraulic conditions up to and including sodium boiling and dryout. As the name SHRS (Shutdown Heat Removal System) implies, a major objective of the program was testing under conditions expected during low-power reactor operation, including low-flow forced convection, natural convection, and forced-to-natural convection transition at various powers. The THORS-SHRS Assembly 1 experimental program was divided up into four phases. Phase 1 included preliminary and shakedown tests, including the collection of baseline steady-state thermal-hydraulic data. Phase 2 comprised natural convection testing. Forced convection testing was conducted in Phase 3. The final phase of testing included forced-to-natural convection transition tests. Phases 1, 2, and 3 have been discussed in previous papers. The fourth phase is described in this paper. 3 refs., 2 figs.

Levin, A.E. (Georgia Inst. of Tech., Atlanta, GA (USA)); Montgomery, B.H. (Oak Ridge National Lab., TN (USA))

1990-01-01

174

International Nuclear Information System (INIS)

The Thermal-Hydraulic Out of Reactor Safety (THORS) Program at Oak Ridge National Laboratory (ORNL) had as its objective the testing of simulated, electrically heated liquid metal reactor (LMR) fuel assemblies in an engineering-scale, sodium loop. Between 1971 and 1985, the THORS Program operated 11 simulated fuel bundles in conditions covering a wide range of normal and off-normal conditions. The last test series in the Program, THORS-SHRS Assembly 1, employed two parallel, 19-pin, full-length, simulated fuel assemblies of a design consistent with the large LMR (Large Scale Prototype Breeder -- LSPB) under development at that time. These bundles were installed in the THORS Facility, allowing single- and parallel-bundle testing in thermal-hydraulic conditions up to and including sodium boiling and dryout. As the name SHRS (Shutdown Heat Removal System) implies, a major objective of the program was testing under conditions expected during low-power reactor operation, including low-flow forced convection, natural convection, and forced-to-natural convection transition at various powers. The THORS-SHRS Assembly 1 experimental program was divided up into four phases. Phase 1 included preliminary and shakedown tests, including the collection of baseline steady-state thermal-hydraulic data. Phase 2 comprised natural convection testing. Forced convection testing was conducted in Phase 3. The final phase of testing included forced-to-natural convection transition tests. Phases 1, 2, and 3 have been discussed in previous papers. The fourth phase is described in this paper. 3 refs., 2 figs

1990-08-12

175

Endwall convective heat transfer for bluff bodies

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 ca...

Wang, Lei; Salewski, Mirko; Sunde?n, Bengt; Borg, Andreas; Abrahamsson, Hans

2012-01-01

176

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)

1961-01-01

177

Convective heat transfer in annular passage with high heat flux

International Nuclear Information System (INIS)

An experimental study was performed on convective heat transfer of concentric annular gas flow with the inner tube heated. The examined region of the Reynolds number ranged from 6000 to 10000 and that of the dimensionless heat flux parameter from 0.2 x 10"-"3 to 5.5 x 10"-"3. Special attention was paid to the examination of the existing correlations for high heat flux heat transfer and also to the criteria for the occurrence of so-called laminarization of the annular flow. No laminarization was detected even for the highest heat flux case although they satisfied the proposed criteria for the occurrence of laminarization for the circular tube flows. It is anticipated, therefore, that the annular flow is less vulnerable to laminarization than is the circular tube flow. Experimental results also show that as the exponent of (T_w/T_i_n) it is recommended to use 0.38 instead of 0.20 or 0.31, which were proposed by the former authors for heat transfer with variable properties. (author)

1987-01-01

178

A Study of Nucleate Boiling with Forced Convection in Microgravity

The ultimate objective of basic studies of flow boiling in microgravity is to improve the understanding of the processes involved, as manifested by the ability to predict its behavior. This is not yet the case for boiling heat transfer even in earth gravity, despite the considerable research activity over the past 30 years. The elements that constitute the nucleate boiling process - nucleation, growth, motion, and collapse of the vapor bubbles (if the bulk liquid is subcooled) - are common to both pool and flow boiling. It is well known that the imposition of bulk liquid motion affects the vapor bubble behavior relative to pool boiling, but does not appear to significantly influence the heat transfer. Indeed, it has been recommended in the past that empirical correlations or experimental data of pool boiling be used for design purposes with forced convection nucleate boiling. It is anticipated that such will most certainly not be possible for boiling in microgravity, based on observations made with pool boiling in microgravity. In earth gravity buoyancy will act to remove the vapor bubbles from the vicinity of the heater surface regardless of how much the imposed bulk velocity is reduced, depending, of course, on the geometry of the system. Vapor bubbles have been observed to dramatically increase in size in pool boiling in microgravity, and the heat flux at which dryout took place was reduced considerably below what is generally termed the critical heat flux (CHF) in earth gravity, depending on the bulk liquid subcooling. However, at heat flux levels below dryout, the nucleate pool boiling process was enhanced considerably over that in earth gravity, in spite of the large vapor bubbles formed in microgravity and perhaps as a consequence. These large vapor bubbles tended to remain in the vicinity of the heater surface, and the enhanced heat transfer appeared to be associated with the presence of what variously has been referred to as a liquid microlayer between the bubble and the heater surface. The enhancement of the boiling process with low velocities in earth gravity for those orientations producing the formation of a liquid macrolayer described above, accompanied by "sliding" vapor bubbles, has been demonstrated. The enhancement was presented as a function of orientation, subcooling, and heated length, while a criterion for the heat transfer for mixed natural/forced convection nucleate boiling was given previously. A major unknown in the prediction and application of flow boiling heat transfer in microgravity is the upper limit of the heat flux for the onset of dryout (or critical heat flux - CHF), for given conditions of fluid-heater surfaces, including geometry, system pressure and bulk liquid subcooling. It is clearly understood that the behavior in microgravity will be no different than on earth with sufficiently high flow velocities, and would require no space experimentation. However, the boundary at which this takes place is still an unknown. Previous results of CHF measurements were presented for low velocity flow boiling at various orientations in earth gravity as a function of flow velocity and bulk liquid subcooling, along with preliminary measurements of bubble residence times on a flat heater surface. This showed promise as a parameter to be used in modeling the CHF, both in earth gravity and in microgravity. The objective of the work here is to draw attention to and show results of current modeling efforts for the CHF, with low velocities in earth gravity at different orientations and subcoolings. Many geometrical possibilities for a heater surface exist in flowing boiling, with boiling on the inner and outer surfaces of tubes perhaps being the most common. If the vapor bubble residence time on and departure size from the heater surface bear a relationship to the CHF, as results to be given indicate, it is important that visualization of and access to vapor bubble growth be conveniently available for research purposes. In addition, it is desirable to reduce the number of variables as much as p

Merte, Herman, Jr.

1999-01-01

179

Numerical solution of staggered circular tubes in two-dimensional laminar forced convection

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english 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.

Carlos Henrique, Marchi; Maykel Alexandre, Hobmeir.

180

Digital Repository Infrastructure Vision for European Research (DRIVER)

Abstract: The entropy generation due to steady laminar forced convection fluid flow through parallel plates microchannel is investigated numerically. The effect of Knudsen, Reynolds, Prandtl, Eckert numbers and the nondimensional temperature difference on entropy generation within the microchannel is discussed. The fraction of the entropy generation due to heat transfer to the total entropy generation within the microchannel is studied in terms of Bejan number. The entropy generation within t...

Osamah Haddad; Mohammad Abuzaid; Mohammad Al-Nimr

2004-01-01

181

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

1996-01-01

182

Experimental investigation of the impact of room/system design on mixed convection heat transfer

Digital Repository Infrastructure Vision for European Research (DRIVER)

Night cooling attracts growing interest. However, architects and engineers still hesitate to apply night cooling because of the important but hard-to-predict convective heat transfer by night. Obviously, this heat transfer mechanism depends on the driving force, fluid motion and heat transfer surface and, thus, on the room and system design. Unfortunately, studies addressing this are scarce. In response, underlying experimental effort intends to instigate global parametric analyses of night c...

2012-01-01

183

Enhancement of convection heat-transfer in a rectangular duct

Energy Technology Data Exchange (ETDEWEB)

Characteristics of wall-to-air heat transfer for a fully developed forced convection have been studied in a large rectangular packed duct with 160cm heated length, 40 cm width, and for low bed equivalent diameter to particle diameter ratio. The separation distance between the top and bottom walls is 10 cm. A uniform heat flux is supplied at the top wall, while the bottom wall is insulated. Raschig rings in two and spherical packing in three sizes have been used in the air flow passage to investigate the enhancement of heat transfer due to packing. Temperature profiles for the steady and unsteady states have been measured. In modeling, the Ergun equation and energy equations are solved to calculate the temperature profile for the steady-state only. It has been found that the introduction of the packing into the air flow passage increases the wall-to-fluid heat transfer approximately three times compared with that of empty bed. This finding can enhance the rational use of energy from solar air heaters, chemical reactors, electronic cooling and many other engineering applications. (Author)

Demirel, Yasar; Al-Ali, Habib H.; Abu-Al-Saud, Basem A. [King Fahd Univ. of Petroleum and Minerals, Dept. of Chemical Engineering, Dhahran (Saudi Arabia)

1999-12-01

184

Details of Exact Low Prandtl Number Boundary-Layer Solutions for Forced and For Free Convection

A detailed report is given of exact (numerical) solutions of the laminar-boundary-layer equations for the Prandtl number range appropriate to liquid metals (0.003 to 0.03). Consideration is given to the following situations: (1) forced convection over a flat plate for the conditions of uniform wall temperature and uniform wall heat flux, and (2) free convection over an isothermal vertical plate. Tabulations of the new solutions are given in detail. Results are presented for the heat-transfer and shear-stress characteristics; temperature and velocity distributions are also shown. The heat-transfer results are correlated in terms of dimensionless parameters that vary only slightly over the entire liquid-metal range. Previous analytical and experimental work on low Prandtl number boundary layers is surveyed and compared with the new exact solutions.

Sparrow, E. M.; Gregg, J. L.

1959-01-01

185

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 internal 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 mode...

Andrade, Cla?udia R.; Zaparoli, Edson L.

2000-01-01

186

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

1993-11-14

187

International Nuclear Information System (INIS)

The thermo fluid dynamic characteristics of natural convection flow depend strongly on thermal boundary condition such as the spatial and temporal variation of heat flux on the pool wall boundaries. In general the natural convection heat transfer phenomena involving the bottom heat generation are represented by the Rayleigh number, Ra, which quantifies the bottom heat source and hence the strength of the buoyancy force. This work focuses on natural convection in which the density gradient is due to a temperature gradient and the body force is gravitational. The presence of a fluid density gradient in a gravitational field does not ensure the existence of natural convection currents, however, in an apparatus enclosed by two horizontal plates of different temperature. The temperature of the lower plate exceeds that of the upper plate, and the density decreases in the direction of the gravitational force. The LIDO (Liquid Internal Dynamics Operation) tests are conducted in a horizontal circular layer 500 mm in diameter and 220 mm in height using fluid, whose thermophysical properties are typified by the Prandtl number, Pr. The tests cover the range of 3x105 10 and 0.02 < Pr < 2.22 Tests are conducted with air, water and Wood's metal (Pb-Bi-Sn-Cd) as simulant to determine the Nusselt number, Nu. The upper and side walls are cooled, while the lower wall is heated at uniform temperatures

2008-05-01

188

Predicting free laminar convection heat transfer on curvilinear surfaces

Energy Technology Data Exchange (ETDEWEB)

Free laminar convection on an isothermal surface of cylindrical and axisymmetric bodies of arbitrary cross section is considered in the Boussinesq approximation. The problem is solved by the method of generalized similarity in its local approximation. Universal formulae are obtained for calculating the local Nusselt number. Prediction of convection on a heated horizontal cylindrical and sphere are given as examples of the method.

Stepanyants, L.G.; Shelukho, S.I.

1989-07-01

189

Heat convection on cylinder at high Prandtl numbers

International Nuclear Information System (INIS)

Natural convection flow on a vertical cylinder is considered here when the Prandtl numbers is large. Little work has been done in this field apart form some experimental studies which are for lower Prandtl numbers. Here, the singular perturbation technique is used to solve this problem. The method adopted is to split the flow into a thin layer close to the surface of the cylinder, surrounded by a much thicker layer where the velocity is reduced to zero. It is shown that at high Prandtl numbers, the velocity boundary layer tends to be somewhat larger due to large kinematic viscosity relative to thermal diffusivity. The motion of the outer layer, however, seems to be caused by the drag force exerted by the inner layer, not due to the buoyancy itself. The basic properties of the flow are evaluated. The heat transfer coefficient is shown to give good prediction for all ranges of Prandtl numbers

2003-04-01

190

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

1980-01-01

191

Aerosol Radiative Effects on Deep Convective Clouds and Associated Radiative Forcing

The aerosol radiative effects (ARE) on the deep convective clouds are investigated by using a spectral-bin cloud-resolving model (CRM) coupled with a radiation scheme and an explicit land surface model. The sensitivity of cloud properties and the associated radiative forcing to aerosol single-scattering albedo (SSA) are examined. The ARE on cloud properties is pronounced for mid-visible SSA of 0.85. Relative to the case excluding the ARE, cloud fraction and optical depth decrease by about 18% and 20%, respectively. Cloud droplet and ice particle number concentrations, liquid water path (LWP), ice water path (IWP), and droplet size decrease significantly when the ARE is introduced. The ARE causes a surface cooling of about 0.35 K and significantly high heating rates in the lower troposphere (about 0.6K/day higher at 2 km), both of which lead to a more stable atmosphere and hence weaker convection. The weaker convection and the more desiccation of cloud layers explain the less cloudiness, lower cloud optical depth, LWP and IWP, smaller droplet size, and less precipitation. The daytime-mean direct forcing induced by black carbon is about 2.2 W/sq m at the top of atmosphere (TOA) and -17.4 W/sq m at the surface for SSA of 0.85. The semi-direct forcing is positive, about 10 and 11.2 W/sq m at the TOA and surface, respectively. Both the TOA and surface total radiative forcing values are strongly negative for the deep convective clouds, attributed mostly to aerosol indirect forcing. Aerosol direct and semi-direct effects are very sensitive to SSA. Because the positive semi-direct forcing compensates the negative direct forcing at the surface, the surface temperature and heat fluxes decrease less significantly with the increase of aerosol absorption (decreasing SSA). The cloud fraction, optical depth, convective strength, and precipitation decrease with the increase of absorption, resulting from a more stable and dryer atmosphere due to enhanced surface cooling and atmospheric heating.

Fan, J.; Zhang, R.; Tao, W.-K.; Mohr, I.

2007-01-01

192

Sedimentation and convective boiling heat transfer of CuO-water/ethylene glycol nanofluids

The convective boiling characteristics of dilute dispersions of CuO nanoparticles in water/ethylene glycol as a base fluid were studied at different operating conditions of (heat fluxes up to 174 kW m-2, mass fluxes range of 353-1,059 kg m-2 s-1 and sub-cooling level of 343, 353 and 363 K) inside the annular duct. The convective boiling heat transfer coefficients of nanofluids in different concentrations (vol%) of nanoparticles (0.5, 1, and 1.5) were also experimentally quantified. Results demonstrated the significant augmentation of heat transfer coefficient inside the region with forced convection dominant mechanism and deterioration of heat transfer coefficient in region with nucleate boiling dominant heat transfer mechanism. Due to the scale formation around the heating section, fouling resistance was also experimentally measured. Experimental data showed that with increasing the heat and mass fluxes, the heat transfer coefficient and fouling resistance dramatically increase and rate of bubble formation clearly increases. Obtained results were then compared to some well-known correlations. Results of these comparisons demonstrated that experimental results represent the good agreement with those of obtained by the correlations. Consequently, Chen correlation is recommended for estimating the convective flow boiling heat transfer coefficient of dilute CuO-water/ethylene glycol based nanofluids.

Sarafraz, M. M.; Hormozi, F.; Kamalgharibi, M.

2014-03-01

193

Predictions of laminar natural convection in heated cavities

International Nuclear Information System (INIS)

Several examples of laminar, natural convection in heated cavities are discussed with illustrative calculations. These include convection in a square cavity at high Rayleigh number; in a narrow cavity at moderate aspect ratio; in a rectangular cavity heated from below; in a trapezoidal cavity, and in a rectangular cavity containing a conducting obstruction. The steady equations for the velocity, pressure and temperature are solved in the Boussinesq approximation, using a standard Galerkin formulation of the finite-element method. (author)

1982-01-01

194

Driving convection by a temperature gradient or a heat current

Digital Repository Infrastructure Vision for European Research (DRIVER)

Bifurcation properties, stability behavior, dynamics, and the heat transfer of convection structures in a horizontal fluid layer that is driven away from thermal equilibrium by imposing a vertical temperature difference are compared with those resulting from imposing a heat current. In particular oscillatory convection that occurs in binary fluid mixtures in the form of travelling and standing waves is determined numerically for the two different driving mechanisms. Conditio...

Matura, P.; Luecke, M.

2006-01-01

195

International Nuclear Information System (INIS)

The void fraction in a forced convective flow boiling is very important information for understanding the characteristics of the boiling two-phase flow. Consequently, many experimental investigations have been carried out to obtain the local void fraction so far, but the detail data among the whole of the test-section has not been enough. Especially, the data under subcooled condition are quite limited. In this study, the void fraction distribution in a forced convective boiling was quantitatively measured by using the thermal neutron radiography. These results were compared with several existing void fraction correlations. Although these correlations show a good agreements with experimental results under low heat flux condition, there is no suitable correlation to estimate the void fraction under non-thermal equilibrium condition. (author)

2012-06-21

196

An experimental investigation of forced convection flat plate solar air heater with storage material

Digital Repository Infrastructure Vision for European Research (DRIVER)

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...

Aissa Walid; El-Sallak Mostafa; Elhakem Ahmed

2012-01-01

197

International Nuclear Information System (INIS)

The helical coiled tube of heat exchanger is used for the evaporator of prototype fast breeder reactor 'Monju'. This report aims at the grasp of two-phase flow phenomena of forced convective boiling of water inside helical coiled tube. A transparent double tube made of the glass is used as a heat exchanger, water flows up inside helical tube and the high temperature oil flows down in the outside tube. The oscillation of the dryout point was observed, that is mainly caused by intensive nucleate boiling near the dryout point and evaporation of thin liquid film flowing along the surface. Also, the oscillation characteristics were experimentally confirmed. (author)

2005-06-06

198

Convective and radiative heat transfer coefficients for individual human body segments

Human thermal physiological and comfort models will soon be able to simulate both transient and spatial inhomogeneities in the thermal environment. With this increasing detail comes the need for anatomically specific convective and radiative heat transfer coefficients for the human body. The present study used an articulated thermal manikin with 16 body segments (head, chest, back, upper arms, forearms, hands, pelvis, upper legs, lower legs, feet) to generate radiative heat transfer coefficients as well as natural- and forced-mode convective coefficients. The tests were conducted across a range of wind speeds from still air to 5.0 m/s, representing atmospheric conditions typical of both indoors and outdoors. Both standing and seated postures were investigated, as were eight different wind azimuth angles. The radiative heat transfer coefficient measured for the whole-body was 4.5 W/m2 per K for both the seated and standing cases, closely matching the generally accepted whole-body value of 4.7 W/m2 per K. Similarly, the whole-body natural convection coefficient for the manikin fell within the mid-range of previously published values at 3.4 and 3.3 W/m2 per K when standing and seated respectively. In the forced convective regime, heat transfer coefficients were higher for hands, feet and peripheral limbs compared to the central torso region. Wind direction had little effect on convective heat transfers from individual body segments. A general-purpose forced convection equation suitable for application to both seated and standing postures indoors was hc=10.3v0.6 for the whole-body. Similar equations were generated for individual body segments in both seated and standing postures.

de Dear, R. J.; Arens, Edward; Hui, Zhang; Oguro, Masayuki

199

Solution of heat removal from nuclear reactors by natural convection

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.

Zitek, Pavel; Valenta, Vaclav

2014-03-01

200

Natural turbulent convection in a partitioned cavity heated from below

Energy Technology Data Exchange (ETDEWEB)

This paper concerns the natural turbulent convective flow inside a cavity which is divided into two compartments separated by a horizontal partition with an opening. Vertical walls are adiabatic; horizontal walls are kept at constant and uniform temperature; the lower wall is warmer than the upper wall. The resulting flow is related to Rayleigh-Benard flows; it is controlled by two main parameters, the Rayleigh number (Ra) and the aperture area to wall area ratio (s/S). In the present work, this flow was investigated both experimentally and numerically. Experiments were conducted in a 3D cavity (1.2 x 10{sup 9} {lt} Ra {lt} 3.7 x 10{sup 9}; s/S = 0.032) while 2D direct numerical simulations were performed (10{sup 8} {lt} Ra {lt} 2 x 10{sup 9}; s/S = 0.15). Results lead to the following conclusions: for large values of s/S, the heat transfer is controlled by the boundary layer and Nu is proportional to Ra{sup 1/3}; for small values of s/S, the heat transfer is controlled by buoyancy forces through the aperture and Nu is proportional to Ra{sup 1/2}. One application of such a flow may concern transport of heat or pollutant inside buildings.

