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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

Transient forced convection heat transfer from an isothermal flat plate.

An analysis is presented for laminar transient forced-convection heat transfer from a horizontal flat plate. The presented analysis results include transient velocity and temperature profiles, boundary-layer growth rate, and heat-transfer rate with time.

Nagendra, H. R.

1973-01-01

3

Liquid sodium forced convection heat transfer for low Pe numbers

International Nuclear Information System (INIS)

The purposes of this study are, first, to obtain the experimental data of sodium forced convection heat transfer from an inner rod surface of a concentric annulus for low Pe numbers ranging from 0.7 to 140, second, to derive a rigorous numerical solution for laminar forced convection heat transfer and, third, to investigate whether the theoretical values can describe the experimental data. (author)

4

Heat transfer intensification at forced convection

International Nuclear Information System (INIS)

The paper presnets the results of an experimental investigation into the principle of convective heat transfer4 intensification based on assumption of a possibility to produce an effect on micro and macrostructure of a gas flow or dropping liquid via artificially produced pressurte heterogeinities by a special shaping of the surface. The principle was implemented through the following design features: 1. Lamellar surface with a subsequent alternation of diffusers and converging tubes; 2. Surface of tube-in-tube type, whose inner tube has sickle-like bulges; 3. Shell-tubular surface whose tube bundle has longitudinal - wave fins; 4. Surfaces with punched longitudinal - wave fins; 5. Surfaces with an angular punched packing. The presented results show that the heat transfer surfaces, which were developed on the basis of pressure heterogeneity introduction into a flow, can be recommended for application in heat exchangers

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

6

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)

7

Topological design of heat dissipating structure with forced convective heat transfer

International Nuclear Information System (INIS)

This paper discusses the use of the topology optimization formulation for designing a heat dissipating structure that utilizes forced convective heat transfer. In addition to forced convection, there is also natural convection due to natural buoyancy forces induced by local heating inside fluid. In the present study, the temperature distribution due to forced convection, neglecting buoyancy and viscous dissipation inside fluid, was simulated and optimized. In order to analyze the heat transfer equation with forced convective heat loss and the Navier-Stokes equation, a common sequential computational procedure for this thermo/hydraulic characteristic was implemented. For topology optimization, four material properties were interpolated with respect to spatially defined density design variables: the inverse permeability in the Navier-Stokes equation, the conductivity, density, and the specific heat capacity of the heat transfer equation. From numerical examples, it was found that the balance between the conduction and convection of fluid is of central importance to the design of heat dissipating structures

8

NANOFLUID PROPERTIES FOR FORCED CONVECTION HEAT TRANSFER: AN OVERVIEW

Directory of Open Access Journals (Sweden)

Full Text Available Nano?uids offer a significant advantage over conventional heat transfer ?uids and consequently, they have attracted much attention in recent years. The engineered suspension of nano-sized particles in a base liquid alters the properties of these nanofluids. Many researchers have measured and modeled the thermal conductivity and viscosity of nanofluids. The estimation of forced convective heat transfer coefficients is done through experiments with either metal or nonmetal solid particles dispersed in water. Regression equations are developed for the determination of the thermal conductivity and viscosity of nanofluids. The parameters influencing the decrease in convection heat transfer, observed by certain investigators, is explained.

W.H.Azmi

2013-06-01

9

Combined natural and forced convection between uniformly heated vertical plates

International Nuclear Information System (INIS)

Combined natural and forced convection between uniformly heated vertical plates was studied experimentally. The followings were revealed; the streak-line of visualizing dye was straight, when the buoyancy didn't have much effect. But the streak-line and the wall temperature began to fluctuate sinusoidally at halfway of channel and the local heat transfer coefficient increased abruptly, when the buoyancy became effective. (author)

10

Forced convection film boiling heat transfer on spheres

International Nuclear Information System (INIS)

This paper presents an experimental study of forced convection film boiling on sphere. To measure the film boiling heat transfer coefficients, a heated sphere with a thermocouple embedded at the center is dropped in (1) a vertical tube filled with water, or (2) a rotating water pool. The rotating pool is for providing higher fluid velocity. The falling speed of the sphere in the vertical stainless steel tube is measured using a set of magnet pickup coils distributed along the tube. The rotating pool is made of Pyrex to make a visualization. The ranges of the experimental conditions are: sphere speed 0.1-1.6 m/s, sphere surface temperature 500 and 700degC. The results show that the forced convection film boiling heat transfer coefficients were 200?400 W/m2K in the present experimental conditions and also show that there are large differences between the data sets or correlations due mainly to experimental difficulties. (author)

11

Mechanistic Multidimensional Modeling of Forced Convection Boiling Heat Transfer

Directory of Open Access Journals (Sweden)

Full Text Available Due to the importance of boiling heat transfer in general, and boiling crisis in particular, for the analysis of operation and safety of both nuclear reactors and conventional thermal power systems, extensive efforts have been made in the past to develop a variety of methods and tools to evaluate the boiling heat transfer coefficient and to assess the onset of temperature excursion and critical heat flux (CHF at various operating conditions of boiling channels. The objective of this paper is to present mathematical modeling concepts behind the development of mechanistic multidimensional models of low-quality forced convection boiling, including the mechanisms leading to temperature excursion and the onset of CHF.

Raf M. Podowski

2008-12-01

12

Forced-Convection Heat Transfer in a Spherical-Annulus Heat Exchanger.

Results are presented of a combined numerical and experimental study of steady forced-convection heat transfer in a spherical annulus heat exchanger with 50 exp 0 C heated water flowing in an annulus formed by an insulated outer sphere and a 0 exp 0 C coo...

D. B. Tuft, H. Brandt

1981-01-01

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

14

Axial heat conduction effects in forced convection along a cylinder

International Nuclear Information System (INIS)

An analytical study is performed to determine the effects of axial heat conduction and transverse curvature on laminar forced convective heat transfer of liquid metals along a circular cylinder. The flow and thermal boundary layers for this problem are nonsimilar, the non-similarity arising both from the transverse curvature xi = (4/R) (?x/u/sub infinity)/sup 1/2/ of the cylindrical surface and from the axial heat conduction effect expressible as ? = 1/Pe/sub x/, where Pe/sub x/ is the local Peclet number. The governing equations are solved by the local nonsimilarity method in which all the terms in the conservation equations are retained and only terms in the derived subsidiary equations are selectively deleted according to the levels of truncation. Numerical results are presented for liquid metals having representative Prandtl numbers of 0.03, 0.008, and 0.003 over a wide range of xi values from 0 (i.e., a flat plate) to 4.0 and ? values from 0 (i.e., without axial heat conduction effect) to 0.20. The results indicate that the local surface heat transfer rate increases with an increase in the transverse curvature of the cylindrical surface, an increase in Prandtl number, and an increase in the axial heat conduction parameter or a decrease in Peclet number. (U.S.)

15

Forced convection over traditional surfaces such as flat plate, cylinder and sphere have been well researched and documented. Data on forced convection over airfoil surfaces, however, remain very scanty in literature. High altitude vehicles that employ airfoils as lifting surfaces often suffer leading edge ice accretions which have tremendous negative consequences on the lifting capabilities and stability of the vehicle. One of the ways of mitigating the effect of ice accretion involves judicious leading edge convective cooling technique which in turn depends on the accuracy of convective heat transfer coefficient used in the analysis. In this study empirical investigation of convective heat transfer measurements on asymmetric airfoil is presented at different angle of attacks ranging from 0° to 20° under subsonic flow regime. The top and bottom surface temperatures are measured at given points using Senflex hot film sensors (Tao System Inc.) and used to determine heat transfer characteristics of the airfoils. The model surfaces are subjected to constant heat fluxes using KP Kapton flexible heating pads. The monitored temperature data are then utilized to determine the heat convection coefficients modelled empirically as the Nusselt Number on the surface of the airfoil. The experimental work is conducted in an open circuit-Eiffel type wind tunnel, powered by a 37 kW electrical motor that is able to generate subsonic air velocities up to around 41 m/s in the 24 square-inch test section. The heat transfer experiments have been carried out under constant heat flux supply to the asymmetric airfoil. The convective heat transfer coefficients are determined from measured surface temperature and free stream temperature and investigated in the form of Nusselt number. The variation of Nusselt number is shown with Reynolds number at various angles of attacks. It is concluded that Nusselt number increases with increasing Reynolds number and increase in angle of attack from 0° to 20° on the upper and lower surface of the airfoil.

Dag, Yusuf

16

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)

17

Forced convection heat transfer with two-phase flow

International Nuclear Information System (INIS)

A number of correlations to represent forced convection heat transfer to two-phase cryogenic fluids are presented. In most cases these were developed from limited data for a single fluid (most often hydrogen). Only one critical evaluation and comparison was made. It should be noted that this evaluation is based only on hydrogen data and that the nucleate boiling portion is based on only 19 data points. Specific recommendations for general use of the equations have not been made because of a lack of generality in their development and the paucity of data. To determine the applicability of any of these correlations with fluids other than those for which they were developed, much more data are required. For cryogens, this should include liquefied methane (liquefied natural gas) nitrogen, oxygen, and helium. While liquid nitrogen and liquid oxygen are already common engineering materials, the rapid growth of the technology of liquefied natural gas and liquid helium presents a real need for more engineering data. (LCL)

18

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

19

Directory of Open Access Journals (Sweden)

Full Text Available In this paper, convective heat transfer coefficient of Indian gooseberry (Emblica officinalis, in three different forms (shreds, slices and pieces, under forced convection mode has been determined. These forms were dried in laboratory drier. Values of constants C and n have been determined using experimental data and regression analysis for calculating values of convective heat transfer coefficient. It was found that the convective heat transfer coefficient varies with form of commodity being dried and decreases as the drying progresses. The value of convective heat transfer coefficient was highest for shredded form (30.39 W/m2oC followed by slices (25.88 W/m2oC and pieces (18.67 W/m2oC when compared at certain final moisture content. The data were also analyzed for per cent uncertainty.

S. I. ANWAR

2012-10-01

20

International Nuclear Information System (INIS)

Under forced convection and atmospheric pressure conditions, heat transfer characteristics were studied using an annulus channel of a heater rod with highly-wettable surface. Improvement of phase change heat transfer under boiling and condensation process requires that either liquid heated or vapor cooled can contact a heat transfer surface effectively, but either a vapor layer or a liquid film can be formed repeatedly on the surface during boiling and condensation processes. As oxide semiconductor materials are known to be highly wettable, TiO2 was coated on a heater rod surface. The Leidenfrost temperature for the TiO2 coated surface was higher than that for the uncoated surface. Under mass flux conditions from 600 to 1700 kg/m2s, the heater rod surface temperature at variable power conditions was measured. A higher heat flux at the point where the forced convection vaporization started and a larger critical heat flux were observed for the TiO2 coated surface than that for the uncoated surface. (author)

21

Under forced convection and atmospheric pressure conditions, heat transfer characteristics were studied using an annulus channel of a heater rod with highly-wettable surface. Improvement of phase change heat transfer under boiling and condensation process requires that either liquid heated or vapor cooled can contact a heat transfer surface effectively, but either a vapor layer or a liquid film can be formed repeatedly on the surface during boiling and condensation processes. As oxide semiconductor materials are known to be highly wettable, TiO2 was coated on a heater rod surface. The Leidenfrost temperature for the TiO2 coated surface was higher than that for the uncoated surface. Under mass flux conditions from 600 to 1700kg/m2s, the heater rod surface temperature at variable power conditions was measured. A higher heat flux at the point where the forced convection vaporization started and a larger critical heat flux were observed for the TiO2 coated surface than that for the uncoated surface.

Mitsutake, Toru; Kano, Fumihisa; Oosato, Tetsuo; Sayano, Akio; Akiba, Miyuki; Morooka, Shinichi

22

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)

23

Digital Repository Infrastructure Vision for European Research (DRIVER)

A two-dimensional steady forced convective flow of a Newtonian fluid past a convectively heated permeable vertically moving plate in the presence of a variable magnetic field and radiation effect has been investigated numerically. The plate moves either in assisting or opposing direction to the free stream. The plate and free stream velocities are considered to be proportional to whilst the magnetic field and mass transfer velocity are taken to be proportional to where is the distance along ...

Uddin, Md Jashim; Khan, Waqar A.; Ismail, A. I. Md

2013-01-01

24

Experimental study on forced convection boiling heat transfer on molten alloy

Energy Technology Data Exchange (ETDEWEB)

This paper describes forced convection boiling heat transfer on molten alloy. Basic experiments were carried out with subcooled water flowing on molten Wood`s alloy pool surface. The Wood`s alloy is pooled in a cavity and heated from a copper conductor located at the bottom of the cavity. A boiling curve for forced convection boiling on molten alloy is obtained and compared with that calculated considering natural convection of molten alloy and assuming the heating surface as a solid. The calculated boiling curves are on the higher superheat side. It would suggest that waving of the surface and fragmentation of molten alloy increase the heat transfer area, resulting in enhancement of heat transfer performance. (author)

Nishimura, Satoshi; Ueda, Nobuyuki; Nishi, Yoshihisa; Furuya, Masahiro; Kinoshita, Izumi [Central Research Inst. of Electric Power Industry, Komae, Tokyo (Japan); Yamaguchi, Takashi

1998-05-01

25

Experimental study on forced convection boiling heat transfer on molten alloy

International Nuclear Information System (INIS)

This paper describes forced convection boiling heat transfer on molten alloy. Basic experiments were carried out with subcooled water flowing on molten Wood's alloy pool surface. The Wood's alloy is pooled in a cavity and heated from a copper conductor located at the bottom of the cavity. A boiling curve for forced convection boiling on molten alloy is obtained and compared with that calculated considering natural convection of molten alloy and assuming the heating surface as a solid. The calculated boiling curves are on the higher superheat side. It would suggest that waving of the surface and fragmentation of molten alloy increase the heat transfer area, resulting in enhancement of heat transfer performance. (author)

26

NUMERICAL ANALYSIS OF FORCED CONVECTIVE HEAT TRANSFER THROUGH HELICAL CHANNELS

Directory of Open Access Journals (Sweden)

Full Text Available Helical ducts are used in a variety of applications including food processing, thermal processing plants and refrigeration. They are advantageous due to their high heat transfer coefficient and compactness compared to straight tubes. The curvature of the coil governs the centrifugal force resulting in development of secondaryflow i.e. the fluid stream in the outer side of the pipe moves faster than the fluid streams in the inner side of the pipe. In the present study, Computational Fluid Dynamics (CFD simulations using Fluent 6.3.26 are carried out for helical rectangular ducts wound over a cylindrical passage. The cylindrical passage is oriented horizontallyand acts as a counterflow heat exchanger. The analysis is done by changing the flow rates of four different fluids like Ethylene Glycol, Kerosene, Nano Fluid and Water. The fluid flow and heat transfer characteristics of the fluids are studied and Nusselt Number correlations with Dean Number are developed.

Dr. K. E. Reby Roy

2012-07-01

27

Experimental and Numerical Study of Steady Forced-Convection Heat Transfer in a Spherical Annulus.

The results of a combined experimental and numerical study of steady forced-convection heat transfer in a spherical annulus with 50 exp 0 C heated water flowing in the annulus, an insulated outer sphere, and a 0 exp 0 C cooled inner sphere are presented. ...

D. B. Tuft

1980-01-01

28

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 101 ? 5 x 105 and Grashof number Gr of 5 x 104 ? 4 x 1011, 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/(Re21/8 Pr1/2) 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 21/8 Pr2/1) -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)

29

International Nuclear Information System (INIS)

In this work, we present an approximate analytical solution of the steady-state conjugate heat transfer of laminar forced convection in a circular pipe with wall axial heat conduction and external convective boundary conditions. Improved lumped-differential approach based on two points Hermite approximation for integrals was applied to reduce the heat conduction equation in the solid into a second-order ordinary differential equation for the radially averaged solid temperature. The energy equation in the fluid was solved by applying the generalized integral transform technique (GITT). The Sturm-Liouville eigenproblem for fluid energy equation in the cylindrical coordinate system was solved by the sign-count method. The truncated system of N ordinary differential equations for transformed potentials of the fluid temperature and the second-order ordinary differential equation for radially averaged solid temperature formed a homogeneous system of N +2 ordinary differential equations, which was solved analytically. The effects of the fluid-solid thermal conductivity ratio on the Nusselt number, the average fluid and solid temperatures, and the fluid-solid interface temperature were investigated. (author)

30

Laminar forced convective heat transfer to near-critical water in a tube

International Nuclear Information System (INIS)

Numerical modeling is carried out to investigate forced convective heat transfer to near-critical water in developing laminar flow through a circular tube. Due to large variations of thermo-physical properties such as density, specific heat, viscosity, and thermal conductivity near thermodynamic critical point, heat transfer characteristics show quite different behavior compared with pure forced convection. With flow acceleration along the tube unusual behavior of heat transfer coefficient and friction factor occurs when the fluid enthalpy passes through pseudocritical point of pressure in the tube. There is also a transition behavior from liquid-like phase to gas-like phase in the developing region. Numerical results with constant heat flux boundary conditions are obtained for reduced pressures from 1.09 to 1.99. Graphical results for velocity, temperature, and heat transfer coefficient with Stanton number are presented and analyzed

31

International Nuclear Information System (INIS)

In order to make clear the forced convection heat transfer phenomena around spherical particles packed in fluid flow, we numerically analyzed the heat transfer and flow pattern of the air using a single sphere and then the closest packed structure arrangement of spherical particles. We used 3-dimensional thermo fluid computation code 'STAR-CCM+'. We calculated the forced convection heat transfer coefficient for spheres of 10 mm diameter with Reynolds number 63 – 6340. Our calculation results of the average heat transfer coefficient for a single sphere agree with the correlation equation proposed by Ranz and Marshall. Local heat transfer coefficient is high at portions where local flow impinges to the surface of spheres for packed spherical particles. Our calculation results of the average heat transfer coefficient for packed spherical particles are close to the correlation equation proposed by Wakao et al.

32

Theoretical solution for forced convection film boiling heat transfer from a horizontal cylinder

International Nuclear Information System (INIS)

The theoretical solutions of forced convection film boiling heat transfer from horizontal cylinders were obtained based on a two-phase laminar boundary layer film boiling model. It was clarified that authors' previous experimental data for the cylinders with the non dimensional diameters, D', of around 1.3 in water and in Freon-113 agreed with the values of theoretical numerical solutions based on the two-phase laminar boundary layer model with the smooth vapor-liquid interface except those for low flow velocities. A forced convection film boiling heat transfer correlation for the cylinders with non dimensional diameters of around 1.3 under saturation conditions was developed based on forced convection two-phase laminar boundary layer film boiling model and the experimental data

33

International Nuclear Information System (INIS)

Heat transfer and CHF (Critical heat flux) characteristics of flow boiling of R-113 in helically coiled tubes were experimentally investigated. Two coiled tubes with coil diameters of 0.165 and 0.32 m, and 10 mm I.D. were tested at a pressure of 0.39 MPa. In the nucleate boiling region, circumferential difference in heat transfer was not clarified qualitatively as well as quantitatively. The ratio of circumferential average boiling heat transfer coefficient to that of the single-phase flow in a curved tube, hTP/hLo, was found to be rather close to the correlations of Pujol-Stenning and Ueda-Kim for a straight tube flow. In the high quality region, the heat transfer coefficient was highest at the coil outside and lowest at the inside, and the average heat transfer coefficient ratio hTP/hLo was expressed in terms of Martinelli parameter, Xtt, only. A correlation applicable to both the nucleate boiling and forced convective evaporation regions was proposed. The CHF of the coiled tube was higher than that of the straight tube when the coil diameter was small, but it became considerably low at low mass velocity when the coil diameter was large. (author)

34

Energy Technology Data Exchange (ETDEWEB)

Heat transfer and CHF (Critical heat flux) characteristics of flow boiling of R-113 in helically coiled tubes were experimentally investigated. Two coiled tubes with coil diameters of 0.165 and 0.32 m, and 10 mm I.D. were tested at a pressure of 0.39 MPa. In the nucleate boiling region, circumferential difference in heat transfer was not clarified qualitatively as well as quantitatively. The ratio of circumferential average boiling heat transfer coefficient to that of the single-phase flow in a curved tube, h{sub TP}/h{sub Lo}, was found to be rather close to the correlations of Pujol-Stenning and Ueda-Kim for a straight tube flow. In the high quality region, the heat transfer coefficient was highest at the coil outside and lowest at the inside, and the average heat transfer coefficient ratio h{sub TP}/h{sub Lo} was expressed in terms of Martinelli parameter, X{sub tt}, only. A correlation applicable to both the nucleate boiling and forced convective evaporation regions was proposed. The CHF of the coiled tube was higher than that of the straight tube when the coil diameter was small, but it became considerably low at low mass velocity when the coil diameter was large. (author)

Kaji, Masou; Mori, Koji; Matsumoto, Tadayosi [Osaka Univ., Suita (Japan). Faculty of Engineering; Oishi, Masaya; Sawai, Tooru; Nakanishi, Shigeyasu

1998-10-01

35

Convective Heat Transfer Coefficients of the Human Body under Forced Convection from Ceiling

DEFF Research Database (Denmark)

The average convective heat transfer coefficient for a seated human body exposed to downward flow from above was determined. Thermal manikin with complex body shape and size of an average Scandinavian female was used. The surface temperature distribution of the manikin’s body was as the skin temperature distribution of an average person. The measurements were performed in a room with controlled thermal environment. Air temperature was set at 26ºC for cooling and at 20ºC for heating. The radiant temperature asymmetry in horizontal and vertical direction was close to zero, i.e. mean radiant temperature was equal to the air temperature. The air velocity of the isothermal downward flow from the ceiling at height of 1.5 m above the floor (above the top of the head) was set in a range between still air and 0.73 m/s. Based on the analyses of the results relationships for determination of the convective heat transfer coefficient of the whole body (hc [W/(m2•K)]) was proposed: hc=4.088+6.592V1.715 for a seated naked body at 20ºC and hc=2.874+7.427V1.345 for a seated naked body at 26ºC. Differences in the convective heat transfer coefficient of the whole body in low air velocity range, V<0.3 m/s, due to the natural convection were found. The results may be useful during design of air distribution in rooms, e.g. low impulse ventilation, diffuse ventilation, etc.

Kurazumi, Yoshihito; Rezgals, Lauris

2014-01-01

36

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

37

Foliage influences forced convection heat transfer in conifer branches and buds.

Conifer foliage structures affect branch and bud temperature by altering the development and convective resistance of the thermal boundary layer. This paper examines foliage effects on forced convection in branches and buds of Picea glauca (Moench) Voss and Pinus contorta Dougl. Ex. Loud., two species that represent the range of variation in foliage structure among conifers. Forced convection is characterized by a power law relating Nusselt (heat transfer) and Reynolds (boundary layer development) numbers. Data were collected in a laminar flow wind tunnel for free stream velocities of 0.16-6.95 m s(-1). Scaling parameters were compared against literature values for silver cast branch replicas, a bed of real foliage, cylinders, and tube banks. Foliage structures reduced Nusselt numbers (heat transfer) relative to cylinders, which are typically used to approximate leafless branches and buds. Significantly different scaling relationships were observed for all foliage structures considered. Forced convection scaling relationships varied with foliage structure. The scaling relationships reported here account for variation within populations of branches and buds and can be used to characterize forced convection in a forest canopy. PMID:16539606

Michaletz, S T; Johnson, E A

2006-01-01

38

International Nuclear Information System (INIS)

A combined experimental and theoretical investigation of heat transfer in a vertical tube and annulus, countercurrent flow heat exchanger is reported. The working fluid was liquid sodium. Included in the range of conditions covered were those which are of interest in connection with the low flow rate operation of fast reactor intermediate heat exchanger systems. The heat transfer process ranged from that of pure forced convection to combined forced and free convection. By changing the direction of fluid flow or the direction of heat flow four different configurations were studied. In two cases the convection process was buoyancy aided and in the other two it was buoyancy opposed. Results are presented showing the influence of flow rate and temperature difference on overall heat transfer coefficient for each case. A theoretical model of turbulent flow and heat transfer incorporating influences of buoyancy was used to produce results for the range of conditions covered in the experiments. The predictions of overall heat transfer coefficient were found to be in reasonable general agreement with the measurements. It was clear from these calculations that the influence of buoyancy on heat transfer stemmed largely, under the conditions of the present experiment, from the modification of the convection process due to the distortion of the velocity field. This led to an enhancement of the heat transfer for the buoyancy-aided process and an impairment for the buoyancy-opposed process. The contribution of the turbulent diffusion of heat was relatively small. (author)

39

Within the present work the dataset of experimental points and the heat transfer correlations available in literature for liquid-metal fully-developed, forced-convective heat transfer in pipes are reviewed and critically analyzed. Over 1,100 data points from 21 different sources are considered for constant heat flux, covering a wide range of operating conditions (velocity, heat flux, diameter, among others). Among 15 evaluated correlations, four appropriate ones are recommended for forced turbulent convection: one covering all the data points and the other three respectively related to alkali liquid metals, lead alloys and mercury. Moreover, a new correlation has been derived as a best fit of the limited number of available data points for constant wall temperature, while an alternative evaluation method is also described for this boundary condition.

Pacio, J.; Marocco, L.; Wetzel, Th.

2014-07-01

40

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)

41

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 tub...

Tahseen, Tahseen A.; Ishak, M.; Rahman, M. M.

2013-01-01

42

Axial heat conduction effects in unsteady forced convection along a cylinder

International Nuclear Information System (INIS)

The aim of the present analysis is to study the effect of the axial heat conduction on the unsteady incompressible laminar forced convective heat transfer of liquid metals along an isothermal circular cylinder in longitudinal flow when the free-stream velocity varies arbitrarily with time. The partial differential equations involving three independent variables have been solved numerically using an implicit finite-difference scheme, The particular cases of the authors results have been compared with the results available in the literature

43

Heat flux at the boundary of a duct is estimated using the inverse technique based on conjugate gradient method (CGM) with an adjoint equation. A two-dimensional inverse forced convection hydrodynamically fully developed turbulent flow is considered. The simulations are performed with temperature data measured in the experimental test performed on a wind tunnel. The results show that the present numerical model with CGM is robust and accurate enough to estimate the strength and position of boundary heat flux.

Parwani, Ajit K.; Talukdar, Prabal; Subbarao, P. M. V.

2014-09-01

44

Experimental and numerical study of steady forced-convection heat transfer in a spherical annulus

International Nuclear Information System (INIS)

The results of a combined experimental and numerical study of steady forced-convection heat transfer in a spherical annulus with 500C heated water flowing in the annulus, an insulated outer sphere, and a 00C cooled inner sphere are presented. The inner sphere radius is 139.7 mm, the outer sphere radius is 168.3 mm and the radius ratio is 1.2. Measurements of inner sphere heat- flux rate distribution, flow separation angle, annulus fluid temperatures and total heat transfer were made for gap Reynolds numbers from 41 to 1086. The angle of separation along the inner sphere was found to vary as a function of Reynolds number. Measured total Nusselt numbers agreed with results reported in the literature to within 2.0% at a gap Reynolds number of 974 and 26.0% at a gap Reynolds number of 66. Results of numerical calculations of laminar incompressible fluid flow and heat transfer in a spherical annulus are presented. Velocity, temperature, pressure and heat-flux rate distributions are presented for gap Reynolds numbers from 4.4 to 440. It is believed that this is the first experimental study of spherical annulus convective heat transfer in which inner sphere heat-fux distribution has been measured and in which sepration angle was measured by non-visual methods and found to vary as a function of Reynolds number. Also, the numerical analysis is the first solution of the full laminar Navier-Stokes equations for forced-convection heat transfer in a sphericaled-convection heat transfer in a spherical annulus. The computations predict well the experimental trends and qualitative aspects of the flow and heat transfer while underpredicting heat-flux rates by a factor of two or more. It is felt that a turbulence model is necessary to predict more accurately the experimental results

45

Mechanistic Multidimensional Modeling of Forced Convection Boiling Heat Transfer

Digital Repository Infrastructure Vision for European Research (DRIVER)

Due to the importance of boiling heat transfer in general, and boiling crisis in particular, for the analysis of operation and safety of both nuclear reactors and conventional thermal power systems, extensive efforts have been made in the past to develop a variety of methods and tools to evaluate the boiling heat transfer coefficient and to assess the onset of temperature excursion and critical heat flux (CHF) at various operating conditions of boiling channels. The objective of this paper is...

Podowski, Raf M.; Podowski, Michael Z.

2008-01-01

46

Conjugate forced convection and heat conduction in circular microchannels

International Nuclear Information System (INIS)

The effects of axial heat conduction in the solid walls of microchannels of circular cross-sections are analyzed here. A systematic approach is adopted, with the aim of pointing out the influence of geometrical parameters and of solid wall thermal conductivity on microchannel heat transfer. The reliability of a commonly adopted criterium, based on the so-called axial conduction number, to assess the relevance of axial heat conduction is also discussed. Numerical simulations concern the simultaneously developing laminar flow of a constant property fluid in microchannels of different length, wall thickness and wall material, heated with a uniform heat flux at the outer surface, for different values of the Reynolds number. Moreover, since often in experimental tests the two end sections of the microchannel wall are not perfectly insulated, the effects of heat losses through these sections are also considered. A hybrid finite element procedure, which implies the step-by-step solution of the parabolized momentum equations in the fluid domain, followed by the solution of the energy equation in the entire domain, corresponding to both the solid and the fluid parts, is used for the numerical simulations.