Blay, D.; Le Quere, P.

1999-07-01

201

Validation of PARET for the modeling of heat transfer under natural convection core cooling

Energy Technology Data Exchange (ETDEWEB)

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.

Ibrahim, J.K.; Kassim, M.S.; Mohammed, F. [Ohio State Univ., Colombus, OH (United States)] [and others

1995-12-31

202

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.

2011-11-01

203

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: Portuguese Abstract in portuguese Este trabalho busca, como funçăo principal, determinar o valor do coeficiente convectivo de transferência de calor através da metodologia de modelagem matemática e simulaçăo numérica associada ao processo experimental previamente realizado, de resfriamento rápido com ar forçado de figos "Roxo de Val [...] inhos". O objetivo foi comparar o resultado com o definido pelo ajuste entre os dados experimentais e o uso de equaçăo empírica da literatura, já realizado. Para a prediçăo da temperatura próximo ao centro das frutas, considerou-se o modelo da lei de Fourier em coordenadas esféricas e se implementaram algoritmos segundo os métodos de diferenças finitas e dos elementos finitos para os modelos unidimensional e tridimensional, respectivamente. O estabelecimento do referido valor se dá a partir da comparaçăo desses dados simulados com os dados experimentais. Os valores resultantes dos dois modelos foram coincidentes e resultaram menores valores residuais que o obtido pela equaçăo empírica; além disso, a representatividade da curva simulada mostra que a atual metodologia é mais precisa que a anteriormente realizada e, portanto, adequada para este e trabalhos futuros. Abstract in english The main purpose of this study is to determine the convective heat transfer coefficient value by using the methodology of mathematical modeling and numerical simulation associated with the experimental process previously performed, of the forced air precooling of the figs type 'Roxo de Valinhos'. Th [...] e objective was to compare the result with the one determined by fitting an empirical equation from the literature to the experimental data. For the prediction of the temperature, near to the center of fruits, the Fourier law model was considered. Algorithms employing the finite differences and finite elements methods were implemented for the one-dimensional and three-dimensional models. The establishment of the referred value was performed through the comparison of these simulated data with the experimental data. The resulting values of the two models were coincident and generated a smaller residual value than the one obtained by the empirical equation. Furthermore, the representation of the simulated curve shows that the methodology of the mathematical modeling and numerical simulation is more accurate than the previous one and suitable for this and future work.

Mariangela, Amendola; Saul, Dussán-Sarria; Anderson A., Rabello.

204

Directory of Open Access Journals (Sweden)

Full Text Available Este trabalho busca, como funçăo principal, determinar o valor do coeficiente convectivo de transferência de calor através da metodologia de modelagem matemática e simulaçăo numérica associada ao processo experimental previamente realizado, de resfriamento rápido com ar forçado de figos "Roxo de Valinhos". O objetivo foi comparar o resultado com o definido pelo ajuste entre os dados experimentais e o uso de equaçăo empírica da literatura, já realizado. Para a prediçăo da temperatura próximo ao centro das frutas, considerou-se o modelo da lei de Fourier em coordenadas esféricas e se implementaram algoritmos segundo os métodos de diferenças finitas e dos elementos finitos para os modelos unidimensional e tridimensional, respectivamente. O estabelecimento do referido valor se dá a partir da comparaçăo desses dados simulados com os dados experimentais. Os valores resultantes dos dois modelos foram coincidentes e resultaram menores valores residuais que o obtido pela equaçăo empírica; além disso, a representatividade da curva simulada mostra que a atual metodologia é mais precisa que a anteriormente realizada e, portanto, adequada para este e trabalhos futuros.The main purpose of this study is to determine the convective heat transfer coefficient value by using the methodology of mathematical modeling and numerical simulation associated with the experimental process previously performed, of the forced air precooling of the figs type 'Roxo de Valinhos'. The objective was to compare the result with the one determined by fitting an empirical equation from the literature to the experimental data. For the prediction of the temperature, near to the center of fruits, the Fourier law model was considered. Algorithms employing the finite differences and finite elements methods were implemented for the one-dimensional and three-dimensional models. The establishment of the referred value was performed through the comparison of these simulated data with the experimental data. The resulting values of the two models were coincident and generated a smaller residual value than the one obtained by the empirical equation. Furthermore, the representation of the simulated curve shows that the methodology of the mathematical modeling and numerical simulation is more accurate than the previous one and suitable for this and future work.

Mariangela Amendola

2009-04-01

205

Convective boiling heat transfer in a single micro-channel

Digital Repository Infrastructure Vision for European Research (DRIVER)

The current industrial need for compact high heat density cooling devices has conveyed an increasing interest on convective boiling in micro-channels. Although there is general agreement that these systems may be able to dissipate potentially very high heat fluxes, their heat transfer characteristics are still unclear and require investigation. The present study aims at providing further insight on two-phase single micro-channel heat transfer, through a sensitivity analysis on the effect of t...

2008-01-01

206

Natural convection heat transfer simulation using energy conservative dissipative particle dynamics.

Dissipative particle dynamics with energy conservation (eDPD) was used to study natural convection via Rayleigh-Bénard (RB) problem and a differentially heated enclosure problem (DHE). The current eDPD model implemented the Boussinesq approximation to model the buoyancy forces. The eDPD results were compared to the finite volume solutions and it was found that the eDPD method predict the temperature and flow fields throughout the natural convection domains properly. The eDPD model recovered the basic features of natural convection, such as development of plumes, development of thermal boundary layers, and development of natural convection circulation cells (rolls). The eDPD results were presented via temperature isotherms, streamlines, velocity contours, velocity vector plots, and temperature and velocity profiles. Further useful quantities, such as Nusselt number was calculated from the eDPD results and found to be in good agreement with the finite volume calculations. PMID:20866351

Abu-Nada, Eiyad

2010-05-01

207

Free convection heat transfer from a vertical cylindrical tube bundle

International Nuclear Information System (INIS)

Free convection heat transfer from verti-tube bundles is encountered in nuclear spent fuel storage pools, energy storage devices, room heaters, vertical thermosyphons and other heat transfer equipment. The first of these situations is of much importance because of the strict regulatory laws. In the work reported here, free convection heat transfer was studied experimentally from a tube bundle under the boundary conditions of the second kind. The results obtained were compared with the closest geometrically similar situation available in the literature. (author)

1985-06-02

208

10,000 - A reason to study granular heat convection

Energy Technology Data Exchange (ETDEWEB)

In sheared granular media, particle motion is characterized by vortex-like structures; here this is demonstrated experimentally for disks system undergoing indefinite deformation during simple shear, as often imposed by the rock masses hosting earthquake fault gouges. In traditional fluids it has been known for years that vortices represent a major factor of heat transfer enhancement via convective internal mixing, but in analyses of heat transfer through earthquake faults and base planes of landslides this has been continuously neglected. Can research proceed by neglecting heat convection by internal mixing? Our answer is astonishingly far from being yes.

Einav, I.; Rognon, P.; Gan, Y.; Miller, T.; Griffani, D. [Particles and Grains Laboratory, School of Civil Engineering, University of Sydney, Sydney, NSW 2006 (Australia)

2013-06-18

209

Calculation of convective heat transfer in complex flow systems

International Nuclear Information System (INIS)

Numerical methods of solving the partial differential equations of fluid and heat flow are now capable of predicting the local conductances in heat-transfer equipment and the convective interactions between different parts of the equipment. Examples are cited and applications to the design of cooling towers, steam condensers, furnaces, and nuclear reactors are discussed. (U.S.)

1974-09-03

210

Multi-scale convection in a geodynamo simulation with uniform heat flux along the outer boundary

Conducting fluids stirring within the Earth and other planets generate magnetic fields through a process known as dynamo action. Numerical simulations of dynamo action provide insight into this process, yet cannot replicate the extreme conditions of planetary turbulence, and so important physics may not be adequately captured. For example, it is generally expected that Earth's magnetic field, which is generated by convecting liquid metal within its core, will produce strong Lorentz forces that substantially alter that convection. In most dynamo models, however, Lorentz forces do very little to change convective flow, which is predominantly fine-scaled (Soderlund et al., 2012; King & Buffett, 2013). An important exception to this observation is in dynamo models that employ uniform heat flux boundary conditions, rather than the usual uniform temperature conditions, in which convection occurs on both small and large scales (Sakuraba & Roberts, 2009; Takahashi & Shimizu, 2012). How, exactly, thermal boundary conditions and magnetic field generation conspire to affect convection is not understood. We investigate the combined influence of thermal boundary conditions and magnetic fields using four simulations: two dynamos and two non-magnetic models, with either uniform temperature or heat flux fixed at the outer boundary. Of the four, only the fixed-heat-flux dynamo simulation produces multi-scale convective flow patterns. Comparison between the models suggests that the fixed-flux dynamo generates large patches of strong toroidal field that suppress convective motions near the outer boundary, giving rise to this observed change in convection scales. Strong toroidal field generation by this particular model is made possible by its relatively strong zonal flow, and its strong zonal flow is owed to a baroclinic response to meridional temperature gradients that persist in models with fixed heat flux boundary conditions. Thus, by allowing temperature to vary along the outer boundary, the fixed-flux dynamo generates stronger azimuthal flow and magnetic field, and the resulting Lorentz forces alter the nature of convective flow. References King, E.M., Buffett, B.A., Flow speeds and length scales in geodynamo models: the role of viscosity, Earth Planet. Sci. Lett., 2013, 371 156-162. Sakuraba, A., Roberts, P.H., Generation of a strong magnetic field using uniform heat flux at the surface of the core. Nature Geosci., 2009, 2 802-805. Soderlund, K.M., King, E.M., Aurnou, J.M., The weak influence of magnetic fields in planetary dynamo models. Earth Planet. Sci. Lett., 2012, 333. 9-20. Takahashi, F., Shimizu, H., A detailed analysis of a dynamo mechanism in a rapidly rotating spherical shell, J. Fluid Mech., 2012, 701, 228-250.

King, E. M.; Matsui, H.; Buffett, B. A.

2013-12-01

211

International Nuclear Information System (INIS)

Heat transfer correlations are developed for forced turbulent and laminar, combined, and natural convections of water in a uniformly heated, square arranged, nine-rod bundle having a P/D ratio of 1.5. In all correlations, the heated equivalent diameter is used in all the dimensionless quantities, and the water physical properties are evaluated at the water bulk temperature. In the experiments, Re is varied from 300 to 2.5 X 104, Pr from 4 to 9, Raq from 3 x 106 to 3 x 108 for natural convection and from 5 x 107 to 7 , 108 for combined convection, and Ri from 0.04 to 100. In both upflow and downflow experiments, the transition from forced turbulent to forced laminar convection occurs at ReT = 6,700; while the transition from forced laminar to buoyancy assisted combined convection occurs at Ri = 2.0. Results show that the rod arrangement in the bundle has little effect on the values of Nu in the forced and natural convection regimes. In general, Nu values for the square arranged rod bundle are less than 8% higher and less than 10% lower than those for a triangularly arranged rod bundle in the forced and natural convection regimes, respectively. 16 refs., 7 figs

1992-08-09

212

Two-dimensional forced convection perpendicular to the outdoor fenestration surface--FEM solution

Energy Technology Data Exchange (ETDEWEB)

Two-dimensional laminar forced convection on an outdoor fenestration surface, with the wind perpendicular to the glazing surface, was analyzed using the penalty function approach finite-element method (FEM). The air far from the fenestration surface (free-stream conditions) was assumed to be at ASHRAE standard fenestration conditions of {minus}18 C (0 F) and 6.7 m/s (15 mph). A prototype fenestration configuration of a typical wood casement window, consisting of a double-step frame and an insulating glazing unit (IGU), was used in defining the outdoor fenestration profile. A flat-plate geometry was also considered for purposes of comparison with other available numerical and experimental results and for validation of the results for the actual fenestration profile. The results are reported in the form of velocity vector plots and local convective surface heat transfer coefficients. Recommendations on the local outdoor surface convective heat transfer coefficient for use in two- and three-dimensional heat transfer analyses of fenestration systems are presented.

Curcija, D.; Goss, W.P. [Univ. of Massachusetts, Amherst, MA (United States). Dept. of Mechanical Engineering

1995-08-01

213

Energy Technology Data Exchange (ETDEWEB)

In contrast to ''classical'' dimensional analysis, whose application is widely described in heat transfer textbooks despite its poor results, the less well-known discriminated dimensional analysis approach provides a deeper insight into the physical problems involved and much better results in all cases where it is applied. The basis of this technique is firstly used to test the dimensional homogeneity of the energy equation for incompressible fluids. It is then applied to the laminar forced convection on flat plates to determine the characteristic lengths of the problem, drag forces and heat transfer coefficient. Neither the classical Reynolds and Nusselt numbers nor the drag coefficient are relevant dimensionless parameters for the discriminated dimensional analysis and they do not play a separate (independent) role in the solution of this kind of problem. Furthermore, the dimensionless groups that really play a separate role are obtained with this technique. The apparent equivalence between dimensional analysis and scale analysis is discussed. (authors)

Madrid, C.N.; Alhama, F. [Applied Physics Department, Technical University of Cartagena (Spain)

2005-04-01

214

The two-dimensional version of the cumulus ensemble model developed by Soong and Ogura is applied both to a prestorm situation and to the mature stage of the extratropical mesoscale convective system (MCS) that developed on 10-11 April 1979 (AVE-SESAME-79 I) over the central United States. The objective is to investigate the statistical properties of convection, developing in response to an imposed large-scale forcing, and the thermodynamic feedback effect of clouds on the large-scale environment in midlatitudes. The result is compared to that recently obtained by Tao for a tropical rainband.The outstanding result of the model integration for 17 h of physical time is that statistical properties of clouds averaged horizontally over 128 km of the model domain undergo temporal variations for a given time-independent large-scale forcing, rather than settling down into a steady state. When applied to a prestorm situation, the model predicts heavy precipitation that continues to fall for the first 5 h, followed by a 4 h period without precipitation. A second burst of deep convection then occurs. An analysis of the result reveals that the pause of precipitation occurs when the subcloud layer is dried up primarily due to the net vertical transport of moisture associated with clouds. Convection again starts developing when the moisture in the subcloud layer is replenished by the imposed large-scale forcing. The precipitation rate averaged over the precipitation period is found to exceed the supply of moisture by the large-scale forcing. The result implies that the fraction of moisture convergence in a vertical air column that contributes to moisten the environmental atmosphere in Kuo's cumulus parameterization scheme can be negative.Further, the result indicates the following: 1) The updraft mass flux increases with height until it reaches the local maximum at 350 mb, indicating that the cloud population is dominated by deep clouds, in contrast to the bimodal or broad spectral distribution of clouds observed in the tropics. 2) The cloud heating effect does not balance the large-scale cooling effect, reflecting the fact that the storage and horizontal advection terms are not negligibly small compared to the vertical advection term in the large-scale heat budget; and 3) The net vertical fluxes of heat and moisture are not negligibly small cormpared to condensation and evaporation processes at upper levels in the heat and moisture budgets, reflecting the fact that the atmosphere considered here is more unstably stratified and updrafts are stronger than the tropical counterparts.

Ogura, Yoshi; Jiang, Jih-Yih

1985-12-01

215

Quantification of convective heat transfer inside tree structures

International Nuclear Information System (INIS)

Convective heat transfer between a vegetal structure and its surrounding medium remains poorly described. However, for some applications, such as forest fire propagation studies, convective heat transfer is one of the main factors responsible for vertical fire transitions, from ground level to the tree crowns. These fires are the most dangerous because their rates of spread can reach high speeds, around one meter per second. An accurate characterization of this transfer is therefore important for fire propagation modelling. This study presents an attempt to formulate a theoretical modelling of the convective heat transfer coefficient for vegetal structures generated using an Iterated Function Systems (IFS). This model depends on the IFS parameters. The results obtained using this approach were compared with previously computed numerical results in order to evaluate their accuracy. The maximal discrepancies were found to be around 12% which proves the efficiency of the present model.

2012-11-26

216

Forced convective air cooling from electronic component arrays in a parallel plate channel

This paper discusses air forced convection heat transfer from inline protruding elements arranged in eight rows. The streamwise and spanwise spacings between elements were varied using a splitter plate that can be positioned at three different modular configurations. A set of empirical formulas was presented to correlate the experimental data for the design of air cooling systems. Arrays of components with one odd- size module have been tested also. Experimental results show that blocks near the entrance and behind the odd- size module have improved performance compared with uniform arrangements. Accordingly, temperature sensitive components are suggested to be arranged in these locations.

Cai, D. Y.; Gan, Y. P.; Ma, C. F.; Li, Q. X.

1994-09-01

217

A perturbation based analysis to investigate forced convection in a porous saturated tube

Fully developed laminar steady forced convection inside a circular tube filled with saturated porous medium and with uniform heat flux at the wall is investigated on the basis of a Brinkman model which is appropriate for hyperporous materials which are of current practical importance. The "WKB method" is applied for small values of the Darcy number. For the case of large Darcy number, the solution for the Brinkman momentum equation is found in terms of an asymptotic expansion. With the velocity distribution determined, the energy equation is solved using the same asymptotic technique. The results for limiting cases were found to be in good agreement with those available in the open literature.

Hooman, K.; Ranjbar-Kani, A. A.

2004-01-01

218

Heat-flux scaling in turbulent Rayleigh-Bénard convection with an imposed longitudinal wind

We present a numerical study of Rayleigh-Bénard convection disturbed by a longitudinal wind. Our results show that under the action of the wind, the vertical heat flux through the cell initially decreases, due to the mechanism of plume sweeping, and then increases again when turbulent forced convection dominates over the buoyancy. As a result, the Nusselt number is a nonmonotonic function of the shear Reynolds number. We provide simple models that capture with good accuracy all the dynamical regimes observed. We expect that our findings can lead the way to a more fundamental understanding of the complex interplay between mean wind and plume ejection in the Rayleigh-Bénard phenomenology.

Scagliarini, Andrea; Gylfason, Ármann; Toschi, Federico

2014-04-01

219

Bounds on Rayleigh Bénard convection with an imposed heat flux

We formulate a bounding principle for the heat transport in Rayleigh Bénard convection with fixed heat flux through the boundaries. The heat transport, as measured by a conventional Nusselt number, is inversely proportional to the temperature drop across the layer and is bounded above according to Nu [less-than-or-eq, slant] cR^1/3, where c NuRa = R^, yielding the bound Nu [less-than-or-eq, slant] c3/2Ra1/2.

Otero, Jesse; Wittenberg, Ralf W.; Worthing, Rodney A.; Doering, Charles R.

2002-12-01

220

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

1995-01-01

221

Extinction transition in bacterial colonies under forced convection

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.

Neicu, T; Larochelle, D A; Kudrolli, A

2000-01-01

222

On forced convection in a circular duct with slug flow and viscous dissipation

Energy Technology Data Exchange (ETDEWEB)

The effect of viscous dissipation in the analysis of fully developed forced convection with slug flow in a circular tube is considered. The fully developed temperature profile is determined for various axial distributions of wallheat flux. It is shown that any axial distribution of wall heat flux which asymptotically vanishes yields a vanishing asymptotic value of the Nusselt number. Moreover, for the wall heat flux distributions which asymptotically tend to a constant value or tend to infinity, the asymptotic value of the Nusselt number is determined analytically as a function of the Brinkman number and of a dimensionless parameter which depends on the asymptotic behavior of the wall heat flux. A comparison with the model in which viscous dissipation is neglected is performed.

Barletta, A. [Univ. di Bologna (Italy)

1996-01-01

223

The problem of thermally driven convection in a rapidly rotating spherical shell with homogeneous boundary conditions has been widely studied owing to its application to planetary cores. The dynamics of this system are determined by the Rayleigh number Ra, measuring the strength of the thermal driving force, the Prandtl number Pr, the ratio of viscous and thermal diffusion, and the Ekman number E, measuring the strength of the Coriolis force. The main challenges are to understand the behaviour of the system in the limits of small E and high Ra that characterise many core regions. An important modification to this homogeneous problem in the geophysical context is the addition of lateral variations in heat-flux at the outer boundary with amplitude measured by the nondimensional parameter q*, the ratio of peak-to-peak heat-flux variations on the boundary to the average outer boundary heat-flux. We study rapidly rotating convection in a spherical shell for Pr=1, 10-4 ? E ? 10-6, 0 ? q* ? 10 and Rayleigh numbers up to 400 times the critical value for the onset of homogeneous convection. Homogeneous (q*=0) and inhomogeneous solutions are compared in terms of their spatio-temporal behaviour and heat transfer properties. We also investigate the extent to which the effects of heat-flux anomalies imposed at the outer boundary penetrate into the deep interior of the shell.