47

Conjugate forced convection and heat conduction in circular microchannels

Energy Technology Data Exchange (ETDEWEB)

The effects of axial heat conduction in the solid walls of microchannels of circular cross-sections are analyzed here. A systematic approach is adopted, with the aim of pointing out the influence of geometrical parameters and of solid wall thermal conductivity on microchannel heat transfer. The reliability of a commonly adopted criterium, based on the so-called axial conduction number, to assess the relevance of axial heat conduction is also discussed. Numerical simulations concern the simultaneously developing laminar flow of a constant property fluid in microchannels of different length, wall thickness and wall material, heated with a uniform heat flux at the outer surface, for different values of the Reynolds number. Moreover, since often in experimental tests the two end sections of the microchannel wall are not perfectly insulated, the effects of heat losses through these sections are also considered. A hybrid finite element procedure, which implies the step-by-step solution of the parabolized momentum equations in the fluid domain, followed by the solution of the energy equation in the entire domain, corresponding to both the solid and the fluid parts, is used for the numerical simulations.

Nonino, C. [Dipartimento di Energetica e Macchine, Universita degli Studi di Udine, Via delle Scienze 208, 33100 Udine (Italy)], E-mail: carlo.nonino@uniud.it; Savino, S.; Del Giudice, S.; Mansutti, L. [Dipartimento di Energetica e Macchine, Universita degli Studi di Udine, Via delle Scienze 208, 33100 Udine (Italy)

2009-10-15

48

Energy Technology Data Exchange (ETDEWEB)

The convective heat transfer characteristics in a periodic converging-diverging heat exchanger channel are investigated experimentally. Experiments were performed for Prandtl number 0.7, for corrugation angle of 30 . In order to determine the channel having the best performance, the channels also compared by considering the flow area goodness factor. (orig.)

Taymaz, I.; Islamoglu, Y. [Sakarya University, Mechanical Engineering Department, Adapazari (Turkey); Koc, I. [Air Force Academy, Department of Aeronautical Engineering, Istanbul (Turkey)

2008-08-15

49

Analysis of transient forced convective heat transfer by boundary layer approximation

International Nuclear Information System (INIS)

Transient heat transfer under forced convection with exponentially increasing heat flux is important associated with accident analysis for nuclear reactor etc.. Based on integral method (boundary layer approximation), transient heat transfer coefficients are obtained for a flat plate in non-boiling region. Approximate but simple and analytic correlations obtained in this analysis cover various flow regimes (laminar, turbulent) and wide range of Prandtl number of fluid. Results indicate that transient heat transfer coefficients increased with increasing heat flux increasing rate, velocity and subcooling. These transient heat transfer coefficients attain the asymptotic values. The ratio between asymptotic heat transfer coefficient and steady state one is correlated in term of dimensionless parameter including heat flux increasing rate. (author)

50

Forced convective heat transfer over ribs at various separation

Digital Repository Infrastructure Vision for European Research (DRIVER)

Flow over transverse ribs is a fundamental problem that has numerous applications in a range of scales from turbine cooling to urban roughness. It can be broadly divided into k-type or d-type flows exhibiting different characteristics. In this study, large-eddy simulation (LES) is used to examine the flows between two ribs at various separation and compare the local heat transfer coefficient (LHTC) on the cavity bottom. Flow instability initiates a dividing streamline at the leeward edge. In ...

Chung, Tnh; Liu, Ch

2012-01-01

51

Experimental and theoretical study on forced convection film boiling heat transfer

International Nuclear Information System (INIS)

Theoretical solutions of forced convection film boiling heat transfer from horizontal cylinders in saturated liquids were obtained based on a two-phase laminar boundary layer film boiling model. It was clarified that author's experimental data for the cylinders with the nondimensional diameters, D, of around 1.3 in water and in Freon-113 agreed with the values of theoretical numerical solutions based on the two-phase laminar boundary layer model with the smooth vapor-liquid interface except those for low flow velocities. A forced convection film boiling heat transfer correlation including the radiation contribution from the cylinders with various diameters in saturated and subcooled liquids was developed based on the two-phase laminar boundary layer film boiling model and the experimental data for water and Freon-113 at wide ranges of flow velocities, surface superheats, system pressures and cylinder diameters. (author)

52

In this article, using Gibson-Ashby constitutive model, we suggest a new method for numerical investigation of forced convection heat transfer in porous foam metal, and try to consolidate the study for mechanical property and that for thermal characteristic. By available experimental data, we simulated to two cases, namely as the transfer in porous media for diameter is 0.6 mm and porosity is 0.402, and for diameter is 1.6 mm and porosity is 0.462. The result, from our constitutive model for single forced convection heat transfer, corresponds well with the experimental data. As for pressure drop prediction in porous is in good agreement with experiment, and the error is only 5% to 10%, but for transfer is less accurate, the error is about 20%, which is acceptable in practice. So it is done that constitutive model is used to simulate the transfer property.

Wang, J. X.; Jia, P. Y.; Wang, Y. S.; Jiang, L.

2010-03-01

53

Heat transfer tests under forced convection conditions with high wettable heater surface

Energy Technology Data Exchange (ETDEWEB)

Under forced convection and atmospheric pressure conditions, heat transfer tests were performed using the annulus channel of a heater rod with highly wettable surface. Improvement of boiling heat transfer requires that the cooling liquid can contact the heating surface, or a high-wettability heating surface, even if a vapor bubble layer is generated on the surface. >From this point of view, high-wettable heating surface was studied. As oxide semiconductor-coated materials are highly-wettable, we made a TiO{sub 2} coated heater rod. TiO{sub 2} coated surface has a high-wettability, in terms of contact angle and Leidenfrost temperature. The boiling curve was measured with and without TiO coated surface. The results showed difference between with and without TiO{sub 2} coating. TiO{sub 2} coating rod showed lower boiling onset heat flux, wider nucleate boiling region and higher critical heat flux than without coating. In summary, high wettablity heater surface produced higher boiling heat transfer characteristics under forced convection conditions. (author)

Mitsutake, Toru; Morooka, Shin-ichi; Miura, Shigeru; Akiba, Miyuki; Sato, Hisaki; Shirakawa, Ken-etsu; Oosato, Tetsuo; Yamamoto, Seiji [Toshiba Co., Kanagawa (Japan)

2002-07-01

54

Heat transfer tests under forced convection conditions with high wettable heater surface

International Nuclear Information System (INIS)

Under forced convection and atmospheric pressure conditions, heat transfer tests were performed using the annulus channel of a heater rod with highly wettable surface. Improvement of boiling heat transfer requires that the cooling liquid can contact the heating surface, or a high-wettability heating surface, even if a vapor bubble layer is generated on the surface. From this point of view, high-wettable heating surface was studied. As oxide semiconductor-coated materials are highly-wettable, we made a TiO2 coated heater rod. TiO2 coated surface has a high-wettability, in terms of contact angle and Leidenfrost temperature. The boiling curve was measured with and without TiO coated surface. The results showed difference between with and without TiO2 coating. TiO2 coating rod showed lower boiling onset heat flux, wider nucleate boiling region and higher critical heat flux than without coating. In summary, high wettablity heater surface produced higher boiling heat transfer characteristics under forced convection conditions. (author)

55

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

56

Digital Repository Infrastructure Vision for European Research (DRIVER)

This paper presents a three-dimensional geometric optimisation of cooling channels in forced convection of a vascularised material with the localised self-cooling property subjected to a heat flux. A square configuration was studied with different porosities. Analytical and numerical solutions were provided. The geometrical configuration was optimised in such a way that the peak temperature was minimised at every point in the solid body. The optimisation was subject to the constra...

Olakoyejo, O. T.; Bello-ochende, Tunde; Meyer, Josua P.

2012-01-01

57

SHAMT, Steady-State Forced Convection Heat Transfer

International Nuclear Information System (INIS)

1 - Description of program or function: SHAMT solves the transient finite-difference equations of mass, momentum, and energy conservation in spherical coordinates for laminar, incompressible, axially-symmetric flow. Computed velocity, temperature, pressure, and heat-flux rate distributions are presented for gap Reynolds numbers from 4.4 to 440. 2 - Method of solution: At each time increment, the momentum equations are solved explicitly for the radial and azimuthal velocity components, and these velocities are iteratively adjusted along with pressure until mass conservation is satisfied. Temperatures are then calculated explicitly from the energy conservation equation which is coupled to the momentum equation by the fluid velocity. Steady results are obtained by allowing the transient solution to reach steady state

58

Boiling and bubbling heat transfer under the conditions of free and forced convection

International Nuclear Information System (INIS)

The present paper gives the main results of studying the hydrogasdynamic aspects of heat transfer and blow-off effect at the developed nucleate boiling by analogy with bubbling under the conditions of free convection. For the first time it proved possible to detect the explicit influence of gas (vapour) phase compressibility on the heat transfer intensity and the effect of liquid blow-off from the surface of heating (bubbling). The existence of regimes automodel with respect to liquid phase viscosity has been found. A description is given of the first forced-circulation loop to study heat transfer and blow-off effect at forced gas-liquid flow in tubes and channels

59

Forced convective subcooled nucleate boiling heat transfer in upflow and downflow

International Nuclear Information System (INIS)

This paper reports on forced convective subcooled nucleate boiling curves in a circular tube (L/D = 44) from ONB to DNB that were obtained experimentally for both upflow and downflow of water by use of a heat-flux- controlled system. The system pressure was changed from 0.1 to 1.0 MPa, mass flux from 25 to 200 Kg/m's and inlet subcooling from 5 to 30 K. The experimental results shows that the effects of system pressure, mass flux, inlet subcooling and flow direction on boiling heat transfer varies in different boiling regions, such as, partial nucleate boiling, fully developed nucleate boiling and high flux nucleate boiling (near DNB). The relation between heat flux and wall superheat is not a simple exponential function. At high heat flux region (near DNB), although the heat flux increases with increasing wall superheat, the increasing rate of heat flux become slow with increasing wall superheat

60

International Nuclear Information System (INIS)

For development of new reactor, supercritical water is expected to be used as coolant to improve thermal efficiency. However, the thermal characteristics of supercritical fluid is not revealed completely because its difficulty for experiment. Specific phenomena tend to occur near the pseudo-boiling point which is characterised by temperature corresponding to the saturation point in ordinary fluid. Around this point, the physic properties such as density, specific heat and thermal conductivity are drastically varying. Although there is no difference between gas and liquid phases in supercritical fluids, phenomena similar to boiling (with heat transfer deterioration) can be observed round the pseudo-boiling point. Experiments of heat transfer have been done for supercritical fluid in forced convective condition. However, these experiments were mainly realised inside stainless steel cylinder pipes, for which flow visualisation is difficult. Consequently, this work has been devoted to the development of method allowing the visualisation of supercritical flows. The experiment setup is composed of main loop and test section for the visualisation. Carbon dioxide is used as test fluid. Supercritical carbon dioxide flows upward in rectangular channel and heated by one-side wall to generate forced convection heat transfer. Through window at mid-height of the test section, shadowgraphy was applied to visualize density gradient distribution. The behavior of the density wave in the channel is visualized and examined through the variation of the heat transfer coefficient. (author)

61

The effect of fouling on thermodynamic performance of forced convective heat transfer through a duct

International Nuclear Information System (INIS)

Based on the first and second thermodynamic laws, a new systematic approach to study in detail the effect of fouling on the thermodynamic performance of forced convective heat transfer through a duct with constant wall temperature and constant wall heat flux for thermally and hydrodynamically fully developed turbulent flow is investigated. When considering fouling exists inside the duct, the local and mean exergy variation coefficient, exergy variation flux, dimensionless exergy variation number and the equation of exergy variation rate of working fluids, etc. have been put forward and their generalized expressions derived. A criterion evaluating the effect of fouling on the exergy variation of working fluids of the forced convective heat transfer process, which is defined as the exergy variation degradation rate, has been put forward. By reference to a duct, the numerical results of the exergy variation of working fluids are obtained (considering fouling or not), the effects of Reynolds number, the thickness of the fouling layer, dimensionless inlet temperature difference and wall heat flux on the exergy variation of working fluids are discussed. The results show that the exergy variation degradation rate increases with the increase of Reynolds number and decreases with the increase of dimensionless inlet temperature and wall heat flux. The exergy variation caused by the heat conduction of fouling plays an important role in the total exergy variation of working fluidhe total exergy variation of working fluids

62

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 aro...

Mohanraj, M.; Chandrasekar, P.

2009-01-01

63

Laminar film condensation under forced convection on surfaces with random variable heat flux

International Nuclear Information System (INIS)

In order to study heat transfer in the condensation process under the forced convection, an analysis for an arbitrary wall-boundary flow being a function of distance X from the front edge of the wall is made. The only assumption is made that this function is continuously differentiable by X. An assumption is also made that the condensation process is determined by a flux in the liquid layer and it is not limited by steam delivery to the interface border. On the basis of functions obtained, a study is made of a number of specific conditions-power and exponential variation of the heat flux

64

Experimental study on forced convection boiling heat transfer on molten alloy

International Nuclear Information System (INIS)

In order to clarify the characteristics of forced convection boiling heat transfer on molten metal, basic experiments have been carried out with subcooled water flowing on molten Wood's alloy pool surface. In these experiments, water flows horizontally in a rectangular duct. A cavity filled with Wood's alloy is present in a portion of the bottom of the duct. Wood's alloy is heated by a copper conductor at the bottom of the cavity. The experiments have been carried out with various velocities and subcoolings of water, and temperature of Wood's alloy. Boiling curves on the molten alloy surface were obtained and compared with that on a solid heat transfer surface. It is observed that the boiling curve on molten alloy is in a lower superheat region than the boiling curve on a solid surface. This indicates that the heat transfer performance of forced convection boiling on molten alloy is enhanced by increase of the heat transfer area, due to oscillation of the surface and fragmentation of molten alloy

65

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

66

A study of forced convective boiling heat transfer under power transients

International Nuclear Information System (INIS)

Experimental investigation has been carried out on forced convective boiling heat transfer under exponentially increasing heat generation rate using Platinum wire heater located parrallel to flow direction. Effects of inlet velocity, subcooling, pressure, exponential period and heater geometry on the transient boiling have been examined. Two types of transient boiling are observed. In one type, the transient boiling curve coincides with steady-state boiling curve and/or its extrapolation (A-type) while in the other type the transient boiling curve does not show the coincidence (B-type). Transient maximum heat flux increases with increasing velocity, subcooling, pressure and with decreasing exponential period for A-type boiling. Finally, the experimental correlation has been obtained for the transient maximum heat flux. (author)

67

Forced convection film boiling drag and heat transfer of a wedge

Energy Technology Data Exchange (ETDEWEB)

Laminar forced convection film boiling flow on a wedge is analyzed considering the streamwise pressure gradient imposed on the flow and the streamwise buoyancy force (important because of the large density difference between the vapor and liquid) acting on the vapor film. A two-phase boundary layer model is proposed, and the local similarity concept is applied to obtain an approximate solution of the governing equations. Parametric trends in this analysis show that, for a water-steam system at atmospheric pressure considered within this study, the density difference between the vapor and liquid is large enough. As a result, both the streamwise pressure gradient and the buoyancy force exert strong influence on the vapor flow dynamics. Wall skin friction results display a strong dependency on the streamwise buoyancy force driving the vapor film and the external pressure gradient. Previously observed skin friction bucket'' type phenomena with increased heating of the wedge are possible when the buoyancy force is small or negligible. Adverse streamwise buoyancy force action on the vapor film, which is the case on the lower surface of a horizontally aligned wedge, may cause vapor flow separation. In contrast to wall skin friction dependency, the wall heat-transfer parameter shows a secondary dependence on the streamwise pressure gradient and the buoyancy force. 16 refs., 9 figs., 3 tabs.

Chappidi, P.R.; Pasamehmetoglu, K.O. (Los Alamos National Lab., NM (USA)); Gunnerson, F.S. (University of Central Florida, Orlando, FL (USA). Dept. of Mechanical Engineering)

1990-01-01

68

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; Sundén, Bengt

1998-01-01

69

Combined free and forced convection heat transfer in magneto fluid mechanic pipe flow

International Nuclear Information System (INIS)

A study is made of fully developed, laminar, free-and-forced convection heat transfer in an electrically conducting fluid flowing in an electrically insulated, horizontal, circular pipe in a vertical transverse magnetic field. The normalized magnetofluidmechanic and energy equations are reduced to three coupled partial differential equations by the introduction of a stream function of the secondary flow. A perturbation solution is generated in inverse powers of the Lykoudis number, Ly = M2/?Gr, which yields the influence of the magnetic field on the stream function of the secondary flow, axial velocity profiles, temperature profiles, and Nusselt number. 6 figures, 1 table

70

Critical heat flux of forced convection boiling in an oscillating acceleration field. Pt. 2

International Nuclear Information System (INIS)

Critical heat flux (CHF) with forced convection generally decreases under an oscillating acceleration condition. Contribution of flow oscillation on the decrease in CHF was investigated experimentally and theoretically. The experiments were performed with a Freon-113 boiling loop. The results showed that, in the high exit quality region deteriotation of CHF could almost wholly be attributed to the variation of inlet flow rate induced by motion. The amplitude of flow oscillation for a given acceleration variation could be predicted from a transfer function derived with linearization technique. Prediction of the transient CHF on the basis of 'local-conditions hypothesis' gave conservative values. (orig.)

71

Forced convection heat transfer from an equilateral triangular cylinder at low Reynolds numbers

An unsteady two-dimensional numerical simulation is performed to investigate the forced convection heat transfer for flow past a long heated equilateral triangular cylinder in an unconfined medium for the low Reynolds number laminar regime. The Reynolds number considered in this study ranges from 50 to 250 with three different values of Prandtl number ( Pr = 0.71, 7 and 100). Fictitious confining boundaries are chosen on the lateral sides of the computational domain that makes the blockage ratio ? = 5 % in order to make the problem computationally feasible. An unstructured triangular mesh is used for the computational domain discretization and the simulation is carried out with the commercial CFD solver Fluent. The flow and heat transfer characteristics are analyzed with the streamline and isotherm patterns at various Reynolds numbers. The dimensionless frequency of vortex shedding (Strouhal number), drag coefficient and Nusselt numbers are presented and discussed. The results obtained are in good agreement with the available results in the literature.

Chatterjee, Dipankar; Mondal, Bittagopal

2012-09-01

72

Numerical study of forced convection in a vertical channel filled with heat-generating porous medium

International Nuclear Information System (INIS)

Steady laminar non-Darcian forced convection in a vertical channel filled with heat-generating porous medium is studied numerically by using the local thermal non-equilibrium model. The heat source generated by solid framework is uniform and kept constant; and the temperature of vertical walls is kept at constant temperature T0. The flow inside porous medium is modelled by using Forchheimer-Brinkman extended Darcy model. The effects of Reynolds number (0.5 ?Re ? 50), effective fluid-to-solid thermal conductivity ratio ?(0.001 ? ? ? 1.0)and Darcy number (10-3 ? Da ? 10-5) are analyzed in detail. It is found that, the effects of Re, ? and Da are remarkable; at low values of Re and ?, and at high value of Da, the effect of local thermal non-equilibrium is significant and the local thermal non-equilibrium model should be adopted for predicting the heat transfer characteristics exactly. (authors)

73

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)

74

Performance of a forced convection solar drier integrated with gravel as heat storage material

Energy Technology Data Exchange (ETDEWEB)

Sun drying is the most common method used in India to dry agricultural products such as grains, fruits and vegetables. The rate of drying depends on solar radiation, ambient temperature, wind velocity, relative humidity, initial moisture content, type of crops, crop absorptivity and mass product per unit exposed area. However, this method of spreading the crop in a thin layer on the ground has several disadvantages. This paper reported on a study that focused on developing a forced convection solar drier integrated with heat storage materials for drying various agricultural crops. The indirect forced convection solar drier, integrated with gravel as a sensible heat material, was used to dry pineapple slices under conditions similar to those found in Pollachi, India. The performance of the system was discussed along with the drying characteristics, drying rate, and specific moisture extraction rate. The results showed that the moisture content (wet basis) of pineapple was reduced from about 87.5 to 14.5 per cent (equilibrium moisture content) in about 29 hours in the bottom tray and 32 hours in the top tray. The thermal efficiency of the solar air heater was also reviewed. 9 refs., 5 figs.

Mohanraj, M. [Dr Mahalingam College of Engineering and Technology, Pollachi (India). Dept. of Mechanical Engineering; Chandrasekar, P. [Swinburne Univ. of Technology, Sarawak (Malaysia). School of Engineering Sciences

2009-07-01

75

Forced convection heat transfer of saturated liquid hydrogen in vertically-mounted heated pipes

Heat transfer from the inner side of vertically-mounted heated pipes to forced flow of saturated liquid hydrogen was measured with a quasi-steady increase of a heat generation rate for wide ranges of flow rate and saturated pressure. The tube heaters have lengths L of 100 mm and 167 mm with the diameter D of 4 mm and lengths of 150 mm and 250 mm with the diameter of 6 mm. The heat fluxes at departure from nucleate boiling (DNB) were higher for higher flow velocity, lower pressures and shorter L/D. The effect of L/D on the DNB heat flux was clarified. It is confirmed that our DNB correlation can describe the experimental data.

Tatsumoto, Hideki; Shirai, Yasuyuki; Shiotsu, Masahiro; Hata, Koichi; Naruo, Yoshihiro; Kobayasi, Hiroaki; Inatani, Yoshifumi

2014-01-01

76

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

77

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

78

International Nuclear Information System (INIS)

The present paper deals with the laminar forced convection in a parallel-plane channel, and is aimed to investigate the effect of conducting walls. On the external boundaries of the duct walls a thermal boundary condition is prescribed, such that the wall heat flux longitudinally varies with sinusoidal law. The local energy balance equation is written separately for the fluid and the solid regions, with reference to the fully developed regime, and then solved both analytically and numerically. Moreover, the local and average Nusselt numbers in a longitudinal period are evaluated. The average Nusselt number, if regarded as a function of the dimensionless pulsation, displays an interesting feature. In fact, for all the considered cases, it has a minimum, so that there exists a value of the dimensionless pulsation such that the heat exchange between the fluid and the solid wall is considerably inhibited. (authors)

79

Investigation of non-Darcian forced convection in an asymmetrically heated sintered porous channel

International Nuclear Information System (INIS)

A study of non-Darcian forced convection in an asymmetric heating sintered porous channel is carried out to investigate the feasibility of using this channel as a heat sink for high-performance forced air cooling in microelectronics. A volume-averaging technique is applied to obtain the macroscopic equations with the non-Darcian effects of no-slip boundary, flow inertia, and thermal dispersion. Local non-thermal-equilibrium is assumed between the solid and the fluid phases. The analysis reveals that the particle Reynolds number significantly affects the solid-to-fluid heat transfer coefficients. A wall function is introduced to model the transverse thermal dispersion process for the wall effect on the lateral mixing of fluid. The local heat transfer coefficient at the inlet is modeled by a modified impinging jet result, and the noninsulated thermal condition is considered at exit. The numerical results are found to be in good agreement with the experimental results in the ranges of 32 ? Red ? 428 and q = 0. 8 ? 3.2 w/cm2 for Pr = 0. 71. 19 refs., 6 figs., 1 tab

80

International Nuclear Information System (INIS)

The general correlation for the critical heat flux in forced convection boiling in vertical, uniformly heated round tubes is presented in dimensionless form, relating to the data of the experiment utilizing the media of water, R12, R144, R21, liquid nitrogen, liquid hydrogen and liquid helium. The analysis is to determine the critical heat flux without entrance subcool enthalpy, as the function of velocity, reference length, vaporizing latent heat, liquid density, vapor density, surface tension and gravity. The experimental values of the special function of critical heat flux are plotted in relation to the experimental values of the function of mass flow rate, and the theoretically calculated curves are also shown for these two functions in the wide range of mass flow rate, in each of four classified partial ranges, H, L, N and HP, divided according to the aspect ratios of test tubes as the parameter. The mass flow rates become smaller successively from HP, N, and H to L ranges. The plotted lines of those functions of critical heat flux have different values and inclinations, and the tolerances of experimental values also disperse in +-20%, +-10%, +-25% and +-10% in the H, L, N and HP ranges, respectively, in relation to the calculated lines. The experimental values of subcooling in the L, H and HP ranges are shown, and the dimensionless formulas concerning subcooling effect are derived. The behavior of critical heat flux in forced convection boiling in vertical, uniformly heated round tubes was evaluated by experimental data and dimensionless analysis, and reported to be regular as the result. (Nakai, Y.)

81

Evaluating a tobacco-curing oven using a forced-convection heat exchanger USCO — MADR

Directory of Open Access Journals (Sweden)

Full Text Available A traditional oven for curing tobacco leaves was redesigned (based on existing infrastructure; a forced-convection heat exchan- ger system was implemented in it which worked with coffee hulls as fuel. This oven (called a forced-convection tobacco leaf curing oven was evaluated during the harvesting season. It was found that temperature and relative humidity inside the furnace could be controlled with this assembly during the three stages involved in curing tobacco leaves. The equipment used performed excellently when using coffee hulls as fuel, having the following approximate consumption during curing: 8.92 kilograms per hour during the yellowing stage, 17.75 kilograms per hour during the leaf drying and color fixation phase and 19.29 kilograms per hour during the stem drying stage. Comparative analysis of the oven’s operating costs along with the proposed adjustments to be made to it would allow its implementation as a promising alternative in the existing tobacco chain.

Néstor Enrique Cerquera Peña

2010-05-01

82

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

83

Analysis of forced convection heat transfer from a circular cylinder embedded in a porous medium

International Nuclear Information System (INIS)

Time-dependent forced convection heat transfer from a single circular cylinder embedded in a horizontal packed bed of spherical particles under local thermal non-equilibrium condition is investigated numerically using the spectral-element method. The non-Darcian effects, i.e. inclusion of the effect of solid boundaries and inertia forces, and the effect of thermal dispersion, are taken into account. The influences of the presence of the porous material and its thermal properties: solid-to-fluid thermal conductivity ratio kr included in [0:01; 1000] and Biot number Bi included in [0:01; 100], on the rates of heat transfer and the hydrodynamic and thermal responses, are examined for the Reynolds number range ReD included in [1; 250]. These effects are quantified. Perhaps not surprisingly, the results show that the presence of the porous particles suppresses significantly the wakes behind the cylinder and enhances considerably the heat transfer. A comparison that is made between the one- and two-equation energy model predictions shows that the former model predicts a continuous increase in Nuf against kr; however, the trend of Nuf with kr, for kr > 10, is governed entirely by Bi when the latter model is used. Also, the increase in Bi decreases Nuf and increases Nus, and high values of kr or Bi lead to establishing a thermal equilibrium status in the porous bed. (al equilibrium status in the porous bed. (authors)

84

Conjugate forced convection heat transfer in a plane channel: Longitudinally periodic regime

International Nuclear Information System (INIS)

The present paper studies the conjugate heat transfer problem in a parallel-plane channel. Laminar and stationary forced convection is studied, with a boundary condition given by a temperature distribution on the external side of the channel wall, which undergoes a sinusoidal longitudinal change. The local energy balance equation is written with reference to the fully developed region, where the temperature distribution can be expressed as a periodic function of the longitudinal coordinate. The temperature field in the solid wall and in the fluid, as well as the local and average Nusselt number, are determined analytically and numerically. A comparison between the values obtained analytically, by employing a complex temperature method, and those evaluated numerically, by employing a Bubnov-Galerkin finite element method, reveals an excellent agreement. (authors)

85

Directory of Open Access Journals (Sweden)

Full Text Available Three dimensional simulations of the single-phase laminar flow and forced convective heat transfer of water in round tube and straight microchannel heat exchangers were investigated numerically. This numerical method was developed to measure heat transfer parameters of round tube and straight microchannel tube geometries. Then, similarities and differences were compared between different geometries. The geometries and operating conditions of those indicated heat exchangers were created using a finite volume-based computational fluid dynamics technique. In this article, at each Z-location variation of dimensionless local temperature, nondimensional local heat flux variation and dimensionless local Nusselt number distribution along the tube length were compared between round tube and straight microchannel heat exchangers. Consequently, averaged computational Nusselt number was obtained for those indicated models and then validation study was performed for round tube counter flow type heat exchanger model. Finally, all of these numerical results for both kind of geometries in counter flow heat exchangers were discussed in details.

P. MOHAJERI KHAMENEH,

2010-11-01

86

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

87

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

88

Convective Heat Transfer with Nanofluids

Digital Repository Infrastructure Vision for European Research (DRIVER)

The present Master thesis is concerned with forced convection heat transfer in laminar and turbulent flowwith nanofluids. Nanofluids are defined as a colloidal suspension of particles in a base fluid, where theparticles have a characteristic length of less than 100 nm. Experiments were conducted to determine thequalification of nanofluids for laminar and turbulent flow forced convection heat transfer. Theexperiments were conducted in two different devices: Firstly, a stainless steel pipe with...

Stro?der, Simon

2011-01-01

89

Laminar forced convection heat transfer to a single layer of ordered and disordered spheres

We study laminar forced convection heat transfer to single layer arrays of equidistantly and non-equidistantly spaced spheres. We report average Nusselt numbers as a function of geometry and flow conditions, for open frontal area fractions between 0.04 and 0.95, Prandtl numbers between 0.7 and 10, and Reynolds numbers (based on sphere diameter and the free stream velocity) between 0.1 and 100. For equidistantly spaced arrays of spheres we propose a general analytical expression for the average Nusselt number as a function of the Reynolds number, Prandtl number and the open frontal area fraction, as well as asymptotic scaling rules for small and large Reynolds. For all studied Prandtl numbers, equidistant arrays exhibit decreasing average Nusselt numbers for decreasing open frontal area fractions at low Reynolds numbers. For high Reynolds numbers, the Nusselt number approaches that of a single spheres in cross-flow, independent of the open frontal area fraction. For equal open frontal area fractions, the Nusselt number in non-equidistant arrays is lower than in equidistant arrays for intermediate Reynolds numbers. For very low and high Reynolds numbers, non-uniformity does not influence heat transfer.

Ambesi, D.; Kleijn, C. R.

2012-11-01

90

Laminar forced convection heat transfer to a single layer of ordered and disordered spheres

International Nuclear Information System (INIS)

We study laminar forced convection heat transfer to single layer arrays of equidistantly and non-equidistantly spaced spheres. We report average Nusselt numbers as a function of geometry and flow conditions, for open frontal area fractions between 0.04 and 0.95, Prandtl numbers between 0.7 and 10, and Reynolds numbers (based on sphere diameter and the free stream velocity) between 0.1 and 100. For equidistantly spaced arrays of spheres we propose a general analytical expression for the average Nusselt number as a function of the Reynolds number, Prandtl number and the open frontal area fraction, as well as asymptotic scaling rules for small and large Reynolds. For all studied Prandtl numbers, equidistant arrays exhibit decreasing average Nusselt numbers for decreasing open frontal area fractions at low Reynolds numbers. For high Reynolds numbers, the Nusselt number approaches that of a single spheres in cross-flow, independent of the open frontal area fraction. For equal open frontal area fractions, the Nusselt number in non-equidistant arrays is lower than in equidistant arrays for intermediate Reynolds numbers. For very low and high Reynolds numbers, non-uniformity does not influence heat transfer.

91

International Nuclear Information System (INIS)

This paper presents the unsteady laminar forced convection heat transfer from a row of five isothermal square cylinders placed in a side-by-side arrangement at a Reynolds number of 150. The numerical simulations are performed using a finite volume code based on the PISO algorithm in a collocated grid system. Special attention is paid to investigate the effect of the spacing between the cylinders on the overall transport processes for the separation ratios (spacing to size ratio) between 0.2 and 10. No significant interaction between the wakes is observed for spacing greater than four times the diameter at this Reynolds number. However, at smaller spacing, the wakes interact in a complicated manner resulting different thermo-hydrodynamic regimes. The vortex structures and isotherm patterns obtained are systematically presented and discussed for different separation ratios. In addition, the mean and instantaneous drag and lift coefficients, mean and local Nusselt number and Strouhal number are determined and discussed for various separation ratios. A new correlation is derived for mean Nusselt number as a function of separation ratio for such flows.

92

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

93

International Nuclear Information System (INIS)

This paper presents a study of the Dual Reciprocity Boundary Element Method (DRBEM) for the laminar heat convection problem in a concentric annulus with constant heat flux boundary condition. DRBEM is one of the most successful technique used to transform the domain integrals arising from the nonhomogeneous term of the poisson equation into equivalent boundary only integrals. This recently developed and highly efficient numerical method is tested for the solution accuracy of the fluid flow and heat transfer study in a concentric annulus. Since their exact solutions are available, DRBEM solutions are verified with different number of boundary element discretization and internal points. The results obtained in this study are discussed with the relative error percentage of velocity and temperature solutions, and potential applicability of the method for the more complicated heat convection problems with arbitrary duct geometries

94

Laminar forced convective/conductive heat transfer by finite element method

International Nuclear Information System (INIS)

The present study is directed at developing a finite element computer program for solution of decoupled convective/conductive heat transfer problems. Penalty function formulation has been used to solve momentum equations and subsequently transient energy equation is solved using modified Crank-Nicolson method. The optimal upwinding scheme has been employed in energy equation to remove oscillations at high Peclet number. (author)

95

In this article a semi-analytical approach is employed to obtain dimensionless heat transfer correlations for forced convection from isothermal circular cylinders with active ends and different aspect ratios (l/d le 8) in laminar axial air flows. Then, using the present results and previous works, the modeling is extended to higher aspect ratios (l/d ge 8) ) as long as the entire flow field remains completely laminar. Validations of the present work are done not only with the available data on drag coefficients but with previous works for long cylinders with inactive ends and long spheroids. Two general correlations are also developed for a rough estimate of forced convection heat transfer from isothermal cylinders with active ends and arbitrary aspect ratios in the range of 1/2 le l/d le 8 and l/d ge 8 .

Hadad, Yaser; Jafarpur, Khosrow

2011-01-01

96

Forced convective heat transfer and pressure drop of a randomly packed HTGR core

International Nuclear Information System (INIS)

Heat and mass transfer have been measured for a randomly packed spherical pebble bed in the range of Reynolds number 30 4. The Prandtl number was Pr = 0.71. For the mass transfer measurements the method of naphthalene sublimation in air (Schmidt number Sc = 2.53) was applied. Effects of free convection, of temperature radiation, and of heat flux due to punctual contact of neighboring spheres are studied. Furthermore the wall effects on heat transfer are considered. For 3 x 102 4 the pressure drop coefficient has been measured. The importance of exact knowledge about the void fraction and its effect on the pressure drop is pointed out

97

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

98

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.

2005-03-01

99

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

100

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.

101

Blade-to-coolant convective heat-transfer coefficients were obtained on a forced-convection water-cooled single-stage turbine over a large laminar flow range and over a portion of the transition range between laminar and turbulent flow. The convective coefficients were correlated by the general relation for forced-convection heat transfer with laminar flow. Natural-convection heat transfer was negligible for this turbine over the Grashof number range investigated. Comparison of turbine data with stationary tube data for the laminar flow of heated liquids showed good agreement. Calculated average midspan blade temperatures using theoretical gas-to-blade coefficients and blade-to-coolant coefficients from stationary-tube data resulted in close agreement with experimental data.

Freche, John C; Schum, Eugene F

1951-01-01

102

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

103

International Nuclear Information System (INIS)

Laminar flow and heat transfer in a cylindrical floating zone are computed for iso- and counter rotation of the feed rod and crystal in the absence of natural and Marangoni convection. For differential iso-rotation at high rotational Reynolds numbers, the velocity field exhibits a core region rotating with the average angular velocity of the rotating ends. For equal counter-rotation, the solution loses symmetry about the midplane at a critical Reynolds number; beyond this, multiple asymmetric solutions are to be found. Local and overall Nusselt numbers are computed for a range of Prandtl and Biot numbers. The computed isotherm patterns agree qualitatively with experiment

104

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)

105

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 ? equation. The secondary flow generation using the RSM model is compared with a non-linear k-? 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; Sundén, Bengt

1997-01-01

106

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 odel is a simple but effective method for safety evaluation of transient heated wall temperature rise caused by rapid decrease in flow rate. (author)

107

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

108

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)

109

In this paper, laminar convective heat transfer of water-alumina nanofluid in a circular tube with uniform heat flux at the tube wall is investigated. The investigation is performed numerically on the basis of two-component model, which takes into account nanoparticle transport by diffusion and thermophoresis. Two thermal regimes at the tube wall, heating and cooling, are considered and the influence of nanoparticle migration on the heat transfer is analyzed comparatively. The intensity of thermophoresis is characterized by a new empirical model for thermophoretic mobility. It is shown that the nanoparticle volume fraction decreases (increases) in the boundary layer near the wall under heating (cooling) due to thermophoresis. The corresponding variations of nanofluid properties and flow characteristics are presented and discussed. The intensity of heat transfer for the model with thermophoresis in comparison to the model without thermophoresis is studied by plotting the dependence of the heat transfer coefficient on the Peclet number. The effectiveness of water-alumina nanofluid is analyzed by plotting the average heat transfer coefficient against the required pumping power. The analysis of the results reveals that the water-alumina nanofluid shows better performance in the heating regime than in the cooling regime due to thermophoretic effect. PMID:25260328

Kozlova, Sofya V; Ryzhkov, Ilya I

2014-09-01

110

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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

111

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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

112

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This paper reports on the augmentation effect of electrohydrodynamically (EHD) induced flow disturbance on forced-convection heat transfer in a channel that was experimentally investigated in order to determine the applicability of the enhanced heat transfer into a low- pressure drop heat exchanger, such as a high-performance oil cooler. The investigation is mainly based on the study carried out on the unique point where the flow is disturbed actively and controllably by applying electric fields between the wall and array of wire electrodes installed near the wall along the main stream. The liquid mixture of refrigerant R113 (96 wt %) and ethanol (4 wt %), called Fronsorubu AE, was selected as a working fluid. Heat transfer was found to be promoted intensely in the turbulent flow as well as in the laminar flow, up to a factor of about twenty-three in the case of laminar flow. It is noteworthy that the rate of increase in heat transfer coefficient is larger compared to that in the pressure drop. From a measurement of velocities by a laser Doppler velocimeter, it was made clear that the electrohydrodynamically induced flow disturbance brings about large heat transfer coefficients

113

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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

114

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Variation of forced convective heat transfer in a rectangular duct flow of a magnetic fluid under a magnetic field was investigated experimentally. Experiments were performed changing the magnetic field intensity, and this magnetic field could be varied from 0 mT to 600 mT. The Reynolds number based on the hydraulic diameter was set to 960, 1900 (laminar flow), and 2830 (turbulent flow). The results of the experiments show that in the case of laminar flow of the magnetic fluid, when a magnetic field is applied to a magnetic fluid flow, heat transfer locally increases in the region where the magnetic field exists. In contrast, in the turbulent flow of the magnetic fluid, heat transfer is not enhanced but reduced. In order to better understand this heat transfer phenomenon, we measured the velocity distribution of magnetic fluid flow by the Ultrasonic Velocity Profile (UVP) method. In the case of laminar flow, the result shows that the flow velocity at the center of the rectangular duct decreases and the velocity gradient in the near-wall region increases. Moreover, we calculated the flow resistance under a magnetic field by measurement of the pressure gradient, and the relationship between heat transfer and flow resistance was discussed.

115

Variation of forced convective heat transfer in a rectangular duct flow of a magnetic fluid under a magnetic field was investigated experimentally. Experiments were performed changing the magnetic field intensity, and this magnetic field could be varied from 0 mT to 600 mT. The Reynolds number based on the hydraulic diameter was set to 960, 1900 (laminar flow), and 2830 (turbulent flow). The results of the experiments show that in the case of laminar flow of the magnetic fluid, when a magnetic field is applied to a magnetic fluid flow, heat transfer locally increases in the region where the magnetic field exists. In contrast, in the turbulent flow of the magnetic fluid, heat transfer is not enhanced but reduced. In order to better understand this heat transfer phenomenon, we measured the velocity distribution of magnetic fluid flow by the Ultrasonic Velocity Profile (UVP) method. In the case of laminar flow, the result shows that the flow velocity at the center of the rectangular duct decreases and the velocity gradient in the near-wall region increases. Moreover, we calculated the flow resistance under a magnetic field by measurement of the pressure gradient, and the relationship between heat transfer and flow resistance was discussed.

Motozawa, M.; Sekine, T.; Sawada, T.; Kawaguchi, Y.

2013-02-01

116

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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)

117

Convection heat transfer is an important topic both for industrial applications and fundamental aspects. It combines the complexity of the flow dynamics and of the active or passive scalar transport process. It is part of many university courses such as Mechanical, Aeronautical, Chemical and Biomechanical Engineering. The literature on convective heat transfer is large, but the present manuscript differs in many aspects from the existing ones, particularly from the pedagogical point of view. Each chapter begins with a brief yet complete presentation of the related topic. This is followed by a

Favre-Marinet, Michel

2009-01-01

118

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An investigation of the thermal hydraulic characteristics in the passive residual heat removal system of the System integrated Modular Advanced ReacTor-P (SMART-P) has been carried out using the MARS code, which is a best estimate system analysis code. The SMART-P is designed to cool the system during accidental conditions by a natural convection. The dominant heat transfer in the steam generator is a boiling mode under a forced convection condition, and it is a single-phase liquid and a boiling heat transfer under a natural convection condition. Most of the heat is removed in the heat exchanger of the passive residual heat removal system by a condensation heat transfer. The passive residual heat removal system can remove the energy from the primary side as long as the heat exchanger is submerged in the refueling water tank. The mass flow is stable under a natural circulation condition though it oscillates periodically with a small amplitude. The parameter study is performed by considering the effects of an effective height between the steam generator and the heat exchanger, a hydraulic resistance, an initial pressure, a non-condensable gas fraction in the compensating tank, and a valve actuation time, which are useful for the design of the passive residual heat removal system. The mass flow in the passive residual heat removal system has been affected by the height between the steam generator and the heat exchanger, and the hydraulic resistance of the loopydraulic resistance of the loop

119

Forced convection heat transfer from two and three isothermal circular cylinders in tandem arrangement is studied numerically. In addition, the flow field and the vortex shedding behavior in the wake of the cylinders are investigated. The governing equations consist of continuity, momentum and energy equations are solved for laminar unsteady flow regime. The numerical simulations are performed with a developed finite volume code using the overset grid method. A general orthogonal boundary fitted coordinate system is used for the grid generation. This simulation is performed for the Prandtl numbers of 0.7 and 7 at the Reynolds numbers of 100 and 200. The spacing ratio L/D is set at 2, 3, 4, 5, 7 and 10. In order to analyze the heat transfer from isothermal cylinders, the mean and local Nusselt numbers and isotherm plots are presented and discussed for different values of the problem parameters. In addition, the mean and instantaneous drag and lift coefficients and Strouhal numbers are computed to elucidate the role of the Reynolds number and spacing ratio. Furthermore, two new correlations for the calculation of the mean Nusselt number, in terms of the spacing ratio and the Reynolds and Prandtl numbers, is proposed. In order to validate the solution, the obtained results are compared with available results in the published literature.

Harimi, Iman; Saghafian, Mohsen

2012-01-01

120

Transient forced convection heat transfer of helium gas at various pressures and temperatures

International Nuclear Information System (INIS)

In this research, to obtain a database of transient heat transfer process at wide experimental conditions for the safety assessment of HTGR, transient heat transfer coefficients for helium gas flowing over a horizontal plate (ribbon) were measured under various pressures and temperatures. The platinum plate with a thickness of 0.1 mm was used as test heater and heated by electric current. The heat generation rate was exponentially increased with a function of Q0exp(t/?). The pressures ranged from 400 kPa to 800 kPa, the gas temperatures ranged from 290 to 353 K, and the gas flow velocities ranged from 4 to 10 m/s, The periods of heat generation rate, ?, ranged from 50 ms to 17 s. The surface superheat and heat flux increase exponentially as the heat generation rate increases with the exponential function. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period ? longer than about 1 s, and it becomes higher for the period shorter than around 1 s. The heat transfer coefficient increases with the increases in pressure and velocity, and it shows some dependence on temperature at the experimental range of this research. Empirical correlations for quasi-steady-state heat transfer and transient one at various pressures were obtained based on the experimental data. (author)

121

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

122

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)

123

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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)

124

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

125

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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, athe 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.

126

An analytical approach for optimal design of heat sinks under forced convection

Saving energy is just as important as generating energy. In this paper, we seek an optimized structure that achieves a certain level of heat transfer rate under a minimum pumping power to drive the fluid stream. Constraints are specified by the flow regime (laminar and turbulent), admissible boundary conditions on the walls (prescribed temperature and constant heat flux), and design standards. The study will help designers with more effective basic tools for the conceptual design of system and in establishing proper operating procedures.

Miguel, Antonio F.

2013-06-01

127

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Mixed convection heat transfer in a ventilated cavity is numerically studied by solving the mixed convection equations with the Boussinesq approximation. Results are presented in terms of streamlines, isotherms and heat transfer for different combinations of the governing parameters namely, the Reynolds number (10 ? Re ? 5000), the Rayleigh number (104 ? Ra ? 106) and the relative height of the openings (B = h'/H' = 1/4). The numerical results show the presence of a maximum interaction between the effects of the forced and natural convection and the existence of different flow regimes. The latter are delineated in the Ra-Re plane and the values of Re separating the different regions are determined and correlated versus Ra

128

Energy Technology Data Exchange (ETDEWEB)

Mixed convection heat transfer in a ventilated cavity is numerically studied by solving the mixed convection equations with the Boussinesq approximation. Results are presented in terms of streamlines, isotherms and heat transfer for different combinations of the governing parameters namely, the Reynolds number (10 {<=} Re {<=} 5000), the Rayleigh number (10{sup 4} {<=} Ra {<=} 10{sup 6}) and the relative height of the openings (B = h'/H' = 1/4). The numerical results show the presence of a maximum interaction between the effects of the forced and natural convection and the existence of different flow regimes. The latter are delineated in the Ra-Re plane and the values of Re separating the different regions are determined and correlated versus Ra.

Raji, A. [Faculty of Sciences and Technologies, Department of Physics, University Sultan Moulay Slimane, Team of Flows and Transfers Modeling (EMET), Laboratory of Physics and Mechanics of Materials, B.P. 523, Beni-Mellal (Morocco)], E-mail: abderaji@fstbm.ac.ma; Hasnaoui, M. [Faculty of Sciences Semlalia, Department of Physics, University Cadi Ayyad, UFR TMF, B.P. 2390 Marrakesh (Morocco); Bahlaoui, A. [Faculty of Sciences and Technologies, Department of Physics, University Sultan Moulay Slimane, Team of Flows and Transfers Modeling (EMET), Laboratory of Physics and Mechanics of Materials, B.P. 523, Beni-Mellal (Morocco)

2008-08-15

129

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

130

Enhancement of forced-convection cooling of multiple heated blocks in a channel using porous covers

Energy Technology Data Exchange (ETDEWEB)

A numerical study was carried out for enhanced heat transfer from multiple heated blocks in a channel by porous covers. The flow field is governed by the Navier-Stokes equation in the fluid region, the Darcy-Brinkman-Forchheimer equation in the porous region, and the thermal field by the energy equation. Solution of the coupled governing equations is obtained using a stream function-vorticity analysis. This study details the effects of variations in the Darcy number, Reynolds number, inertial parameter, and two pertinent geometric parameters, to illustrate important fundamental and practical results. The results show that the recirculation caused by porous-covering block will significantly enhance the heat transfer rate on both top and right faces of second and subsequent blocks. (Author)

Huang, P.C.; Yang, C.F. [National Taipei Univ. of Technology, Taiwan (China). Dept. of Air Conditioning and Refrigerating Engineering; Hwang, J.J.; Chiu, M.T. [National Sun Yat Sen Univ., Kaohsiung, Taiwan (China). Dept. of Mechanical Engineering

2005-02-01

131

Sensitivity studies of heat transfer: forced convection across a cylindrical pipe and duct flow

We consider two common heat transfer processes and perform a through sensitivity study of the variables involved. We derive and discuss analytical formulas for the heat transfer coefficient in function of film velocity, air temperature and pipe diameter. The according plots relate to a qualitative analysis of the multi-variable function $h$, according to functional optimization. For each process, we provide with graphs and tables of the parameters of interest, such as the Reynolds number. This method of study and the specific values can constitute a useful reference for didactic purposes.

Ferrantelli, Andrea; Viljanen, Martti

2013-01-01

132

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mbined convection regime could be bounded by a suitable selection of Re number ranges and heat flux ranges. The obtained Richardson numbers (Ri) ranged approximately from 0.1171 to 7.125. The average Nusselt numbers were correlated with the Rayleigh numbers/Reynolds numbers

133

Supercritical droplet gasification experiments with forced convection

Preliminary results of a comprehensive experimental program are presented which offer the first direct observations of suspended n-heptane droplet gasifications in pure nitrogen with forced convection without the interference to optical probing associated with a flame. Measurements show attainment of a wet-bulb temperature until reduced pressures exceed about 1.0 under supercritical gas temperatures. Thereafter, temperature measurements indicate fully transient heat-up through the critical temperature. The surface is found to regress in a continuous manner with the measured temperature approaching the critical value at the end of the droplet lifetime under supercritical conditions with very mild level of convection. At increased level of convection for the same ambient conditions, similar sized droplets will undergo significant deformation during the gasification process until partially convected away as a dense vapor cloud as the critical temperature is approached.

Litchford, Ron; Parigger, Chris; Jeng, San-Mou

1992-01-01

134

A convective heat flow probe device is provided which measures heat flow and fluid flow magnitude in the formation surrounding a borehole. The probe comprises an elongate housing adapted to be lowered down into the borehole; a plurality of heaters extending along the probe for heating the formation surrounding the borehole; a plurality of temperature sensors arranged around the periphery of the probe for measuring the temperature of the surrounding formation after heating thereof by the heater elements. The temperature sensors and heater elements are mounted in a plurality of separate heater pads which are supported by the housing and which are adapted to be radially expanded into firm engagement with the walls of the borehole. The heat supplied by the heater elements and the temperatures measured by the temperature sensors are monitored and used in providing the desired measurements. The outer peripheral surfaces of the heater pads are configured as segments of a cylinder and form a full cylinder when taken together. A plurality of temperature sensors are located on each pad so as to extend along the length and across the width thereof, with a heating element being located in each pad beneath the temperature sensors. An expansion mechanism driven by a clamping motor provides expansion and retraction of the heater pads and expandable packer-type seals are provided along the probe above and below the heater pads.

Dunn, James C. (Albuquerque, NM); Hardee, Harry C. (Albuquerque, NM); Striker, Richard P. (Albuquerque, NM)

1985-01-01

135

Directory of Open Access Journals (Sweden)

Full Text Available The present work is an experimental study of steady state convective heat transfer of de-ionized water with a low volume fraction (0.003% by volume of copper oxide (CuO nanoparticles dispersed to form a nanofluid that flows through a copper tube. The effect of mass flow rate ranging from (0.0113 kg/s to 0.0139 kg/s and the effect of inlet temperatures at 100C and 17 0C on the heat transfer coefficient are studied on the entry region under laminar flow condition. The results have shown 8% enhancement of the convective heat transfer coefficient of the nanofluid even with a low volume concentration of CuO nanoparticles. The heat transfer enhancement was increased considerably as the Reynolds number increased. Possible reasons for the enhancement are discussed. Nanofluid thermo-physical properties and chaotic movement of ultrafine particles which accelerate the energy exchange process are proposed to be the main reasons for the observed heat transfer enhancement. A correlation for convective heat transfer coefficient of nanofluids, based on transport property and D/x for 8 mm tube has been evolved. The correlation predicts variation in the local Nusselt number along the flow direction of the nanofluid. A good agreement (±10% is seen between the experimental and predicted results.

Lazarus Godson Asirvatham

2009-03-01

136

International Nuclear Information System (INIS)

Degradation of the thermal performance of steam generators(SGs) is a serious problem in nuclear power stations throughout the world (Lovett and Dow, 1991). In pressurized-heavy-water reactors (PWHRs), the reduced thermal performance of the SGs is manifested by an increase of the primary coolant reactor inlet header temperature (RIHT). In pressurized-light-water reactors(PWRs), which operate with fixed primary coolant temperature, the loss of thermal performance is manifested by a reduction of the steam pressure. Degradation mechanisms that may contribute to the loss of SG thermal performance include: fouling of the boiler tube inner surfaces (primary-side fouling); fouling of the boiler tube outer surfaces (secondary-side fouling); divider and thermal plate leakage that causes the coolant to bypass either the SG or the integral preheater and fouling of the steam separators. The relative contribution of these various degradation mechanisms to the overall loss of thermal performance is still under investigation. Soulard et al. (1990) examined the relative contributions of tube bundle fouling, divider plate leakage, and thermal plate leakage to the increase in RIHT at the Point Lepreau Generating Station, and concluded that tube fouling contributes to a significant fraction of the loss of thermal performance. Corrosion products deposit on both the inner and outer surfaces of the boiler tubes. Thus a complete understanding of the reasons fro the loss of thermal performance and the development of strategies to mitigate this loss requires a knowledge of the thermal resistance of tube deposits under primary and secondary side heat transfer conditions. We present here the results of measurements of the thermal resistance of primary-side and secondary-side boiler tube deposits performed under single-phase forced convection and flow-boiling conditions, respectively. The results are discussed in terms of the physical properties of the deposit and the mode of heat transfer

137

Digital Repository Infrastructure Vision for European Research (DRIVER)

The last decade has witnessed very fast development in microfabrication technologies. The increasing industrial applications of microfluidic systems call for more intensive and systematic knowledge on this newly emerging field. Especially for gaseous flow and heat transfer at microscale, the applicability of conventional theories developed at macro scale is not yet completely validated; this is mainly due to scarce experimental data available in literature for gas flows. The objective of this...

Yang, Yahui

2013-01-01

138

International Nuclear Information System (INIS)

In this paper, results of an experimental investigation of mixed convection in laminar air flow across a tungsten wire are presented. Results are correlated by the 3-D surface equation of heat transfer, and compared with traditional equations for mixed convection which have been suggested by several investigators. Experimental results are given for Reynolds numbers in the range of 0.8 ? Re ? 17.5, Grashof numbers in the range of 1.5 · 10-3 ? Gr ? 25 and buoyancy parameter in the range of 6 · 10-5 ? Gr/Re2 ? 11.5

139

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

140

International Nuclear Information System (INIS)

A model describing the thermal behaviour of a slurry of phase change material flow in a circular duct is presented. Reactors connected in series are considered for the representation of the circular duct with constant wall temperature. A phenomenological equation is formulated to take account of the heat generation due to phase change in the particles. Results of the simulation present a plateau of temperature along the longitudinal direction, characteristic of the phase change. The effect of different parameters such as the Reynolds number, the weight fraction and the temperature of the cold spring on the length of the plateau is analysed. A correlation resulting from numerical results is proposed for use in the determination of the characteristics of the exchanger for a phase change material slurry

141

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)

142

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

143

Steady, three-dimensional, internally heated convection

International Nuclear Information System (INIS)

Numerical calculations have been carried out of steady, symmetric, three-dimensional modes of convection in internally heated, infinite Prandtl number, Boussinesq fluids at a Rayleigh number of 1.4x104 in a spherical shell with inner/outer radius of 0.55 and in a 3x3x1 rectangular box. Multiple patterns of convection occur in both geometries. In the Cartesian geometry the patterns are dominated by cylindrical cold downflows and a broad hot upwelling. In the spherical geometry the patterns consist of cylindrical cold downwellings centered either at the vertices of a tetrahedron or the centers of the faces of a cube. The cold downflow cylinders are immersed in a background of upwelling within which there are cylindrical hot concentrations (plumes) and hot halos around the downflows. The forced hot upflow return plumes of internally heated spherical convection are fundamentally different from the buoyancy-driven plumes of heated from below convection

144

Driving force for magnetospheric convection

International Nuclear Information System (INIS)

The most popular concept for the driving force for magnetospheric convection has been that merging of the interplanetary magnetic field with the geomagnetic field at the front of the magnetosphere establishes a good electrically conducting path between the solar wind and the polar ionosphere; the flow of electrical energy then provides the driving force. This model has had impressive success in predicting some phenomena, but it also has some serious defects. The alternative view for the driving force has been that a viscous interaction exists between the solar wind and the outer layer of the magnetosphere. Although mechanisms of viscous interaction have been recognized, they have proved quantitatively inadequate to provide the required driving force. This concept has experienced a recent resurgence of support based on observation, but the conceptual problem in understanding the magnitude of the effect remains. A third possibility suggested by recent observations is that solar wind plasma enters the magnetosphere rather readily around the neutral points, where the flow is turbulent and the geomagnetic field is weak, and then spreads throughout a boundary layer covering virtually the entire inner surface of the magnetopause. This plasma can expand and drive magnetospheric convection by moving tailward around the flanks of the magnetosphere until it escapes out the open tail. The picture is rather similar to the viscously driven magnetosphere, but the driving force is dimagnetosphere, but the driving force is different

145

International Nuclear Information System (INIS)

A method for determining the heat transfer characteristics in the thermal entrance region for a developed axial laminar flow through a circular shell containing a uniform-heat-flux rod bundle is presented. It utilizes a small parameter series solution, employing the Leveque similarity variable, and accounts for both the curvature of the rods and the circumferential non-linearity of the velocity profile. The solution provides a lower bound to the turbulent Nusselt moduli and an upper bound to the rod temperature variation. Nusselt moduli for the rod bundle of several configurations are reported and discussed. Results compare favorably with published solutions to annuli and experimental data for rod bundles. (U.S.)