Davies, C. J.

2013-12-01

224

Developing convective heat transfer in deep rectangular microchannels

International Nuclear Information System (INIS)

Experimental results have been obtained for single-phase forced convection in deep rectangular microchannels. Two configurations were tested, a single channel system and a multiple channel system. The two systems are identical except for the lack of extended surfaces in the single channel system. In the case of the multiple channel system the channels are 251 ?m wide and the channel walls are 119 ?m thick. In both systems the channels are approximately 1000 ?m deep and define a projected area of 2.5cmx2.5cm. All tests were performed with deionized water as the working fluid, where the Reynolds number ranged from 173 to 12 900. The experimentally obtained local Nusselt number agrees reasonably well with classical developing channel flow theory. Furthermore, the results show that, in terms of flow and heat transfer characteristics, our microchannel system designed for developing laminar flow outperforms the comparable single channel system designed for turbulent flow. (Copyright (c) 1999 Elsevier Science B.V., Amsterdam. All rights reserved.)

1999-04-01

225

Developing convective heat transfer in deep rectangular microchannels

Energy Technology Data Exchange (ETDEWEB)

Experimental results have been obtained for single-phase forced convection in deep rectangular microchannels. Two configurations were tested, a single channel system and a multiple channel system. The two systems are identical except for the lack of extended surfaces in the single channel system. In the case of the multiple channel system the channels are 251 {mu}m wide and the channel walls are 119 {mu}m thick. In both systems the channels are approximately 1000 {mu}m deep and define a projected area of 2.5cmx2.5cm. All tests were performed with deionized water as the working fluid, where the Reynolds number ranged from 173 to 12 900. The experimentally obtained local Nusselt number agrees reasonably well with classical developing channel flow theory. Furthermore, the results show that, in terms of flow and heat transfer characteristics, our microchannel system designed for developing laminar flow outperforms the comparable single channel system designed for turbulent flow. (Copyright (c) 1999 Elsevier Science B.V., Amsterdam. All rights reserved.)

Harms, T.M.; Kazmierczak, M.J.; Gerner, F.M. [Department of Mechanical, Industrial, and Nuclear Engineering, University of Cincinnati, Cincinnati, OH (United States)

1999-04-01

226

Turbulence Modeling Experience in Ducts with Forced Convective Flow

DEFF Research Database (Denmark)

This study presents a general computational method for calculating turbulent quantities in arbitrary three-dimensional ducts. Four different turbulence models for the turbulent Reynolds stresses are compared, namely, a standard K-epsilon model, a nonlinear K- epsilon model, an explicit algebraic stress model (EASM), and a full Reynolds stress model (RSM). The turbulent heat fluxes are modeled by the simple eddy diffusivity concept, the generalized gradient diffusion hypothesis, and the wealth alpha earnings time methods. A finite volume technique for nonstaggered grids combined with the SIMPLEC algorithm is applied. A modified strongly implicit procedure is implemented for solving the equations. The van Leer scheme is applied for the convective terms except for the K and epsilon equations, where the hybrid scheme is used.

Rokni, Masoud; SundĂ©n, Bengt

1999-01-01

227

Directory of Open Access Journals (Sweden)

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.

Sumon Saha

2006-10-01

228

A multiple-relaxation-time lattice Boltzmann model for convection heat transfer in porous media

Digital Repository Infrastructure Vision for European Research (DRIVER)

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...

Liu, Qing; He, Ya-ling; Li, Qing; Tao, Wen-quan

2013-01-01

229

Electro-magneto convective heat transfer in magnetic liquids

International Nuclear Information System (INIS)

Magneto convective and electro-magneto convective heat transfer coefficients have been measured in a magnetic liquid (kerosene) and a super-paramagnetic colloid (hematite in distilled water) from a single platinum wire (diameter 0.025 mm) mounted along the axis of a copper cylinder. For this study, we used two copper cylinders (diameters = 20.5 mm and 53 mm). An inhibition in heat transfer coefficient was observed in the presence of magnetic field. A similar effect was also noticed when electric and magnetic fields were applied simultaneously. The heat transfer coefficient was found to be independent of the size of the cylinder over the range of the space ratio 820 to 2120 but an enhancement of heat transfer coefficient was observed when the inclination of the cylinder was increased from the horizontal to vertical positions

1995-12-01

230

Natural convection heat transfer: Revised boundary layer relations

International Nuclear Information System (INIS)

For a flat vertical surface, the heat transfer equation for the laminar regime in natural convection, derived from the integral boundary layer equations, is modified on the basis that the boundary layer ratio (?/?/sub T/) ? 1 at all Prandtl numbers. For transfer in the turbulent boundary layer, an equation Nu = (GrPr)/sup 3/8/, f(Pr) has been formulated

1985-01-01

231

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)

1993-01-01

232

Second Law Analysis in Convective Heat and Mass Transfer

Digital Repository Infrastructure Vision for European Research (DRIVER)

This paper reports the numerical determination of the entropy generation due to heat transfer, mass transfer and fluid friction in steady state for laminar double diffusive convection, in an inclined enclosure with heat and mass diffusive walls, by solving numerically the mass, momentum, species conservation and energy balance equations, using a Control Volume Finite-Element Method. The influences of the inclination angle, the thermal Grashof number and the buoyancy ratio on total entropy gen...

Magherbi, M.; Abbassi, H.; Hidouri, N.; Ben Brahim, A.

2006-01-01

233

Convective heat transfer modelling in offices with night cooling

Digital Repository Infrastructure Vision for European Research (DRIVER)

Night ventilation to cool buildings attracts growing interest. For, it can improve the summer comfort and can lower the cooling need. However, the extent to which building designers succeed in finding an optimal night cooling design depends strongly on the simulation tool they use. Today, stand-alone building energy simulation (BES) programs are quite popular, but the way they model the convective heat transfer raises questions. They model the complex heat transfer in the boundary layer and t...

2012-01-01

234

Effect of radiation on the laminar convective heat transfer through a layer of highly porous medium

International Nuclear Information System (INIS)

A numerical investigation is reported of the coupled forced convective and radiative transfer through a highly porous medium. The porosity range investigated is high enough that the fluid inertia terms in the momentum equation cannot be neglected; i.e., the simple form of Darcy's law is invalid. The geometry studied is a plane layer of highly porous medium resting on one impermeable boundary and exposed to a two-dimensional laminar external flow field. The objective is to determine the effective overall heat transfer coefficients for such a geometry. The results are applicable to diverse situations, including insulation batts exposed to external flow, the heat loss and drying rates of grain fields and forest areas, and the drying of beds of porous material exposed to convective and radiative heating

1986-01-01

235

Convective heat transfer near the entrance region of a tube for water near the critical point

Energy Technology Data Exchange (ETDEWEB)

Numerical modeling is carried out to investigate the characteristics of convective heat transfer for the turbulent developing flow of water near the critical point in a tube with or without buoyancy force for constant wall temperature. The modeling includes the effects of the thermodynamic and transport property variations. The effects of proximity to the critical point and wall temperature variation on the buoyancy force are considered as well as varying properties near the pseudocritical temperature. For turbulent diffusivity, a modified mixing length model including density fluctuation is used. Emphasis is on effects very close to the inlet (z/D < 10). The results show heat transfer and fluid flow characteristics which include velocity profiles, heat transfer coefficient and friction factor distribution along the tube. Effects of gravity force on turbulent eddy viscosity distribution inside the tube are also studied. Results using several models for turbulent Prandtl number are compared.

Lee, S.H.; Howell, J.R.

1997-07-01

236

Critical heat flux in natural convection cooled TRIGA reactors with hexagonal bundle

Energy Technology Data Exchange (ETDEWEB)

A three-rod bundle Critical Heat Flux (CHF) study at low flow, low pressure, and natural convection condition has been conducted, simulating TRIGA reactors with the hexagonally configured core. The test section is a custom-made trefoil shape tube with three identical fuel pin heater rods located symmetrically inside. The full scale fuel rod is electrically heated with a chopped-cosine axial power profile. CHF experiments were carried out with the following conditions: inlet water subcooling from 30 K to 95 K; pressure from 110 kPa to 230 kPa; mass flux up to 150 kg/m{sup 2}s. About 50 CHF data points were collected and compared with a few existing CHF correlations whose application ranges are close to the testing conditions. Some tests were performed with the forced convection to identify the potential difference between the CHF under the natural convection and forced convection. The relevance of the CHF to test parameters is investigated. (authors)

Yang, J.; Avery, M.; De Angelis, M.; Anderson, M.; Corradini, M. [Univ. of Wisconsin-Madison, 1500 Engineering Drive, Madison, WI 53706 (United States); Feldman, E. E.; Dunn, F. E.; Matos, J. E. [Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439 (United States)

2012-07-01

237

Numerical analysis of laminar forced convection in a spherical annulus

Energy Technology Data Exchange (ETDEWEB)

Calculations of steady laminar incompressible fluid-flow and heat transfer in a spherical annulus are presented. Steady pressures, temperatures, velocities, and heat transfer coefficients are calculated for an insulated outer sphere and a 0/sup 0/C isothermal inner sphere with 50/sup 0/C heated water flowing in the annulus. The inner sphere radius is 13.97 cm, the outer sphere radius is 16.83 cm and the radius ratio is 1.2. The transient axisymmetric equations of heat, mass, and momentum conservation are solved numerically in spherical coordinates. The transient solution is carried out in time until steady state is achieved. A variable mesh is used to improve resolution near the inner sphere where temperature and velocity gradients are steep. It is believed that this is the first fully two-dimensional analysis of forced flow in a spherical annulus. Local and bulk Nusselt numbers are presented for Reynolds numbers from 4.4 to 440. Computed bulk Nusselt numbers ranged from 2 to 50 and are compared to experimental results from the literature. Inlet flow jetting off the inner sphere and flow separation are predicted by the analysis. The location of wall jet separation was found to be a function of Reynolds number, indicating the location of separation depends upon the ratio of inertia to viscous forces. Wall jet separation has a pronounced effect on the distribution of local heat flux. The area between inlet and separation was found to be the most significant area for heat transfer. Radial distributions of azimuthal velocity and temperature are presented for various angles beginning at the inlet. Inner sphere pressure distribution is presented and the effect on flow separation is discussed.

Tuft, D.B.

1980-07-21

238

Numerical analysis of laminar forced convection in a spherical annulus

International Nuclear Information System (INIS)

Calculations of steady laminar incompressible fluid-flow and heat transfer in a spherical annulus are presented. Steady pressures, temperatures, velocities, and heat transfer coefficients are calculated for an insulated outer sphere and a 0"0C isothermal inner sphere with 50"0C heated water flowing in the annulus. The inner sphere radius is 13.97 cm, the outer sphere radius is 16.83 cm and the radius ratio is 1.2. The transient axisymmetric equations of heat, mass, and momentum conservation are solved numerically in spherical coordinates. The transient solution is carried out in time until steady state is achieved. A variable mesh is used to improve resolution near the inner sphere where temperature and velocity gradients are steep. It is believed that this is the first fully two-dimensional analysis of forced flow in a spherical annulus. Local and bulk Nusselt numbers are presented for Reynolds numbers from 4.4 to 440. Computed bulk Nusselt numbers ranged from 2 to 50 and are compared to experimental results from the literature. Inlet flow jetting off the inner sphere and flow separation are predicted by the analysis. The location of wall jet separation was found to be a function of Reynolds number, indicating the location of separation depends upon the ratio of inertia to viscous forces. Wall jet separation has a pronounced effect on the distribution of local heat flux. The area between inlet and separation was found to be the most significant area for heat transfer. Radial distributions of azimuthal velocity and temperature are presented for various angles beginning at the inlet. Inner sphere pressure distribution is presented and the effect on flow separation is discussed

1980-10-24

239

Optimization of fin geometry in heat convection with entransy theory

International Nuclear Information System (INIS)

The entransy theory developed in recent years is used to optimize the aspect ratio of a plate fin in heat convection. Based on a two-dimensional model, the theoretical analysis shows that the minimum thermal resistance defined with the concept of entransy dissipation corresponds to the maximum heat transfer rate when the temperature of the heating surface is fixed. On the other hand, when the heat flux of the heating surface is fixed, the minimum thermal resistance corresponds to the minimum average temperature of the heating surface. The entropy optimization is also given for the heat transfer processes. It is observed that the minimum entropy generation, the minimum entropy generation number, and the minimum revised entropy generation number do not always correspond to the best heat transfer performance. In addition, the influence factors on the optimized aspect ratio of the plate fin are also discussed. The optimized ratio decreases with the enhancement of heat convection, while it increases with fin thermal conductivity increasing. (general)

2013-02-01

240

A Numerical Study of Turbulent Forced Convection in a Square Duct Using Different Turbulence Models

DEFF Research Database (Denmark)

The present work concerns development and application of turbulence models for forced convective heat transfer in ducts. The numerical approach is based on the finite volume technique and a non-staggered arrangement is employed. The SIMPLEC-algorithm is used for handling the pressure-velocity coupling. Cyclic boundary conditions are imposed in the main flow direction. The standard k-* model with wall functions is used as a reference. The non-linear k-* model of Speziale is applied to calculate the turbulent shear stresses. The turbulent heat fluxes are calculated by the simple eddy diffusivity concept, the GGDH method and the WET method. The overall comparison between the methods is presented in terms of the friction factor and average Nusselt number. In particular the secondary flow field is investigated.

Rokni, Masoud; SundĂ©n, Bengt

1995-01-01

241

Enhancement of convective heat transfer by using microencapsulated PCM slurry

Energy Technology Data Exchange (ETDEWEB)

As a method to make use of a high latent heat of a liquid-solid phase change material in a confined convective heat transfer, a phase change material, lauric acid, was encapsulated in a tiny hollow sphere, and mixed with water, forming a slurry of microencapsulated phase-change material (MCPCM slurry). Four different sizes (200, 100, 50, and 25 {mu}m ID) of MCPCMs were tested in this study. Thermal characteristics of the pure lauric acid and lauric acid MCPCMs were tested by a differential scanning calorimeter. Viscosity of the slurries of water and the capsules was measured by a rotating viscometer. The slurries were also tested in a convective-heat-transfer-test apparatus. (author). 11 refs., 5 figs., 1 tab.

Choi, E.S. [Myongji University, Yongin (Korea); Yoo, J.H. [Graduate School, Myong Ji University, Yongin (Korea)

1998-11-01

242

Convective heat conductivity and diffusion in one-dimensional hydrodynamics

International Nuclear Information System (INIS)

Impurity concentration in the localized structures is described on the basis of the analytical solutions of the convective diffusion model equations in unidimensional hydrodynamic approach. In case of the Prandtl integral numbers, the solution of problems of nonstationary convective diffusion in the field of flow of evenly moving shock wave is boiled down to the solution of problems for heat conductivity equation. Relations between problems with integral-valued differences in the Prandtl numbers are determined. Various representations of the Green functions of the convective diffusion equations are analyzed. In case of the Prandtl integral numbers, they are expressed through the error integrals. The asymptotic nature of the solutions depends essentially on the observance of the integral laws of conservation. In case of the integral conservation of the impurity mass, the effect of coalescence of impurity solitons, their clusterization corresponds to the effect of coalescence of shock waves

1999-11-01

243

Single-phase convective heat transfer in rod bundles

International Nuclear Information System (INIS)

The convective heat transfer for turbulent flow through rod bundles representative of nuclear fuel rods used in pressurized water reactors is examined. The rod bundles consist of a square array of parallel rods that are held on a constant pitch by support grids spaced axially along the rod bundle. Split-vane pair support grids, which create swirling flow in the rod bundle, as well as disc and standard support grids are investigated. Single-phase convective heat transfer coefficients are measured for flow downstream of support grids in a rod bundle. The rods are heated using direct resistance heating, and a bulk axial flow of air is used to cool the rods in the rod bundle. Air is used as the working fluid instead of water to reduce the power required to heat the rod bundle. Results indicate heat transfer enhancement for up to 10 hydraulic diameters downstream of the support grids. A general correlation is developed to predict the heat transfer development downstream of support grids. In addition, circumferential variations in heat transfer coefficients result in hot streaks that develop on the rods downstream of split-vane pair support grids

2008-04-01

244

Second Law Analysis in Convective Heat and Mass Transfer

Directory of Open Access Journals (Sweden)

Full Text Available This paper reports the numerical determination of the entropy generation due to heat transfer, mass transfer and fluid friction in steady state for laminar double diffusive convection, in an inclined enclosure with heat and mass diffusive walls, by solving numerically the mass, momentum, species conservation and energy balance equations, using a Control Volume Finite-Element Method. The influences of the inclination angle, the thermal Grashof number and the buoyancy ratio on total entropy generation were investigated. The irreversibilities localization due to heat transfer, mass transfer and fluid friction is discussed for three inclination angles at a fixed thermal Grashof number.

A. Ben Brahim

2006-02-01

245

Second Law Analysis in Convective Heat and Mass Transfer

This paper reports the numerical determination of the entropy generation due to heat transfer, mass transfer and fluid friction in steady state for laminar double diffusive convection, in an inclined enclosure with heat and mass diffusive walls, by solving numerically the mass, momentum, species conservation and energy balance equations, using a Control Volume Finite-Element Method. The influences of the inclination angle, the thermal Grashof number and the buoyancy ratio on total entropy generation were investigated. The irreversibilities localization due to heat transfer, mass transfer and fluid friction is discussed for three inclination angles at a fixed thermal Grashof number.

Magherbi, M.; Abbassi, H.; Hidouri, N.; Brahim, A. B.

2006-03-01

246

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.

Olson, Sandra

2011-01-01

247

Energy Technology Data Exchange (ETDEWEB)

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)

Oosthuizen, P.H.; Sun, L. [Queen' s Univ., Dept. of Mechanical Engineering, Kingston, Ontario (Canada)]. E-mail: oosthuiz@me.queensu.ca; sun@me.queensu.ca; Naylor, D. [Ryerson Univ., Dept. of Mechanical, Aerospace and Industrial Engineering, Toronto, Ontario (Canada)]. E-mail: dnaylor@ryerson.ca

2003-07-01

248

DEFF Research Database (Denmark)

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.

Taherian, Hessam; Yazdanshenas, Eshagh

2006-01-01

249

Energy Technology Data Exchange (ETDEWEB)

This progress report describes the thermodynamic testing and modeling of a thermosyphon heat exchanger used in solar water heating systems. Testing of a four tube-in-shell thermosyphon heat exchanger was performed in two parts. The first portion of the test increased the collector fluid while the storage tank remained isothermal. After the collector fluid temperature was raised to 95 C, the second part of the test allowed the storage tank to gain heat. The test was performed for two collector flow rates. Measured values included collector side forced flow rate, temperature differences across the heat exchanger, vertical temperature distribution in the storage tank, vertical water temperature profile in the heat exchanger, and pressure drop on the thermosyphon side of the heat exchanger. The overall heat transfer coefficient-area product (UA) values obtained confirmed that models which assume UA depends solely on thermosyphon flow rate do not adequately characterize thermosyphon heat exchangers. This is because heat transfer in thermosyphon exchangers occurs in the mixed convection, rather than forced flow, regime. A linear regression equation was developed to better predict UA using the Prandtl, Reynolds, and Grashof numbers and dimensionless parameters based on fluid properties calculated for the average hot and cold leg temperatures. 9 figs.

Davidson, J.H.

1998-06-01

250

Mixed convection boundary layer flow over a vertical cylinder with prescribed surface heat flux

Energy Technology Data Exchange (ETDEWEB)

The steady mixed convection boundary layer flow along a vertical cylinder with prescribed surface heat flux is investigated in this study. The free stream velocity and the surface heat flux are assumed to vary linearly with the distance from the leading edge. Both the case of the buoyancy forces assisting and opposing the development of the boundary layer are considered. Similarity equations are derived, their solutions being dependent on the mixed convection parameter, the curvature parameter, as well as of the Prandtl number. Dual solutions are found to exist for both buoyancy assisting and opposing flows. It is also found that the boundary layer separation is delayed for a cylinder compared to a flat plate.