146

International Nuclear Information System (INIS)

The differences in the single-phase forced-convection heat transfer characteristics between upflow and downflow were investigated experimentally with a narrow vertical rectangular channel. The objectives of the experiment were to investigate in both laminar and turbulent flow regions the applicability of existing correlations to and the effects of buoyant force on the heat transfer characteristics in the narrow vertical rectangular channel, which is simulating a subchannel of 2.25mm in gap and 750mm in length in the fuel element of the research reactor, JRR-3 to be upgraded at 20 MWt. As the results, it was revealed that (1) by use of equivalent hydraulic diameter, existing correlations are applicable to a channel as narrow as 2.25mm in gap for turbulent flow though the precision and critical Reynolds number are different among the correlations, and (2) in the laminar flow, the difference in heat transfer characteristics arises between upflow and downflow with Reynolds number less than about 700 and Grashof number larger than about 1,000, giving smaller Nusselt number for downflow than for upflow as the effect of buoyant force. New heat transfer correlations for channel heated from both sides are proposed as lower limits for upflow and downflow, respectively, in the laminar flow. (author)

147

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-? model, the non-linear k-? 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

148

Mixed convection heat transfer from discrete heat sources mounted in a rectangular duct

Digital Repository Infrastructure Vision for European Research (DRIVER)

Experiments and supporting 3-D numerical computations have been performed for water and FC-77 to investigate mixed convection heat transfer from a four-row, in-line array of twelve, square heat sources which are flush mounted to the lower wall of a horizontal, rectangular channel. The experimental data encompass heat transfer regimes characterized by natural convection, mixed convection, laminar forced convection, and the initiation of transition to turbulence. The variation of the row-averag...

Mahaney, Howard Victor

1989-01-01

149

Laminar Mixed Convection Heat Transfer Correlation for Horizontal Pipes

International Nuclear Information System (INIS)

This study aimed at producing experimental results and developing a new heat transfer correlation based upon a semi-empirical buoyancy coefficient. Mixed convection mass transfers inside horizontal pipe were investigated for the pipe of various length-to-diameters with varying Re. Forced convection correlation was developed using a very short cathode. With the length of cathode increase and Re decrease, the heat transfer rates were enhanced and becomes higher than that of forced convection. An empirical buoyancy coefficient was derived from correlation of natural convection and forced convection with the addition of L/D. And the heat transfer correlation for laminar mixed convection was developed using the buoyancy coefficient, it describes not only current results, but also results of other studies. Mixed convection occurs when the driving forces of both forced and natural convections are of comparable magnitude (Gr/Re2?1). It is classical problem but is still an active area of research for various thermal applications such as flat plate solar collectors, nuclear reactors and heat exchangers. The effect of buoyancy on heat transfer in a forced flow is varied by the direction of the buoyancy force. In a horizontal pipe the direction of the forced and buoyancy forces are perpendicular. The studies on the mixed convections of the horizontal pipes were not investigated very much due to the lack of practical uses compared to those of vertical pipes. Even the definitions on the buoyancy coefficient that presents the relative influence of the forced and the natural convections, are different by scholars. And the proposed heat transfer correlations do not agree

150

Laminar Mixed Convection Heat Transfer Correlation for Horizontal Pipes

Energy Technology Data Exchange (ETDEWEB)

This study aimed at producing experimental results and developing a new heat transfer correlation based upon a semi-empirical buoyancy coefficient. Mixed convection mass transfers inside horizontal pipe were investigated for the pipe of various length-to-diameters with varying Re. Forced convection correlation was developed using a very short cathode. With the length of cathode increase and Re decrease, the heat transfer rates were enhanced and becomes higher than that of forced convection. An empirical buoyancy coefficient was derived from correlation of natural convection and forced convection with the addition of L/D. And the heat transfer correlation for laminar mixed convection was developed using the buoyancy coefficient, it describes not only current results, but also results of other studies. Mixed convection occurs when the driving forces of both forced and natural convections are of comparable magnitude (Gr/Re{sup 2}?1). It is classical problem but is still an active area of research for various thermal applications such as flat plate solar collectors, nuclear reactors and heat exchangers. The effect of buoyancy on heat transfer in a forced flow is varied by the direction of the buoyancy force. In a horizontal pipe the direction of the forced and buoyancy forces are perpendicular. The studies on the mixed convections of the horizontal pipes were not investigated very much due to the lack of practical uses compared to those of vertical pipes. Even the definitions on the buoyancy coefficient that presents the relative influence of the forced and the natural convections, are different by scholars. And the proposed heat transfer correlations do not agree.

Chae, Myeong Seon; Chung, Bum Jin [Kyung Hee Univ., Yongin (Korea, Republic of)

2013-10-15

151

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

152

Design of an R-134a loop for subcritical and supercritical forced-convection heat transfer studies

International Nuclear Information System (INIS)

The R-134a test loop is a forced-flow experimental facility for the study of heat transfer properties of R-134a under subcritical and supercritical thermodynamic conditions. The loop is designed to operate with pressures as high as 6 MPa and temperatures up to 140 °C. The intended mass flux is in the range of 500-6000 kg/m2s for the experiments with subcritical thermodynamic states and 500-4000 kg/m2s for supercritical conditions. The loop has been designed to accommodate a variety of test-section geometries, ranging from a straight circular tube to a 7-rod bundle, achieving heat fluxes up to 2.5 MW/m2 depending on the test section geometry. The design of the loop allows for easy reconfiguration of the test-section orientation relative to the gravitational direction and adjustment to the length of the test section. (author)

153

International Nuclear Information System (INIS)

This paper provides information on heat transfer enhancement due to jet mixing inside a cylindrical enclosure. The work addresses conservative heat transfer assumptions regarding mixing and condensation that have typically been incorporated into passive containment design analyses. The current research presents an interesting possibility for increasing decay heat removal of passive containment systems under combined natural and forced convection. Eliminating these conservative assumptions could provide the basis for a change of containment design and reduce the construction cost. It is found that the ratio of forced- and free convection Nusselt numbers can be predicted as a function of the Archimedes number and a correlated factor accounting for jet orientation and enclosure geometry. To use the small-scale tests for large containment design, scale-up methods and criteria are important for matching the key governing parameters and fluid properties. In the present experiment, a cylindrical enclosure was constructed with a vertical wall of 2.29-m diameter and 0.8-m height and a vertically adjustable ceiling. A horizontal copper plate was installed at the bottom to provide an isothermal heating surface. Cold air was injected at several positions with varying pipe diameters and injecting orientations and was removed from the top of the enclosure. The experiment was performed with an extensive set of tests to study the combined natural- and forced convection heat transfer ural- and forced convection heat transfer in a cylindrical enclosure mixed by an injected jet. The goals are to evaluate the key parameters governing the heat transfer augmentation by a forced jet and to investigate the effect of geometric factors, including jet diameter, jet injection orientation, and enclosure geometry (aspect ratio). Flow velocity measurement further provides a better understanding of the flow patterns developed inside the enclosure, which will determine the effectiveness of the whole volume mixing process. An additional experiment with a vertically cooled wall, similar to the condition of a passive containment cooling surface, is under construction. It will be used for further investigation of similar phenomena for steam condensation in the presence of non-condensable gases. From the current study with the cylindrical enclosure, it was found that the augmentation of natural-convection heat transfer by a forced jet is primarily determined by the jet Reynolds number, while the injecting nozzle diameter has a relatively weak effect. The jet orientation also plays an important role in determining the augmentation ratio, and of the four different jet orientations studied here, vertical downward injection at the center of the enclosure gives the highest augmentation, while azimuthal injection gives the lowest. The enclosure geometry and aspect ratio are also important factors, depending on the jet orientation. A combining rule is employed with a weighted relation to balance the contributions from separate heat-transfer correlations representing natural and forced dominated convections. It was found that under natural convection without the jet, the mean Nusselt number inside the large enclosure can be correlated by the enclosure Rayleigh number, RaD1/3, and under forced convection with a strong jet, the data are well represented as a function of the jet Reynolds number, Rej2/3. According to these relationships, a correlation form predicting mixed convection inside the enclosure was developed by a function that uses the ratio of forced- and free-convection heat transfer coefficients. This form can then be further reduced to predict the augmentation ratio as a function of the Archimedes number and a correcting factor accounting for jet orientation and enclosure geometry. Figure 1 presents the heat transfer augmentation as the Nusselt number ratio of mixed convection to natural convection (NuD/Nunc) versus the Archimedes number. The data, including four injecting orientations and three different jet diameters, are well correlated for each injecti

154

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.

155

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 103Remodelled.

Julia, J. E.; Hernández, L.; Martínez-Cuenca, R.; Hibiki, T.; Mondragón, R.; Segarra, C.; Jarque, J. C.

2012-11-01

156

Suppression of saturated nucleate boiling by forced convective flow

International Nuclear Information System (INIS)

Tube-side forced convective boiling nitrogen and oxygen and thin film shell-side forced convective boiling R-11 data demonstrate a reduction in the heat transfer coefficient associated with nucleate boiling as the two-phase friction pressure drop increases. Techniques proposed in the literature to account for nucleate boiling during forced convective boiling are discussed. The observed suppression of nucleate boiling for the tube-side data is compared against the Chen correlation. Although general agreement is exhibited, supporting the interactive heat transfer mechanism theory, better agreement is obtained by defining a bubble growth region within the thermal boundary layer. The data suggests that the size of the bubble growth region is independent of the friction drop, but is only a function of the physical properties of the boiling liquid. 15 refs

157

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.

2011-10-01

158

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.

159

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

160

International Nuclear Information System (INIS)

Forced air convection heat pipe cooling systems play an essential role in the thermal management of electronic and power electronic devices such as microprocessors and IGBT's (Integrated Gate Bipolar Transistors). With increasing heat dissipation from these devices, novel methods of improving the thermal performance of fin stacks attached to the heat pipe condenser section are required. The current work investigates the use of a wing type surface protrusions in the form of 3-D delta wing tabs adhered to the fin surface, thin wings punched-out of the fin material and TiNi shape memory alloy delta wings which changed their angles of attack based on the fin surface temperature. The longitudinal vortices generated from the wing designs induce secondary mixing of the cooler free stream air entering the fin stack with the warmer fluid close to the fin surfaces. The change in angle of the attack of the active delta wings provide heat transfer enhancement while managing flow pressure losses across the fin stack. A heat transfer enhancement of 37% compared to a plain fin stack was obtained from the 3-D tabs in a staggered arrangement. The punched-out delta wings in the staggered and inline arrangements provided enhancements of 30% and 26% respectively. Enhancements from the active delta wings were lower at 16%. However, as these devices reduce the pressure drop through the fin stack by approximately 19% in the de-activate position, over the activated position, a reduction in fan operating cost may be achieved for systems operating with inlet air temperatures below the maximum inlet temperature specification for the device. CFD analysis was also carried out to provide additional detail of the local heat transfer enhancement effects. The CFD results corresponded well with previously published reports and were consistent with the experimental findings. - Highlights: ? Heat transfer enhancements of heat pipe fin stacks was successfully achieved using fixed and active delta wing vortex generators. ? The active vortex generators, made from Ti-Ni, protruded into the flow stream at high temperatures and resume a low profile position at a low temperature set point. ? By considering wing spacing and the distance from the leading edge of the fin stack, heat transfer enhancements of up to 37%, compared to plane fin stacks, were achieved. ? By replacing the fixed delta wings with the active vortex generators, heat transfer enhancements of up to 16% was achieved and the pressure loss associated with flow obstructions was effectively managed.

161

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

162

Modelling of natural-convection driven heat exchangers

Digital Repository Infrastructure Vision for European Research (DRIVER)

Abstract: A lumped model is developed for shell-and-tube heat exchangers driven by natural convection, which is based on a one-dimensional approximation. The heat flux is driven by the logarithmic mean temperature difference. The volumetric air flow rate is driven by the buoyant force. Based on the Boussinesq aproximation, this force is characterised by an analytic expression of the inflow and outflow temperatures. The lumped model is finished by relating the buoyant force to the friction...

Dirkse, M. H.; Loon, W. K. P.; Stigter, J. D.; Bot, G. P. A.

2007-01-01

163

International Nuclear Information System (INIS)

Within the project 'Convection in a Cylinder' (CiC) heat transfer enhancement is studied for the case of two concentric, vertically aligned cylinders. The cylindrical gap is filled with a dielectric liquid, which viscosity is just few times higher than that of water. The inner cylinder is heated and the outer one is cooled. This setup in a gravitational buoyancy field leads to a fluid movement in a single convective cell with hot fluid rising at the inner boundary and cold fluid sinking at the outer boundary. The top and bottom part of the system shows horizontal movement, again in boundary layers. The strengthening of temperature gradient induces instabilities of that convective motion. If we vary the buoyancy force by means of electro-hydrodynamic effects, the patterns of convection differ from those instabilities rising only from variation of the temperature gradient.

164

Digital Repository Infrastructure Vision for European Research (DRIVER)

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. T...

Rajesh Khatri; Pankaj Agarwal

2012-01-01

165

International Nuclear Information System (INIS)

Distributions of heat transfer coefficients in axial heat exchangers, consisting of closely spaced tube bundles, were measured with a holographic interferometer. For the transition from laminar to turbulent flow (Re 500 to 5000) six different spacings were examined. In addition to the local differences, temporal variations could be observed. A theoretical calculation of the local heat transfer, using the finite difference method, is in good agreement with the experimental results. (U.S.)

166

Modeling of laminar forced convection in spherical- pebble packed beds

International Nuclear Information System (INIS)

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

167

Numerical investigation of nanofluids forced convection in circular tubes

Digital Repository Infrastructure Vision for European Research (DRIVER)

Abstract In this paper, developing laminar forced convection flow of a water–Al2O3 nanofluid in a circular tube, submitted to a constant and uniform heat flux at the wall, is numerically investigated. A single and two-phase model (discrete particles model) is employed with either constant or temperature-dependent properties. The investigation is accomplished for size particles equal to 100 nm. The maximum difference in the average heat transfer coefficient between sing...

2009-01-01

168

Energy Technology Data Exchange (ETDEWEB)

This experimental study comparatively examined the two-phase flow structures, pressured drops and heat transfer performances for the cocurrent air-water slug flows in the vertical tubes with and without the spiky twisted tape insert. The two-phase flow structures in the plain and swirl tubes were imaged using the computerized high frame-rate videography with the Taylor bubble velocity measured. Superficial liquid Reynolds number (Re{sub L}) and air-to-water mass flow ratio (AW), which were respectively in the ranges of 4000-10000 and 0.003-0.02 were selected as the controlling parameters to specify the flow condition and derive the heat transfer correlations. Tube-wise averaged void fraction and Taylor bubble velocity were well correlated by the modified drift flux models for both plain and swirl tubes at the slug flow condition. A set of selected data obtained from the plain and swirl tubes was comparatively examined to highlight the impacts of the spiky twisted tape on the air-water interfacial structure and the pressure drop and heat transfer performances. Empirical heat transfer correlations that permitted the evaluation of individual and interdependent Re{sub L} and AW impacts on heat transfer in the developed flow regions of the plain and swirl tubes at the slug flow condition were derived. (author)

Chang, Shyy Woei [Thermal Fluids Laboratory, National Kaohsiung Marine University, No. 142, Haijhuan Road, Nanzih District, Kaohsiung City 81143 (China); Yang, Tsun Lirng [Department of Marine Engineering, National Kaohsiung Marine University, No. 142, Haijhuan Road, Nanzih District, Kaohsiung City 81143 (China)

2009-10-15

169

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

170

Burnout conditions in BSR cores under forced convection flow

International Nuclear Information System (INIS)

The BSR normally operates at a power level of two megawatts with a downward forced convection cooling flow of about 1000 gallons per minute. Because of the relatively low downward coolant velocity, one or two feet per second, the bouyancy produced by the heating of the water as it passes through the core may result in a significant reduction in the critical heat flux as determined by the usual forced convection correlation methods. Previous estimates of the critical heat flux have not taken this bouyancy effect into consideration. It is the purpose of this study to develop a suitable method for estimating the relationship between flow and power level which permits the establishment of limits on operating parameters adequate to prevent burnout and to apply these to the operation of the BSR

171

International Nuclear Information System (INIS)

Liquid nitrogen was used as working fluid in a tube heated in cosine distribution to study burn-out phenomena in the present experiment. Two types of burn-out were observed. One occurred when flow pattern changed from churn flow or slug flow to annular flow. Another one is DNB phenomena when the flow was unstable. (author)

172

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 “V-shaped orifice tubulators, VOT”. The effects of Blockage Ratios (BR = 0.05-0.20, flow attack angles (? = 20°, 30° and 45° and flow directions (V-Downstream and V-Upstream with a single Pitch Ratio (PR = 1 are investigated for Reynolds number based on the hydraulic diameter of the square channel (Dh, Re = 100-2000. The fully developed periodic flow and heat transfer are applied for the computational domain. The SIMPLE algorithm and the finite volume method are used in the current study. The numerical results show that the use of VOT not only increasing heat transfer rate, but also rise up very enlarge pressure loss due to reducing the flow area of the cross sectional area. In addition, the maximum thermal enhancement factors are found around 2.4 and 2.5 for BR = 0.10, ? = 30° at the highest Reynolds number of V-Downstream and V-Upstream, respectively.

Amnart Boonloi

2014-01-01

173

Energy Technology Data Exchange (ETDEWEB)

The goal of this NERI project was to perform research on high temperature fluoride and chloride molten salts towards the long-term goal of using these salts for transferring process heat from high temperature nuclear reactor to operation of hydrogen production and chemical plants. Specifically, the research focuses on corrosion of materials in molten salts, which continues to be one of the most significant challenges in molten salts systems. Based on the earlier work performed at ORNL on salt properties for heat transfer applications, a eutectic fluoride salt FLiNaK (46.5% LiF-11.5%NaF-42.0%KF, mol.%) and a eutectic chloride salt (32%MgCl2-68%KCl, mole %) were selected for this study. Several high temperature candidate Fe-Ni-Cr and Ni-Cr alloys: Hastelloy-N, Hastelloy-X, Haynes-230, Inconel-617, and Incoloy-800H, were exposed to molten FLiNaK with the goal of understanding corrosion mechanisms and ranking these alloys for their suitability for molten fluoride salt heat exchanger and thermal storage applications. The tests were performed at 850Ã?Â?Ã?Â?Ã?Â?Ã?Â?C for 500 h in sealed graphite crucibles under an argon cover gas. Corrosion was noted to occur predominantly from dealloying of Cr from the alloys, an effect that was particularly pronounced at the grain boundaries Alloy weight-loss due to molten fluoride salt exposure correlated with the initial Cr-content of the alloys, and was consistent with the Cr-content measured in the salts after corrosion tests. The alloysÃ?Â?Ã?Â¢Ã?Â?Ã?Â?Ã?Â?Ã?Â? weight-loss was also found to correlate to the concentration of carbon present for the nominally 20% Cr containing alloys, due to the formation of chromium carbide phases at the grain boundaries. Experiments involving molten salt exposures of Incoloy-800H in Incoloy-800H crucibles under an argon cover gas showed a significantly lower corrosion for this alloy than when tested in a graphite crucible. Graphite significantly accelerated alloy corrosion due to the reduction of Cr from solution by graphite and formation on Cr-carbide on the graphite surface. Ni-electroplating dramatically reduced corrosion of alloys, although some diffusion of Fe and Cr were observed occur through the Ni plating. A pyrolytic carbon and SiC (PyC/SiC) CVD coating was also investigated and found to be effective in mitigating corrosion. The KCl-MgCl2 molten salt was less corrosive than FLiNaK fluoride salts for corrosion tests performed at 850oC. Cr dissolution in the molten chloride salt was still observed and consequently Ni-201 and Hastelloy N exhibited the least depth of attack. Grain-boundary engineering (GBE) of Incoloy 800H improved the corrosion resistance (as measured by weight loss and maximum depth of attack) by nearly 50% as compared to the as-received Incoloy 800H sample. Because Cr dissolution is an important mechanism of corrosion, molten salt electrochemistry experiments were initiated. These experiments were performed using anodic stripping voltammetry (ASV). Using this technique, the reduction potential of Cr was determined against a Pt quasi-reference electrode as well as against a Ni(II)-Ni reference electrode in molten FLiNaK at 650 oC. The integrated current increased linearly with Cr-content in the salt, providing for a direct assessment of the Cr concentration in a given salt of unknown Cr concentration. To study heat transfer mechanisms in these molten salts over the forced and mixed convection regimes, a forced convective loop was constructed to measure heat transfer coefficients, friction factors and corrosion rates in different diameter tubes in a vertical up flow configuration in the laminar flow regime. Equipment and instrumentation for the forced convective loop was designed, constructed, and tested. These include a high temperature centrifugal pump, mass flow meter, and differential pressure sensing capabilities to an uncertainty of < 2 Pa. The heat transfer coefficient for the KCl-MgCl2 salt was measured in two different diameter channels (0.083Ã?Â?Ã?Â¢Ã?Â?Ã?Â?Ã?Â

Sridharan, Kumar; Anderson, Mark; Allen, Todd; Corradini, Michael

2012-01-30

174

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

175

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)

176

Laminar free-convective heat transfer in vertical uniform heat flux ducts

International Nuclear Information System (INIS)

A numerical analysis is made of laminar free convection in vertical ducts with Uniform Heat Flux (UHF), in consideration of temperature-dependence of fluid thermophysical properties. The local Nusselt numbers of Constant Property Solutions (CPS) can be well correlated with the dimensionless parameters derived by the authors, regardless of the exit Rayleigh number, Prandtl number, and circular tube or parallel plates. The CPS of free-convective heat transfer is close to that of forced convective heat transfer in UHF-ducts. The local Nusselt numbers of Variable Property Solutions (VPS) can also be well correlated by the usual correction method used for forced convective heat transfer in duct. The difference in the heat transfer characteristics of UHF-ducts between CPS and VPS is not so large as those of Uniform Wall Temperature ducts. (author)

177

FORCED CONVECTION GREENHOUSE PAPAD DRYING: AN EXPERIMENTAL STUDY

Directory of Open Access Journals (Sweden)

Full Text Available In this research paper, the behavior of heat and mass transfer phenomenon during greenhouse papad drying under forced convection mode has been investigated. Various experiments were performed during the month of April 2010 at Guru Jambheshwar University of Science and Technology Hisar (29o5’5” N 75o45’55” E. Experimental data obtained for forced convection greenhouse drying of papad were used to determine the constants in the Nusselt number expression by using the simple linear regression analysis and, consequently, the values of convective and evaporative heat transfer coefficients were evaluated. The average values of experimental constants C and n were determined as 0.996 and 0.194 respectively. The average values of convective and evaporative heat transfer coefficients were determined as 0.759 W/m2 oC and 23.48 W/m2 oC respectively. The experimental error in terms of percentage uncertainty was also evaluated.

MAHESH KUMAR

2013-04-01

178

EXPERIMENTAL AND NUMERICAL STUDY OF FORCED CONVECTION IN ENGINE BLOCK

Directory of Open Access Journals (Sweden)

Full Text Available Experiments were conducted to study forced convection in three different engine blocks. Current study uses three engine blocks machined from plain Aluminium. Plain Cylinder Block, Cylinder Block with fins & Square Block with fins is used. Inside the cylinder block fine heaters were fitted to mimic heating at different locations. Temperatures at various locations were measured. Few experimental data is compared with CFD results by Fluent.

BELSARE S.N.

2014-04-01

179

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

180

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

181

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

182

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

183

Energy Technology Data Exchange (ETDEWEB)

An experiment of heat transfer to CO{sub 2}, which flows upward and downward in a circular tube with an inner diameter of 6.32 mm, was carried out with mass flux of 285-1200 kg/m{sup 2} s and heat flux of 30-170 kW/m{sup 2} at pressures of 7.75 and 8.12 MPa, respectively. The corresponding Reynolds number at the tube test section inlet ranges from 1.8 x 10{sup 4} to 3.8 x 10{sup 5}. The tube inner diameter corresponds to the equivalent hydraulic diameter of the fuel assembly sub-channel, which is being studied at KAERI. Among the tested correlations, the Bishop correlation predicted the experimental data most accurately, but only 66.9% of normal heat transfer data were predicted within {+-}30% error range. The Watts and Chou correlation, which is claimed to be valid for both the normal and deteriorated heat transfer regime, showed unsatisfactory performance. A significant decrease in Nusselt number was observed in the range of 10{sup -6}

Bae, Yoon-Yeong; Kim, Hwan-Yeol [Korea Atomic Energy Research Institute, 1045 Daedeokdaero, Yuseong, Daejeon 305-353 (Korea); Kang, Deog-Ji [Korea Hydro and Nuclear Power Co. Ltd., 216 Kori, Jangan-eup, Gijang-gun, Busan 619-711 (Korea)

2010-11-15

184

Double tube heat exchanger with novel enhancement: part II—single phase convective heat transfer

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.

Tiruselvam, R.; Chin, W. M.; Raghavan, Vijay R.

2012-08-01

185

International Nuclear Information System (INIS)

Experiments of upflow- and downflow-forced turbulent and laminar convection, natural convection and buoyancy-assisted combined convection of water are performed in uniformly heated, square arrayed, nine-rod bundles having P/D ratios of 1.25, 1.38 and 1.5. The heat transfer data are correlated in the respective convective regimes, where the heated equivalent diameter is used as the characteristic length in all dimensionless quantities and water properties are evaluated at the mean bulk temperature. The forced convection data fall into two distinct regimes: forced turbulent and forced laminar; the Reynolds number at the transition between these regimes is a linear function of P/D. In the forced convection experiments the flow is hydrodynamically developing, but thermally fully developed. The natural convection data are correlated to within ± 10% in terms of Raq and the combined convection data are correlated to within ± 15% by superimposing the Nusselt numbers, raised to the fourth power, of forced laminar and natural convection. For all P/D values, the transition from forced laminar to combined convection occurs at Ri = 2.0. A comparison with triangularly arrayed rod bundles shows that for the same flow area per rod, the rod arrangement negligibly affects Nu in both forced and natural convection regimes. (author)

186

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

187

Convective heat transfer in buildings: recent research results

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 compare with existing ASHRAE correlations and differences of as much as 20% are observed. It is shown that such differences can have a significant impact on the accuracy of building energy analysis computer simulations. Interzone coupling correlations obtained from experimental work reported 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. Finally, future research needs are suggested.

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

1982-04-01

188

Unsteady radiative-convective heat transfer on a radiating surface

Research of radiation-convective heat exchange on radiating surfaces at natural and forced convection is complex mathematical task and here we obtain approximate analytical formulations for this process. We consider two dimensional unsteady heat transfer between solid surface and fluid under the natural laminar convection within optically transparent grey media. Also we assume constant thermo-physical properties except density which is decreasing linearly with temperature. Complex radiative-convective unsteady heat transfer approximately can be considered as a multi-stage process. At the beginning heat transfer coefficient is time dependent but almost independent on longitudinal coordinate. Afterwards heat transfer coefficient becomes dependent on longitudinal coordinate but does not change over time. Analytic formulations obtained for those two stages could be merged along the "time-space" characteristic basing on the equality of heat flows and temperatures there. Solutions are constructed using asymptotic expansions. Theoretical analysis of the solutions revealed the following: effect of radiation leads to a change in the heat transfer coefficient from the values that are characteristic to the second order boundary conditions to the values that are characteristic for the first order boundary conditions. The rate of this transition depends on ß radiation coefficient. Experimental research confirmed correctness of the simplifications introduced.

Salomatov, Vasily; Puzyrev, Evgeniy; Salomatov, Vladimir

2012-11-01

189

Natural convection heat transfer in SIGMA experiment

International Nuclear Information System (INIS)

A loss-of-coolant accident (LOCA) results in core melt formation and relocation at various locations within the reactor core over a considerable period of time. If there is no effective cooling mechanism, the core debris may heat up and commence natural circulation. The high temperature pool of molten core material will threaten the structural integrity of the reactor vessel. The extent and urgency of this threat depend primarily upon the intensity of the internal heat sources and upon the consequent distribution of the heat fluxes on the vessel walls in contact with the molten core material pools. In such a steady molten pool convection state, the thermal loads against the vessel would be determined by the in-vessel heat transfer distribution involving convective and conductive heat transfer from the decay-heated core material pool to the lower head wall in contact with the core material. In this study, upward and downward heat transfer fraction ratio is focused on

190

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.

191

Heat transfers in porous media. Conduction, convection, radiant transfer

International Nuclear Information System (INIS)

Multiple physico-chemical and transport phenomena take place in porous media. The study of these phenomena requires the knowledge of fluid storage, transfer and mechanical properties of these media. Like all polyphasic heterogenous systems, these properties depend on the morphology of the matrix and of the phenomena interacting in the different phases. This makes the heat transfers in porous media a particularly huge field of researches. This article makes a synthesis of these researches. Content: 1 - classification and characterization of porous media; 2 - modeling of transfer phenomena; 3 - heat transfer by conduction: concept of equivalent thermal conductivity (ETC), modeling of conduction heat transfer, ETC determination; 4 - heat transfer by convection: modeling of convection heat transfer, natural convection (in confined media, along surfaces or impermeable bodies immersed in a saturated porous medium), forced and mixed convection; 5 - radiant heat transfer: energy status equation, approximate solutions of the radiant transfer equation, use of the approximate solutions: case of fibrous insulating materials; 6 - conclusion. (J.S.)