Ishak, Anuar [School of Mathematical Sciences, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor (Malaysia)], E-mail: anuar_mi@ukm.my

2009-05-15

251

Energy Technology Data Exchange (ETDEWEB)

The objective of the present work is to investigate the characteristics of the airflow and heat/contaminant transport structures in the indoor air environment by means of a convection transport visualization technique. Laminar double-diffusive mixed convection in a two-dimensional displacement ventilated enclosure with discrete heat and contaminant sources are numerically studied. Based on the governing equations, the fluid, heat, and contaminant transport processes are respectively described by the corresponding streamfunction, heatfunction, and massfunction. Attentions are given to analyze the effects of the main factors - the strength of heat source indicated by the Grashof number (Gr), the strength of contaminant source by the buoyancy ratio (Br), the strength of ventilation by the Reynolds number (Re), and the ventilation mode - on the indoor air environment. Numerical results, presented by the contour function lines, namely, streamlines, heatlines, and masslines, illustrated that the indoor air, heat and contaminant transport structures are mainly determined by the interaction between the internal buoyancy natural convection induced by the discrete heat/contaminant sources and the external forced convection driven by the mechanical ventilation. It is found that the convection transport method could explicitly disclose the complicated philosophy of indoor air environment, and thus provides a simple but practical approach to see the indoor airflow and heat and contaminant transport structures. (author)

Qi-Hong Deng; Jiemin Zhou; Chi Mei [Central South University, Changsha (China). School of Energy and Power Engineering; Yong-Ming Shen [Dalian University of Technology (China). State Key Laboratory of Coastal and Offshore Engineering

2004-11-01

252

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 ...

Dore, Valentina

2010-01-01

253

Analysis of natural convection heat transfer and flows in internally heated stratified liquid pools

Energy Technology Data Exchange (ETDEWEB)

In this paper, natural convection flows and heat transfer in a liquid pool, with two superposed immiscible fluid layers, are analyzed. The objective of the study is to examine the effect of interfacial hydrodynamics and to develop a method which enables energy splitting to be evaluated in a stratified liquid pool. The thermal convection, with and without an internal heat source, in a rectangular cavity with different pairs of fluids was numerically simulated by a CFD code FLOW-3D. It was found that the code performs very well for prediction of heat transfer coefficients for different conditions. The hydrodynamic coupling between immiscible layers was found to have minor, if any, impact on the natural convection heat transfer for the conditions examined. Calculated results were used to develop, and validate, a new correlation for energy splitting and for heat transfer in stratified liquid pools.

Gubaidullin, A.A. Jr.; Dinh, T.N.; Sehgal, B.R.

1999-07-01

254

Experimental Study of Forced Convection over Equilateral Triangle Helical Coiled Tubes

Directory of Open Access Journals (Sweden)

Full Text Available

This study presents an experimental investigation of an equilateral triangular cross-sectioned helical tube under uniform heat flux boundary condition. The experiments are carried out for nine helical coiled-tubes of different parameters. Different diameter ratio (D/a ranged from 6.77 to 15.43 and pitch ratio (P/a ranged from 1.127 to 3.062 are employed in the present study, The experiments covered a range of Reynolds number from 5.3X10^{2} to 2.2X10^{3}. Uniform heat flux is applied to the inside surface of the helical coil and air is selected as tested fluid. The experimental results obtained from the equilateral triangular cross-sectioned helical tube indicated that the parameters of the coil diameter and pitch of helical coil have important effects on the heat transfer coefficient. The Nusselt number increases with the increase of Reynolds number and coil diameter at constant pitch of the helical coil. Also, Nusselt number increases with the increase of Reynolds number and Pitch of helical coil at constant coil diameter tube. A comparison between the present experimental data with a previous work with circular cross-sectioned helical tubes have the same test conditions was achieved. From this comparison, it is clear that the average enhancement of Nusselt number for equilateral triangular cross-sectioned helical is about 1.12 ~ 1.25 times the circular cross-sectioned helical for all tested conditions. A general correlation of the average Nusselt number as a function in Re, D/a and P/a ratios is obtained to describe the forced convection from the equilateral triangle cross sectioned coiled tube.

** Key words: ** Forced convection; Helical coiled tubes; Coil diameter ratio; Pitch ratio

E. El-Kashif

2012-06-01

255

Numerical prediction of natural convection heat transfer in horizontal annulus

International Nuclear Information System (INIS)

The numerical prediction of natural convection heat transfer in a horizontal annulus in which the inner cylinder is hotter than the outer cylinder is considered. A modified SIMPLE procedure is used for this purpose and it is shown that this procedure yields faster convergence. Results have been obtained for L/Dsub(i) = 0.8 and 0.15. This latter value is of interest in Horizontal Pressurised Heavy Water Reactors. (author)

1986-01-01

256

Natural Convection in Parabolic Enclosure Heated from Below

Digital Repository Infrastructure Vision for European Research (DRIVER)

The effects of vertical parabolic walls on natural convection in a parabolic enclosure have investigated numerically in this paper. The bottom wall is heated isothermally, while the other vertical parabolic walls are maintained at constant cold temperature and the top wall is well insulated. The flow and temperature fields are studied numerically for three values (C = 0.1, 0.5, 1.0) of the parabolic equation constant. The laminar flow field is analyzed numerically by solving the stea...

Mustafa, Ahmed W.

2011-01-01

257

Mixed convection flow of nanofluid with Newtonian heating

This article explores the flow of viscous nanofluid over a stretching sheet with mixed convection. The main objective here is to examine the Newtonian heating effect. The partial differential equations are reduced to the ordinary differential equations (ODE). The resulting ODEs are computed for the convergent series solutions. Plots for physical quantities of interest are displayed and discussed. Local Nusselt and Sherwood numbers are computed and analyzed.

Imtiaz, Maria; Hayat, T.; Hussain, Majid; Shehzad, S. A.; Chen, G. Q.; Ahmad, B.

2014-05-01

258

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

2013-03-01

259

Aerial measurements of convection cell elements in heated lakes

Power plant-heated lakes are characterized by a temperature gradient in the thermal plume originating at the discharge of the power plant and terminating at the water intake. The maximum water temperature discharged by the power plant into the lake depends on the power generated at the facility and environmental regulations on the temperature of the lake. Besides the observed thermal plume, cloud-like thermal cells (convection cell elements) are also observed on the water surface. The size, shape and temperature of the convection cell elements depends on several parameters such as the lake water temperature, wind speed, surfactants and the depth of the thermocline. The Savannah River National Laboratory (SRNL) and Clemson University are collaborating to determine the applicability of laboratory empirical correlations between surface heat flux and thermal convection intensity. Laboratory experiments at Clemson University have demonstrated a simple relationship between the surface heat flux and the standard deviation of temperature fluctuations. Similar results were observed in the aerial thermal imagery SRNL collected at different locations along the thermal plume and at different elevations. SRNL will present evidence that the results at Clemson University are applicable to cooling lakes.

Villa-Aleman, E.; Salaymeh, S. R.; Brown, T. B.; Garrett, A. J.; Nichols, L. S.; Pendergast, M. M.

2008-03-01

260

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)

2011-01-01

261

The influence of the Rayleigh and Prandtl numbers on convective heat transport in stars

International Nuclear Information System (INIS)

The two important parameters which essentially determine the nature of the convective regime, and conseqently the total heat transported by convective processes, across an unstable layer in a star are the Prandtl and Rayleigh numbers. Their influence on convective heat transport is discussed

1980-01-01

262

Convective Heat and Mass Transfer in Rotating Disk Systems

The book describes results of investigations of a series of convective heat and mass transfer problems in rotating-disk systems, namely, over free rotating disks, under conditions of transient heat transfer, solid- body rotation of fluid, orthogonal flow impingement onto a disk, swirl radial flow between parallel co-rotating disks, in cone-disk systems and for Prandtl and Schmidt numbers larger than unity. Methodology used included integral methods, self-similar and approximate analytical solutions, as well as CFD. The book is aimed at the professional audience of academic researchers, industr

Shevchuk, Igor V

2009-01-01

263

Using magnetic fluids to simulate convection in a central force field in the laboratory

Digital Repository Infrastructure Vision for European Research (DRIVER)

Large-scale convection in planetary or stellar interiors plays a significant role but it is difficult to reproduce the central force field of those systems in experimental studies. A technique to approximate a central force field through the magnetic field from magnets acting on a magnetic liquid is presented. The thermomagnetic convection in a spherical shell filled with a magnetic liquid is analyzed in the context of a terrestrial laboratory using a 2D Finite Element model. Two configuratio...

Fru?h, W. -g

2005-01-01

264

Experimental study of convective heat transfer of compressed air flow in radially rotating ducts

Energy Technology Data Exchange (ETDEWEB)

The convective heat transfer of pressurized air flow in radially rotating serpentine channel is investigated experimentally in the present study. The main governing parameters are the Prandtl number, the Reynolds number for forced convection, the rotation number for the Coriolis force induced cross stream secondary flow and the Grashof number for natural convection. To simulate the operation conditions of a real gas turbine, the present study kept the parameters in the test rig approximately the same as those in a real engine. The air in the present serpentine channel was pressurized to increase the air density for making up the low rotational speed in the experiment. Before entering the rotating ducts, the air was also cooled to gain a high density ratio of approximately 1/3 in the ducts. This high density ratio will give a similar order of magnitude of Grashof number in a real operation condition. The local heat transfer rate on the four channel walls are present and compared with that in existing literature.

Hwang, G.J,; Tzeng, S.C.; Mao, C.P.

1999-07-01

265

Development of convective heat transfer correlations for common designs of solar dryer

International Nuclear Information System (INIS)

Highlights: ? Separate experimental methods of hcpf evaluation are proposed for different dryers. ? Correlation for hcpf in terms of dimensionless numbers for each dryer is proposed. ? Single correlation for hcpf representing different dryer designs is also developed. ? Levenberg–Marquardt algorithm is used to develop temperature dependent correlation. ? Close agreement of experimental and predicted hcpf validates proposed correlations. - Abstract: The knowledge of convective heat transfer coefficient hcpf (absorber plate to flowing air) is necessary to predict or evaluate thermal performance of any solar dryer. In order to determine hcpf, laboratory models of direct (cabinet), indirect and mixed mode solar dryer are designed and constructed to perform no-load steady state experiments for natural and forced air circulation. The dryers are operated under indoor simulation conditions for absorbed thermal energy and air flow rate for the range of 300–800 W/m2 and 1–3 m/s, respectively. Separate methods depending on mode of heat utilisation are proposed for determination of hcpf for different dryers. Correlations of hcpf in terms of dimensionless numbers are developed for each dryer operating under natural and forced convection. Levenberg–Marquardt algorithm is used to develop temperature dependent correlations. A close agreement between experimental and predicted hcpf values obtained from proposed correlations for natural convection dryers demonstrates their reliability. However, for forced convection dryers, there is a need to use temperature dependent Nu–Re correlation for more accurate results. The low uncertainty ranging from 0.3% to 0.8% in the determination of hcpf confirms the accuracy of experimental data obtained for various dryer designs operated under different conditions.

2012-12-01

266

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.

Neshat, E.; Hossainpour, S.; Bahiraee, F.

2014-06-01

267

Convective Heat Transfer Analysis in Fluid Flow with Turbulence Promoters with Heat Pipes

Digital Repository Infrastructure Vision for European Research (DRIVER)

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.

2007-01-01

268

Numerical predictions of natural convection in a uniformly heated pool

International Nuclear Information System (INIS)

In the event of a core meltdown accident, one of the accident progression paths is fuel relocation to the lower reactor plenum. In the heavy water new production reactor (NPR-HWR) design the reactor cavity is flooded with water. In such a design, decay heat removal to the water in the reactor cavity and thence to the containment may be adequate to keep the reactor vessel temperature below failure limits. If this is the case, the accident progression can be arrested by retaining a coolable corium configuration in the lower reactor plenum. The strategy of reactor cavity flooding to prevent reactor vessel failure from molten corium relocation to the reactor vessel lower head has also been considered for commercial pressurized water reactors. Previously, the computer code COMMIX-LAR/P was used to determine if the heat removal rate from the molten cerium in the lower plenum to the water in the cavity was adequate to keep the reactor vessel temperature in the NPR-HWR design below failure limits. It was found that natural convection in the molten pool resulted in heat removal rates that kept the peak reactor vessel temperature about 400 degrees C below the steel melting point. The objective of the work presented in this paper was to determine whether COMMIX adequately predicts natural convection in a pool heated by a uniform heat source. For this purpose, the experiments of free convection in a semicircular cavity of Jahn and Reeneke were analyzed with COMMIX and code predictions were compared with experimental measurements. COMMIX is a general purpose thermalhydraulics code based on finite differencing by the first order upwind scheme

1993-06-20

269

Aqueous Al2O3 nanofluids: the important factors impacting convective heat transfer

A high accuracy, counter flow double pipe heat exchanger system is designed for the measurement of convective heat transfer coefficients with different nanofluids. Both positive and negative enhancement of convective heat transfer of alumina nanofluids are found in the experiments. A modified equation was proposed to explain above phenomena through the physic properties of nanofluids such as thermal conductivity, special heat capacity and viscosity.

Cao, Jianguo; Ding, Yulong; Ma, Caiyun

2014-05-01

270

The entropy generation due to steady laminar forced convection fluid flow through parallel plates microchannel is investigated numerically. The effect of Knudsen, Reynolds, Prandtl, Eckert numbers and the nondimensional temperature difference on entropy generation within the microchannel is discussed. The fraction of the entropy generation due to heat transfer to the total entropy generation within the microchannel is studied in terms of Bejan number. The entropy generation within the microchannel is found to decrease as Knudsen number increases, and it is found to increase as Reynolds, Prandtl, Eckert numbers and the nondimensional temperature difference increase. The contribution of the viscous dissipation in the total entropy generation increases as Knudsen number increases over wide ranges of the flow controlling parameters.

Haddad, Osamah; Abuzaid, Mohammad; Al-Nimr, Mohammad

2004-12-01

271

International Nuclear Information System (INIS)

The current study presents a numerical computation of combined gas radiation and forced convection through two parallel plates. A laminar flow of a temperature-dependent and non-grey gas in the entrance region of the channel was investigated. Over-heated water vapor was chosen as a gas because of its large absorption bands. Some special attention was given to entropy generation and its dependence on geometrical and thermodynamic parameters. The radiative part of the study was solved using the 'Ray Tracing' method through S4 directions, associated with the 'statistical narrow band correlated-k' (SNBCK) model. The temperature fields were used to calculate the distributions of local and global entropy generation

2008-07-01

272

Directory of Open Access Journals (Sweden)

Full Text Available Abstract: The entropy generation due to steady laminar forced convection fluid flow through parallel plates microchannel is investigated numerically. The effect of Knudsen, Reynolds, Prandtl, Eckert numbers and the nondimensional temperature difference on entropy generation within the microchannel is discussed. The fraction of the entropy generation due to heat transfer to the total entropy generation within the microchannel is studied in terms of Bejan number. The entropy generation within the microchannel is found to decrease as Knudsen number increases, and it is found to increase as Reynolds, Prandtl, Eckert numbers and the nondimensional temperature difference increase. The contribution of the viscous dissipation in the total entropy generation increases as Knudsen number increases over wide ranges of the flow controlling parameters.

Mohammad Al-Nimr

2004-12-01

273

Optimization of a Radial Flow Heat Sink Under Natural Convection

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.

Bhowmik, Himangshu

2014-01-01

274

Local instantaneous convective heat transfer characteristics of radial reattaching nozzles

Energy Technology Data Exchange (ETDEWEB)

An experimental investigation of pulsating radial reattaching (PRJR) nozzles was made to determine local instantaneous heat flux and surface temperature characteristics for different geometric, flow, and pulsation conditions. Local instantaneous heat flux and surface temperature measurements were made using a heat flux microsensor with a time constant of 6 microseconds. The experimental apparatus consisted of air flowing through an annular feeding tube, which exited via a mechanically pulsed nozzle diverter and impinged upon a heated plate at a specified distance below. cases were run which varied the gap based Reynolds number 1,936 to 3,983, pulsation rate 5 to 21 Hz, Strouhal number 0.02 to 0.09, non dimensional nozzle-to-plate ratio 1.161 to 1.3, gap height variation to nozzle radius ratio of 0.31, and exit angle 0 and 20{degree}. The heated plate onto which the flow impinged was supplied with a constant heat flux of 4.0 kW/m{sup 2}. Local instantaneous convective heat transfer coefficients varied by as much as 40% when compared to local time average coefficients. Fast Fourier transform analysis identified that the dominant frequency observed in power spectrum plots of heat flux corresponded to the pulsation frequency with other subharmonics also present. Also the data showed a phase lag in local heat transfer and surface temperature minima and maxima as a function of radial distance from the nozzle centerline.

Furlow, J.S.; James, D.L.

1999-07-01

275

Inverse convection problem for determining wall heat flux in annular duct flow

Energy Technology Data Exchange (ETDEWEB)

An inverse problem for unsteady forced convection in an annular duct is presented. The simulated temperature data taken at the outer wall are used to estimate the time-dependent, axially-varying surface heat flux distribution at the inner wall of the annular passage. No prior information on the functional form of the unknown wall heat flux is needed in the inverse method. The effects of the functional form of the wall heat flux and the measurement errors on the accuracy of the estimation are investigated. It is shown that the reconstruction of the timewise and spatial variations of the wall heat flux is satisfactory even if the estimated function is not smooth.

Li, H.Y.; Yan, W.M.

1999-07-01

276

International Nuclear Information System (INIS)

The forced crucible rotation technique has been applied to the solidification of Nd-Fe-B alloys. Specially sealed samples were subjected to well-defined forced rotation during induction heating and solidification. The resulting microstructure of the Nd-Fe-B alloys in consideration of melt convection has been investigated using scanning electron probe microscopy. The determination of the ?-Fe volume fraction by measuring the magnetic moment in a vibrating sample magnetometer (VSM) resulted in a distinct reduction of the ?-Fe volume fraction in samples with high crucible rotation frequencies. Furthermore, a new category of experiment has been started where a tailored magnetic field was applied in order to study the microstructure evolution due to an enhancement or suppression of the melt convection by additional alternating magnetic fields. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

2006-09-01

277

Laser heating of uncoated optics in a convective medium.

Powerful, long-pulse lasers have a variety of applications. In many applications, optical elements are employed to direct, focus, or collimate the beam. Typically the optic is suspended in a gaseous environment (e.g., air) and can cool by convection. The variation of the optic temperature with time is obtained by combining the effects of laser heating, thermal conduction, and convective loss. Characteristics of the solutions in terms of the properties of the optic material, laser beam parameters, and the environment are discussed and compared with measurements at the Naval Research Laboratory, employing kW-class, 1 µm wavelength, continuous wave lasers and optical elements made of fused silica or BK7 glass. The calculated results are in good agreement with the measurements, given the approximations in the analysis and the expected variation in the absorption coefficients of the glasses used in the experiments. PMID:22614476

Hafizi, B; Ting, A; Gordon, D F; Sprangle, P; Peñano, J R; Fischer, R F; DiComo, G P; Colombant, D C

2012-05-10

278

Energy Technology Data Exchange (ETDEWEB)

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.

Favre, E.

1997-09-26

279

Natural convection of heat generating fluid within horizontal cylinder

International Nuclear Information System (INIS)

The natural convection of a heat generating fluid within a horizontal cylinder has been studied analytically by solving the governing equations by a finite difference method. The rate of heat generation was assumed constant and distributed uniformly in the fluid. The surface temperature of the cylinder was considered isothermal. Computations were carried out on the case of Prandtl number Pr = 0.1, 1.0, 10 and 1000, and Rayleigh number from 10"2 to 10"8. It was revealed that the velocity and temperature fields were little affected by Prandtl number in the range of Pr ? 1.0. The effect of natural convection on the heat transfer between the wall and fluid appeared at beyond Rayleigh number Ra = 10"4, and became dominant at Ra ? 10"6. Heat transfer experiments were also performed for the range of Ra = 3 x 10"3 - 10"9 by using a NaCl solution of 0.05 mol/kg water in concentration. The calculated average Nusselt numbers were in good agreement with the experiments. (author)

280

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.)

1984-01-01

281

Natural convection heat transfer from cylinders of arbitrary cross section

Energy Technology Data Exchange (ETDEWEB)

Analytical, numerical, and experimental studies of free convection heat transfer from the external surface of isothermal two-dimensional bodies, especially circular cylinders and vertical flat plates, have been conducted by many investigators. A review of the existing literature shows that most workers have focused mainly on experimental data of circular cylinders and vertical plates with air (Pr[approximately]0.71) as working fluid. The results of these studies have been empirical correlations restricted either to some simple geometries or to a narrow range of Rayleigh numbers. Only a few scientists have carried out numerical techniques, which unfortunately, are also restricted to simple geometries such as circular cylinders. There are a few analytical solutions for cylinders of fairly arbitrary cross section, but, unfortunately, those solutions are restricted to a very narrow range of Rayleigh were the boundary layer is thin compared to the local radius of curvature of the cylinder. The approximate method suggested by Raithby and Hollands, however, is capable of predicting heat transfer from cylinders of various cross sections and for wide ranges of Prandtl and Rayleigh numbers. Unfortunately, following the steps of their approximate method is complex, and engineers might be discouraged from using it. Therefore it is the goal of this study to simplify this approximate method and introduce an expression for predicting natural convection heat transfer from isothermal two-dimensional bodies of arbitrary cross section over a wide range of Rayleigh and Prandtl numbers. In this paper, following a similar procedure to that outlined by Hassani and Hollands, an expression for predicting the convection heat transfer from two-dimensional bodies (cylinders of different convex cross sections) is derived.

Hassani, A.V. (National Renewable Energy Lab., Golden, CO (United States))

1992-08-01

282

Simplified method of estimating efficiency of radiant and convective heating systems

Energy Technology Data Exchange (ETDEWEB)

This paper investigates the thermal performance of radiant and convective heating systems in a house and proposes a simplified method for evaluating the efficiency of these systems. The characteristics of the systems--heat transfer on enclosed surfaces, temperature and velocity distribution--were examined both by experiment in a full-scale experimental room and by numerical analysis. Convective heat transfer was calculated using a two-equation turbulent model along with radiant analysis. The following conclusions were obtained: (1) With the convective heating system, convective heat transfer amounts to more than 70% of the total heat exchange. With radiant heating, the convective heat exchange is approximately half of the total heat exchange; (2) When convective heat exchange is dominant, the heat loss through poorly insulated windows is larger than when radiant heat exchange is dominant. (3) A simplified method to estimate the efficiency of radiant and convective heating systems is proposed. The simplified formula, which uses operative temperature as a criterion for thermal comfort, can give the ratio of heat load between convective and radiant systems that attains the same operative temperature.

Hanibuchi, Haruo; Hokoi, Shuichi

2000-07-01

283

Convective and radiative heat transfer in MHD radiant boilers

International Nuclear Information System (INIS)

A combined convection-gas radiation, two-zone flow model is formulated for study of the heat transfer characteristics of MHD radiant boilers. The radiative contributions of carbon dioxide, water vapor, potassium atoms, and slag particles are included in the formulation, and are determined by solving the radiation transport equation using the P1 approximation. The scattering and absorption cross section of slag particles are calculated from Mie theory. The model is used to analyze the scale-up of heat transfer in radiant boilers with refractory thickness, wall emissivity, and boiler size, under conditions of a gas composition and slag particle spectrum typical of coal-fired MHD combustion. A design procedure is suggested for sizing radiant boilers so as to achieve required heat extraction rate and to provide a flow residence time that is adequate for decomposition of NO/sub x/ to acceptable levels

1981-01-01

284

A multiple-relaxation-time lattice Boltzmann model for convection heat transfer in porous media

In this paper, a 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 forcing term to the MRT-LB equation of the flow field in the moment space. The proposed MRT-LB model is validated by numerical simulations of several two-dimensional convection problems in porous media. The numerical results predicted by the present MRT-LB model agree well with those reported in the literature.

Liu, Q; Li, Q

2013-01-01

285

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…

Wang, Liang-Bi; Zhang, Qiang; Li, Xiao-Xia

2009-01-01

286

Natural convection heat transfer in volumetrically heated spherical pools

International Nuclear Information System (INIS)

Results of experiments conducted to determine the heat transfer to the bounding walls of partially filled spherical cavities (a pyrex bell jar) are reported. The pools are volumetrically heated (magnetron) and the bounding wall is cooled from outside (subcooled water). Three types of boundary conditions, free surface, nearly insulated rigid wall and a cooled rigid wall, were employed at the pool surface. Freon 113 is used. Pool depth and pool radius are varied parametrically; as such, pool Rayleigh number is varied between 10"1"1 and 10"1"4. Correlations for local and average heat transfer coefficients along the curved boundary of the pool have been developed. 13 figs., 1 tab., 10 refs

1994-03-09

287

International Nuclear Information System (INIS)

In the present paper, the entropy generation and optimal Reynolds number for developing forced convection in a double sine duct with various wall heat fluxes, which frequently occurs in plate heat exchangers, are studied based on the entropy generation minimization principle by analytical thermodynamic analysis as well as numerical investigation. According to the thermodynamic analysis, a very simple expression for the optimal Reynolds number for the double sine duct as a function of mass flow rate, wall heat flux, working fluid and geometric dimensions is proposed. In the numerical simulations, the investigated Reynolds number (Re) covers the range from 86 to 2000 and the wall heat flux (q'') varies as 160, 320 and 640 W/m2. From the numerical simulation of the developing laminar forced convection in the double sine duct, the effect of Reynolds number on entropy generation in the duct has been examined, through which the optimal Reynolds number with minimal entropy generation is detected. The optimal Reynolds number obtained from the analytical thermodynamic analysis is compared with the one from the numerical solutions and is verified to have a similar magnitude of entropy generation as the minimal entropy generation predicted by the numerical simulations. The optimal analysis provided in the present paper gives worthy information for heat exchanger design, since the thermal system could have the least irreversibility and best exergy utilization if the optimal Re can be used according to practical design conditions

2006-04-01

288

Natural-convection heat transfer of a spherical lighting fixture

Energy Technology Data Exchange (ETDEWEB)

The surface temperatures of the inner lamp and the outer globe of a spherical lighting fixture, the surfaces of which are painted black, were measured. From the results, the average convective heat-transfer coefficients between the inner lamp and the outer globe and on the outer surface of the globe were obtained. These data are correlated with the aid of existing equations for two concentric spheres and the outer surface of a single sphere. The relationships between the maximum and mean temperatures on the lamp and the globe were also obtained. By the use of these equations, a method for the optimal thermal design of spherical lighting fixtures is proposed.

Ikeda, Takamasa; Fujii, Tetsu

1994-09-01

289

Convective Heat Transfer in Impinging- Gas- Jet Arrangements

Directory of Open Access Journals (Sweden)

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.

J.M. Buchlin

2011-01-01

290

International Nuclear Information System (INIS)

The natural convection heat transfer from a heated vertical plate with wall temperature decreasing linearly from the lower end and embedded in a water saturated porous medium (coarse sand of 23% porosity) has been investigated experimentally. The steady state isothermal lines in the porous medium were developed with the aid of a computer program from the measured temperature data for several rates of heat input. The computed values of the local Nusselt number and modified Rayleigh number were used to obtain the empirical correlations. In order to check the accuracy of the measuring instruments and experimental procedures in simulating the thermal field, experimental investigations of heat transfer by natural convection from a vertical heated plate with constant wall temperature and embedded in the water saturated sand were conducted and the experimental results were compared with those previous investigations. The agreement was very good indicating that the instrumentation and experimental set-up used in the present study were reasonably satisfactory

1987-01-01

291

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)

2006-04-01

292

Mixed Convection Heat Transfer on the Outside of a Vertical Cylinder

International Nuclear Information System (INIS)

An experimental study was made of turbulent heat transfer from a vertical cylinder placed in a square channel. The flow medium was water flowing upwards. Basic differential equations governing the mixed flow heat transfer phenomena in a vertical annulus are presented. A dimensional analysis is done to find the dimensionless variables affecting the relative magnitude of the effect of buoyancy on forced convection heat transfer. Dimensionless equations correlating the experimental data ana incorporating a buoyancy parameter of the form Gr/Re2 are presented. Reynolds number range covered is 690 to 129,500 and the Rayleigh num- ber range covered is 109 to 4.2 x 1013 . Effect of different length parameters, like hydraulic diameter and distance of the measuring point from the inlet of the test section, on dimensionless equations are studied

1965-01-01

293

Unsteady natural convection heat and mass transfer in a saturated porous enclosure

International Nuclear Information System (INIS)

A detailed numerical study has been performed to investigate transient natural convection heat and mass transfer in a porous enclosure. Major dimensionless groups governing the present problem are Ra, N, Le, ? and Ar. Results are particular presented to illustrate the effects of the combined thermal and solutal buoyancy forces on the temporal evolution of local/average Nusselt and Sherwood numbers. The results show that with the increase in the Rayleigh number, the heat and mass transfer is enhanced as a result of greater buoyancy effect. Additionally, the increase in buoyancy ratio N results in an improvement in the heat and mass transfer rates and in the mean time causes a short time duration for the flow to approach the steady-state condition. (orig.)

1992-12-01

294

Energy Technology Data Exchange (ETDEWEB)

The numerical analysis of thermal stratification in the cylindrical hot water storage tank with using different kind of obstacles has been presented. The heat transfer is carried out by forced convections in the tank. To obtain higher thermal stratification inside the tank, optimum obstacle type has been determined between 30 different kinds of obstacles. The forced convection effect into the temperature distribution inside the tank has also obtained for different obstacle types. The temperature relations between water exit from the tank and, the water enter to the collector and the water return to the collector have presented according to obstacle type in the graphs. The temperature differences of these waters have also presented to determine thermal stratification according to the obstacle types and angles. The temperature distributions between tank with using obstacle and smooth tank have also been analyzed to compare to get high thermal stratification. At the results, Obstacle type 11 has been found that it has best thermal stratification inside the tank between all investigated cases in forced convection. (orig.)

Arslan, M.; Altuntop, N.; Ozceyhan, V. [Erciyes University, Kayseri (Turkey). Dept. of Mechanical Engineering

2004-07-01

295

Forced convection flow boiling and two-phase flow phenomena in a microchannel

The present study was performed to numerically analyze the evaporation phenomena through the liquid-vapor interface and to investigate bubble dynamics and heat transfer behavior during forced convective flow boiling in a microchannel. Flow instabilities of two-phase flow boiling in a microchannel were studied as well. The main objective of this research is to investigate the fundamental mechanisms of two-phase flow boiling in a microchannel and provide predictive tools to design thermal management systems, for example, microchannel heat sinks. The numerical results obtained from this study were qualitatively and quantitatively compared with experimental results in the open literature. Physical and mathematical models, accounting for evaporating phenomena through the liquid-vapor interface in a microchannel at constant heat flux and constant wall temperature, have been developed, respectively. The heat transfer mechanism is affected by the dominant heat conduction through the thin liquid film and vaporization at the liquid-vapor interface. The thickness of the liquid film and the pressure of the liquid and vapor phases were simultaneously solved by the governing differential equations. The developed semi-analytical evaporation model that takes into account of the interfacial phenomena and surface tension effects was used to obtain solutions numerically using the fourth-order Runge-Kutta method. The effects of heat flux 19 and wall temperature on the liquid film were evaluated. The obtained pressure drops in a microchannel were qualitatively consistent with the experimental results of Qu and Mudawar (2004). Forced convective flow boiling in a single microchannel with different channel heights was studied through a numerical simulation to investigate bubble dynamics, flow patterns, and heat transfer. The momentum and energy equations were solved using the finite volume method while the liquid-vapor interface of a bubble is captured using the VOF (Volume of Fluid) technique. The effects of different constant heat fluxes and different channel heights on the boiling mechanisms were investigated. The effects of liquid velocity on the bubble departure diameter were analyzed. The obtained results showed that the wall superheats at the position of nucleate boiling are relatively independent of the mass flow rates at the same channel height. The obtained results, however, showed that the heat flux at the onset of nucleate boiling strongly depends on the channel height. With a decrease of the channel height and an increase of the liquid velocity at the channel inlet, the departure diameter of a bubble was smaller. The periodic flow patterns, such as the bubbly flow, elongated slug flow, and churn flow were observed in the microchannel. Flow instabilities of two-phase flow boiling in a trapezoidal microchannel using a three-dimensional model were investigated. Fluctuation behaviors of flow boiling parameters such as wall temperature and inlet pressure caused by periodic flow patterns were studied at different heat fluxes and mass fluxes. The numerical results showed large amplitude and short period oscillations for wall temperature and inlet pressure fluctuations. Stable and unstable flow boiling regime with short period oscillations were investigated. Those flow boiling regimes were not listed in stable and unstable boiling regime map proposed by Wang et al. (2007).

Na, Yun Whan

296

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 ...

Grooms, Ian

2014-01-01

297

International Nuclear Information System (INIS)

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 data are outlined. Correction methods to accurately account for secondary heat transfer mechanisms were developed and results show that as convective heat transfer coefficients and length scales decrease, the importance of accounting for errors increases. Combined with flow patterns that produce large temperature gradients, the influence of heat flow within the foil on the resultant heat transfer becomes significant. Substantial errors in the heat transfer coefficient are apparent by neglecting corrections to the measured data for the cases examined. Methods to account for these errors are presented here, and demonstrated to result in an accurate measurement of the local heat transfer map on the surface

2009-10-01

298

Convective heat and mass transfer in rotating disk systems

Energy Technology Data Exchange (ETDEWEB)

The book describes results of investigations of a series of convective heat-and-mass transfer problems in rotating-disk systems, namely, over free rotating disks, under conditions of transient heat transfer, solid-body rotation of fluid, orthogonal flow impingement onto a disk, swirl radial flow between parallel co-rotating disks, in cone-disk systems and for Prandtl and Schmidt numbers larger than one. Methodology used included integral methods, self-similar and approximate analytical solutions, as well as CFD. The book is aimed at the professional audience of academic researchers, industrial R and D engineers, university lecturers and graduate/postgraduate students working in the area of rotating-disk systems. (orig.)

Shevchuk, Igor V. [MBtech Powertrain GmbH, Fellbach-Schmiden (Germany)

2009-07-01

299

A highly stable microchannel heat sink for convective boiling

International Nuclear Information System (INIS)

To develop a highly stable two-phase microchannel heat sink, we experimented with convective boiling in diverging, parallel microchannels with different distributions of laser-etched artificial nucleation sites. Each microchannel had a mean hydraulic diameter of 120 µm. The two-phase flow visualization and the magnitudes of pressure drop and inlet temperature oscillations under boiling conditions demonstrated clearly the merits of using artificial nucleation sites to further stabilize the flow boiling in diverging, parallel microchannels. The stability map showed the plane of subcooling number versus phase change number. It illustrated that diverging, parallel microchannels with artificial nucleation cavities have a much wider stable region than parallel microchannels with uniform cross-sections or diverging, parallel microchannels without artificial nucleation cavities. In addition, the results revealed that the design with cavities distributed uniformly along the downstream half of the channel presented the best stability performance among the three distributions of nucleation sites. This particular design can be regarded as a highly stable microchannel heat sink for convective boiling

2009-05-01

300

Mixed-convective, conjugate heat transfer during molten salt quenching of small parts

Energy Technology Data Exchange (ETDEWEB)

It is common in free quenching immersion heat treatment calculations to locally apply constant or surface-averaged heat-transfer coefficients obtained from either free or forced steady convection over simple shapes with small temperature differences from the ambient fluid. This procedure avoids the solution of highly transient, non-Boussinesq conjugate heat transfer problems which often involve mixed convection, but it leaves great uncertainty about the general adequacy of the results. In this paper we demonstrate for small parts (dimensions of the order of inches rather than feet) quenched in molten salt, that it is feasible to calculate such nonuniform surface heat transfer from first principles without adjustable empirical parameters. We use literature physical property salt data from the separate publications of Kirst et al., Nissen, Carling, and Teja, et al. for T<1000 F, and then extrapolate it to the initial part temperature. The reported thermal/chemical breakdown of NaNO{sub 2} for T>800 F is not considered to be important due to the short time the surface temperature exceeds that value for small parts. Similarly, for small parts, the local Reynolds and Rayleigh numbers are below the corresponding critical values for most if not all of the quench, so that we see no evidence of the existence of significant turbulence effects, only some large scale unsteadiness for brief periods. The experimental data comparisons from the open literature include some probe cooling-rate results of Foreman, as well as some cylinder thermal histories of Howes.

Chenoweth, D.R.

1997-02-01

301

Estimation of heat loss during transient study of natural convection heat transfer in path facility

International Nuclear Information System (INIS)

Post accident heat removal in Fast Breeder Reactors (FBR) is a very important issue for assuring public safety. Core meltdown accident results in fragmented debris. Accumulation of these core debris on to the main vessel bottom is to be avoided. Hence, as defense-in-depth, an in-vessel Core Catcher (CC) system is provided in FBR just below the strong back of the core support structure to collect, support and maintain the debris in coolable and sub-critical configuration. Heat generated by the debris settled on CC is removed by natural convection so that main vessel temperature does not exceed the allowable value. At SED, Safety Group, experimental and numerical studies are taken up related to core-catcher with the objectives to investigate continuous heat removal capability of core catcher plate through natural convection and to generate data for validation of mathematical models developed. To carry out the experiments on natural convection a Post Accident Thermal Hydraulic (PATH) facility is design and commissioned. In this paper the evaporative loss along with convective and evaporative heat losses from the second phase of experiments, with closed top lid have been estimated and compared with the analytical solution based on the experimental condition at different time intervals. (author)

2011-03-01

302

International Nuclear Information System (INIS)

One strategy for preventing the failure of lower head of a nuclear reactor vessel is to flood the concrete cavity with subcooled water in accidents in which relocation of core material into the vessel lower head occurs. After the core material relocates into the vessel, a crust of solid material forms on the inner wall of the vessel, however, most of the pool remains molten and natural convection exists in the pool. At present, uncertainty exists with respect to natural convection heat transfer coefficients between the pool of molten core material and the reactor vessel wall. In the present work, experiments were conducted to examine natural convection heat transfer in internally heated partially filled spherical pools with external cooling. In the experiments, Freon-113 contained in a Pyrex bell jar was used as a test liquid. The pool was bounded with a spherical segment at the bottom, and was heated with magnetrons taken from a conventional microwave oven. The vessel was cooled from the outside with natural convection of water or with nucleate boiling of liquid nitrogen

1998-01-01

303

Directory of Open Access Journals (Sweden)

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.

Zeinali Heris Saeed

2011-01-01

304

Forced convective boiling in vertical tube for binary refrigerant mixtures of R11 and R113

International Nuclear Information System (INIS)

An experimental study was carried out on convective boiling heat transfer for mixtures of R11 and R113 flowing in a uniformly heated vertical tube by measuring the wall and bulk temperatures, and the results were compared with an existing correlation. A reduction of the average heat transfer coefficient for mixtures was verified for flow boiling. It was observed that two kinds of boiling behavior existed depending on mass flux. It was also found that the Chen's correlation was particularly successful for the case of high mass rate flow in which convective boiling prevailed. However in the case of low mass rate flow where nucleate boiling was dominant, the Chen's correlation was found to be inappropriate. Mass transfer resistance in the liquid film played a vital role for determining the heat transfer coefficient of refrigerant mixtures. It has been also found that the equilibrium assumption was hardly applicable to the convective boiling phenomena

1998-06-01

305

International Nuclear Information System (INIS)

The effects of forced convection of electrolyte on efficiency of alkaline water electrolysis were experimentally investigated, and forced convective two-phase flows between electrodes were visualized. Electrode was made of Ni with Pt plating (? 1 ?m thickness), with 100 mm height and 30 mm width. KOH solution (8.5 wt%, 20 C) was used for electrolyte. The experimental conditions were as follows; electrodes space ?=1-30 mm, current density ?=0.1-2.0 A/cm2, flow velocity ul=0.1-2.0 m/s. The obtained experimental results show that flow velocity of forced convection clearly affects the efficiency of water electrolysis, i.e. as flow velocity becomes larger, the efficiency of water electrolysis becomes higher. The optimum condition of water electrolysis in the forced convective system was also found, that was function of current density, electrode space and flow velocity. This tendency was found to be the same qualitatively with two-phase flow modeling for natural convective system. (authors)

2006-06-13

306

Energy Technology Data Exchange (ETDEWEB)

The effects of forced convection of electrolyte on efficiency of alkaline water electrolysis were experimentally investigated, and forced convective two-phase flows between electrodes were visualized. Electrode was made of Ni with Pt plating ({approx} 1 {mu}m thickness), with 100 mm height and 30 mm width. KOH solution (8.5 wt%, 20 C) was used for electrolyte. The experimental conditions were as follows; electrodes space {delta}=1-30 mm, current density {phi}=0.1-2.0 A/cm{sup 2}, flow velocity u{sub l}=0.1-2.0 m/s. The obtained experimental results show that flow velocity of forced convection clearly affects the efficiency of water electrolysis, i.e. as flow velocity becomes larger, the efficiency of water electrolysis becomes higher. The optimum condition of water electrolysis in the forced convective system was also found, that was function of current density, electrode space and flow velocity. This tendency was found to be the same qualitatively with two-phase flow modeling for natural convective system. (authors)

Niro Nagai; Masanori Takeuchi [University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, (Japan); Tetsuya Furuta [Graduate School of University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, (Japan)

2006-07-01

307

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...