192

Southern Ocean Deep Convection Forcing of North Atlantic Centennial Variability

We present an oceanic teleconnection associated with the Southern Ocean Centennial Variability (SOCV). The SOCV is driven by internal multi-centennial variability of open ocean deep convection in the Atlantic sector of the Southern Ocean, which forces significant variations in the strength of the Atlantic Meridional Overturning Circulation (AMOC) in control simulations with the Kiel Climate Model employing present climate greenhouse gas concentrations. The deep convection is stimulated by a strong built-up of heat at mid-depth and lasts until this heat reservoir is virtually depleted. The heat originates from relatively warm deep water formed in the North Atlantic. The several decades lasting recharge process sets a minimum time between convection events. Stochastically occurring, favorable sea ice conditions as well as coincidental strong surface freshening are further factors influencing the timing of convection onset and shutdown. The deep convection flip-flop has intriguing similarities to the Weddell Polynya observed during the 1970s. The state of Weddell Sea deep convection strongly influences the northward extent of Antarctic Bottom Water (AABW). The retreat of AABW results in an enhanced meridional density gradient that drives an increase in the strength and vertical extent of the North Atlantic Deep Water (NADW) cell. This shows, for instance, as a peak in AMOC strength at 30°N about a century after Weddell Sea deep convection has ceased. The stronger southward flow of NADW is compensated by an expansion of the North Atlantic subpolar gyre and an acceleration of the North Atlantic Current, indicating greater deep water formation. Contractions of the North Atlantic subpolar gyre enable anomalously warm water to penetrate farther to the north, eventually weakening the AMOC and closing a quasi-centennial cycle. In our simulation the SOCV is associated with sea level variations in the order of 10 to 30 cm/century in the North Atlantic and Southern Ocean, which is due to subpolar gyre extent and oceanic heat content changes, respectively. This suggests that internal variability on long time scales cannot be neglected a priori in assessments of 20th and 21st century AMOC and regional sea level change.

Martin, Torge; Park, Wonsun; Latif, Mojib

2014-05-01

193

International Nuclear Information System (INIS)

Combined convection between vertical plates with asymmetric and uniform heating section is investigated numerically and experimentally in the case when forced convection aids natural convection. The experiments are carried out by using water as a working fluid at wide ranges of Re and Gr numbers. Numerical solutions are presented for Pr = 7.64 (water) and Pr = 0.005 (liquid sodium) or various numerical combinations of Gr and Re numbers. In addition, numerical analysis has been done for both pure natural convection and pure forced convection. Influences of Gr and Re numbers on velocity, temperature and pressure drop are discussed, and comparisons between theoretical results and experimental ones show good agreement. (orig.)

194

In the second part of review, we have considered the problems related to momentum and heat transfer in nanofluids. Results on hydrodynamic friction, forced and free convection in the laminar and turbulent flows are analysed; heat transfer at boiling is considered. The available models describing heat transfer intensification and suppression in nanofluids are studied. It is shown that for some problems on convective heat transfer there is a contradiction in data of different authors; possible reasons for this contradiction are analysed.

Terekhov, V. I.; Kalinina, S. V.; Lemanov, V. V.

2010-06-01

195

A meshless method for modeling convective heat transfer

Energy Technology Data Exchange (ETDEWEB)

A meshless method is used in a projection-based approach to solve the primitive equations for fluid flow with heat transfer. The method is easy to implement in a MATLAB format. Radial basis functions are used to solve two benchmark test cases: natural convection in a square enclosure and flow with forced convection over a backward facing step. The results are compared with two popular and widely used commercial codes: COMSOL, a finite element model, and FLUENT, a finite volume-based model.

Carrington, David B [Los Alamos National Laboratory

2010-01-01

196

Effect of buoyancy on forced convection in a cusped duct

International Nuclear Information System (INIS)

In the event of a loss of coolant accident in a pressurized water reactor, swelling of the fuel rod cladding will lead to reduction of the subchannel flow area and worsening of the core heat transfer in the region of the blockage. The four-cusped duct is an ideal geometry for the simulation of such a channel blockage. Understanding the characteristics of flow and heat transfer in the cusped duct is essential for better design of the emergency core cooling system. Thus, in this paper, combined natural and forced convection in a vertical cusped duct has been investigated in the region of both hydrodynamically and thermally fully developed flow. The thermal boundary condition imposed on the cusped duct is the axial uniform heat flux with peripheral uniform temperature. The results indicate that the fluid flow and heat transfer in the corner region of the cusped duct are improved because of the influence of natural convection. As the Rayleigh number increases, the friction factor and Nusselt number increase accordingly. It was also found that the critical Rayleigh number is 1200, at which flow reversal occurs in the buoyancy-assisted flow (heated upflow). The velocity, temperature, and local Nusselt number distribution are presented for range of Rayleigh numbers

197

A Study on the Mixed Convection Heat Transfer in a Vertical Cylinder Using Electroplating System

International Nuclear Information System (INIS)

us with heat transfer coefficient, can be directly obtained from the information of the bulk concentration and electric current between electrodes. This study simulated the mixed convective heat transfer phenomena in a vertical cylinder using copper electroplating system. The mixed convection phenomenon is observed when the forced and natural convections are of comparable magnitudes in one system. The mixed convection is classified as laminar and turbulent flows depending on the exchange mechanism and also as buoyancy aided and buoyancy opposed flows depending on the directions of forced flows with respect to the buoyancy forces. For a laminar flow, the heat transfer rate of buoyancy aided flow is larger than the corresponding forced convection heat transfer due to the increased flow velocity while the heat transfer rate of buoyancy opposed flow is smaller. However for a turbulent flow, the buoyancy opposed flow shows higher heat transfer rates than corresponding forced convective flow due to the increased turbulent production and the buoyancy aided flow shower lower heat transfer rates due to laminarization. Mixed convection heat transfer in a vertical cylinder with aiding flow and opposing flow studied experimentally for Reynolds numbers ranging from 4,000 to 10,000 with a constant Grashof number 6.2x109 and Prandtl number about 2000. The experimental results reproduced the trend of mixed convection heat transfer phenomena in a turbulent situation and agree well with the study performed by Y. Parlatan(1996). The analogy experimental method successfully simulates the mixed convection heat transfer system and seems to be a useful tool for heat transfer studies for HTGR as well as the systems with high buoyancy condition and high Prandtl number fluid, as the electroplating method not only provides useful information regarding heat transfer but also has a cost-effective advantage over any other comparable experimental method

198

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

199

Prandtl Number Dependent Natural Convection with Internal Heat Sources

Energy Technology Data Exchange (ETDEWEB)

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.

Kang Hee Lee; Seung Dong Lee; Kune Y. Suh; Joy L. Rempe; Fan-Bill Cheung; Sang B. Kim

2004-06-01

200

Two-phase forced-convective fouling under steam generator operating conditions

International Nuclear Information System (INIS)

thickness peaks are noted, approximately equally spaced circumferentially. It is hypothesized that the fouling pattern is developed due to the cross-flow pattern present in the tube bundle. The possible interactions between the force-convective and nucleate-boiling fouling streams are briefly discussed. A method is presented for the superposition of the forced-convective and nucleate boiling fouling components. This method is based on the Chen heat transfer correlation. (author)

201

Natural convection in porous media heated from above

Transient natural convection in horizontal porous media is analyzed for geometries with permeable boundaries, and with upper boundary heating. A mathematical model is postulated for porous layers in which continuously stable density gradients exclude the problem of marginal stability. In natural convection, temperature coupling through buoyancy is exhibited in the governing partial differential equations of the mathematical model for mixed mode heat transfer. Corresponding asymptotic solutions for small and large Rayleigh number exhibit negligible momentum energy coupling. The system model is descriptive of conduction opposed natural convection heat transfer, with a continuous applicable range of driving force. Effectiveness of the one dimensional system model is demonstrated in numerical simulation of heat transfer in a packed bed of uniform spheres. Time dependent heating and interface radiation to the environment are considered in the mathematical model of a physical system with large solid to fluid phase conductivity ratio. It is indicated that effective conductivity should be determined by nothing the initial stages of transient response, whereas effective interface emissivity should be inferred from the large time response. This method internal convection and radiant exchange with the environment.

Leith, J. R.

1980-11-01

202

International Nuclear Information System (INIS)

The loss of cooling accident (LOCA) in a nuclear reactor is a case where the heat transfer by forced convection is of the same order of magnitude as the heat transfer by natural convection. Two simple theoretical solutions for evaluating the heat transfer coefficient are presented, assuming a combined velocity and temperature profile of natural and forced convection, within the boundary layer. The following mathematical form is proposed: Y= [1+Xsup(n)]sup(1/n) which is a general expression for a combined effect of wo limiting solutions. (author)

203

Electrohydrodynamic convective heat transfer in a square duct.

Laminar to weakly turbulent forced convection in a square duct heated from the bottom is strengthened by ion injection from an array of high-voltage points opposite the heated strip. Both positive and negative ion injection are activated within the working liquid HFE-7100 (C(4)F(9)OCH(3)), with transiting electrical currents on the order of 0.1 mA. Local temperatures on the heated wall are measured by liquid crystal thermography. The tests are conducted in a Reynolds number range from 510 to 12,100. In any case, heat transfer is dramatically augmented, almost independently from the flow rate. The pressure drop increase caused by the electrohydrodynamically induced flow is also measured. A profitable implementation of the technique in the design of heat sinks and heat exchangers is foreseen; possible benefits are pumping power reduction, size reduction, and heat exchange capability augmentation. PMID:19426338

Grassi, Walter; Testi, Daniele

2009-04-01

204

Free convection film flows and heat transfer

Presents development of systematic studies for hydrodynamics and heat and mass transfer in laminar free convection, accelerating film boiling and condensation of Newtonian fluids, and accelerating film flow of non-Newtonian power-law fluids. This book provides a system of analysis models with a developed velocity component method.

Shang, Deyi

2010-01-01

205

Forced and mixed convection heat and mass transfer are studied numerically for water containing metallic corrosion products in a heated or cooled vertical tube with variable thermophysical properties at super-critical pressures. The fouling mechanisms and fouling models are presented. The influence of variable properties at super-critical pressures on forced or mixed convection has been analyzed. The differences between heat and mass transfer under heating and cooling conditions are discussed. It is found that variable properties, especially buoyancy, greatly influence the fluid flow and heat mass transfer.

Jiang, Pei-Xue; Ren, Ze-Pei; Wang, Bu-Xuan

1995-01-01

206

Heat Flux Sensors for Infrared Thermography in Convective Heat Transfer

Directory of Open Access Journals (Sweden)

Full Text Available This paper reviews the most dependable heat flux sensors, which can be used with InfraRed (IR thermography to measure convective heat transfer coefficient distributions, and some of their applications performed by the authors’ research group at the University of Naples Federico II. After recalling the basic principles that make IR thermography work, the various heat flux sensors to be used with it are presented and discussed, describing their capability to investigate complex thermo-fluid-dynamic flows. Several applications to streams, which range from natural convection to hypersonic flows, are also described.

Giovanni Maria Carlomagno

2014-11-01

207

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)

208

Acoustical Convective Cooling Or Heating

Small, efficient ultrasonic device circulates fluid. Vibrating at ultrasonic frequency, piezoelectric driver sets up vortexes transfering heat to or from object in space. Used on Earth to apply localized or concentrated cooling to individual electronic components or other small parts.

Trinh, Eugene H.; Robey, Judith L.

1988-01-01

209

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

210

Approximate convective heating equations for hypersonic flows

Laminar and turbulent heating-rate equations appropriate for engineering predictions of the convective heating rates about blunt reentry spacecraft at hypersonic conditions are developed. The approximate methods are applicable to both nonreacting and reacting gas mixtures for either constant or variable-entropy edge conditions. A procedure which accounts for variable-entropy effects and is not based on mass balancing is presented. Results of the approximate heating methods are in good agreement with existing experimental results as well as boundary-layer and viscous-shock-layer solutions.

Zoby, E. V.; Moss, J. N.; Sutton, K.

1979-01-01

211

Melting of phase change fluid in heat storage capsules by convection

Energy Technology Data Exchange (ETDEWEB)

Melting of ice in cylindrical capsules by natural and forced convections in outer region was studied by experiments including flow visualization technique and numerical calculations. When a cylindrical diameter was small, natural convection melting was well simulated by axisymmetric heat conduction analysis. In case of a cylinder with large diameter, surface temperatures were affected by the effect of natural convection in a cylindrical capsule. Melting time became smaller by the natural convection in the cylindrical capsule for natural convection melting. In case of forced convection melting, surface temperatures were considerably scattered and the stagnation temperature showed the highest value. The shape of the solid liquid boundary became asymmetric. Non-uniform distribution of heat transfer coefficient should be considered in treating melting with large size capsules. (author)

Iwasa, Kaoru; Shiina, Yasuaki [Japan Atomic Energy Research Inst., Tokyo (Japan); Inagaki, Terumi

1999-10-01

212

Melting of phase change fluid in heat storage capsules by convection

Energy Technology Data Exchange (ETDEWEB)

Melting of ice in cylindrical capsules by natural and forced convections was studied by flow visualization technique. Surface temperatures were changed by the effect of natural convection in a cylindrical capsule with large diameter. In case of natural convection melting, the process was well simulated by axisymmetric heat conduction analysis when capsule diameters were small. In case of forced convection melting, surface temperatures were considerably scattered and the stagnation point showed the highest value. The shape of the solid liquid boundary became considerably asymmetric. (author)

Iwasa, Kaoru; Inagaki, Terumi [Ibaraki Univ., Mito (Japan); Shiina, Yasuaki

1998-07-01

213

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

214

On the transfer of heat by free convection of air in heated vertical channels

International Nuclear Information System (INIS)

The heat transfer in upward flow in vertical channels is investigated in this work. The object of the study is to determine the dependence of the heat transfer on the channel dimensions and on the characteristic temperature difference in order to determine the connection between free and forced convection for through-flow channels and in order to be able to give generally applicable rules for optimum design from the technically interesting viewpoint. (orig.)

215

Thermal Performance Of Convective-Radiative Heat Transfer In Porous Fins

Directory of Open Access Journals (Sweden)

Full Text Available Forced and natural convection in porous fin with convective coefficient at the tip under radiation and convection effects are investigated in this letter. Aluminum and copper as materials of fins are determined. In forced and natural convection, the air and water, are applied as working fluids, respectively. In order to solution of this nonlinear equation, HPM and VIM has been used . For verifying the accuracy of the solution methods, compare them with exact solutions (BVP. In this work the effects of porosity parameter ( , Radiation parameter (? and Temperature-Ratio parameter (µ on temperature distribution for both of the flows have been shown. The results shows that the effects of (? and (µ on temperature distribution in natural convection are based on porosity and in forced convection are uniform, approximately. Also, its shown that both VIM and HPM are capables to solving this nonlinear heat transfer equation.

Majid SHAHBABAEI

2014-11-01

216

Experimental Study of Combined Forced and Free Laminar Convection in a Vertical Tube

An apparatus was built to verify an analysis of combined forced and free convection in a vertical tube with uniform wall heat flux and to determine the limits of the analysis. The test section was electrically heated by resistance heating of the tube wall and was instrumented with thermocouples in such a way that detailed thermal entrance heat-transfer coefficients could be obtained for both upflow and downflow and any asymmetry in wall temperature could be detected. The experiments showed that fully developed heat-transfer results, predicted by a previous analysis, were confirmed over the range of Rayleigh numbers investigated. The concept of "locally fully developed" heat transfer was established. This concept involves the assumption that the fully developed heat-transfer analysis can be applied locally even though the Rayleigh number is varying along the tube because of physical-property variations with temperature. Thermal entrance region data were obtained for pure forced convection and for combined forced and free convection. The analysis of laminar pure forced convection in the thermal entrance region conducted by Siegel, Sparrow, and Hallman was experimentally confirmed. A transition to an eddy motion, indicated by a fluctuation in wall temperature was found in many of the upflow runs. A stability correlation was found. The fully developed Nusselt numbers in downflow were below those for pure forced convection but fell about 10 percent above the analytical curve. Quite large circumferential variations in wall temperature were observed in downflow as compaired with those encountered in upflow, and the fully developed Nussalt numbers reported are based on average wall temperatures determined by averaging the readings of two diametrically opposite wall thermocouples at each axial position. With larger heating rates in downflow the wall temperature distributions strongly suggested a cell flow near the bottom. At still larger heating rates the wall temperatures varied in a periodic way.

Hallman, Theodore M.

1961-01-01

217

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.

2008-09-01

218

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...

Sumon Saha; Goutam Saha; Mohammad Ali; Md. Quamrul Islam

2006-01-01

219

Experimental studies on mixed convection heat transfer in laminar flow through a plain square duct

This paper reports the findings of experimental studies on combined free and forced convection through a plain square duct in laminar region. The test fluid flows through an inner square duct, hot water at high flow rate circulated through a annular channel formed between square duct and circular tube, in counter current fashion to attain a nearly uniform wall temperature conditions. The importance of mixed convection is judged by the value of the Richardson number ( Ri). It was observed that at low Reynolds number, heat transfer was mainly governed by mixed convection. However at higher values of Reynolds number, heat transfer was significantly dominated by forced convection. It was found that Reynolds number higher than 1050 for water and 480 for ethylene glycol resulted in laminar forced convention heat transfer. The empirical correlation developed for Nusselt number in terms of Grashoff number and Graez number, was found to fit with experimental Nusselt number within ±11 and ±12 % for water and ethylene glycol respectively.

Patil, S. V.; Vijay Babu, P. V.

2012-12-01

220

Convective and radiative heating of a Saturn entry probe

The extent of convective and radiative heating for a Saturn entry probe is investigated in the absence and presence of ablation mass injection. The flow in the shock layer is assumed to be axisymmetric, viscous and in local thermodynamic equilibrium. The importance of chemical nonequilibrium effects for both the radiative and convective nonblowing surface heating rates is demonstrated for prescribed entry conditions. Results indicate that the nonequilibrium chemistry can significantly influence the rate of radiative heating to the entry probes. With coupled carbon-phenolic ablation injection, the convective heating rates are reduced substantially. Turbulence has little effect on radiative heating but it increases the convective heating considerably.

Tiwari, S. N.; Szema, K. Y.; Moss, J. N.; Subramanian, S. V.

1984-01-01

221

Experimental study of mixed convective heat transfer in narrow vertical rectangular channel, (2)

International Nuclear Information System (INIS)

During some anticipated operational occurrences and accident conditions, which were posturated in the JRR-3 safety assessment, there is a case that core flow decreases from steady-state downward forced convective flow to zero flow and at last becomes upward flow due to the natural circulation induced between the core and the reactor pool, that is, core flow reversal occurs. During the core flow reversal, the mixed convective heat transfer becomes significant. In this case it is important to understand the heat transfer characteristics of the mixed convection to evaluate the heat transfer of the fuel plates. To investigate the heat transfer characteristics of the mixed convection, heat transfer experiments were carried out using a vertical rectangular channel with water gap of 2.5 mm, which was nearly equal to that of the subchannels of the standard fuel elements of the upgraded JRR-3. In conclusion the heat transfer correlations which can be applied for a narrow vertical channel in free, mixed and forced convections were obtained. The range of the mixed convection region was identified by the non dimensional parameter Grx/Rex21/8Pr1/2. It was confirmed that in the mixed convection in a narrow channel, the influence of the acceleration of the main flow owing to the development of the boundary layer played a important part in the promotion of the heat transfer, compared with the case along a flat plate or in a wide channel. (author)

222

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

223

International Nuclear Information System (INIS)

Coupled heat and mass transfer in porous media has many important applications in engineering. These include the migration of moisture in fibrous insulation, the spreading of chemical pollutants in saturated soil, underground disposal of nuclear wastes and the extraction of geothermal energy. Here, similarity solutions are reported for coupled heat and mass transfer by free, forced, and mixed convection from a horizontal surface in a saturated porous medium. The present analysis shows that similarity solutions are possible for a surface maintained at a constant heat flux and mass flux condition. The governing parameters for the problem under consideration are the Lewis number Le, the buoyancy ratio N and mixed convection parameter Ra/Pe3/2. Depending on the values of N and Le, the heat and mass transfer results may range from the asymptotic free convection limit to that of the forced convection limit. The results are presented in terms of the relation between the transfer coefficients and the governing parameters

224

Energy Technology Data Exchange (ETDEWEB)

Based on the method we suggested in a previous paper [Int.J. Heat Mass Transfer 45 (2002) pp.2373-2385] the present work is to investigate the mixed convection problem. A two-dimensional, steady, laminar displacement ventilation model is adopted here for the interaction between the buoyancy driven natural convection and the external forced convection is important to achieve the goal of ventilation effectiveness. The solution is determined by the non-dimensional parameters Gr and Gr/Re{sup 2}, the influences of which on the resulting heat and fluid flow are discussed. To optimize the ventilation system, different outlet locations are investigated. Results and comparisons show that the displacement ventilation guarantees a high indoor air quality (IAQ) and is therefore a desired air-conditioning system. (author)

Deng, Q.-H.; Tang, G.-F. [Hunan Univ., Changsha (China). College of Civil Engineering

2002-05-01

225

Multiscale convection in a geodynamo simulation with uniform heat flux along the outer boundary

is generally expected that Earth's magnetic field, which is generated by convecting liquid metal within its core, will substantially alter that convection through the action of Lorentz forces. In most dynamo simulations, however, Lorentz forces do very little to change convective flow, which is predominantly fine-scaled. 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 multiscale convection is observed. We investigate the combined influence of thermal boundary conditions and magnetic fields using four simulations: two dynamos and two nonmagnetic models, with either uniform temperature or heat flux fixed at the outer boundary. Of the four, only the fixed-heat-flux dynamo simulation produces multiscale convective flow patterns. Comparison between the models suggests that the fixed-flux dynamo generates large patches of strong azimuthal magnetic field that suppress small-scale convective motions. By allowing temperature to vary along the outer boundary, the fixed-flux dynamo generates stronger azimuthal flow and, in turn, stronger magnetic field, and the resulting Lorentz forces alter the nature of convective flow. Extrapolation of the analyses presented here suggests that magnetic fields may also suppress small-scale convection in the Earth's core.

Matsui, Hiroaki; King, Eric; Buffett, Bruce

2014-08-01

226

International Nuclear Information System (INIS)

An analysis is performed to study the effects of transverse thermal dispersion and inertia on the mixed convection boundary layer flow in the thermal plume above a horizontal line source in a porous medium of infinite extent. A local similarity solution is obtained where the dependent and independent variables are scaled to a mixed convection parameter applicable at both the free- and forced-convection limits. The solution is valid from the free-convection dominated region near the heat source to that of the forced-convection dominated region away from the source. Numerical computations of the temperature and vertical velocity profiles were carried out for the cases of air, water and silicon oil for a wide range of the mixed convection parameter. The effects of the transverse thermal dispersion and inertia on the heat transfer and fluid flow characteristics from free- to forced-convection limits are illustrated

227

Combined forced and free laminar convection in verticale rod bundles with longitudinal flow

International Nuclear Information System (INIS)

The heat-transfer problem of combined forced and free convection by fully developed laminar flow in a vertically mounted reactor fuel rod assembly has been solved analytically for both upflow and downflow. Results reveal the strong augmentation of buoyancy effects by radial gradients in the power generation across the assembly. 12 refs

228

Combined free and forced convection MHD flow in vertical circular porous channel

International Nuclear Information System (INIS)

The combined free and forced convection MHD flow in a saturated vertical porous tube of circular cross-section with a uniform heat source is examined in this paper. The governing equations are solved for velocity and temperature fields in the form of Fourier Bessel series. (author). 12 refs

229

Laminar convective heat transfer of non-Newtonian nanofluids with constant wall temperature

Nanofluids are obtained by dispersing homogeneously nanoparticles into a base fluid. Nanofluids often exhibit higher heat transfer rate in comparison with the base fluid. In the present study, forced convection heat transfer under laminar flow conditions was investigated experimentally for three types of non-Newtonian nanofluids in a circular tube with constant wall temperature. CMC solution was used as the base fluid and ?-Al2O3, TiO2 and CuO nanoparticles were homogeneously dispersed to create nanodispersions of different concentrations. Nanofluids as well as the base fluid show shear thinning (pseudoplastic) rheological behavior. Results show that the presence of nanoparticles increases the convective heat transfer of the nanodispersions in comparison with the base fluid. The convective heat transfer enhancement is more significant when both the Peclet number and the nanoparticle concentration are increased. The increase in convective heat transfer is higher than the increase caused by the augmentation of the effective thermal conductivity.

Hojjat, M.; Etemad, S. Gh.; Bagheri, R.; Thibault, J.

2011-02-01

230

Thermally Developing Forced Convection in a Horizontal Equilateral Triangular Channel

Directory of Open Access Journals (Sweden)

Full Text Available An experimental investigation was carried out to study thermally developing fully developed laminar forced convection in a horizontal equilateral triangular channel where the channel surface was heated uniformly. The channel length was (1.5 m long and constructed from three plane (100 mm walls to form the equilateral triangular cross section. The experiments were conducted for three mass flow rates (1.91×10-3, 2.54×10-3 and 3.03×10-3 kg/s and four heat fluxes (91, 171, 272 and 406 W/m2. Reynolds number range was (1198 Re 1988. It was found that local Nusselt number increasing as the heat fluxes and the mass flow rate increased. Also it was found that the greatest local Nusselt number value was in the beginning of the channel then it decreased accompanied by growing the thermal boundary layer along the channel. Present experimental results have a good agreement with previous results obtained for similarly configured channels.

Manar Mahdi

2013-04-01

231

Convective heat transfer in a closed two-phase thermosyphon

A numerical analysis of heat transfer processes and hydrodynamics in a two-phase closed thermosyphon in a fairly wide range of variation of governing parameters has been investigated. A mathematical model is formulated based on the laws of mass conservation, momentum and energy in dimensionless variables "stream function - vorticity vector velocity - temperature". The analysis of the modes of forced and mixed convection for different values of Reynolds number and heat flows in the evaporation zone, the possibility of using two-phase thermosyphon for cooling gas turbine blades, when the heat is coming from the turbine blades to the thermosyphon is recycled a secondary refrigerant has been studied with different values of the centrifugal velocity. Nusselet Number, streamlines, velocity, temperature fields and temperature profile has been calculated during the investigation.

Al-Ani, M. A.

2014-08-01

232

Convective heat transfer and infrared thermography.

Infrared (IR) thermography, because of its two-dimensional and non-intrusive nature, can be exploited in industrial applications as well as in research. This paper deals with measurement of convective heat transfer coefficients (h) in three complex fluid flow configurations that concern the main aspects of both internal and external cooling of turbine engine components: (1) flow in ribbed, or smooth, channels connected by a 180 degrees sharp turn, (2) a jet in cross-flow, and (3) a jet impinging on a wall. The aim of this study was to acquire detailed measurements of h distribution in complex flow configurations related to both internal and external cooling of turbine components. The heated thin foil technique, which involves the detection of surface temperature by means of an IR scanning radiometer, was exploited to measure h. Particle image velocimetry was also used in one of the configurations to precisely determine the velocity field. PMID:12496015

Carlomagno, Giovanni M; Astarita, Tommaso; Cardone, Gennaro

2002-10-01

233

Heat transfer by natural convection in an internally heated reactor materials melt. Rev. 0

International Nuclear Information System (INIS)

The report is structured as follows: Heat flux calculations and estimates for In-Vessel Retention (IVR); Rayleigh-Benard convection and the Rayleigh number (criterion); Free convection in the horizontal layer of a liquid which is heated by internal heating sources, and a modified Rayleigh criterion; and Turbulent convection at the reactor vessel bottom during IVR. (P.A.)

234

Numerical Study Of Magneto-Fluid-Mechanics Forced Convection Pipe Flow

Digital Repository Infrastructure Vision for European Research (DRIVER)

The problem of fully developed, laminar, steady, forced convection heat transfer in an electrically conducting fluid flowing in an electrically insulated, horizontal, circular pipe (whose wall is subjected to a uniform heat flux) in a vertical uniform transverse magnetic field has been considered numerically. The central difference scheme is employed in the analysis. For high Hartmann numbers (100 •& M •& 500), the refinement of the mesh in the radial direction is necessary in order to ha...

Al Khawaja, M. J.

1994-01-01

235

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 distance 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 geom...

Carlos Henrique Marchi; Maykel Alexandre Hobmeir

2007-01-01

236

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

237

Transient natural convection in heated inclined tubes

Energy Technology Data Exchange (ETDEWEB)

To simulate natural convection flow patterns in directionally drilled wellbores, experiments and analyses were conducted for a circular tube with length-to-diameter (L/D) ratio of 36 at angles of 0{degree}, 20{degree}, and 35{degree} from the vertical. The tube was heated at the bottom and cooled at the top, and the insulation was adjusted so that approximately one- to two-thirds of the power dissipated was transferred through the tube wall to the surroundings. An aqueous solution of polyvinyl alcohol was employed as the working fluid in order to obtain low Rayleigh numbers corresponding to conditions in geothermal wellbores. Results were primarily qualitative but were useful in providing insight into the phenomena occurring. Steady-state temperature distributions were measured for the three orientations and for several heating rates to demonstrate the effects of tube angle and Rayleigh number. transient measurements of the temperature distribution were obtained during cooling from a higher temperature without a heat source to calibrate the heat losses. With the electrical heat source, temporal data were taken during heating to examine the approach to steady state. Quasi-steady flow conditions were approached rapidly, but the overall time constant of the apparatus was of the order of one-third of a day. Predictions with the three-dimensional TEMPEST code were first tested by comparison with simple conduction analyses. Comparison with actual data showed good agreement of the predicted temperature levels for the maximum inclination, 35{degree}, and slightly poorer agreement for the other limit, a vertical tube. Trends of temperature level and Nusselt number with heating rate or Rayleigh number were reasonable, but the predicted variation of the end Nusselt number versus inclination was in the opposite direction from the experiment. 75 refs., 20 figs., 8 tabs.

McEligot, D.M. (Westinghouse Electric Corp., Middletown, RI (USA). Oceanic Div.); Denbow, D.A. (Software AG of North America, Inc., Lakewood, CO (USA)); Murphy, H.D. (Los Alamos National Lab., NM (USA))

1990-05-01

238

International Nuclear Information System (INIS)

The vertical upward flow of water in a heated tube at a supercritical pressure was numerically simulated by means of a commercially available computational fluid dynamics code, FLUENT. To examine the reliability of the embedded turbulence models at a supercritical pressure, a series of simulations was performed with several two-equation turbulence models: a representative low-Reynolds number k-? model, k-? model, RNG k-? model as well as a standard k-? model. The turbulence models were evaluated by comparing the results of the simulations with the experimental data published by Yamagata et al. (1972). In normal heat transfer enhancement regions, the RNG k-? model with an enhanced wall treatment option reproduces best the experiment. But, in heat transfer deterioration regions, the difference between the simulations and the experiment are remarkable. The heat transfer trend predicted by the calculation with the k-? model and low-Reynolds number k-? model is qualitatively similar to that of the experiment. Therefore, in the heat transfer deterioration regions, the k-? models could be a candidate for a reasonable solution if some modifications are made. (author)

239

International Nuclear Information System (INIS)

A numerical investigation was conducted on the transient behavior of a hydrodynamically, fully developed, laminar flow of power-law fluids in the thermally developing entrance region of circular ducts taking into account the effect of viscous dissipation but neglecting the effect of axial conduction. In this regard, the unsteady state thermal energy equation was solved by using a finite difference method, whereas the steady state thermal energy equation without wall heat flux was solved analytically as the initial condition of the former. The effects of the power-law index and wall heat flux on the local Nusselt number and thermal entrance length were investigated. Moreover, the local Nusselt number of steady state conditions was correlated in terms of the power-law index and wall heat flux and compared with literature data, which were obtained by an analytic solution for Newtonian fluids. Furthermore, a relationship was proposed for the thermal entrance length

240

International Nuclear Information System (INIS)

This paper presents an extensive study of heat-transfer correlations applicable to supercritical-water flow in vertical bare tubes. A comprehensive dataset was collected from 33 papers by 27 authors, including more than 125 graphs and wide range of parameters. The parameters range was as follows: pressures 22.5 - 34.5 MPa, inlet temperatures 85 - 350oC, mass fluxes 250 - 3400 kg/m2s, heat fluxes 75 - 5,400 kW/m2, tube heated lengths 0.6 - 27.4 m, and tube inside diameters 2 - 36 mm. This combined dataset was then investigated and analyzed by calculating Heat Transfer Coefficients (HTCs) and wall temperatures using various correlations and comparing them with the corresponding experimental results. Three correlations were used in this comparison: original Bishop et al., Mokry et al. (modified Bishop et al.) and Gupta et al. (modified Swenson et al). The main objectives of this study were a selection of the best supercritical-water bare-tube correlation for HTC calculations in: 1) fuel bundles of SuperCritical Water-cooled Reactors (SCWRs) as a preliminary and conservative approach; 2) heat exchangers in case of indirect-cycle SCW Nuclear Power Plants (NPPs); and 3) heat exchangers in case of hydrogen co-generation at SCW NPPs from SCW side. The comparison showed that in most cases, the Bishop et al. correlation deviates significantly from the experimental data within the pseudocritical region and actually, underestimates the temperatured actually, underestimates the temperature in the most cases. On the other hand, the Mokry et al. and Gupta et al. correlations showed a relatively better fit within the most operating conditions. In general, the Gupta et al. correlation showed slightly better fit with the experimental data than the Mokry et al. correlation. (author)

241

Aerosol indirect effects, i.e., the interactions of aerosols with clouds by serving as cloud condensation nuclei or ice nuclei constitute the largest uncertainty in climate forcing and projection. Previous IPCC reported negative aerosol indirect forcing, which does not account for aerosol-convective cloud interactions because the complex processes involved are poorly understood and represented in climate models. Here we elucidated how aerosols change convective intensity, diabatic heating, and regional circulation under different environmental conditions. We found that aerosol indirect effect on deep convective cloud systems could lead to enhanced regional convergence and a strong top-of-atmosphere warming. Aerosol invigoration effect occurs mainly in warmed-based convection with weak shear. This could result in a strong radiative warming in the atmosphere (up to +5.6 W m-2), a lofted latent heating, and a reduced diurnal temperature difference, all of which could potentially impact regional circulation and modify weather systems. The positive aerosol radiative forcing on deep clouds could offset the negative aerosol radiative forcing on low clouds to an unknown extent.

Fan, Jiwen; Rosenfeld, Daniel; Ding, Yanni; Leung, L. Ruby; Li, Zhanqing

2012-05-01

242

Energy Technology Data Exchange (ETDEWEB)

A numerical study is conducted on time-dependent double-diffusive natural convection heat transfer in a horizontal annulus. The inner cylinder is heated with sinusoidally-varying temperature while the outer cylinder is maintained at a cold constant temperature. The numerical procedure used in the present work is based on the Galerkin weighted residual method of finite-element formulation by incorporating a non-uniform mesh size. Comparisons with previous studies are performed and the results show excellent agreement. In addition, the effects of pertinent dimensionless parameters such as the thermal Rayleigh number, Buoyancy ratio, Lewis number, and the amplitude of the thermal forcing on the flow and heat transfer characteristics are considered in the present study. Furthermore, the amplitude and frequency of the heated inner cylinder is found to cause significant augmentation in heat transfer rate. The predictions of the temporal variation of Nusselt and Sherwood numbers are obtained and discussed. (orig.)

Al-Amiri, Abdalla M. [United Arab Emirates University, Mechanical Engineering Department, Al-Ain (United Arab Emirates); Khanafer, Khalil [University of Michigan, Biomedical Engineering Department, Ann Arbor, MI (United States); Lightstone, Marilyn F. [McMaster University, Mechanical Engineering Department, Hamilton, ON (Canada)

2006-09-15

243

A numerical study is conducted on time-dependent double-diffusive natural convection heat transfer in a horizontal annulus. The inner cylinder is heated with sinusoidally-varying temperature while the outer cylinder is maintained at a cold constant temperature. The numerical procedure used in the present work is based on the Galerkin weighted residual method of finite-element formulation by incorporating a non-uniform mesh size. Comparisons with previous studies are performed and the results show excellent agreement. In addition, the effects of pertinent dimensionless parameters such as the thermal Rayleigh number, Buoyancy ratio, Lewis number, and the amplitude of the thermal forcing on the flow and heat transfer characteristics are considered in the present study. Furthermore, the amplitude and frequency of the heated inner cylinder is found to cause significant augmentation in heat transfer rate. The predictions of the temporal variation of Nusselt and Sherwood numbers are obtained and discussed.

Al-Amiri, Abdalla M.; Khanafer, Khalil; Lightstone, Marilyn F.

2006-09-01

244

Directory of Open Access Journals (Sweden)

Full Text Available Experimental and CFD analysis is conducted in order to establish effect of geometrical fin parameters for natural convection heat transfer from vertical rectangular fin arrays.Natural convective heat transfer from rectangular vertical plates has been reviewed. Study revealed that most of the work was carried out considering various configurations. Experimental work carried on steady state natural convection heat transfer from vertical rectangular fins made of aluminum. Experimental work carried investigates the effect of fin spacing, fin height, fin length on the performance of heat dissipation from the fin arrays. It is found that convection heat transfer rate depends on fin height and fin length. For a given fin spacing, the convection heat transfer rate from fins increases with fin height. For a given fin spacing, the convection heat transfer rate from fins increases with fin length .This trend is observed for every fin configuration. It is found that convection heat transfer rate is more for less aspect ratio fin array for same power input.

V.S.Daund*

2014-08-01

245

Laminar forced convection and flow characteristics for the multiple plate porous insulation

International Nuclear Information System (INIS)

A numerical study of steady state flow and heat transfer has been conducted for the multiple plate porous insulation used in the reactor pressure vessels of 'Magnox' nuclear power stations. The insulation pack studied, consisting of seven dimpled stainless steel sheets and six plane stainless steel sheets, was of the type installed in the Sizewell A plant. In the reactor application the fluid within the insulation pack is carbon dioxide at 20 bar but in the numerical investigation the insulation performance was examined in air at lower pressures. A three-dimensional computation model with a periodicity condition was used in the numerical investigation. Result was obtained for laminar forced convection with constant wall temperatures. Numerical results are presented to show the flow and thermal fields in a single flow passage. In forced convection it is shown that mid-dimple 'peaking' of the Nusselt number distribution may be related directly to the convective influence of distorted velocity profiles

246

Forced Convection and Sedimentation Past a Flat Plate

The steady laminar flow of a well-mixed suspension of monodisperse solid spheres, convected steadily past a horizontal flat plate and sedimenting under the action of gravity, is examined. It is shown that, in the limit as Re approaches infinity and epsilon approaches 0, where Re is the bulk Reynolds number and epsilon is the ratio of the particle radius a to the characteristic length scale L, the analysis for determining the particle concentration profile has several aspects in common with that of obtaining the temperature profile in forced-convection heat transfer from a wall to a fluid stream moving at high Reynolds and Prandtl numbers. Specifically, it is found that the particle concentration remains uniform throughout the O(Re(exp -1/2)) thick Blasius boundary layer except for two O(epsilon(exp 2/3)) thin regions on either side of the plate, where the concentration profile becomes non-uniform owing to the presence of shear-induced particle diffusion which balances the particle flux due to convection and sedimentation. The system of equations within this concentration boundary layer admits a similarity solution near the leading edge of the plate, according to which the particle concentration along the top surface of the plate increases from its value in the free stream by an amount proportional to X(exp 5/6), with X measuring the distance along the plate, and decreases in a similar fashion along the underside. But, unlike the case of gravity settling on an inclined plate in the absence of a bulk flow at infinity considered earlier, here the concentration profile remains continuous everywhere. For values of X beyond the region near the leading edge, the particle concentration profile is obtained through the numerical solution of the relevant equations. It is found that, as predicted from the similarity solution, there exists a value of X at which the particle concentration along the top side of the plate attains its maximum value phi(sub m) and that, beyond this point, a stagnant sediment layer will form that grows steadily in time. This critical value of X is computed as a function of phi(sub s), the particle volume fraction in the free stream. In contrast, but again in conformity with the similarity solution, for values of X sufficiently far removed from the leading edge along the underside of the plate, a particle-free region is predicted to form adjacent to the plate. This model, with minor modifications, can be used to describe particle migration in other shear flows, as, for example, in the case of crossflow microfiltration.

Pelekasis, Nikolaos A.; Acrivos, Andreas

1995-01-01

247

International Nuclear Information System (INIS)

An experimental investigation has been conducted to determine the local condensation heat transfer coefficient (HTC) of steam in the presence of air or helium flowing downward inside a 46-mm-i.d. vertical tube. The gas-steam mixture flow rate was measured with a calibrated vortex flowmeter before it entered the 2.54-m-long test condenser. Cooling water flow rate in an annulus around the tube was measure with a calibrated rotameter. Temperatures of the cooling water, the gas-steam mixture, and the tube inside and outside surfaces were measured at 0.3-m intervals in the test condenser. Inlet and exit pressures and temperatures of the gas-steam mixture and of the cooling water were also measured. The local heat flux was obtained from the slope of the coolant axial temperature profile and the coolant mass flow rate. It was found that for the same mass fraction of the noncondensable gas, compared with air, helium has a more inhibiting effect on the heat transfer, but for the same molar ratio, air was found to be more inhibiting. An application where there is important is the proposed advanced passive boiling water reactor design (Simplified Boiling Water Reactor), which utilizes the isolation condenser as a main component of the passive containment cooling system (PCCS)

248

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

249

Absolute-convective instability of mixed forced-free convection boundary layers

International Nuclear Information System (INIS)

A spatio-temporal inviscid instability of a mixed forced-free convection boundary layer is investigated. The base flow considered is the self-similar flow with free-stream velocity ue ? xn. Such a boundary-layer flow presents the unusual behaviour of generating a region of velocity overshoot, in which the streamwise velocity within the boundary layer exceeds the free-stream speed. A linear stability analysis has been carried out. Saddle points have been located and a critical value for the buoyancy parameter, G0c ? 3.6896, has been determined below which the flow is convectively unstable and above which the flow becomes absolutely unstable. Two spatial modes have been obtained, one mode being convective in nature and the other absolute. The convective-type spatial mode shows mode crossing behaviour at lower frequencies. Thermal buoyancy is shown to be destabilizing to the absolutely unstable spatial mode.

250

Visualization of bubble behaviors in forced convective subcooled flow boiling

International Nuclear Information System (INIS)

Condensation characteristics of vapor bubble after the departure from a heated section in forced convective subcooled flow boiling were studied visually by using a high speed camera. The purpose of the present study was to measure two-phase flow parameters in subcooled flow boiling. These two-phase flow parameters are void fraction, interfacial area concentration and Sauter mean diameter, which express bubble interface behaviors. The experimental set-up was designed to measure the two-phase flow parameters necessary for developing composite equations for the two fluid models in subcooled flow boiling. In the present experiments, the mass flux, liquid subcooling and the heater were varied within 100-1000kg/m2s, 2-10K and 100-300kW/m2 respectively. Under these experimental conditions, the bubble images were obtained by a high-speed camera, and analyzed paying attention to the condensation of vapor bubbles. These two-phase parameters were obtained by the experimental data, such as the bubble parameter, the bubble volume and the bubble surface. In the calculation process of the two phase flow parameters, it was confirmed that these parameters are related to the void fraction. (author)

251

Conjugate parallel-flowing free and forced convection boundary layers on vertical wall sides

Energy Technology Data Exchange (ETDEWEB)

The paper describes an analytical study of two parallel-flowing boundary layers of free and forced convection modes on the facing sides of a vertical thin wall. The two layers are analyzed separately within the framework of boundary layer theory, and coupled by the matching conditions at wall. Numerical data are obtained for a wide range of a dimensionless conjugation parameter {zeta} relating the heat transfer effectiveness of two convection modes. Based on these data, an expression for calculating the conjugate mean Nusselt number as a function of {zeta}-parameter is found by means of a curve-fitting method. (orig.)

Mosaad, M.; Ben-Nakhi, Abdullatif [Faculty of Technological Studies, Mechanical Power and Refrigeration Department (Kuwait)

2003-02-01

252

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

253

Convective boiling heat transfer of water in the transition region

International Nuclear Information System (INIS)

An experimental study of forced convective boiling heat transfer for upflow of water in a circular tube has been performed using a heat transfer system with temperature-controlled indirect Joule heating. In this way, complete boiling curves from incipience of boiling to film boiling could be measured including the transition boiling regime. Usually, the test were performed in a quasi-steady mode by increasing the set-point wall temperature average at a constant time rate. The main body of the results covers the pressure range from 0.1 to 1.0 MPa, mass flux range from 25 to 200 kg/(m2s) and inlet subcooling from 5 to 30 K. The empirical data in the transition region were correlated in terms of a heat flux/surface superheat relationship that was normalized by the maximum heat flux and its corresponding wall superheat, respectively, to anchor the transition boiling curve to its low temperature limit. The exponent n in this power law relation was assumed to depend on the ratio of phase densities, normalized wall superheat and inlet subcooling as well as a dimensionless mass flux with a rms error of 17.6 % if compared to 1094 data points within the ranges of system parameters as noted above. (author)

254

Time evolution simulation of heat removal in a small water tank by natural convection

International Nuclear Information System (INIS)

One of the cooling modes for any source of heat such as in a shutdown nuclear core is the natural convection. The design specifications of any cooling pool can only be done when the removal heat rate and the corresponding mass flow rate is reasonably established. In our simulation scheme, we assumed that the body forces acting in the cubic water cell are: the weight, the drag force and the integrated pressure forces on the horizontal surfaces, the viscosity shear forces on the vertical surfaces and also a special viscosity drag force due to the mass dislocation along a Bernoulli type current tube outside the motive region. For a suitable time step, the uprising convection velocity is determined by an implicit and also by an explicit solution algorithm. The resulting differential equation depends on updating specific mass, dynamic viscosity and constant pressure heat coefficient with the last known temperature in the cell that absorbed heat. Numerical calculation software was performed using MATLAB’s technical computing language and then applied for a heat generation plate simulating a spent fuel assembler from a shutdown nuclear core. The results show time evolution of convection, terminal velocity and water temperature distribution. Pool dimension as well as pool level decrement are also determined for various air exhausting system conditions and heat rate of the spent fuel plate being cooled. (author)

255

International Nuclear Information System (INIS)

A mechanistic model for forced convective transition boiling has been developed to investigate transition boiling mechanisms and to predict transition boiling heat flux realistically. This model is based on a postulated multi-stage boiling process occurring during the passage time of the elongated vapor blanket specified at a critical heat flux (CHF) condition. Between the departure from nucleate boiling (DNB) and the departure from film boiling (DFB) points, the boiling heat transfer is established through three boiling stages, namely, the macrolayer evaporation and dryout governed by nucleate boiling in a thin liquid film and the unstable film boiling characterized by the frequent touches of the interface and the heated wall. The total heat transfer rates after the DNB is weighted by the time fractions of each stage, which are defined as the ratio of each stage duration to the vapor blanket passage time. The model predictions are compared with some available experimental transition boiling data. The parametric effects of pressure, mass flux, inlet subcooling on the transition boiling heat transfer are also investigated. From these comparisons, it can be seen that this model can identify the crucial mechanisms of forced convective transition boiling, and that the transition boiling heat fluxes including the maximum heat flux and the minimum film boiling heat flux are well predicted at low qualities/high pressures near 10 bar. In future, this model will be improved in In future, this model will be improved in the unstable film boiling stage and generalized for high quality and low pressure situations

256

Natural and forced convection film boiling over axisymmetric bodies at high temperatures

International Nuclear Information System (INIS)

Natural and forced convection film boiling over axisymmetric bodies is analysed in the case of high surface temperatures. In these conditions, the global heat transfer is no more driven by simple conduction through the vapour film, an assumption that is commonly made in film boiling modelling for simplicity reasons, but rather by convection through this film. Therefore a mathematical method was developed which enables a full description of the vapour flow by including the inertia and convection terms of the momentum and energy equations. It is based on classical two-phase boundary layer integral methods where polynomial functions of order 5 are used to describe the velocity and the temperature profiles in the vapour flow. Also, a simple scaling analysis is described in order to understand when inertial and convective effects in the vapour flow become important. It is shown for example that for any given fluid, these effects will become predominant even at a low surface superheat when the fluid pressure is increased near its critical pressure. The developed models are then compared with three simpler models: a similar model which uses order 3 polynomial functions, a model where the convective effects are modelled by just using an effective latent heat, and an even simpler model where convection and inertia in the vapour film are not considered at all. It is shown on some examples with very high surface temperatures that if convective and inertial effects are totally neglected, the global heat transfer is clearly underestimated compared to the ones calculated with the two other models, which are quite similar. However, if other important parameters such as vapour production or vapour film thickness at the front stagnation point are calculated, the results given by the model with the effective latent heat diverge from those given by the developed models. (author)

257

Endwall convective heat transfer for bluff bodies

DEFF Research Database (Denmark)

The endwall heat transfer characteristics of forced flow past bluff bodies have been investigated using liquid crystal thermography (LCT). The bluff body is placed in a rectangular channel with both its ends attached to the endwalls. The Reynolds number varies from 50,000 to 100,000. In this study, a single bluff body and two bluff bodies arranged in tandem are considered. Due to the formation of horseshoe vortices, the heat transfer is enhanced appreciably for both cases. However, for the case of two bluff bodies in tandem, it is found that the presence of the second bluff body decreases the heat transfer as compared to the case of a single bluff body. In addition, the results show that the heat transfer exhibits Reynolds number similarity. For a single bluff body, the Nusselt number profiles collapse well when the data are scaled by Re0.55; for two bluff bodies arranged in tandem, the heat transfer scaling is changed to Re0.51, indicating that the power index of Reynolds number is flow dependent.

Salewski, Mirko

2012-01-01

258

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)

259

Effects of convection instability due to incompatibility between ocean dynamics and surface forcings

Directory of Open Access Journals (Sweden)

Full Text Available The study demonstrates that an incompatibility between a surface temperature climatology and a given ocean model, into which the climatology is assimilated via Haney restoration, can cause model ocean climate drift and interdecadal oscillations when the ocean is switched to a weaker restoration. This is made using an idealized Atlantic Ocean model driven by thermal and wind forcing only. Initially, the temperature climatology is forcefully assimilated into the model, and an implied heat flux field is diagnosed. During this stage any incompatibility is suppressed. The restoring boundary condition is then switched to a new forcing consisting of a part of the diagnosed flux and a part of the restoring forcing in such a way that at the moment of the switching the heat flux is identical to that prior to the switching. Under this new forcing condition, the incompatibility becomes manifest, causing changes in convection patterns, and producing drift and interdecadal oscillations. The mechanisms are described.

W. Cai

260

Electrode mass transfer under conditions of natural and forced convection

In order to investigate the mass transfer to a cathode under conditions of natural and forced convection, silver was plated under conditions where the deposition rate was controlled by mass transfer. Rather than analyzing chemically for the deposited silver, it was determined locally by electrochemical stripping. This enabled the mass transfer coefficients to be determined accurately over the whole surface of the electrode under conditions of natural convection, electrolyte jetting, and gas sparging. In general, the mass transfer coefficient was enhanced significantly only in the very close vicinity of the agitation source. An improved sparger design with closely spaced multiple bubble sources was found to enhance overall mass transfer.

Cooke, A. V.; Chilton, J. P.; Fray, D. J.

1989-02-01

261

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

Directory of Open Access Journals (Sweden)

Full Text Available This paper aims to demonstrate the importance of adequately estimating the discretization error intrinsic in the result of any numerical simulation. The problem under consideration is forced convection in a staggered circular tube heat exchanger. The problem is solved to analyze the effect of the distance 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

2007-03-01

262

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.

2007-03-01

263

Laminar forced convection inside externally finned tubes

International Nuclear Information System (INIS)

Analytical solutions are obtained for thermal entry region problems inside ducts with axially varying heat transFer coefficient, by making use of the ideas in the recently advanced generalized integral transform technique. The analysis is applied to stepwise variations of Biot number that simulate intermitent rows of external fins. Numerical results are obtained for different fin arrangements, in a systematic manner, so as to critically examine the relative marits of wider and/or more numerous fin rows. (author)

264

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...

Mohammad Al-Nimr; Mohammad Abuzaid; Osamah Haddad

2004-01-01

265

International Nuclear Information System (INIS)

Pressure drop is significantly affected by heat transfer in mixed convection situations. A semi-empirical correlation for pressure drop is developed from a theoretical base by first making use of the momentum integral solution to the heat and momentum equations for natural convection on a vertical surface. The boundary condition on the free-stream side of the boundary layer is changed to reflect the shear on that surface due to the forced convection, and empirical data are used to develop a formula useful in design and applications. The equation may be used with bulk fluid properties or film properties. The equation is valid for laminar, mixed convection conditions in vertical, internal, aiding flows with constant wall temperature boundary condition

266

When a heated solid sphere is introduced into an ambient fluid, a natural convective flow occurs which results in a drag force on the sphere. This study involves the numerical calculation of both the steady-state and the transient natural convective drag force around spheres at low Grashof numbers. Numerical techniques are taken from Geoola and Cornish. An empirical expression is suggested for the total drag coefficient for Grashof numbers ranging from 4 x 10 to 0.5 and Prandtl number = 0.72: log C sub DT = 1.25 + 0.31 log Gr - 0.097(log Gr). The dimensionless time required to reach 90% of the steady-state drag force can be approximated by the second-order polynomial: log t sub 90% = 1.32 - log Gr - 0.11(Gr).

Dudek, D.; Fletcher, T. H.

1987-02-01

267

Interaction of forced and free convection in case of an accident in a nuclear reactor

International Nuclear Information System (INIS)

This paper presents an experimental investigation of the interactions of forced and free convection in a vertical tube in case of reduction of flow. The parameters studies range from fully turbulent forced convection conditions to strongly buoyancy influenced conditions. Both upward and downward flows are considered. The tube is electrically heated to simulate a nuclear reactor channel with constant heat flux boundary condition. The results show that for upward flow the heat transfer could be either impaired or enhanced depending on the extent of buoyancy influence. A localized temperature peak developed on the wall temperature distribution under buoyancy influenced condition. The peak shifts towards the tube entrance as the buoyancy effect is increased. For downward flow, buoyancy forces always cause an enhancement of the heat transfer coefficient relative to the forced convection value. The wall temperature distribution stays well behaved over the entire channel length and no localized peaks were observed. The experimental data is correlated using a semi-empirical model which accounts for the modification of shear stress due to buoyancy forces. Correlation equations for the heat transfer coefficient and the temperature peaks are presented in terms of a buoyancy parameter which suitably combine the Reynolds, Grashof and Prandtl numbers in a manner suggested by theory. These equations represent the experimental data very well over a wide range of parameters. It can be concluded from the results of this study that in case of a reduction of flow in a reactor channel where the flow is in the downward direction, buoyancy always works in our favor. However, if the flow is in the upward direction the cladding surface temperature may develop localized peaks as a result of buoyancy. The temperature at these peaks may exceed safe limits under certain conditions.(author)

268

Directory of Open Access Journals (Sweden)

Full Text Available The paper presented the most aspects of convective circulate mode of heat transfer : heat transfer through the boundary layer formed at the surface of the heat generator; heat transfer in the heat carrier and heat transfer through the boundary layer formed at the heated surface

Nadia Potoceanu

2007-10-01

269

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.

2014-09-01

270

Natural convective heat transfer from short inclined cylinders

Natural Convective Heat Transfer from Short Inclined Cylinders examines a heat transfer situation of significant, practical importance not adequately dealt with in existing textbooks or in any widely available review papers. Specifically, the book introduces the reader to recent studies of natural convection from short cylinders mounted on a flat insulated base where there is an “exposed” upper surface. The authors considers the effects of the cylinder cross-sectional shape, the cylinder inclination angle, and the length-to-cross sectional size of the cylinder. Both numerical and experimental studies are discussed and correlation equations based on the results of these studies are reviewed. This book is ideal for professionals involved with thermal management and related systems, researchers, and graduate students in the field of natural convective heat transfer, instructors in graduate level courses in convective heat transfer.

Oosthuizen, Patrick H

2014-01-01

271

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

272

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)

273

Predictions of laminar natural convection in heated cavities

International Nuclear Information System (INIS)

This chapter discusses, with illustrative calculations, several examples of laminar, natural convection in heated cavities. These include convection in the following types of cavity: a square cavity at high Rayleigh number; a narrow cavity at moderate aspect ratio; a rectangular cavity heated from below; a trapezoidal cavity; 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

274

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

275

Solution of heat removal from nuclear reactors by natural convection

Directory of Open Access Journals (Sweden)

Full Text Available This paper summarizes the basis for the solution of heat removal by natural convection from both conventional nuclear reactors and reactors with fuel flowing coolant (such as reactors with molten fluoride salts MSR.The possibility of intensification of heat removal through gas lift is focused on. It might be used in an MSR (Molten Salt Reactor for cleaning the salt mixture of degassed fission products and therefore eliminating problems with iodine pitting. Heat removal by natural convection and its intensification increases significantly the safety of nuclear reactors. Simultaneously the heat removal also solves problems with lifetime of pumps in the primary circuit of high-temperature reactors.