Shang, De-Yi

2012-01-01

308

Modelling unsteady turbulent forced convection inside tubes using a low-Reynolds-number k ? model

International Nuclear Information System (INIS)

The study of turbulent forced convection in unsteady flow regimes is of both fundamental interest and considerable practical importance because of the widespread occurrence of such flow regimes in engineering systems. Since the complexity of the equations governing turbulent flow, obviously precludes direct analysis, even for steady flow, investigators must turn to experimentation and modelling in order to study this problem. Whilst a considerable body of work does exist on the hydrodynamics of unsteady turbulent shear flow, very few studies also consider heat transfer and in consequence it is not well understood. The present study was motivated by unsteady heat transfer measurements made in water pipe flow undergoing harmonic oscillation. Although the experiment was complicated by variable viscosity and density effects, the results suggested that measured Nusselt numbers could differ significantly from those obtained using a pseudo-steady correlation (a correlation strictly applicable to steady flow evaluated at the instantaneous conditions of an unsteady flow). This study primarily aims to assess the degree to which the widely used low-Reynolds-number k-? turbulence model reproduces the previous experimental data. (Author)

1992-01-01

309

An experimental investigation of forced convection flat plate solar air heater with storage material

Directory of Open Access Journals (Sweden)

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.

Aissa Walid

2012-01-01

310

Convective heat transfer correlations for fenestration glazing cavities: A review

Energy Technology Data Exchange (ETDEWEB)

Convective heat transfer in insulated glazing unit (IGU) cavities is a major component of the overall heat transfer in fenestration systems. Accurately quantifying the heat-transfer coefficient within the cavity is of great significance in calculating the center-of-glass U factor, the edge-of-glass U factor, and therefore the overall U factor. Over the past 40 years, along with the rapid development of experimental techniques and numerical methods as well as the powerful computer systems, the heat-transfer correlations have been updated either from experimental data or from numerical data from time to time. The literature reviewed here covers correlations based on simplified analytical studies, on the experimental data, and on the numerical results. In addition, most of the correlation equations cover the typical design range of fenestration glazing cavities, e.g., Ra < 20,000. Inconsistencies and discrepancies existing between different experimental observations, and between derived numerical/analytical correlations and available experimental data, are discussed.

Zhao, Y.; Curcija, D.; Goss, W.P.

1999-07-01

311

The eddy heat-flux in rotating turbulent convection

The three components of the heat-flux vector $F =\\rho C_p are numerically computed for a stratified rotating turbulent convection using the NIRVANA code in a flat box. The latitudinal component $F_\\theta$ proves to be negative (positive) in the northern (southern) hemisphere so that the heat always flows towards the poles. As a surprise, the radial heat-flux $F_r$ peaks at the equator rather than at the poles (Taylor numbers O(10^6)). The same behavior is observed for the radial turbulence intensity $$ which for \\emph{free} turbulence is also believed to peak at the poles (see Eq. (19) below). As we can show, however, the consequences of this unexpected result (also obtained by Kaepylae, Korpi and Tuominen 2004) for the theory of differential rotation are small as mainly the $F_\\theta$ is responsible to solve the `Taylor number puzzle'. In all our simulations the azimuthal component $F_\\phi$ proves to be negative so that the rotating turbulence produces an westwards directed azimuthal heat-flux which should ...

Rüdiger, G; Kitchatinov, L L; Küker, M

2004-01-01

312

Studies on convective heat transfer through helical coils

An experimental investigation on steady state convection heat transfer from vertical helical coiled tubes in water was performed for laminar flow regime. Three coils with curvature ratios as 0.0757, 0.064, 0.055 and range of Prandtl number from 3.81 to 4.8, Reynolds number from 3,166 to 9,658 were considered in this work. The heat transfer data were generated from 30 experiments conducted at constant water bath temperature (60 °C) for different cold water flow rates in helical coils. For the first time, an innovative approach of correlating Nusselt number with ‘M’ number is proposed which is not available in the literature and the developed correlations are found to be in good agreement with the work of earlier researchers. Thus, dimensionless number ‘M’ was found to be significant to characterize the hydrodynamics of fluid flow and heat transfer correlations in helical coils. Several other correlations based on experimental data are developed. To cover wide range of industrial applications, suitable generalized correlations based on extended parameters beyond the range of present experimental work are also developed. All these correlations are developed by using least-squares power law fit and multiple-regression analysis of MATLAB software. Correlations so developed were compared with published correlations and were found to be in good agreement. Comparison of heat transfer coefficients, friction factor and Nusselt number for different geometrical conditions is presented in this paper.

Pawar, S. S.; Sunnapwar, Vivek K.

2013-12-01

313

Weight and water loss in the neonate in natural and forced convection.

Digital Repository Infrastructure Vision for European Research (DRIVER)

We describe a simple method of determining weight loss and hence water loss of infants in incubators. Unlike previously reported methods, it does not interfere with the microenvironment surrounding the infant. Weight loss of 16 term and 32 preterm infants was measured in both forced and natural convection. No significant increase in water loss was observed in the term infants but in the preterm infants the mean loss in natural convection was 0.85 g/kg/hour compared with 1.26 g/kg/hour in forc...

Thompson, M. H.; Stothers, J. K.; Mclellan, N. J.

1984-01-01

314

The Arctic Mediterranean Sea - Deep convection, oceanic heat transport and freshwater

The speculations about the driving forces behind the oceanic meridional circulation and the importance of the northward transports of oceanic heat for the ice conditions in the Arctic Ocean have a long history, but only after the Fram expedition 1893-1896 and from the studies by Nansen, Helland-Hansen and Sandström in the early 1900s did these speculations attain observational substance. In the late 1970s and onward these questions have again risen to prominence. A study of deep convection in the Greenland Sea, then assumed to drive the global thermohaline circulation, started with the Greenland Sea Project (GSP), while the investigation of the exchanges of volume and heat through Fram Strait had a more hesitant start in the Fram Strait Project (FSP). Not until 1997 with the EC project VEINS (Variation of Exchanges in the Northern Seas) was a mooring array deployed across Fram Strait. This array has been maintained and has measured the exchanges ever since. Eberhard Fahrbach was closely involved in these studies, as a secretary for the GSP and as the major driving force behind the Fram Strait array. Here we shall examine the legacy of these projects; How our understanding of these themes has evolved in recent years. After the 1980s no convective bottom water renewal has been observed in the Greenland Sea, and the Greenland Sea deep waters have gradually been replaced by warmer, more saline deep water from the Arctic Ocean passing through Fram Strait. Small-scale convective events penetrating deeper than 2500m but there less dense than their surroundings were, however, observed in the early 2000s. The Fram Strait exchanges have proven difficult to estimate due to strong variability, high barotropic and baroclinic eddy activity and short lateral coherence scales. The fact that the mass transports through Fram Strait do not balance complicates the assessment of the heat transport through Fram Strait into the Arctic Ocean and mass (volume) and salt (freshwater) balances for the entire Arctic Ocean are needed. The waters exiting the Arctic Ocean through Fram Strait are colder than those entering and with reasonable assumptions about the origin of the waters providing the net outflow it is possible to deduce the amount of the entering oceanic heat going to the atmosphere (>50%), to ice melt (20%). Almost all of this heat loss occurs in the Nansen Basin. The rest of the heat is used for heating the net outflow. It also becomes clear that freshwater, with its phase changes and its multiple transport pathways, plays a crucial role in the climate, not just of the Arctic Ocean but of the Arctic as a whole.

Rudels, Bert

2014-05-01

315

Directory of Open Access Journals (Sweden)

Full Text Available 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.

Bhaskar Kalita

2012-12-01

316

International Nuclear Information System (INIS)

Highlights: ? We study the effect of radiation on natural convection in a cavity with a heated plate. ? The heated plate is placed horizontally or vertically at the center of the cavity. ? The surface radiation makes the temperature distribution uniform inside the cavity. ? The average Nusselt number increases with the emissivity and the Rayleigh number. ? The heat transfer rate is higher when the plate is oriented vertically. -- Abstract: This paper reports a fundamental theoretical study made to understand the interaction of surface radiation and natural convection in an air filled cavity with a centrally placed thin heated plate. The vertical walls of the cavity are cooled while the horizontal ones are insulated. The thin plate is assumed to be isothermal and is placed horizontally or vertically. The governing equations were solved using a finite volume method on a uniformly staggered grid system. The effects of the pertinent parameters, viz., Rayleigh number (105 ? Ra ? 107), plate length (0.25 ? D ? 0.75) and emissivity (0 ? ? ? 1) are investigated in detail. In general the results indicate a better homogenization of temperature field within the cavity by radiation. It is also found that the contribution of the convective mechanism to the overall heat transfer increases with emissivity when the plate is horizontally placed whereas decreases when it is vertically placed. This study demonstrates that any model representing a situation of this kind that ignores surface radiation leads to erroneous predictions

2013-04-01

317

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

1996-01-01

318

Energy Technology Data Exchange (ETDEWEB)

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.

Triplett, C.E.

1996-12-01

319

Study of thermoelectric module's cooling performance in forced convection

International Nuclear Information System (INIS)

The thermal performance of a thermoelectric module in a thermoelectric cooling system with a heat sink is analytically and experimentally examined. The thermoelectric cooling system is composed of a thermoelectric module and two heat sinks. The thermal performance of a thermoelectric module (Seepel, Crystal) is analytically taken into account. The results show that COP (Coefficient Of Performance) of a thermoelectric module is reduced and the heat absorbed increases as the input voltage increases. The thermoelectric cooling system with 12V provides 73.3% lower COP and 46.7% higher heat absorbed compared to 5V, experimentally. The theoretical predicted result is in good agreement with the measured data.

2009-04-23

320

Mixed convection heat transfer in rotating vertical elliptic ducts

Directory of Open Access Journals (Sweden)

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 ascending 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<820.

Olumuyiwa A. Lasode

2007-06-01

321

Mixed convection heat transfer in rotating vertical elliptic ducts

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english 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

Olumuyiwa A., Lasode.

322

Energy Technology Data Exchange (ETDEWEB)

The air-side forced convective heat transfer of a plate fin-tube heat exchanger is investigated by experimental measurement and numerical computation. The heat exchanger consists of a staggered arrangement of refrigerant pipes with a diameter of 10.2 mm and a fin pitch of 3.5 mm. In the experimental study, the forced convective heat transfer was measured at Reynolds numbers of 1082, 1397, 1486, 1591 and 1649 based on the diameter of the refrigerant piping and on the maximum velocity. The average Nusselt number for the convective heat transfer coefficient was also computed for the same Reynolds number by using the commercial software STAR-CD with the standard k .{epsilon} turbulent model. It was found that the relative errors of the average Nusselt numbers between the experimental and numerical data were less than 6 percent in a Reynolds number range of 1082{approx}1649. The errors between the experiment and other correlations from literature ranged from 7% to 32.4%. However, the literature correlation of Kim et al. is closest to the experimental data within a relative error of 7%

Paeng, Jin Gi; Kim, Kyung Hwan; Yoon, Young Hwan [Changwon National University, Changwon (Korea, Republic of)

2009-02-15

323

International Nuclear Information System (INIS)

The air-side forced convective heat transfer of a plate fin-tube heat exchanger is investigated by experimental measurement and numerical computation. The heat exchanger consists of a staggered arrangement of refrigerant pipes with a diameter of 10.2 mm and a fin pitch of 3.5 mm. In the experimental study, the forced convective heat transfer was measured at Reynolds numbers of 1082, 1397, 1486, 1591 and 1649 based on the diameter of the refrigerant piping and on the maximum velocity. The average Nusselt number for the convective heat transfer coefficient was also computed for the same Reynolds number by using the commercial software STAR-CD with the standard k .? turbulent model. It was found that the relative errors of the average Nusselt numbers between the experimental and numerical data were less than 6 percent in a Reynolds number range of 1082?1649. The errors between the experiment and other correlations from literature ranged from 7% to 32.4%. However, the literature correlation of Kim et al. is closest to the experimental data within a relative error of 7%

2009-02-01

324

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-Boel...

D. Mendez

1994-01-01

325

Energy Technology Data Exchange (ETDEWEB)

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.

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

326

Forced laminar convection in an array of stacked plates

Energy Technology Data Exchange (ETDEWEB)

A numerical study of laminar flow and heat transfer in an array of stacked rectangular plates is presented. The array is placed in a uniform stream, and the plates are subjected to a constant surface heat flux. This flow configuration is relevant to a number of practical heat transfer devices with finned surfaces. The computations were performed using a finite volume solution of the steady, two-dimensional Navier-Stokes equations and energy equation. A numerical scheme that reduces numerical diffusion is used to discretize the equations. The dominant feature of the flow is the separation, and subsequent reattachment of, the boundary layer, which takes place at Reynolds numbers greater than about 75. The separation first occurs downstream of the leading edge of the plate; then as Re increases, the separation point moves upstream and remains fixed at the leading edge, and the reattachment length increases linearly with Re. The appearance and growth of the separation bubble are accompanied by a local thinning of the thermal boundary layer and a substantial heat transfer augmentation in the reattachment region, with local maximum heat transfer rates occurring slightly downstream of reattachment. The heat transfer augmentation is attenuated at higher blockage ratios (reduced spacing between plates) as a result of the reduction of the separation bubble size.

Djilali, N. (Univ. of Victoria, British Columbia (Canada). Dept. of Mechanical Engineering)

1994-04-01

327

Roughness induced forced convective laminar-transitional micropipe flow: energy and exergy analysis

Variable fluid property continuity, Navier Stokes and energy equations are solved for roughness induced forced convective laminar-transitional flow in a micropipe. Influences of Reynolds number, heat flux and surface roughness, on the momentum-energy transport mechanisms and second-law of thermodynamics, are investigated for the ranges of Re = 1 2,000, Q = 5 100 W/m2 and ? = 1 50 ?m. Numerical investigations put forward that surface roughness accelerates transition with flatter velocity profiles and increased intermittency values (?); such that a high roughness of ? = 50 ?m resulted in transitional character at Re tra = 450 with ? = 0.136. Normalized friction coefficient (C {f/*}) values showed augmentation with Re, as the evaluated C {f/*} are 1.006, 1.028 and 1.088 for Re = 100, 500 and 1,500, respectively, at ? = 1 ?m, the corresponding values rise to C {f/*} = 1.021, 1.116 and 1.350 at ? = 50 ?m. Heat transfer rates are also recorded to rise with Re and ?; moreover the growing influence of ? on Nusselt number with Re is determined by the Nu ?=50 ?m/Nu ?=1 ?m ratios of 1.086, 1.168 and 1.259 at Re = 500, 1,000 and 1,500. Thermal volumetric entropy generation (bar S_{Updelta {text{T}}}^{'''} ) values decrease with Re and ? in heating; however the contrary is recorded for frictional volumetric entropy generation (bar S_{Updelta {text{P}}}^{'''} ) data, where the augmentations in bar S_{Updelta {text{P}}}^{'''} are more considerable when compared with the decrease rates of bar S_{Updelta {text{T}}}^{'''} .

Ozalp, A. Alper

2008-05-01

328

System-wide response of tropical deep convection to aerosol forcing (Invited)

In this talk, I will present results of modeling studies of the response of deep convection to aerosol forcing from dust and black carbon. Focus will be on the system-wide non-local response of deep convections to aerosol forcing in tropical climate systems on subseasonal time scales and beyond. On these time scales, the impacts of aerosols are often intertwined with influences from atmospheric dynamics and boundary forcing from the earth surface. We found that the systemwide response of deep convection and rainfall is complex, involving non-local responses that are functions of the changing dynamical system, and cannot be explained in terms of elemental aerosol direct, semi-direct and indirect effects, where atmospheric dynamics are assumed constant. In many cases, the maximum response is far remote from the region of maximum aerosol forcing in space and time, depending on seasonally varying stability conditions, prevailing circulation, and feedback from atmospheric dynamics. Examples will be drawn from effects of dust, absorbing aerosols on the evolution of deep convections from the South Asia, Southeast Asia, and the West African monsoon systems.

Lau, W. K.; Tao, W.; Kim, K.; Shi, J. J.

2013-12-01

329

Energy Technology Data Exchange (ETDEWEB)

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.

Ma, R.Y. [California State Polytechnic Univ., Pomoma, CA (United States). Dept. of Mechanical Engineering

1993-09-01

330

Transient characteristics of He II forced flow heated at the center of a pipe line

Numerical analysis is performed with the two-fluid model and the simplified model of Kashani et al. to study one-dimensional forced convection heat transfer of He II in a pipe heated at the midpoint along its length. An entropy transport equation derived from the theory of Khalatnitkov is incorporated into the two-fluid model. The numerical results obtained with the two models are compared with each other and with experimental data to investigate the validity of the models. In the range of parameters of concern in this paper, good agreement is observed for temperature distributions and internal convection heat fluxes obtained numerically and experimentally. Explanation is provided for the relationship between the two-fluid model and the simplified model. Conditions are discussed under which the simplified model will produce results that are in close agreement with the two-fluid model. Some interesting characteristics of transient flow and heat transfer are also presented.

Rao, Y. F.; Inaba, Y.; Noda, T.; Fukuda, K.

331

International Nuclear Information System (INIS)

Unsteady laminar mixed convection flow (combined free and forced convection flow) along a vertical slender cylinder embedded in a porous medium under the combined buoyancy effect of thermal and species diffusion has been studied. The effect of the permeability of the medium as well as the magnetic field has been included in the analysis. The partial differential equations with three independent variables governing the flow have been solved numerically using an implicit finite difference scheme in combination with the quasilinearization technique. Computations have been carried out for accelerating, decelerating and oscillatory free stream velocity distributions. The effects of the permeability of the medium, buoyancy forces, transverse curvature and magnetic field on skin friction, heat transfer and mass transfer have been studied. It is found that the effect of free stream velocity distribution is more pronounced on the skin friction than on the heat and mass transfer. The permeability and magnetic parameters increase the skin friction, but reduce the heat and mass transfer. The skin friction, heat transfer and mass transfer are enhanced due to the buoyancy forces and curvature parameter. The heat transfer is strongly dependent on the viscous dissipation parameter and the Prandtl number, and the mass transfer on the Schmidt number. (orig.)