Zitek Pavel

2014-03-01

276

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

277

SIMULATION OF MIXED CONVECTIVE HEAT TRANSFER USING LATTICE BOLTZMANN METHOD

Directory of Open Access Journals (Sweden)

Full Text Available In this paper, mixed (forced–natural convective heat transfer around a heated square cylinder located inside a lid driven cavity has been studied numerically using the lattice Boltzmann method in the range of 100? Re ? 1000 with the corresponding Richardson number 0.01?Ri?10. The double-population lattice Boltzmann formulation is used as the governing equation. Two dimensional nine-velocity models are used for the computation of the velocity field while a four-velocity model is used for the computation of the temperature field. We found that the combination of nine- and four-velocity models can be applied to the calculation without losing its accuracy. The results are presented in the form of streamline and isotherm plots as well as the variation of local Nusselt number at the top surface of the heated square. The computational results demonstrate that the flow pattern, formation of vortex and also the Nusselt number are influence by the Reynolds number and Richardson number.

A. R. M. Rosdzimin

2010-12-01

278

Digital Repository Infrastructure Vision for European Research (DRIVER)

A numerical study of steady state laminar forced and free convective and radiative heat transfer in an inclined rotating rectangular duct with a centered circular tube is reported for an hydrodynamically fully developed flow. The two heat transfer mechanisms of convection and radiation are treated independently and simultaneously. The coupled equations of momentum and energy transports are solved using Gauss-Seidel iteration technique subject to given boundary constraints. A thermal boundary ...

Adegun, I. K.; Bello-ochende, F. L.

2004-01-01

279

Slow forced and free convection in inclined channels

International Nuclear Information System (INIS)

Mixed convection at low Reynolds numbers in tilted rectangular channels of intermediate aspect ratios is dealt with. A laminar flow enters into an inclined enclosure whose upper plate is adiabatic whereas a constant heat flux is supplied to the bottom plate. The analytic solution of the linearised problem is first obtained. Subsequently the numerical solution of the pertinent equations is carried on by a computer's time saving iterative procedure. (Author)

280

A study of heat transfer in the case of single-phase convection in a duct with a porous insert

The effect of a porous matrix on heat transfer in a plane channel under conditionss of forced convection is investigated by the integral method using a two-temperature model, with allowance made for coolant mixing. Analytical expressions for the heat transfer coefficient are obtained which are in good agreement with experimental data. It is shown that heat flow inhomogeneity leads to a significant reduction in the local heat transfer coefficient, particularly at a large distance from the heat flow maximum.

Gortyshov, Iu. F.; Ashikhmin, S. R.; Nadyrov, I. N.

281

Latent heat energy storage systems with both annular and countercurrent flows are modeled numerically. The change of phase of the phase-change material (PCM) and the transient forced convective heat transfer for the transfer fluid are solved simultaneously as a conjugate problem. A parametric study and a system optimization are conducted. It is found that the energy storage system with the countercurrent flow is an efficient way to absorb heat energy in a short period for pulsed power load space applications.

Cao, Y.; Faghri, A.; Juhasz, A.

1991-02-01

282

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

283

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...

Consolini, Lorenzo

2008-01-01

284

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)

285

Convective heat transfer around vertical jet fires: An experimental study

Energy Technology Data Exchange (ETDEWEB)

Highlights: Black-Right-Pointing-Pointer Experiments were carried out to analyze convection around a vertical jet fire. Black-Right-Pointing-Pointer Convection heat transfer is enhanced increasing the flame length. Black-Right-Pointing-Pointer Nusselt number grows with higher values of Rayleigh and Reynolds numbers. Black-Right-Pointing-Pointer In subsonic flames, Nusselt number increases with Froude number. Black-Right-Pointing-Pointer Convection and radiation are equally important in causing a domino effect. - Abstract: The convection heat transfer phenomenon in vertical jet fires was experimentally analyzed. In these experiments, turbulent propane flames were generated in subsonic as well as sonic regimes. The experimental data demonstrated that the rate of convection heat transfer increases by increasing the length of the flame. Assuming the solid flame model, the convection heat transfer coefficient was calculated. Two equations in terms of adimensional numbers were developed. It was found out that the Nusselt number attains greater values for higher values of the Rayleigh and Reynolds numbers. On the other hand, the Froude number was analyzed only for the subsonic flames where the Nusselt number grows by this number and the diameter of the orifice.

Kozanoglu, Bulent, E-mail: bulentu.kozanoglu@udlap.mx [Universidad de las Americas, Puebla (Mexico); Zarate, Luis [Universidad Popular Autonoma del Estado de Puebla (Mexico); Gomez-Mares, Mercedes [Universita di Bologna (Italy); Casal, Joaquim [Universitat Politecnica de Catalunya (Spain)

2011-12-15

286

Convective heat transfer around vertical jet fires: An experimental study

International Nuclear Information System (INIS)

Highlights: ? Experiments were carried out to analyze convection around a vertical jet fire. ? Convection heat transfer is enhanced increasing the flame length. ? Nusselt number grows with higher values of Rayleigh and Reynolds numbers. ? In subsonic flames, Nusselt number increases with Froude number. ? Convection and radiation are equally important in causing a domino effect. - Abstract: The convection heat transfer phenomenon in vertical jet fires was experimentally analyzed. In these experiments, turbulent propane flames were generated in subsonic as well as sonic regimes. The experimental data demonstrated that the rate of convection heat transfer increases by increasing the length of the flame. Assuming the solid flame model, the convection heat transfer coefficient was calculated. Two equations in terms of adimensional numbers were developed. It was found out that the Nusselt number attains greater values for higher values of the Rayleigh and Reynolds numbers. On the other hand, the Froude number was analyzed only for the subsonic flames where the Nusselt number grows by this number and the diameter of the orifice.

287

Transient convective heat transfer to laminar flow from a flat plate with constant heat capacity

International Nuclear Information System (INIS)

Most basic transient heat transfer problem is the transient response characteristics of forced convection heat transfer in the flow along a flat plate or in a tube. In case of the laminar flow along a flat plate, the profile method using steady temperature distribution has been mostly adopted, but its propriety has not been clarified yet. About the unsteady heat transfer in the laminar flow along a flat plate, the analysis or experiment evaluating the heat capacity of the flat plate exactly was never carried out. The purpose of this study is to determine by numerical calculation the unsteady characteristics of the boundary layer in laminar flow and to confirm them by experiment concerning the unsteady heat transfer when a flat plate with a certain heat capacity is placed in parallel in uniform flow and given a certain quantity of heat generation suddenly. The basic equation and the solution are given, and the method of numerical calculation and the result are explained. The experimental setup and method, and the experimental results are shown. Both results were in good agreement, and the response of wall temperature, the response of Nusselt number and the change of temperature distribution in course of time were able to be determined by applying Laplace transformation and numerical Laplace inverse transformation to the equation. (Kako, I.)

288

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english In this paper, the natural convective flow of nanofluids over a convectively heated vertical plate in a saturated Darcy porous medium is studied numerically. The governing equations are transformed into a set of ordinary differential equations by using appropriate similarity variables, and they are [...] numerically solved using the fourth-order Runge-Kutta method associated with the Gauss-Newton method. The effects of parametric variation of the Brownian motion parameter (Nb), thermophoresis parameter (Nt) and the convective heating parameter (Nc) on the boundary layer profiles are investigated. Furthermore, the variation of the reduced Nusselt number and reduced Sherwood number, as important parameters of heat and mass transfer, as a function of the Brownian motion, thermophoresis and convective heating parameters is discussed in detail. The results show that the thickness of the concentration profiles is much lower than the temperature and velocity profiles. For low values of the convective heating parameter (Nc), as the Brownian motion parameter increases, the non-dimensional wall temperature increases. However, for high values of Nc, the effect of the Brownian motion parameter on the non-dimensional wall temperature is not significant. As the Brownian motion parameter increases, the reduced Sherwood number increases and the reduced Nusselt number decreases.

M., Ghalambaz; A., Noghrehabadi; A., Ghanbarzadeh.

2014-06-01

289

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english In this paper, the natural convective flow of nanofluids over a convectively heated vertical plate in a saturated Darcy porous medium is studied numerically. The governing equations are transformed into a set of ordinary differential equations by using appropriate similarity variables, and they are [...] numerically solved using the fourth-order Runge-Kutta method associated with the Gauss-Newton method. The effects of parametric variation of the Brownian motion parameter (Nb), thermophoresis parameter (Nt) and the convective heating parameter (Nc) on the boundary layer profiles are investigated. Furthermore, the variation of the reduced Nusselt number and reduced Sherwood number, as important parameters of heat and mass transfer, as a function of the Brownian motion, thermophoresis and convective heating parameters is discussed in detail. The results show that the thickness of the concentration profiles is much lower than the temperature and velocity profiles. For low values of the convective heating parameter (Nc), as the Brownian motion parameter increases, the non-dimensional wall temperature increases. However, for high values of Nc, the effect of the Brownian motion parameter on the non-dimensional wall temperature is not significant. As the Brownian motion parameter increases, the reduced Sherwood number increases and the reduced Nusselt number decreases.

M., Ghalambaz; A., Noghrehabadi; A., Ghanbarzadeh.

290

Formulation of nano fluids for natural convective heat transfer applications

International Nuclear Information System (INIS)

The paper is concerned about formulation of aqueous based nanofluids and its application under natural convective heat transfer conditions. Titanium dioxide nanoparticles are dispersed in distilled water through electrostatic stabilization mechanisms and with the aid of a high shear mixing homogenizer. Nanofluids formulated in such a way are found very stable and are used to investigate their heat transfer behaviour under the natural convection conditions. The preliminary results are presented in this paper. Both transient and steady heat transfer coefficients are measured and the results show a systematic decrease in the natural convective heat transfer coefficient with increasing particle concentration. This is in contradiction to the initial expectation. Possible reasons for the observations are discussed

291

Laminar mixed convection heat and mass transfer in an isothermally cooled channel

Energy Technology Data Exchange (ETDEWEB)

Many studies have been performed to investigate the combined effects of thermal and mass buoyancy forces in channel convection. This paper presented a numerical study of a laminar mixed convection associated with phase change in an isothermally cooled vertical parallel-plate channel. The plates were wetted by a thin liquid water film and maintained at a constant temperature lower than that of the air entering the channel. The paper discussed the use of an elliptical model, including axial diffusion of momentum, heat and mass transfer. The paper also presented the solution of the governing equations using the finite volume method with the simpler algorithm for handling the velocity-pressure coupling. Cases of film evaporation and vapor condensation were also considered. In order to to bring out the effects of buoyancy forces, results from mixed convection case and forced convection were compared. A comparative evaluation showed that these forces, mostly of thermal origin, decelerate the flow near the walls and induce flow reversal for high temperatures, thus reducing heat and mass transfer. 17 refs., 1 tab., 6 figs.

Oulaid, O. [Laboratoire de Mecanique des Fluides et d Energetique, Marrakech (Morocco). Dept. of Physics; Sherbrooke Univ., PQ (Canada). Dept. of Mechanical Engineering; Benhamou, B. [Laboratoire de Mecanique des Fluides et d Energetique, Marrakech (Morocco). Dept. of Physics; Galanis, N. [Sherbrooke Univ., PQ (Canada). Dept. of Mechanical Engineering

2009-07-01

292

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: Portuguese Abstract in portuguese Este trabalho busca, como função principal, determinar o valor do coeficiente convectivo de transferência de calor através da metodologia de modelagem matemática e simulação 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 equação empírica da literatura, já realizado. Para a predição 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 comparação desses dados simulados com os dados experimentais. Os valores resultantes dos dois modelos foram coincidentes e resultaram menores valores residuais que o obtido pela equação 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.

293

Reynolds stress and heat flux in spherical shell convection

Context. Turbulent fluxes of angular momentum and heat due to rotationally affected convection play a key role in determining differential rotation of stars. Aims. We compute turbulent angular momentum and heat transport as functions of the rotation rate from stratified convection. We compare results from spherical and Cartesian models in the same parameter regime in order to study whether restricted geometry introduces artefacts into the results. Methods. We employ direct numerical simulations of turbulent convection in spherical and Cartesian geometries. In order to alleviate the computational cost in the spherical runs and to reach as high spatial resolution as possible, we model only parts of the latitude and longitude. The rotational influence, measured by the Coriolis number or inverse Rossby number, is varied from zero to roughly seven, which is the regime that is likely to be realised in the solar convection zone. Cartesian simulations are performed in overlapping parameter regimes. Results. For slow ...

Käpylä, P J; Guerrero, G; Brandenburg, A; Chatterjee, P

2010-01-01

294

International Nuclear Information System (INIS)

Results on numerical investigation of the local opposing mixed convection heat transfer in a vertical flat channel with symmetrical heating in laminar airflow are presented. A numerical two-dimensional simulation was performed using the FLUENT 6.1 code. Investigations were performed in airflow of 0.1, 0.2 and 0.4 MPa absolute pressure at Reynolds numbers from 1500 up to 4310 with Grq number variation from 1.65 * 105 to 3.1 * 109 in order to define the effect of the influence of buoyancy on heat transfer. Numerical calculations demonstrated that under the effect of small buoyancy there were only small transformations in the velocity profile, but the flow was oriented downward (direction of forced flow). With increasing the buoyancy forces, flow separation occurred at some distance from the beginning of the heated channel section. With a further increase of buoyancy, the position of flow separation point moved towards the beginning of the heated section. The channel wall temperature noticeably decreased at the flow separation point. Correlations for calculation of heat transfer in the laminar mixed convection region and for the determination of the position of flow separation from the wall are suggested. (author)

295

Convective Heat Transfer in the Reusable Solid Rocket Motor of the Space Transportation System

This simulation involved a two-dimensional axisymmetric model of a full motor initial grain of the Reusable Solid Rocket Motor (RSRM) of the Space Transportation System (STS). It was conducted with CFD (computational fluid dynamics) commercial code FLUENT. This analysis was performed to: a) maintain continuity with most related previous analyses, b) serve as a non-vectored baseline for any three-dimensional vectored nozzles, c) provide a relatively simple application and checkout for various CFD solution schemes, grid sensitivity studies, turbulence modeling and heat transfer, and d) calculate nozzle convective heat transfer coefficients. The accuracy of the present results and the selection of the numerical schemes and turbulence models were based on matching the rocket ballistic predictions of mass flow rate, head end pressure, vacuum thrust and specific impulse, and measured chamber pressure drop. Matching these ballistic predictions was found to be good. This study was limited to convective heat transfer and the results compared favorably with existing theory. On the other hand, qualitative comparison with backed-out data of the ratio of the convective heat transfer coefficient to the specific heat at constant pressure was made in a relative manner. This backed-out data was devised to match nozzle erosion that was a result of heat transfer (convective, radiative and conductive), chemical (transpirating), and mechanical (shear and particle impingement forces) effects combined.

Ahmad, Rashid A.; Cash, Stephen F. (Technical Monitor)

2002-01-01

296

Forced air heat sink apparatus

A high efficiency forced air heat sink assembly employs a split feed transverse flow configuration to minimize the length of the air flow path through at least two separated fin structures. Different embodiments use different fin structure material configurations including honeycomb, corrugated and serpentine. Each such embodiment uses a thermally conductive plate having opposed exterior surfaces; one for receiving a component to be cooled and one for receiving the fin structures. The serpentine structured fin embodiment employs a plurality of fin supports extending from the plate and forming a plurality of channels for receiving the fin structures. A high thermal conductivity bondant, such as metal-filled epoxy, may be used to bond the fin structures to either the plate or the fin supports. Dip brazing and soldering may also be employed depending upon the materials selected.

Rippel, Wally E. (Inventor)

1989-01-01

297

International Nuclear Information System (INIS)

The objective of this paper is to numerically investigate the cooling performance of electronic devices with an emphasis on the effects of the arrangement and number of electronic components. The analysis uses a two dimensional rectangular enclosure under combined natural and forced convection flow conditions and considers a range of Raleigh numbers. Heat sources in the enclosure generate the natural convection flow and an externally sourced air stream through the enclosure generates the forced convection flow. The results show that increasing the Raleigh number significantly improves the enclosure heat transfer process. At low Raleigh numbers, placing more heat sources within the enclosure reduces the heat transfer rate from the sources and consequently increases their overall maximum temperature. The arrangement and the number of heat sources have a considerable contribution to the cooling performance. However, increasing the Raleigh number reduces this contribution. (author)

298

Computation of combined turbulent convective and impingement heat transfer

International Nuclear Information System (INIS)

Full text: Among various cooling methods of gas turbine components, impingement and forced convection cooling is preferable due to improved cycle efficiency and reduced emission levels. However, influences of various design parameters like crossflow and surface enlargements (like ribs) are not well understood. Reliable engineering design methods for complex geometries and flow systems are not available and only a very limited amount of experimental data exist. In addition, experiments on real applications are cumbersome, very costly and not attractable. Thus there is a request for reliable and cost-effective computational prediction. Such methods could be based on the numerical solution of the Reynolds-averaged Navier-Stokes equations (RANS), the energy equation and models for the turbulence field. Turbulence modeling is a critical issue and it is known that the widely used linear two-equation models suffer from a too high generation of turbulence and thus heat transfer in stagnating flow fields. This problem may be eliminated or reduced by using more advanced formulations like full Reynolds stress equations or by application of a realizability constraint on the linear two-equation models. In recent years nonlinear formulations of the constitutive relations have emerged and the performance of the two-equation models has been improved. The main reasons for this improvement are the incorporation of variable coefficients in the stress-strain relationship (constitutive retress-strain relationship (constitutive relation) and the ability to capture anistropy in the turbulent normal stresses. The geometries selected for the investigation are idealized to reveal the fundamental issues and enable validation of the considered models with available experimental data. Thus single unconfined round air jets, confined jets with crossflow are considered. The numerical approach is based on the finite volume method and uses a co-located computational grid. Various number of grid points have been used and the grid influence is discussed. The wall adjacent grid points are always placed at a dimensionless distance (y+) less than 0.5 from the heated wall. The considered turbulence models are all so-called low Reynolds number models (both linear and nonlinear ones). A realizability constraint is applied on the linear models to prevent severe over-prediction of the heat transfer at stagnation points. The constraint puts a limit on the time scale for the tubulence field. Anisotropic formulations of the turbulent heat fluxes are discussed some comparative results are considered. Our recent investigations show that linear and non-linear two-equations turbulence models can be used for impinging jet heat transfer predictions with reasonable success. However, the computational results also suggest that an application of a realizability constraint is necessary to avoid overprediction of the stagnation point heat transfer coefficients. For situations with combined forced convection and impingement cooling it was revealed that as the crossflow is squeezed under the jet, the heat transfer coefficient is reduced. (author)

299

Lox droplet vaporization in a supercritical forced convective environment

A systematic investigation has been conducted to study the effects of ambient flow conditions (i.e. pressure and velocity) on supercritical droplet gasification in a forced-convective environment. The model is based on the time-dependent conservation equations in axisymmetric coordinates, and accommodates thermodynamic nonidealities and transport anomalies. In addition, an efficient scheme for evaluating thermophysical properties over the entire range of fluid thermodynamic states is established. The analysis allows a thorough examination of droplet behavior during its entire lifetime, including transient gasification, dynamic deformation, and shattering. A parametric study of droplet vaporization rate in terms of ambient pressure and Reynolds number is also conducted.

Hsiao, Chia-Chun; Yang, Vigor

1994-01-01

300

Convective heat transfer around vertical jet fires: an experimental study.

The convection heat transfer phenomenon in vertical jet fires was experimentally analyzed. In these experiments, turbulent propane flames were generated in subsonic as well as sonic regimes. The experimental data demonstrated that the rate of convection heat transfer increases by increasing the length of the flame. Assuming the solid flame model, the convection heat transfer coefficient was calculated. Two equations in terms of adimensional numbers were developed. It was found out that the Nusselt number attains greater values for higher values of the Rayleigh and Reynolds numbers. On the other hand, the Froude number was analyzed only for the subsonic flames where the Nusselt number grows by this number and the diameter of the orifice. PMID:21962859

Kozanoglu, Bulent; Zárate, Luis; Gómez-Mares, Mercedes; Casal, Joaquim

2011-12-15

301

International Nuclear Information System (INIS)

A collection of papers is presented on mixed convection, pool boiling, and flow boiling and two-phase flow. A unified similarity analysis is presented for turbulent convection next to vertical surfaces. Topics of interest include free-forced convection from a heated cone, decay of vertical buoyant jets in uniform environment, minimum heat flux during film boiling, and the dynamics of two-phase flow in a duct

302

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

303

The experimental investigations were consisting of two parts. The first part was carried out to study the effect of corner geometry on the steady-state forced convection inside horizontal isosceles triangular ducts with sharp corners. The electrically-heated triangular duct was used to simulate the triangular passage of a plate-fin compact heat exchanger. The isosceles triangular ducts were manufactured with duralumin, and fabricated with the same length of 2.4m and hydraulic diameter of 0.44m, but five different apex angles (i.e. ?a=15?,30?, 40?,60?, and 90?) respectively. The investigation was performed under turbulent flow condition covering a wide range of Reynolds number (i.e. 7000heat transfer performance. Non-dimensional expressions for the determination of the heat transfer coefficient of the triangular ducts with different apex angles and surface roughnesses were also developed.

Leung, C. W.; Wong, T. T.; Kang, H. J.

304

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

305

Investigating Convective Heat Transfer with an Iron and a Hairdryer

A simple experimental set-up to study free and forced convection in undergraduate physics laboratories is presented. The flat plate of a domestic iron has been chosen as the hot surface, and a hairdryer is used to generate an air stream around the plate. Several experiments are proposed and typical numerical results are reported. An analysis and…

Gonzalez, Manuel I.; Lucio, Jesus H.

2008-01-01

306

Calculational method for combined natural circulation and forced-convection flow in a channel

International Nuclear Information System (INIS)

This paper presents a finite element solution for combined natural circulation and forced convection flow in a channel. Because the buoyancy force plays an important role in a mixed convection flow, an iteration scheme was used in solving the coupled energy-momentum equations. The momentum equations and the pressure equation are solved to calculate velocity profiles instead of solving the momentum equations with the continuity equation. Though the pressure equation is obtained by using the continuity equation, the continuity principle is reinforced into the momentum equations and the pressure equation at each iteration. Calculations are performed for the combined natural circulation and forced convection case and the forced convection only case

307

International Nuclear Information System (INIS)

Some of the papers in this volume address complex flow situations, covering the separation and reattachment mechanisms--a common feature of heat transfer augmentation schemes--rotation, combustion, and helical pipe geometries. Other topics include the application of the probability density function method for predicting heat transfer in turbulent flows and mechanical constraints of high heat flux. Several papers deal with the fundamentals of jet impingement and spray cooling heat transfer. Other topics covered are forced and mixed convection. A broad coverage of the subject in terms of fundamentals of natural convection, numerical and experimental applications in laminar and turbulent flows, and of mixed heat and mass transfer is presented. Four papers emphasized aspects of natural circulation. Separate abstracts were prepared for most papers in this volume

308

Radiative effects on forced convection flows in micropolar fluids with variable viscosity

International Nuclear Information System (INIS)

The interaction of forced convection and thermal radiation during the flow of a surface moving continuously in a flowing stream of micropolar fluid with variable viscosity is studied. Two cases are considered: one corresponding to a plane surface moving in parallel with the free stream, the other to a surface moving in the opposite direction to the free stream. The Rosseland approximation is used to describe the radiative heat flux in the energy equation. The viscosity of the fluid is taken as a function of temperature. (author)

309

The influence of surface radiation on forced-convection film boiling

International Nuclear Information System (INIS)

In this paper a new approach to address the effect of surface radiation on a laminar film boiling flow over a horizontal flat plate is suggested. Previously developed approximate expressions of the thermal boundary layer thickness and the wall shear stress of a moving surface in a flowing liquid are used to circumvent the complicated simultaneous solution of the vapor and liquid flow. Approximate closed-form expressions to predict the wall heat transfer and skin friction are obtained. For a water-steam system at atmospheric pressure within the wall temperature range considered (Tw ? = 20 degrees C) forced convection film boiling

310

Modeling and control of transport phenomena in a forced convection flow

We analyze dynamics of momentum, heat and moisture transport in forced convection using the Boussinesq approximation, including both the effects of humidity and temperature. Analytical aspects and an exact solution of the resulting equations are presented. Stability of the exact solution is analyzed. A numerical simulation is performed using a finite element discretization. Stability analysis and simulations reveal a sequence of transitions with increasing inlet air velocity. Effects of the resulting recirculation zones and unsteady flows on the indoor environment (comprised of temperature and humidity distributions) are discussed, and scenarios for indoor climate control are presented.

John, Thomas; Mezic, Igor; Narayanan, Satish; Myers, Mark

2002-11-01

311

Forced convective transition boiling: review of literature and comparison of prediction methods

International Nuclear Information System (INIS)

This report reviews the published information on transition boiling heat transfer under forced convective conditions. It was found that transition boiling data have been obtained only within a limited range of conditions and many data are considered unreliable. The data do not permit the derivation of a correlation; however the parametric trends can be isolated from the data. Several authors have proposed correlations valid in the transition boiling region. Most of the correlations are valid only within a narrow range of conditions. A comparison with the data shows that in general agreement is poor. Hsu's correlation is tentatively recommended for low flows and pressures. (author)

312

International Nuclear Information System (INIS)

In order to understand the influence of a semispherical crucible geometry combined with different convection modes as a thermocapillary convection, natural convection and forced convection, induced by crystal rotation, on melt flow pattern in silicon Czochralski crystal growth process, a set of numerical simulations are conducted using Fluent Software. We solve the system of equations of heat and momentum transfer in classical geometry as cylindrical and modified crystal growth process geometry as cylindro-spherical. In addition, we adopt hypothesis adapted to boundary conditions near the interface and calculations are executed to determine temperature, pressure and velocity fields versus Grashof and Reynolds numbers. The analysis of the obtained results led to conclude that there is advantage to modify geometry in comparison with the traditional one. The absence of the stagnation regions of fluid in the hemispherical crucible corner and the possibility to control the melt flow using the crystal rotation enhances the quality of the process comparatively to the cylindrical one. The pressure field is strongly related to the swirl velocity.

313

Digital Repository Infrastructure Vision for European Research (DRIVER)

Experiments were performed to determine secondary flow development and Nusselt number distributions for laminar mixed convection in the thermal entry region of a parallel plate channel heated uniformly from below. Flow visualization showed the onset of secondary flow on the heater surface and revealed the developing longitudinal plumes and vortices. Longitudinal distributions of the local Nusselt number initially followed forced convection. Subsequent mixing associated with the development of...

Maughan, James R.

1989-01-01

314

Heat transfer mechanisms in bubbly Rayleigh-Benard convection

The heat transfer mechanism in Rayleigh-Benard convection in a liquid with a mean temperature close to its boiling point is studied through numerical simulations with point-like vapor bubbles, which are allowed to grow or shrink through evaporation and condensation and which act back on the flow both thermally and mechanically. It is shown that the effect of the bubbles is strongly dependent on the ratio of the sensible heat to the latent heat as embodied in the Jacob number Ja. For very small Ja the bubbles stabilize the flow by absorbing heat in the warmer regions and releasing it in the colder regions. With an increase in Ja, the added buoyancy due to the bubble growth destabilizes the flow with respect to single-phase convection and considerably increases the Nusselt number.

Oresta, Paolo; Lohse, Detlef; Prosperetti, Andrea

2008-01-01

315

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

316

Measurement of the Convective Heat-Transfer Coefficient

We propose an experiment for investigating how objects cool down toward the thermal equilibrium with their surroundings. We describe the time dependence of the temperature difference of the cooling objects and the environment with an exponential decay function. By measuring the thermal constant t, we determine the convective heat-transfer…

Conti, Rosaria; Gallitto, Aurelio Agliolo; Fiordilino, Emilio

2014-01-01

317

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

318

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

319

Flow excursion-induced dryout at low heat flux natural convection boiling

International Nuclear Information System (INIS)

Flow excursion-induced dryout at low heat flux natural convection boiling, typical of liquid metal fast breeder reactors, is addressed. Steady-state calculations indicate that low-quality boiling is possible up to the point of the Ledinegg instability, leading to flow excursion and subsequent dryout in agreement with experimental data. A flow regime -dependent critical heat flux relationship based upon a saturated boiling criterion is also presented. Transient analyses indicate that premature flow excursion cannot be ruled out and the boiling process is transient dependent. Analysis of a loss-of-flow transient at high heat flux forced convection shows a significantly faster flow excursion leading to dryout, which is in excellent agreement with the results of the two-dimensional THORAX code

320

Natural convection in porous media with heat generation

International Nuclear Information System (INIS)

The heat transfer characteristics of fluid-saturated porous media are investigated for the case of uniform internal heat generation with cooling from above. Analytical models of conduction and single-phase cellular convection show good agreement with previous Rayleigh number correlations and with experimental data obtained by Joule heating of saltwater in a sand bed. An approximate dryout criterion is also derived for two-phase boiling heat transfer in a fixed bed which is neither channeled nor fluidized. Correlation of dryout data using this criterion is encouraging, especially considering the analytical rather than correlational basis of the criterion

321

Convective heat transfer in the presence of an obstructing medium

International Nuclear Information System (INIS)

The manuscripts in this volume were submitted and reviewed by the General Papers Committee of the Heat Transfer Division for presentation at the 1990 ASME Winter Annual Meeting in Dallas, Texas. One of the main charges of this Committee is to consider papers which cannot be easily included in the various technical sessions sponsored by the Heat Transfer Division K- Committees. General Papers Sessions provide an opportunity to survey a broad range of heat transfer topics covering both traditional and new and emerging areas. The papers in this volume address convective heat transfer in the presence of an obstructing medium

322

Time-dependent mixed convection heat transfer from a sphere in a micro-gravity environment

A fundamental problem of interest for crystal growth in micro-gravity applications involves the mixed convection heat transfer from a sphere in a uniform flow of fluid at a differing temperature. Under the combined influence of the imposed free stream as well as an induced buoyancy force due to thermal expansion of the fluid, the heat transfer from the sphere will be different from that of either the pure forced convection flow or the pure free convection flow. For the present study, the method of matched asymptotic expansions is applied to the laminar flow problem of an impulsively heated, impulsively started sphere in an originally quiescent fluid. Time series expansions are developed for the dependent variables by acknowledging the existence of two distinct regions: one, an inner region, near the sphere, in which viscous effects are significant; and two, an outer region in which the fluid may be treated as inviscid. The time series expansions are developed in terms of the Reynolds number and Richardson number (Buoyancy Parameter), and the relevant heat transfer and drag coefficients are calculated and plotted.