1992-12-01

332

A semi-empirical model of supercritical pressure convective heat transfer

International Nuclear Information System (INIS)

Full text of publication follows: During the late fifties and throughout the sixties, considerable interest was stimulated in heat transfer to fluids at supercritical pressure by the introduction in the USA, the USSR and Japan of conventional thermal power plant operating at such pressures. Much experimental work was carried out at that time in support of the development of the once-through steam generators needed for such plant. Since then there has been very little research on this topic. Recently, however, there has been a renewed interest in it. The Supercritical Pressure Water-cooled Reactor, which will supply high temperature 'steam' to turbines at pressures well above the supercritical value, is now receiving active consideration. In the SPWR application the requirements are very different to those in conventional steam generators; the equivalent diameters of the fuel element sub-channels are quite small (? 3 mm). Also there is not only a need for data covering normal operating conditions but also for fault conditions, which include operation at reduced flow rates and near-critical pressures. Some experimental results have been reported recently by the author (ICAPP 2004) which highlight the fact that severe localized impairment of heat transfer can be experienced under such conditions due to the influence of buoyancy. The implications of this could be important for SPWR, especially for operation under fault conditions. The particular characteristic of fluids just above the critical pressure which makes them of special interest is that their physical properties vary rapidly with temperature. The non-uniformity of thermal conductivity, density and specific heat affects the basic processes involved in convective heat transfer and the dependence of density on temperature can lead to influences of buoyancy and thermal expansion on mean flow and turbulence which also affect heat transfer. In this paper a semi-empirical model of turbulent variable property mixed convection heat transfer will be reported which utilizes a well-established forced convection correlation equation combined with semi-empirical multipliers to account for effects of buoyancy and flow acceleration due to heating. It has been fitted to the author's data for CO2 and validated using data for water. It will enable predictions to be made for conditions relevant to SPWR operation under normal and fault conditions. (author)

2005-10-02

333

A study on sodium pool combustion phenomena under forced or natural convection airflow

International Nuclear Information System (INIS)

Full text of publication follows: Sodium pool fire is a design basis accident of sodium-cooled fast reactor. In this study, a numerical method for axisymmetric two-dimensional modeling of sodium pool fire has been developed. Flow dynamics calculation is based on SIMPLE method. It considers coupling of thermal-hydraulics, chemical reaction and aerosol dynamics equations. One assumes chemical equilibrium is valid for the chemical reaction because the reaction rate seems to be higher than mass transfer. The aerosol dynamic equation is solved for multi-group diameters classes. Also low Reynolds number two equation turbulence model for fluid flow and zero equation turbulence model for temperature field are used in the computer code. From the present two-dimensional computation, phenomena of sodium pool fire are understood such as flow and temperature fields and aerosol mass distribution of various sizes for forced and natural convection airflow situations. It has been found that the burning rate and aerosol release fraction calculated by the numerical methodology are in agreement with experimental data. The burning rate varies along with the radial direction by a factor of 20 and the mass and heat transfer around the pool edges is maximum and most influential. The thermal-hydraulic phenomena in the near-surface region are very important to determine sodium pool fire consequence such as burning rate and aerosol emission. (authors)

2005-10-02

334

Controlling the structure of forced convective flow by means of rotating magnetic-field inductors

International Nuclear Information System (INIS)

The forced convective flow generated by a rotating magnetic-field inductor is used in a melt as a means of controlling the transfer of mass and heat in the case of directed crystallization. An obvious advantage in using a rotating field is the generation of azimuthal twisting of the fluid, this providing for an evening out of the crystallization conditions in the azimuthal direction under nonsymmetrical boundary conditions in an actual technological process. From the standpoint of affecting the crystallization processes it would be preferable to use an inductor which would allow alteration of the intensity and of the direction of the meridional flow. Mixing in the form of velocity pulsations generated by the inductor within the melt would be if interest from the standpoint of affecting the crystallization processes, in particular to intensify the crystallization purification. The authors propose the use of a double magnetohydrodynmic rotator which consists of two rotating magnetic-field inductors, separated in altitude, with separate power supplies. The supply of power to the inductors with various current loads allows the generation of a controllable nonuniformity in field distribution and in the azimuthal velocity through the altitude and thus allows control of both the intensity and configuration of the meridional flows. The dual rotator makes it possible to purposefully control the structure of the meridional flows and the pulsation component of velocity and can be recommended for use in processes of directed crystallization as well as in crystallization purification. 4 refs., 3 figs

1993-10-01

335

Effect of a circular cylinder on separated forced convection at a backward-facing step

Energy Technology Data Exchange (ETDEWEB)

The current study investigates the augmentation in the laminar forced convection characteristics of the backward-facing step flow in a two-dimensional channel by means of introducing an adiabatic circular cylinder in the domain. The effects of various cross-stream positions (i.e., y{sub c} = 0-1.5) of the circular cylinder on the flow and heat transfer characteristics of the backward-facing step flow has been numerically explored for the Reynolds number range 1-200 and Prandtl number of 0.71 (air). The governing continuity, Navier-Stokes and energy equations along with appropriate boundary conditions are solved by using FLUENT. The flow and thermal fields have been explained by streamline and isotherm profiles, respectively; however, no temperature dependency effects are considered for the flow viscosity and thermal conductivity. The engineering parameters like wake/recirculation length, total drag coefficient and average Nusselt number, etc. are calculated for the above range of conditions. The present results show an enhancement in the peak Nusselt value of up to 155% using a circular cylinder as compared to the unobstructed case (i.e., without cylinder). Finally, simple correlations for total drag coefficient and peak Nusselt number are obtained for the above range of conditions. (authors)

Kumar, Ankit; Dhiman, Amit K. [Department of Chemical Engineering, Indian Institute of Technology, Roorkee 247667 (India)

2012-02-15

336

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

1980-01-01

337

Natural Convection in Parabolic Enclosure Heated from Below

Directory of Open Access Journals (Sweden)

Full Text Available The effects of vertical parabolic walls on natural convection in a parabolic enclosure have investigated numerically in this paper. The bottom wall is heated isothermally, while the other vertical parabolic walls are maintained at constant cold temperature and the top wall is well insulated. The flow and temperature fields are studied numerically for three values (C = 0.1, 0.5, 1.0 of the parabolic equation constant. The laminar flow field is analyzed numerically by solving the steady, two-dimensional incompressible Navier-Stokes and energy equations. The Cartesian velocity components and pressure on a collocated (non-staggered grid are used as dependent variables in the momentum equations, which discretized by finite volume method, body fitted coordinates are used to represent the complex parabolic wall geometry accurately, and grid generation technique based on elliptic partial differential equations is employed. SIMPLE algorithm is used to adjust the velocity field to satisfy the conservation of mass. The range of Rayleigh number is (103? Ra ?105 and Prandtl number is 0.7. The results show that the heat transfer rates decrease with increase the parabolic equation constant.

Ahmed W. Mustafa

2011-06-01

338

Natural convection heat transfer analysis of ATR fuel elements

Energy Technology Data Exchange (ETDEWEB)

Natural convection air cooling of the Advanced Test Reactor (ATR) fuel assemblies is analyzed to determine the level of decay heat that can be removed without exceeding the melting temperature of the fuel. The study was conducted to assist in the level 2 PRA analysis of a hypothetical ATR water canal draining accident. The heat transfer process is characterized by a very low Rayleigh number (Ra {approx} 10{sup {minus}5}) and a high temperature ratio. Since neither data nor analytical models were available for Ra < 0.1, an analytical approach is presented based upon the integral boundary layer equations. All assumptions and simplifications are presented and assessed and two models are developed from similar foundations. In one model, the well-known Boussinesq approximations are employed, the results from which are used to assess the modeling philosophy through comparison to existing data and published analytical results. In the other model, the Boussinesq approximations are not used, thus making the model more general and applicable to the ATR analysis.

Langerman, M.A.

1992-05-01

339

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)

2006-01-01

340

ENHANCEMENT OF NATURAL CONVECTION HEAT TRANSFER FROM RECTANGULAR FINS BY CIRCULAR PERFORATIONS

Digital Repository Infrastructure Vision for European Research (DRIVER)

The importance of heat transfer by natural convection in enclosures can be found in many engineering applications, such as energy transfer in buildings, solar collectors, nuclear reactors and electronic packaging. An experimental study was conducted to investigate heat transfer by natural convection in a rectangular fin plate with circular perforations as heat sinks. The patterns of the perforations included 24 circular perforations (holes) for the first fin; the number of perforations increa...

2011-01-01

341

The effect of Coriolis force on nonlinear convection in a porous medium

Directory of Open Access Journals (Sweden)

Full Text Available Nonlinear convection in a porous medium and rotating about vertical axis is studied in this paper. An upper bound to the heat flux is calculated by the method initiated first by Howard [6] for the case of infinite Prandtl number.

D. H. Riahi

1994-09-01

342

Convective heat transfer from molten salt droplets in a direct contact heat exchanger

This paper presents a new predictive model of droplet flow and heat transfer from molten salt droplets in a direct contact heat exchanger. The process is designed to recover heat from molten CuCl in a thermochemical copper-chlorine (Cu-Cl) cycle of hydrogen production. This heat recovery occurs through the physical interaction between high temperature CuCl droplets and air. Convective heat transfer between droplets and air is analyzed in a counter-current spray flow heat exchanger. Numerical results for the variations of temperature, velocity and heat transfer rate are presented for two cases of CuCl flow. The optimal dimensions of the heat exchanger are found to be a diameter of 0.13 m, with a height of 0.6 and 0.8 m, for 1 and 0.5 mm droplet diameters, respectively. Additional results are presented and discussed for the heat transfer effectiveness and droplet solidification during heat recovery from the molten CuCl droplets.

Jaber, O.; Naterer, G. F.; Dincer, I.

2010-10-01

343

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english 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.

Mariani, V. C.; Coelho, L. S..

344

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

1986-01-01

345

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

Cebeci, Tuncer

1989-01-01

346

Experience of Modeling of Turbulent Forced Convection in Ducts

DEFF Research Database (Denmark)

The need for a reliable and reasonable accurate turbulence model without convergence problem for calculating duct flows for industrial applications has become more important. In this study three different turbulence models for predicting the turbulent Reynolds stresses namely; standard k-Îµ model, the non-linear k-Îµ model of Speziale and a full Reynolds Stress Model (RSM) are compared with each other. The advantages, disadvantages and accuracy of these models are discussed. The turbulent heat fluxes are modeled by the SED concept, the GGDH and the WET methods. The advantages of GGDH and WET compared to SED are discussed and the limitations of these models are clarified.

Rokni, Masoud; SundĂ©n, B.

1997-01-01

347

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

1979-01-01

348

Energy Technology Data Exchange (ETDEWEB)

Fibrous materials are some of the most widely used materials for thermal insulation. In this project the focus of interest has been on fibrous materials for building application. Interest in improving the thermal properties of insulation materials is increasing as legislation is being tightened to reduce the overall energy consumption. A knowledge of the individual heat transfer mechanisms - whereby heat is transferred within a particular material is an essential tool to improve continuously the thermal properties of the material. Heat is transferred in fibrous materials by four different transfer mechanisms: conduction through air, conduction through fibres, thermal radiation and convection. In a particular temperature range the conduction through air can be regarded as a constant, and conduction through fibres is an insignificant part of the total heat transfer. Radiation, however, constitutes 25-40% of the total heat transfer in light fibrous materials. In Denmark and a number of other countries convection in fibrous materials is considered as non-existent when calculating heat transmission as well as when designing building structures. Two heat transfer mechanisms have been the focus of the current project: radiation heat transfer and convection. The radiation analysis serves to develop a model that can be used in further work to gain a wider knowledge of the way in which the morphology of the fibrous material, i.e. fibre diameter distribution, fibre orientation distribution etc., influences the radiation heat transfer under different conditions. The convection investigation serves to examine whether considering convection as non-existent is a fair assumption to use in present and future building structures. The assumption applied in practically is that convection makes a notable difference only in very thick insulation, at external temperatures below -20 deg. C, and at very low densities. For large thickness dimensions the resulting heat transfer through the fibrous material will be relatively small, which means that a relatively small increase in heat loss by convection may counterbalance part of the savings achieved by increasing the thickness. (au) 34 refs.

Dyrboel, Susanne

1998-05-01

349

Mixed convection in a vertical double pipe heat exchanger

International Nuclear Information System (INIS)

A numerical study of steady state, simultaneously developing, laminar mixed convection in a vertical double pipe heat exchanger has been conducted for upward parallel flow. The model is elliptic and takes into account conduction in the solid walls as well as dissipation in the two streams. The viscosity and the density of the fluids depend on the temperature while all other thermophysical properties are constant. Results have been calculated for fixed inlet temperatures, a Richardson number equal to 1 for the annular space and three different values in the cylinder (4.85, 1 and 0.34). Flow reversal occurs in the warm fluid for Ric ?1 and affects significantly the thermal and hydrodynamic fields. In particular, the temperature of the wall separating the two fluids varies considerably in the flow direction despite its high conductivity. The results include temperature and velocity profiles at different cross sections as well as the axial evolution of bulk and wall temperatures, Nusselt numbers and friction factors in the cylinder and the annular region. The effect of Ric on the asymptotic values of the Nusselt numbers is analyzed. (authors)

2007-06-01

350

Mixed convection in a vertical double pipe heat exchanger

Energy Technology Data Exchange (ETDEWEB)

A numerical study of steady state, simultaneously developing, laminar mixed convection in a vertical double pipe heat exchanger has been conducted for upward parallel flow. The model is elliptic and takes into account conduction in the solid walls as well as dissipation in the two streams. The viscosity and the density of the fluids depend on the temperature while all other thermophysical properties are constant. Results have been calculated for fixed inlet temperatures, a Richardson number equal to 1 for the annular space and three different values in the cylinder (4.85, 1 and 0.34). Flow reversal occurs in the warm fluid for Ri{sub c}{>=}1 and affects significantly the thermal and hydrodynamic fields. In particular, the temperature of the wall separating the two fluids varies considerably in the flow direction despite its high conductivity. The results include temperature and velocity profiles at different cross sections as well as the axial evolution of bulk and wall temperatures, Nusselt numbers and friction factors in the cylinder and the annular region. The effect of Ri{sub c} on the asymptotic values of the Nusselt numbers is analyzed. (author)

Voicu, I. [Laboratoire GCGM, INSA Rennes, 20 Av. Buttes des Coesmes, CS 14315, 35043 Rennes cedex (France); Facultatea de Instalatii, UTCB, 66 Blv. Pache Protopopescu, cod 73232, Bucuresti (Romania); Mare, T.; Miriel, J. [Laboratoire GCGM, INSA Rennes, 20 Av. Buttes des Coesmes, CS 14315, 35043 Rennes cedex (France); Galanis, N. [Genie mecanique, Universite de Sherbrooke, Sherbrooke, Qc (Canada); Colda, I. [Facultatea de Instalatii, UTCB, 66 Blv. Pache Protopopescu, cod 73232, Bucuresti (Romania)

2007-06-15

351

Explicit finite element analysis of convective-conductive heat transfer

International Nuclear Information System (INIS)

The present paper discusses an explicit finite element approach to problems in transient convective-conductive heat transfer in a fluid region. The governing equations are the incompressible Navier-Stokes equations coupled with the thermal energy equation. Plane and axisymmetric problems are considered in terms of the primitive variables: velocity, pressure and temperature. The space discretization is based on 4-node or 9-node quadrilateral finite elements, while a finite difference method is used for time integration. Due to the complexity of the governing equations, an explicit time discretization method is choosen in connection with a diagonal mass representation. To deal with the necessarily implicit incompressibility constraint and the associated pressure terms, a fractional-step method is developed for marching in time. In this way, the pressure field is fully decoupled from and solved alternatively with the momentum equations. A weak treatment of the prescribed tangential components of velocity is introduced in order to avoid the spurious phenomenon of chequerboard splitting of the discrete pressure field encountered in other studies. To illustrate the proposed fractional-step method, numerical examples are presented in plane and axisymmetric configurations using both bilinear and biquadratic local approximations. The solutions obtained are found in good agreement with previously published results. (orig.)

1981-08-21

352

International Nuclear Information System (INIS)

The laminar combined convection heat transfer of the liquid sodium which flows through a single horizontal row of cooling tubes in the direction of gravity are studied using numerical analysis. The heat transfer characteristics at large Reynolds numbers are improved when Richardson numbers (=Gr/Re2) are increased and the improvement rate is enlarged with an increase in p/d value. The temperature field at small Reynolds numbers does not exhibit much change even when the Richardson number reaches a high value. Consequently the Nusselt numbers do not differ from those of forced convection. In other words, in a decay heat removal system at a low velocity, there is a possibility that an improvement in the heat transfer characteristics by combined convection cannot be expected even in a system with a large Richardson number. (orig.)

1990-09-01

353

Energy Technology Data Exchange (ETDEWEB)

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)

Oosthuizen, P.H.; Sun, L. [Queen' s Univ., Dept. of Mechanical Engineering, Kingston, Ontario (Canada); Naylor, D. [Ryerson Univ., Dept. of Mechanical, Aerospace and Industrial Engineering, Toronto, Ontario (Canada)

2002-07-01

354

Simulation of Convective Heat-Transfer Coefficient in a Buried Exchanger

Directory of Open Access Journals (Sweden)

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.

Taoufik Mnasri

2008-01-01

355

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)

2006-12-05

356

Convective heat transfer coefficients in a full-scale room with and without furniture

Energy Technology Data Exchange (ETDEWEB)

The convective heat transfer coefficient at an outer ambient wall with a window exposed to natural climate was measured in a room with and without furniture. The method used was to estimate the heat flow from measured temperatures and solar radiation. The convective heat transfer was calculated as the difference between the heat flow through the building element and the calculated long-wave radiation. Even though the accuracy was at best {+-} 15%, the effect of different heating and ventilation strategies could clearly be detected. Local coefficients may be more than 10 times the expected, due to ventilation or position of the radiator. (author)

Wallenten, P. [Lund Institute of Technology (Sweden). Dept. of Building Science

2001-07-01

357

Energy Technology Data Exchange (ETDEWEB)

Highlights: > We investigate laminar convective heat transfer in channels with periodic cavities. > Heat transfer rates are lower than for the flat channel. > This is ascribed to the steady circulating motion within the cavities. > Diffusion in a low Prandtl number fluid can locally overcome the heat transfer decrease due to advection only for isothermal boundary conditions. - Abstract: Convective heat transfer in laminar conditions is studied numerically for a Prandtl number Pr = 0.025, representative of liquid lead-bismuth eutectic (LBE). The geometry investigated is a channel with a periodic series of shallow cavities. Finite-volume simulations are carried out on structured orthogonal curvilinear grids, for ten values of the Reynolds number based on the hydraulic diameter between Re{sub m} = 24.9 and Re{sub m} = 2260. Flow separation and reattachment are observed also at very low Reynolds numbers and wall friction is found to be remarkably unequal at the two walls. In almost all cases investigated, heat transfer rates are smaller than the corresponding flat channel values. Low-Prandtl number heat transfer rates, investigated by comparison with Pr = 0.71 results, are large only for uniform wall temperature and very low Re. Influence of flow separation on local heat transfer rates is discussed, together with the effect of different thermal boundary conditions. Dependency of heat transfer performance on the cavity geometry is also considered.

Stalio, E., E-mail: enrico.stalio@unimore.it [Dipartimento di Ingegneria Meccanica e Civile, Universita degli Studi di Modena e Reggio Emilia, Via Vignolese 905/B, 41125 Modena (Italy); Angeli, D., E-mail: diego.angeli@unimore.it [Dipartimento di Ingegneria Meccanica e Civile, Universita degli Studi di Modena e Reggio Emilia, Via Vignolese 905/B, 41125 Modena (Italy); Barozzi, G.S., E-mail: giovanni.barozzi@unimore.it [Dipartimento di Ingegneria Meccanica e Civile, Universita degli Studi di Modena e Reggio Emilia, Via Vignolese 905/B, 41125 Modena (Italy)

2011-10-15

358

International Nuclear Information System (INIS)

Highlights: ? We investigate laminar convective heat transfer in channels with periodic cavities. ? Heat transfer rates are lower than for the flat channel. ? This is ascribed to the steady circulating motion within the cavities. ? Diffusion in a low Prandtl number fluid can locally overcome the heat transfer decrease due to advection only for isothermal boundary conditions. - Abstract: Convective heat transfer in laminar conditions is studied numerically for a Prandtl number Pr = 0.025, representative of liquid lead-bismuth eutectic (LBE). The geometry investigated is a channel with a periodic series of shallow cavities. Finite-volume simulations are carried out on structured orthogonal curvilinear grids, for ten values of the Reynolds number based on the hydraulic diameter between Rem = 24.9 and Rem = 2260. Flow separation and reattachment are observed also at very low Reynolds numbers and wall friction is found to be remarkably unequal at the two walls. In almost all cases investigated, heat transfer rates are smaller than the corresponding flat channel values. Low-Prandtl number heat transfer rates, investigated by comparison with Pr = 0.71 results, are large only for uniform wall temperature and very low Re. Influence of flow separation on local heat transfer rates is discussed, together with the effect of different thermal boundary conditions. Dependency of heat transfer performance on the cavity geometry is also considered.

2011-10-01

359

Heat transfer of a two-layer fluid system has been of great importance in a variety of industrial applications. For example, the phenomena of immiscible fluids can be found in materials processing and heat exchangers. Typically in solidification from a melt, the convective motion is the dominant factor that affects the uniformity of material properties. In the layered flow, thermocapillary forces can come into an important play, which was first emphasized by a previous investigator in 1958. Under extraterrestrial environments without gravity, thermocapillary effects can be a more dominant factor, which alters material properties in processing. Control and optimization of heat transfer in an immiscible fluid system need complete understanding of the flow phenomena that can be induced by surface tension at a fluid interface. The present work is focused on understanding of the magnetic field effects on thermocapillary convection, in order to optimize material processing. That is, it involves the study of the complicated phenomena to alter the flow motion in crystal growth. In this effort, the Marangoni convection in a cavity with differentially heated sidewalls is investigated with and without the influence of a magnetic field. As a first step, numerical analyses are performed, by thoroughly investigating influences of all pertinent physical parameters. Experiments are then conducted, with preliminary results, for comparison with the numerical analyses.

Ramachandran, N.; Ludovisis, D.; Cha, S. S.

2006-01-01

360

Characterization of Fuego for Laminar and Turbulent Natural Convection Heat Transfer.

A computational fluid dynamics (CFD) analysis is conducted for internal natural convection heat transfer using the low Mach number code Fuego. The flow conditions under investigation are primarily laminar, transitional, or low-intensity level turbulent fl...

N. D. Francis

2005-01-01

361

International Nuclear Information System (INIS)

A comprehensive separate effects study has been performed with the one-dimensional code LOOP-1 on powers and times for sodium boiling initiation and dryout in a closed loop system. Two different kinds of transients were considered: loss-of-flow and loss-of-heat-sink. Loss-of-flow transients were studied under both forced- and natural-convection conditions. Loss-of-heat-sink transients were studied under natural-convection conditions. The results for loss-of-flow transients indicate that the boiling initiation time was reduced by a small amount, and the dryout time was reduced very significantly by increasing either the input power or the inlet temperature, or by decreasing the test section pressure for both forced- and natural-convection conditions. Under forced-convection conditions, a stabilizing effect occurred by either increasing the test section valve setting or by decreasing the bypass ratio with a pump head adjusted to provide the same steady state and initial transient flows; thus, longer boiling times could be maintained before dryout occurred. For natural-convection loss-of-flow conditions, increasing the test section valve setting or decreasing the bypass ratio reduced the test section inlet flow, which resulted in boiling inception and dryout occurring more rapidly. A larger flow before the loss of flow transient starts yielded longer boiling initiation and dryout times. Under loss-of-heat-sink conditions, the higher the inlet temperature, the lower the boiling and the dryout powers. The margin between boiling and dryout powers increases with increasing inlet temperature. Results have been verified with experimental data. These results indicate that a margin between several seconds and several hours (depending on the type of transient) is available before core damage may occur in an actual reactor. (orig.)

1985-03-01

362

Energy Technology Data Exchange (ETDEWEB)

Natural convection in anisotropic heat generating porous medium enclosed inside a rectangular cavity has been studied. A 3D finite volume based code is developed using the Darcy approximation and validated using experimental results of natural convection around an enclosed rod bundle. Subsequently, detailed simulation is carried out for a cavity, filled with orthotropic porous medium. The effects of heat generation, geometry and anisotropy are studied. Anisotropy is found to be of significant importance for both maximum value and distribution of temperature. (orig.)

Suresh, Ch.S.Y.; Krishna, Y.Vamsee; Sundararajan, T.; Das, Sarit K. [Indian Institute of Technology Madras, Department of Mechanical Engineering, Chennai (India)

2005-07-01

363

Energy Technology Data Exchange (ETDEWEB)

In this note, the evaporation rate from a vertical wet fabric sheet is calculated using a free convection heat transfer correlation. Chilton-Colburn analogy is used to derive a mass transfer correlation from a heat transfer correlation proposed by Churchill and Chu for free convection from a vertical isothermal plate. The mass transfer rate obtained from this expression has shown excellent agreement with experimental data. (orig.)

Tafreshi, H. Vahedi; Ercan, E.; Pourdeyhimi, B. [North Carolina State University, Nonwovens Cooperative Research Center, Raleigh, NC (United States)

2006-07-15

364

International Nuclear Information System (INIS)

Experimental data are presented on heat transfer from a horizontal cylinder 0.126 mm in diameter under conditions of free convection of helium with large temperature heads (the dimensionless temperature head, equal to the ratio of the temperature difference to the absolute temperature of the gas, ranged from 0 to 50). An empirical relation is used which reliably describes the experimental data. A condition is found for ignoring convection compared to conduction when evaluating heat transfer from small-diameter cylinders

1986-11-01

365

Joule-Thomson effect and internal convection heat transfer in turbulent He II flow

The temperature rise in highly turbulent He II flowing in tubing was measured in the temperature range 1.6-2.1 K. The effect of internal convection heat transport on the predicted temperature profiles is calculated from the two-fluid model with mutual friction. The model predictions are in good agreement with the measurements, provided that the pressure gradient term is retained in the expression for internal convection heat flow.

Walstrom, P. L.

1988-03-01

366

Convective and boiling heat transfer from a heated surface to an impinging, planar jet of water

Energy Technology Data Exchange (ETDEWEB)

Cooling of hot rolled steel strip is often accomplished with planar, impinging jets of water. Liquid jet cooling is also well suited for dissipating the heat generated by microelectronic circuits. Although thorough studies of transport processes in impinging gas jets have been performed, the data base and predictive methods for impinging liquid jets are relatively undeveloped. The important heat transfer mechanism for liquid jet cooling have not been identified. Boiling studies have been limited to the region directly beneath the jet. Methods and equipment were developed for measuring the heat transfer to an impinging liquid jet. The rationale for each approach was documented. Alternative methods and designs are also described. The results of this study include local heat transfer coefficients for single phase convection, partial boiling and fully developed nucleate boiling. Heat transfer in the laminar boundary layer was shown to be sensitive to free stream turbulence. The response of boiling and nonboiling heat transfer processes to changes in the heat flux, flow rate and fluid temperature are described on the basis of surface temperature data and the results of a photographic study.

Vader, D.T.

1988-01-01

367

Energy Technology Data Exchange (ETDEWEB)

This paper addresses the problem of natural convection cooled channels heated with flush-mounted discrete heat sources: this configuration is more realistic for the modelization of the cooling of electronic devices. The two-dimensional, laminar and steady-state problem is investigated numerically by the full elliptic Navier-Stokes and energy equations in a composite domain. The computational domain is I-shaped and is made by the channel and two rectangular reservoirs. The thermal behavior of a channel symmetrically heated by a single flush-mounted strip as function of the strip position with respect to the inlet section is investigated. Wall temperature profiles as well as the velocity and local Nusselt numbers as functions of the nondimensional coordinates are shown for channels heated by five strips per each wall. The analysis is carried out in the aspect ratio, height of plates/channel gap, and the Rayleigh numbers in ranges 1--10 and 10{sup 3}--10{sup 5}, respectively. The wall temperature profiles show useful indications on the correct position of the heated strips with respect to the channel inlet in order to attain higher heat transfer rate coefficients. The velocity profiles at the inlet and exit sections are strongly influenced by the Rayleigh numbers. The local Nusselt values show that, for different values of the aspect ratio, the channels behave as uniformly heated walls at lower Rayleigh numbers. A correlation for the average Nusselt number in terms of Rayleigh channel number is given.

Campo, A. [Idaho State Univ., Pocatello, ID (United States). Nuclear Engineering Dept.; Manca, O. [Seconda Univ. degli studi di Napoli, Aversa (Italy). Dipt. di Ingegneria Aerospaziale; Morrone, B. [Univ. degli studi Federico II Napoli (Italy)

1996-12-31

368

Investigations into FBR-emergency decay heat removal under natural convection and boiling conditions

International Nuclear Information System (INIS)

Sodium boiling experiments have been performed in two 37-pin bundles to assess the amount of heat which can be removed under natural convection in a system consisting of test section, bypass and cold leg. The results showed, that the amounts of heat which can be removed under boiling conditions correspond broadly to reactor decay heat levels. (author)

1978-10-19

369

Directory of Open Access Journals (Sweden)

Full Text Available In Indian Coal mines, underground mines, mineral ore mines syngases at high pressure and high temperature are found in large quantities. This syngas is highly toxic, harmful and flammable gas which will be present in atmosphere which may cause many accidents. Hence it is necessary to reduce the content of syngas from mines in the atmosphere. This paper describes literature review of Convective heat transfer characteristics of high pressure gas used in mines. Heat transfer in convection cooling section of pressurized coal gasifier with the membrane helical coils and membrane serpentine tubes under high pressure is studied. This review covers the status and perspectives of syngas used in heat exchanger. It also covers the work carried out by different research scholar in this promising area. Some suggestions in the form of new concept are also suggested to reduce the percentage of high pressure syngas in the atmosphere. This paper will be useful for those who are working in the field of coal mine, underground mines and mineral ore mines.

MR.S.S.GADDAMWAR

2012-04-01

370

Laboratory grey cast iron continuous casting line with electromagnetic forced convection support

Directory of Open Access Journals (Sweden)

Full Text Available The article describes the construction of a 20 mm diameter grey cast iron ingots continuous casting laboratory line. This line is made ofthree main units: melting unit (induction furnace, casting unit and the pulling unit. In order to improve the homogeneity of themicrostructure of ingots (by applying forced convection of liquid metal during the crystallization process in this case a crystallizer system generating the forced movement of liquid metal based on a system of electrical power windings of the AC specific frequency. Thissolution allowed to obtain a homogeneous microstructure of the continuous casting of cast iron EN-GJL-200 species.

J. Szajnar

2010-07-01

371

Directory of Open Access Journals (Sweden)

Full Text Available The steady, laminar axisymmetric convective heat and mass transfer in boundary layer flow over a vertical thin cylindrical configuration in the presence of significant surface heat and mass flux is studied theoretically and numerically. The governing boundary-layer equations for momentum, energy and species conservation are transformed from a set of partial differential equations in a (x,r coordinate system to a ( system using a group of similarity transformations. The resulting equations are solved using the Network Simulation Method (NSM for the buoyancy-assisted pure free convection and also the pure forced convection cases, wherein the effects of Schmidt number, Prandtl number and surface mass parameter on velocity, temperature and concentration distributions in the regime are presented graphically and discussed. For the buoyancy-assisted pure free convection case, nondimensional velocity (f/ is found to increase with a rise in surface mass transfer (S but decrease with increasing Prandtl number (Pr, particularly in the vicinity of the cylinder surface (small radial coordinate, . Dimensionless temperature ( decreases however with increasing S values from the cylinder surface into the free stream; with increasing Prandtl number, temperature is strongly reduced, with the most significant decrease at the cylinder surface. Dimensionless concentration ( is decreased continuously throughout the boundary layer regime with an increase in S; conversely is enhanced for all radial coordinate values with an increase in Prandtl number. For the pure forced convection case, velocity increases both with dimensionless axial coordinate ( and dimensionless radial coordinate ( but decays smoothly with increasing Prandtl number and increasing Schmidt number, from the cylinder surface to the edge of the boundary layer domain. The model finds applications in industrial metallurgical processes, thermal energy systems, polymer processing, etc.

J. Zueco

2011-01-01

372

Unsteady heat transfer by natural convection in the cavity of a passive heating room

Energy Technology Data Exchange (ETDEWEB)

In this study, three dimensional unsteady state equations of heat transfer and flow have been solved numerically for a passive heating room using values of hourly averaged radiation during winter in Elazig region in Turkey. For this purpose, a room having volume of 44.8 m{sup 3} (4 x 4 x 2.8 m) was considered. Variable heat flux boundary condition depending on time was applied on absorber surface using values of hourly averaged radiation. Convection boundary condition was used on glass surface, the walls and ceiling of the room by using overall heat transfer coefficient. Constant surface temperature (6 C) was used for floor of the room. Experiments were carried out manufacturing a model room having the size of 1 x 1.2 x 1.2 m dimensions in order to maintain the corroboration of numerical solution. Governing equations for the model room built were solved numerically and compared to experimental data. It was seen that the model predictions agreed quite well with experimental data. The effect of overall heat transfer coefficient of glass on Nusselt number was also investigated. It was revealed that the overall heat transfer coefficient for low Rayleigh number affected the average Nusselt number more than that of high Rayleigh number. (author)

Kurtbas, irfan; Durmus, Aydin [Department of Mechanical Education, University of Firat, 23119 Elazig (Turkey)

2008-08-15

373

Energy Technology Data Exchange (ETDEWEB)

A problem of mass-transfer induced in multiphase liquid systems by thermo-capillary convection is considered as a new method for extraction of water from a binary mixture by solar light. The basis of consideration is a way to attract opaque drops in transparent liquids with velocities up to about 10 cm/sec due to thermocapillary convection that has been found by using of continuous wave argon laser light in experiments on mass-transfer in absorbing liquids. Such a high velocity of propulsion was called light-channeled phoresis. Motive forces for propulsion of a drop are explained by the self-organization of a heat cycle. This process occurs between illuminated and dark parts of any opaque drop (disperse phase) involved in mass-transfer in a transparent liquid (disperse medium) owing to separating surface of the phase discontinuity. The fluid surface is considered to be a work substance in a heat cycle to transform chaotic thermal energy of liquid into its more ordered (synergetic) hydrodynamic form of motion. The non-equilibrium Gibbs' surface is considered as both a source of motive forces to put a liquid droplet in motion and a necessary element to satisfy the law of momentum conservation in dynamic boundary conditions for solving hydrodynamic problem. The dissipative motion of a droplet under external gravitation and the motive motion under internal thermal energy powered by light has been compared. It is shown that being powered by light, the self-organization of heat cycles generates the vortexes in liquids with much higher density of kinetic energy then in case of droplet's motion with the same velocity in external gravitation field. In order to demonstrate a new possibility of optical liquid extraction owing to motive forces induced by light, butylcillusolve with water was chosen as a model mixture in laser experiments. As a absorbing pollution, a dye Rodamin 6G was used. Such solution has a lower point of phase transition, when the initial homogeneous liquid becomes two-phase system by heating up to about only 42 C. As far as light is able to heat the mixture up to this point, the extraction of water is observed as flow of water propelled by motive forces in center of illuminated zone. The analysis of the experimental and calculated data leads to the conclusion regarding the rule as to which of the phases in stratifying solutions should be extracted by light. In a system with the lower critical point the component of disperse phase will be collected by light at the maximum of its intensity; on the contrary, if a system has a upper critical point, the component of the disperse medium is extracted by light. The experimental results obtained in model laser experiments are discussed as a prerequisite to apply solar light to power supply of the new engineering constructions, for example in space, for liquid extraction based on motive forces by self-organization of heat cycles.

Sukhodolsky, A.T.

1998-07-01

374

Scale/Analytical Analyses of Freezing and Convective Melting with Internal Heat Generation

Energy Technology Data Exchange (ETDEWEB)

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.

Ali S. Siahpush; John Crepeau; Piyush Sabharwall

2013-07-01

375

Consideration about the utilization of a heat function in problems of natural convection in cavities

International Nuclear Information System (INIS)

The heat line, as defined by Kimura and Bejan does not involve, explicitly, a reference temperature for the energy converted by the flow. As a result different flow patterns may be obtained for the same physical problem. In this paper the heat function is re-defined and the temperature of the cold wall, in natural convection flow problem inside cavities, is used as reference temperature. The heat function, as defined here, is also written in a generalized coordinate system and is employed to visualize the heat flow in some natural convection problems inside irregular cavities. (author)

1989-12-05

376

On the periodic conditions to simulate mixed convection heat transfer in horizontal channels

Energy Technology Data Exchange (ETDEWEB)

Mixed convection heat transfer in open-ended channels with discrete heating from below is numerically studied when the convection cells remain attached to the heating elements. Two approaches are used: in method A, the computation domain covers the whole channel, while in method B, periodic conditions are imposed at the entrance and exit of a computation domain that is one wavelength long. Nu is calculated as functions of Ra (5 {times} 10{sup 3} {<=} Ra {<=} 20 {times} 10{sup 3} {<=} J) and of Pe (0.1 {<=} Pe {<=} 2.5). The results from two approaches show that method B can adequately be used to calculate heat transfer.

Bilgen, E.; Wang, X.; Vasseur, P.; Meng, F.; Robillard, L. [Ecole Polytechnique Univ. of Montreal, Quebec (Canada). Mechanical Engineering Dept.

1995-04-01

377

Application of a square function heat pulse to forced flow He II

International Nuclear Information System (INIS)

This paper comprises the third report on a series of experiments involving forced convection heat transfer in He II. A square function heat pulse is applied to the middle of a copper flow tube. Pulse width and amplitude were varied in the experimental runs. Bath temperature was also varied between 1.65, 1.80, and 1.95 K. Bath pressure was kept under saturation pressure. He II flow rate was varied between zero and 80 cm per second. Fluid temperature was monitored at eight locations along the tube. Experimental temperature profiles were compared with a numerical solution to the He II energy equation

1988-01-01

378

Study on natural convection heat transfer in vertical annular space of a double coaxial cylinder

International Nuclear Information System (INIS)

Water cooling panels are adopted as a vessel cooling system of a high temperature-engineering test reactor (HTTR) to cool the reactor core indirectly by natural convection and thermal radiation. In this study, we carried out experiments on natural convection heat transfer coupled with thermal radiation in vertical annular space of a double coaxial cylinder in order to investigate heat transfer characteristics in vertical annular space between the reactor pressure vessel and the cooling panels of the HTTR. In the present experiments, Rayleigh number based on the width of the vertical space was set to be 6.8 x 10"5 < Ra < 1.8 x 10"6 for helium and 4.2 x 10"7 < Ra < 1.0 x 10"8 for nitrogen. This report described about the heat transfer coefficient of natural convection in the vertical space and the effect of thermal radiation of the transferred heat. As a result, a heat transfer coefficient of natural convection coupled with thermal radiation was obtained as functions of Rayleigh number, aspect ratio of the space, temperature and emissivities on the heated and cooled walls. In addition to the experiments, numerical analyses were performed on the combined phenomena of natural convection and thermal radiation in the space. The numerical results were in good agreement with the experimental ones regarding the temperature on the heated and cooled walls. (author)

2001-01-01

379

Turbulent mixed convection heat transfer experiments in a vertical cylinder using analogy concept

Energy Technology Data Exchange (ETDEWEB)

A series of the turbulent mixed convection heat transfer experiments in a vertical cylinder was carried out. In order to achieve high Grashof number easily, the analogy concept was adopted and the heat transfer system was simulated by a mass transfer system. With reasonable facility heights, large Grashof numbers could be achieved using a copper electroplating system. The tests in buoyancy-aided and opposed flow configurations, were performed for Reynolds numbers from 4000 to 10,000 with a constant Grashof number of 6.2 x 10{sup 9} and Prandtl number of about 2000. The test results reproduced the typical of the mixed convection heat transfer phenomena in a turbulent situation and agreed well with the study performed by Palratan et al. The analogy experimental method simulated the mixed convection heat transfer phenomena successfully and proved itself to be a useful tool for the draft estimation of the heat transfer rate in the highly buoyant systems such as the VHTR.

Ko, Bong-Jin [Department of Nuclear and Energy Engineering, Jeju National University, 66 Jejudaehak-no, Jeju 690-756 (Korea, Republic of); Lee, Won Jae [Korea Atomic Energy Research Institute, P.O. Box 105, Yuseong, Daejeon 305-353 (Korea, Republic of); Chung, Bum-Jin, E-mail: bjchung@jejunu.ac.k [Department of Nuclear and Energy Engineering, Jeju National University, 66 Jejudaehak-no, Jeju 690-756 (Korea, Republic of)

2010-12-15

380

Convection in layered porous media: A comparison of boundary heating methods

Energy Technology Data Exchange (ETDEWEB)

Convection in a horizontal, doubly layered porous medium has been investigated numerically. A two-dimensional, time dependent model has been developed to compute heat transfer in a saturated porous medium that is locally heated from either above or below. The primary objective is to ascertain how these modes of heating can be differentiated via an examination of the heat transfer results. Both natural and mixed convection are considered. For mixed convection in which a uniform horizontal flow is assumed to enter the domain, the qualitative relation between the Rayleigh and Peclet numbers is obtained over a large range for each. The effect of the length of the heating zone on the flow structure is also examined. The permeability ratio and the ratio of the thermal conductivity of the two layers is also allowed to vary, thus giving the computing Nusselt numbers a broad range of applicability in geophysical and engineered systems.

Jendoubi, S.; Kulacki, F.A.

1999-07-01

381

International Nuclear Information System (INIS)

Accurate mass diffusivity values are important in mass transfer processes. Diffusion experiments conducted on earth are typically convectively contaminated due to either thermal or solutal gradients. Liquid metals and semiconductors have high electrical conductivities, and applied magnetic fields may suppress buoyant convection in these liquids. In this paper, an axisymmetric self-diffusivity model is considered in the presence of a steady, strong, uniform axial magnetic field with liquid indium. An isopycnic (radioisotope) tracer is used so that only thermal differences drive the convection. Five different combinations of a steady, uniform heat flux and a steady, periodic heat flux are imposed along the vertical wall while uniform heat loss is allowed through the top and bottom walls of the cylinder. The addition of periodic heat flux with the reduced uniform heat flux has a positive impact on the output diffusivity results for the same applied magnetic field of 5.24 T

2005-02-25

382

Convective heat transfer from a heated elliptic cylinder at uniform wall temperature

Energy Technology Data Exchange (ETDEWEB)

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.

Kaprawi, S.; Santoso, Dyos [Mechanical Department of Sriwijaya University, Jl. Raya Palembang-Prabumulih Km. 32 Inderalaya 50062 Ogan Ilir (Indonesia)

2013-07-01

383

Digital Repository Infrastructure Vision for European Research (DRIVER)

This paper analyses the transient heat transfer across multi-layer floors subjected to multiple heat sources. The formulation that is proposed to solve this problem uses analytical expressions handling both conduction and convection phenomena. The Green's functions (analytical expressions) for the layered formation are established by imposing the continuity of temperatures and heat fluxes at the medium interfaces between the various layers. The heat field inside a layer is obtained by adding ...

Simo?es, Nuno; Tadeu, Anto?nio

2006-01-01