Hommel, Mark J.

1986-01-01

323

Turbulent forced convection of nanofluid in a wavy channel using two phase model

Two phase mixture model is used to numerically simulate the turbulent forced convection of Al2O3-Water nanofluid in a channel with corrugated wall under constant heat flux. Both mixture and single phase models are implemented to study the nanofluid flow in such a geometry and the results have been compared. The effects of the volume fraction of nanoparticles, Reynolds number and amplitude of the wavy wall on the rate of heat transfer are investigated. The results showed that with increasing the volume fraction of nanoparticles, Reynolds number and amplitude of wall waves, the rate of heat transfer increases. Also the results showed that the mixture model yields to higher Nusselt numbers than the single phase model in a similar case.

Manavi, Seyed Alborz; Ramiar, Abas; Ranjbar, Ali Akbar

2014-05-01

324

International Nuclear Information System (INIS)

Numerical analysis has been conducted for combined free and forced laminar convection of liquid metals in a horizontal pipe which is isothermally heated from a certain axial location. The steady-state solutions have been obtained in consideration of axial conduction for Peclet numbers of 0.3 ? 7.5 and Rayleigh numbers of 15 ? 500. The results reveal marked effects of axial conduction on the flow and heat transfer characteristics of liquid metals. At the entrance to the heated section, the secondary flow has already developed and a reverse flow occurs near the pipe top as the buoyancy effect becomes large. Consequently, the circumferential average Nusselt number decreases with increasing secondary flow in comparison with that observed in moderate and large Prandtl-number flows. The regime of reverse flow is clearly identified in the Pe-Ra (Peclet-Rayleigh) coordinates. (author)

325

Experimental-theoretical analysis of laminar internal forced convection with nanofluids

Energy Technology Data Exchange (ETDEWEB)

This work reports fundamental experimental-theoretical research related to heat transfer enhancement in laminar channel flow with nanofluids, which are essentially modifications of the base fluid with the dispersion of metal oxide nanoparticles. The theoretical work was performed by making use of mixed symbolic-numerical computation (Mathematica 7.0 platform) and a hybrid numerical-analytical methodology (Generalized Integral Transform Technique - GITT) in accurately handling the governing partial differential equations for the heat and fluid flow problem formulation with temperature dependency in all the thermophysical properties. Experimental work was also undertaken based on a thermohydraulic circuit built for this purpose, and sample results are presented to verify the proposed model. The aim is to illustrate detailed modeling and robust simulation attempting to reach an explanation of the controversial heat transfer enhancement observed in laminar forced convection with nanofluids. (author)

Cerqueira, Ivana G.; Cotta, Renato M. [Lab. of Transmission and Technology of Heat-LTTC. Mechanical Eng. Dept. - POLI and COPPE/UFRJ, Rio de Janeiro, RJ (Brazil)], E-mail: cotta@mecanica.coppe.ufrj.br; Mota, Carlos Alberto A. [Conselho Nacional de Pesquisas - CNPq, Brasilia, DF (Brazil)], e-mail: carlosal@cnpq.br; Nunes, Jeziel S. [INPI, Rio de Janeiro, RJ (Brazil)], e-mail: jeziel@inpi.gov.br

2010-07-01

326

A unidirectional free convective flow driven by internal heating

e We describe an exact solution of the Boussinesq equations for a fluid having uniform kinematic viscosity ?, but zero thermal conductivity. This solution models flow near the centre of a long convection cell, like that occurring in the mantle beneath the Pacific plate. The flow is driven purely by a uniform internal heating of intensity q W/kg; specifically, the shear stress is required to vanish at the top and bottom of the layer, and there is no net horizontal transport. With x being a horizontal unit vector, and with the y--coordinate increasing upwards, the Boussinesq equations admit a solution in which the velocity is given by v= v(y)x, and the temperature, by T=(x-ct)?(y). So the velocity field is steady, but the thermal field has the form of a travelling wave. The wave speed c is a parameter in the solution. For c>0, the temperature gradient ?(y) proves to be negative; since T/t is then positive, the solution describes a rightward propagating wave of reheating. The governing equations admit a solution of this type if c> c*= 0.31 (?g q D^3/?cp)^1/2. Here ?, cp, D, and g are respectively the thermal expansion coefficient, specific heat, layer depth and gravitational acceleration. Though the model contains no cold plume (subducting slab), it predicts velocities close to that of the Pacific plate. Simplified mantle flow models that assume the motion to be driven purely by density differences across slabs may therefore be leaving out an essential part of the forcing.

Morris, S. J. S.

2007-03-01

327

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

328

Heat transfer and horizontally averaged temperature of convection with large viscosity variations

It is pointed out that the understanding of convection in large-Prandtl-number Boussinesq fluids with uniform properties and contained in simple geometries is virtually complete. Present efforts are typically directed towards relaxing some of the original assumptions by going to lower Prandtl number, more complicated geometries, variable material properties, or introducing new dynamical processes such as the Lorentz forces. A description is given of experiments which are concerned with the effect on convection of relaxing the assumption of a uniform viscosity. The reported experiments were designed to measure both the horizontally averaged temperature as a function of depth and the heat transfer of convection over a range of viscosity variations up to 100,000.

Richter, F. M.; Nataf, H.-C.; Daly, S. F.

1983-01-01

329

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)

330

Forced convection in channels of small cross sectional dimensions has been recommended as an effective heat removal method for electronic components and packages. Many of the experimental results reported in the literature on the heat transfer performance of small-cross section channels are of contradicting nature, even though some generally agreeing results are also found. One of the probable reasons for the deviations, as suggested by investigators, is the intrusive nature of measurement techniques, which interferes with the flow field. Hence a non intrusive measurement technique is preferable for temperature measurement in small channels. The present work is aimed at developing an interferometric method for convective heat transfer measurement in a liquid medium flowing through channels of small cross sectional dimensions, with hydraulic diameters ranging from 12 to 3 mm and characterizing the nature of fluid flow and heat transfer in these channels. Mach-Zehnder interferometric arrangement is used to obtain the temperature distributions in water flowing through the channels, which are further analyzed digitally to obtain the local heat transfer coefficients and Nusselt numbers. The results are compared and contrasted with classical results for channel flow and heat transfer, and attempt has been made to interpret the variations and deviations observed. The experimental study has been performed under different fluid velocities in the laminar flow regime, and under various wall heat fluxes corresponding to the heat dissipation range expected in microelectronic devices. Parametric variations for the heat transfer performance have been obtained and correlated using the experimental results.

Sajith, V.; Sobhan, C. B.

2011-05-01

331

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

332

Experimental investigation of forced convection and desiccant integrated solar dryer

Energy Technology Data Exchange (ETDEWEB)

An indirect forced convection and desiccant integrated solar dryer is designed and fabricated to investigate its performance under the hot and humid climatic conditions of Chennai, India. The system consists of a flat plate solar air collector, drying chamber and a desiccant unit. The desiccant unit is designed to hold 75kg of CaCl{sub 2}-based solid desiccant consisting of 60% bentonite, 10% calcium chloride, 20% vermiculite and 10% cement. Drying experiments have been performed for green peas at different air flow rate. The equilibrium moisture content M{sub e} is reached in 14h at an air flow rate of 0.03kg/m{sup 2}s. The system pickup efficiency, specific moisture extraction rate, dimensionless mass loss, mass shrinkage ratio and drying rate are discussed in this paper. (author)

Shanmugam, V. [Department of Mechanical Engineering, Sathyabama Deemed University, Chennai 600 119 (India); Natarajan, E. [Institute for Energy Studies, College of Engineering, Anna University, Chennai 600 025 (India)

2006-07-15

333

Evaporation of a binary liquid film by forced convection

Directory of Open Access Journals (Sweden)

Full Text Available This paper deals with a numerical analysis of the evaporation of a thin binary liquid film by forced convection inside a channel constituted by two parallel plates. The first plate is externally insulated and wetted by a thin water ethylene glycol film while the second is dry and isothermal. The liquid mixture consists of water (the more volatile component and ethylene glycol while the gas mixture has three components: dry air, water vapour and ethylene-glycol vapour. The set of non linear and coupled equations expressing the conservation of mass, momentum, energy and species in the liquid and gas mixtures is solved numerically using a finite difference method. Results concerns with the effects of inlet ambience conditions and the inlet liquid concentration of ethylene glycol on the distribution of the temperature, concentrations profiles and the axial variation of the evaporation rate of species i.

Nasr Abdelaziz

2011-01-01

334

Hydrodynamic simulation of forced convection in Czochralski melts

Systematic hydrodynamic simulation experiments at room temperature were carried out in order to enhance the understanding of flow phenomena in Czochralski melts. An apparatus has been designed which allows the (three-dimensional) observation of hydrodynamic effects for various crucible shapes with adjustable rotation rates for crystal and crucible for iso- and counter-rotation. Also the effect of the accelerated crucible (and crystal) rotation technique (ACRT) on melt homogenization can be investigated. Flow visualization and measurement was done by ink injection, by light scattering of suspended particles and by the Schlieren technique, and laser doppler anemometry is foreseen. In this first report the results on forced convection in Czochralski melts and the occurrence ranges of specific flow regimes (like Cochran and Ekman flow, Couette and Taylor cells) and the transition from stable mixing to flow instability are discussed.

Rappl, Paulo H. O.; Matteo Ferraz, Luiz F.; Scheel, Hans J.; Barros, Miriam R. X.; Schiel, Dietrich

1984-12-01

335

Free and forced convective-diffusion solutions by finite element methods

International Nuclear Information System (INIS)

Several free and forced convective-diffusion examples are solved and compared to either laboratory experiment or closed-form analysis. The problems solved illustrate the application of finite element methods to both strongly-coupled and weakly-coupled velocity and temperature fields governed by the steady-state momentum and energy equations. Special attention is given to internal forced convection with temperature-dependent viscosity and free convection within an enclosure

336

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

337

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; Sundén, Bengt

1999-01-01

338

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

339

Energy Technology Data Exchange (ETDEWEB)

A numerical analysis was carried out to study the detailed heat and mass transfer processes between a condensation liquid film and mixed turbulent moist airflow. Results show that the condensation latent heat transfer is more important for a system with higher inlet relative humidity or lower inlet Reynolds number of a moist airstream. The heat and mass transfer coefficients are higher for a system with higher inlet relative humidity and inlet Reynolds number of moist air. In addition, the aiding-buoyancy forces cause diminution in heat and mass transfer results compared with the corresponding results of forced convection.

Tsay, Y.L. [National Yuenlin Polytechnic Inst., Huwei, Yuenlin (Taiwan, Province of China). Dept. of Aeronautics Engineering Technology

1996-05-10

340

International Nuclear Information System (INIS)

The report describes an experiment on natural convective heat transfer of silicone oil and water around a horizontal cylinder in a rectangular box and on the visualization of temperature distribution by the liquid-crystal suspension method. The natural convection around the heated cylinder in the box is effected by the thermal stratification and plume. Experimental results are summarized as follows. The Nusselt numbers on the heated horizontal cylinder are correlated with the Ra numbers. The reference fluid temperature on the heat transfer is determined from the thermal stratification in the box. The swaying motion of the thermal plume is effected by the stratification. (author)

341

Simulation of Convective Heat-Transfer Coefficient in a Buried Exchanger

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 propag...

Taoufik Mnasri; Younes, Rached B. .; Maher Raddaoui; Salem Elouragini

2008-01-01

342

International Nuclear Information System (INIS)

Heat transfer coefficients and pressure drop of gaseous ammonia in forced convection are experimentally determined. The fluid flows (mass flow rate 0.6 to 2.4 g/s) in a long tungsten tube (di = 2.8 mm, de = 5.1 mm, L = 700 mm) electrically heated. The temperature of the wall reaches 3000 deg. K and the fluid 2500 deg. K; maximum heat flux 530 w/cm2. Ammonia is completely dissociated and the power necessary for dissociation reaches 30 per cent of the total power exchanged. Inlet pressure varies between 6 and 16 bars and the maximum pressure drop in the tube reaches 15 bars. Two regimes of dissociation have been shown: catalytic and homogeneous and the variation of dissociation along the length of the tube is studied. The measured heat transfer coefficients may be about 10 times these calculated by the means of classical formulae. A correlation of experimental results using enthalpy as a driving force for heat transmission is presented. Pressure drops may be calculated by the means of a classical friction factor. (authors)

343

Energy Technology Data Exchange (ETDEWEB)

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 (a modified version of the COBRA-IV code).

Bates, J.M.; Khan, E.U.

1980-10-01

344

Laminar free convection over two-dimensional bodies with uniform surface heat flux

A general analysis is presented of the steady nondissipative constant-property laminar boundary layer flow over a two-dimensional body of uniform surface heat flux situated in an infinite ambient fluid of undisturbed temperature. The analysis is then applied to a long horizontal circular cylinder. Numerical solutions to the universal functions associated with the first two terms in the derived series are given for Prandtl numbers 0.7, 1, 3, 5, 7, and 10. The results are compared with those obtained by Koh (1964) whose method is patterned after the Blasius-Frossling procedure for forced convection flow. The study reveals that Wilks' (1972) analysis concerning the external natural convection about two-dimensional bodies with constant heat flux is in error.

Lin, F. N.

1976-01-01

345

International Nuclear Information System (INIS)

The electro-thermo-convective motion in a plane horizontal dielectric liquid layer subjected to simultaneous action of electric field and thermal gradient is numerically investigated. We consider the case of a strong unipolar charge injection C = 10 from above or below. Therefore in this context, we only take into account the Coulomb force, disregarding the dielectric one. The effect of the electric field on the heat transfer is analyzed through the characterization of the time history of the Nusselt number as well as its evolution according to the characteristic dimensionless electric parameter T. It is demonstrated that the electric effects dominate the buoyancy ones resulting in an electrically induced convection which significantly enhance the heat transfer.

346

Silk cocoon drying in forced convection type solar dryer

International Nuclear Information System (INIS)

The thin layer silk cocoon drying was studied in a forced convection type solar dryer. The drying chamber was provided with several trays on which the cocoons loaded in thin layer. The hot air generated in the solar air heater was forced into drying chamber to avoid the direct exposure of sunlight and UV radiation on cocoons. The drying air temperature varied from 50 to 75 oC. The cocoon was dried from the initial moisture content of about 60-12% (wb). The drying data was fitted to thin layer drying models. Drying behaviour of the silk cocoon was best fitted with the Wang and Singh drying model. Good agreement was obtained between predicted and experimental values. Quality of the cocoons dried in the solar dryer was at par with the cocoons dried in the conventional electrical oven dryer in term of the silk yield and strength of the silk. Saving of electrical energy was about 0.75 kWh/kg cocoons dried. Economic analysis indicated that the NPV of the solar dryer was higher and more stable (against escalation rate of electricity) as compare to the same for electrical oven dryer. Due to simplicity in design and construction and significant saving of operational electrical energy, solar cocoon dryer seems to be a viable option.

347

Convective heat transfer in airflow through a duct with wall thermal radiation

This paper presents a numerical investigation on airflow through a heated horizontal rectangular duct wherein the model considers the combined modes of natural and forced convection heat transfer and the thermal radiation from duct walls. The duct periphery is differentially heated with known temperature profiles imposed on the two opposite vertical sidewalls while the other two walls are treated as adiabatic. The air enters into the duct hydrodynamically fully developed and flows steadily under laminar conditions undergoing thermal development within the duct. Considering several temperature profiles on the two vertical sidewalls, the numerical simulation generates the heat transfer rates and associated fluid flow patterns in the duct for a range of airflow rates, duct aspect ratios and surface emissivity. The variation of local Nusselt number at duct walls and the fluid flow patterns are critically examined to identify thermal instabilities and the significance of wall thermal radiation effects on the overall heat transfer rates.

Chandratilleke, T. T.; Narayanaswamy, R.; Wangdhamkoom, P.

2010-06-01

348

Energy Technology Data Exchange (ETDEWEB)

Heating associated with tropical cloud systems drive the global circulation. The overall research objectives of this project were to i) further quantify and understand the importance of heating in tropical convective cloud systems with innovative observational techniques, and ii) use global models to determine the large-scale circulation response to variability in tropical heating profiles, including anvil and cirrus cloud radiative forcing. The innovative observational techniques used a diversity of radar systems to create a climatology of vertical velocities associated with the full tropical convective cloud spectrum along with a dissection of the of the total heating profile of tropical cloud systems into separate components (i.e., the latent, radiative, and eddy sensible heating). These properties were used to validate storm-scale and global climate models (GCMs) and were further used to force two different types of GCMs (one with and one without interactive physics). While radiative heating was shown to account for about 20% of the total heating and did not have a strong direct response on the global circulation, the indirect response was important via its impact on convection, esp. in how radiative heating impacts the tilt of heating associated with the Madden-Julian Oscillation (MJO), a phenomenon that accounts for most tropical intraseasonal variability. This work shows strong promise in determining the sensitivity of climate models and climate processes to heating variations associated with cloud systems.

Schumacher, Courtney

2012-12-13

349

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; Sundén, Bengt

1995-01-01

350

Numerical investigation of turbulent forced convection in a duct with a trapezoidal cross section

DEFF Research Database (Denmark)

The present work concerns development and application of turbulence models for forced convective heat transfer in trapezoidal ducts. 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. Cyclic conditions in the main flow direction are imposed. The standard k-? model with wall functions is used as a reference. The non-linear k-? of Speziale is applied to calculate the turbulent shear stresses. The turbulent heat fluxes are calculated by the simple eddy diffusivity (SED) 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; Sunden, Bengt

1995-01-01

351

Lagrangian dispersion and heat transport in convective turbulence

Digital Repository Infrastructure Vision for European Research (DRIVER)

Lagrangian studies of the local temperature mixing and heat transport in turbulent Rayleigh-Benard convection are presented, based on three-dimensional direct numerical simulations. Contrary to vertical pair distances, the temporal growth of lateral pair distances agrees with the Richardson law, but yields a smaller Richardson constant due to correlated pair motion in plumes. Our results thus imply that Richardson dispersion is also found in anisotropic turbulence. We find t...

Schumacher, Joerg

2008-01-01

352

A preliminary study on the mixed convection heat transfer in a horizontal pipe

Energy Technology Data Exchange (ETDEWEB)

Mixed convection phenomena have applications in heat exchangers designed for viscous liquids, pipelines used for transporting oil, and heat exchangers for gas flows and have been investigated for numerical study. The experimental studies on the topic are rare due to the limited practical uses. The definitions on the buoyancy coefficient that represent the relative influence of the forced and the natural convection are different by scholars. When the fluid passes through a horizontal heated pipe, the secondary flow patterns of the flows moving upward along inner wall and downward through the centerline of the pipe, appear. Two symmetric counter rotating, spiraling axial flows around the vertical centerline and the heat transfer of the bottom is higher due to the unstable stratification. This study is preliminary experiments for laminar and turbulent mixed convection for horizontal pipe geometries. The diameter of the pipe was 0.026m and lengths were ranging from 0.03m to 0.50m varying the Reynolds number. Experiments were carried out using a copper sulfate electroplating system was adopted for mass transfer based on the analogy concept.

Chae, Myeong Seon; Chung, Bum Jin [Jeju Nat' l Univ., Jeju (Korea, Republic of)

2012-10-15

353

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.

Yazdanshenas, Eshagh

2006-01-01

354

Energy Technology Data Exchange (ETDEWEB)

Latent heat energy storage systems with both annular and countercurrent flows are modeled numerically. The change of phase of the phase-change material(PCM) and the transient forced convective heat transfer for the transfer fluid are solved simultaneously as a conjugate problem. A parametric study and a system optimization are conducted. It is found that the energy storage system with the counter-current flow is an efficient way to absorb heat energy in a short period for pulsed power load space applications.

Cao, Y.; Faghri, A. (Wright State Univ., Dayton, OH (USA)); Juhasz, A. (NASA Lewis Research Center, Cleveland, OH (USA))

1991-02-01

355

AERIAL MEASUREMENTS OF CONVECTION CELL ELEMENTS IN HEATED LAKES

Energy Technology Data Exchange (ETDEWEB)

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; Saleem Salaymeh, S; Timothy Brown, T; Alfred Garrett, A; Malcolm Pendergast, M; Linda Nichols, L

2007-12-19

356

Digital Repository Infrastructure Vision for European Research (DRIVER)

The thermal entry region in laminar forced convection of Herschel-Bulkley fluids is solved analytically through the integral transform technique, for both circular and parallel-plates ducts, which are maintained at a prescribed wall temperature or at a prescribed wall heat flux. The local Nusselt numbers are obtained with high accuracy in both developing and fully-developed thermal regions, and critical comparisons with previously reported numerical results are performed.

Quaresma, J. N. N.; Mace?do, E. N.

1998-01-01

357

Digital Repository Infrastructure Vision for European Research (DRIVER)

Magnetic field effect on local entropy generation due to steady two-dimensional laminar forced convection flow past a horizontal plate was numerically investigated. This study was focused on the entropy generation characteristics and its dependency on various dimensionless parameters. The effect of various dimensionless parameters, such as Hartmann number (Ha), Eckert number (Ec), Prandtl number (Pr), Joule heating parameter (R) and the free stream temperature parameter (ÃŽÂ¸Ã¢ÂˆÂž)...

Al-nimr, Moh D. A.; Damseh, Renhe A.; Al-odat, Mohammed Q.

2004-01-01

358

Directory of Open Access Journals (Sweden)

Full Text Available The thermal entry region in laminar forced convection of Herschel-Bulkley fluids is solved analytically through the integral transform technique, for both circular and parallel-plates ducts, which are maintained at a prescribed wall temperature or at a prescribed wall heat flux. The local Nusselt numbers are obtained with high accuracy in both developing and fully-developed thermal regions, and critical comparisons with previously reported numerical results are performed.

J. N. N. QUARESMA

1998-03-01

359

Periodic heat transfer by forced laminar boundary layer flow over a semi-infinite flat plate

Digital Repository Infrastructure Vision for European Research (DRIVER)

The paper reports a study of periodic convection in a steady forced laminar boundary layer flow over a semi-infiniteimpermeable .at plate due to periodical variation of the wall heat flux. The Fourier transform based approach allows to obtain a transfer function for the boundary layer that can be used to solve also transient (non-periodic) heating problems, and examples are reported comparing with available studies in the open literature. The effect of periodic heating on the value of the ave...

Cossali, Gianpietro

2005-01-01

360

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

361

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

362

International Nuclear Information System (INIS)

A penalty finite element analysis with bi-quadratic elements is performed to investigate the influence of uniform and non-uniform heating of bottom wall on mixed convection lid driven flows in a square cavity. In the present investigation, bottom wall is uniformly and non-uniformly heated while the two vertical walls are maintained at constant cold temperature and the top wall is well insulated and moving with uniform velocity. A complete study on the effect of Gr shows that the strength of circulation increases with the increase in the value of Gr irrespective of Re and Pr. As the value of Gr increases, there occurs a transition from conduction to convection dominated flow at Gr=5.103 and Re=1 for Pr=0.7. A detailed analysis of flow pattern shows that the natural or forced convection is based on both the parameters Ri (Gr/Re2) and Pr. As the value of Re increases from 1 to 102, there occurs a transition from natural convection to forced convection depending on the value of Gr irrespective of Pr. Particularly for higher value of Grashof number (Gr=105), the effect of natural convection is dominant up to Re=10 and thereafter the forced convection is dominant with further increase in Re. As Pr increases from 0.015 to 10 for a fixed Re and Gr (Gr=103), the inertial force gradually becomes stronger and the intensity of secondary circulation gradually weakens. The local Nusselt number (Nub) plot shows that the heat tra number (Nub) plot shows that the heat transfer rate is very high at the edges of the bottom wall and then decreases at the center of the bottom wall for the uniform heating and that contrasts lower heat transfer rate at the edges for the non-uniform heating of the bottom wall. It is also observed that Nul shows non-monotonic behavior with both uniform and non-uniform heating cases for Re=10 at higher value of Pr. The average Nusselt number plot for the left or right wall shows a kink or inflexion at Gr=104 for highest value of Pr. Thus the overall power law correlation for average Nusselt number may not be obtained for mixed convection effects at higher Pr. (authors)

363

Forced and combined convection of water in a vertical seven-rod bundle with P/D = 1.38

International Nuclear Information System (INIS)

Heat transfer experiments of forced turbulent and laminar, and combined laminar downflows of water are conducted in a uniformly heated, triangularly arranged, seven-rod bundle having a pitch-to-diameter ratio of 1.38. In the forced flow experiments Reynolds number (Re) ranged from 1200 to 24 800 and Prandtl number (Pr) from 6.8 to 9.0, while in the combined convection experiments Re varied from 148 to 3800, Grashof number (Grq) from 1.3 x 105 to 3 x 106, and Richardson number (Ri) from 0.01 to 9. The data in the forced turbulent and the laminar flow regimes are in good agreement with the upflow correlations (within ±10%). Also, the transition between these two regimes, occurring at Re = 3800, is the same as that for the upflow condition. In the laminar flow regime, the flow entering the heated section is hydrodynamically developing while the flow in the heated section is thermally developed. The transition from forced laminar to combined convection occurred at Ri = 0.1, which is an order of magnitude lower than that for upflow. The combined convection data are correlated by superimposing the correlations for forced laminar and natural laminar flows as: NuC,L =[NuF,L3 + NuN,L3]1/3, for upflow and NuC,L=[NuF,L2 -NuN,L2]1/2, for downflow, where NuC,L, NuF,L and NuN,L are the Nusselt number for combined laminar flow, forced laminar flow and natural laminar flow respectively. These correlations are within ±11 and ±15% of the upflow and downflow data, respectively. (author)

364

Convective transfers; Transferts convectifs

Energy Technology Data Exchange (ETDEWEB)

This session about convective transfers gathers 31 articles dealing with: numerical study of the hydrodynamic stability of a bottom heated supercritical fluid layer; establishment of laminar-turbulent transition criteria of free convection dynamic and thermal boundary layers; heat transfer changes in free convection by mechanical and thermal disturbances; natural convection stability in partially porous horizontal layers; experimental characterization of the dynamic and thermal aspects of a natural convection flow inside a confined space; determination of transitions towards non-stationary natural convection inside a differentially heated inclined cavity; interface temperatures for the convection of fluids with variable viscosity; influence of the height of a vertical cylinder on the flow resulting from a plume-thermosyphon interaction; simultaneous measurement of dynamic and thermal fields by thermo-chromic liquid crystals in natural convection; numerical simulation of turbulent natural convection flows inside a heated room; numerical and experimental study of mixed convection heat transfer inside an axisymmetrical network; analysis of laminar flow instabilities in assisted mixed convection; entropy generation in mixed convection; thermal and mass convection in non-stationary regime inside a ventilated cavity; study of a low Reynolds number mixed convection flow; numerical study of a convective flow inside a rotating annular cavity; study of the dynamical behaviour of a transient mixed convection flow inside a thick vertical duct; internal laminar convection: selection criteria for the identification of natural, mixed or forced regimes; turbulent flow and convection heat transfer inside a channel with corrugated walls; study of the impact of an axisymmetrical jet on a concave wall; modeling of volume irreversibilities of turbulent forced convection; numerical study of forced convection irreversibilities around a network of cylindrical tubes; estimation of the exchange coefficient of a mobile cylinder impacted by a water jet - study of single-phase forced convection; second order modeling of the thermal field of an homogenous turbulence; numerical study of the effect of a periodical disturbance on the dynamical structure of the flow downstream of a descending step; numerical study of flows and heat transfers inside the air gap of a rotating machine; dynamical and thermal characteristics of boundary layers inside a turbulent Poiseuille flow with low flow rate ratio downstream of a T-junction; study of convective transfers at the inlet of a cylindrical tube with a low shape ratio (L/D = 8); experimental study of convective transfers in a rotor/stator system subjected to a air flux; correction of the strength and heat flux transferred by a moving cylinder between two parallel planes in Stokes-type regime; algebraic model for the forecasting of turbulent heat fluxes. (J.S.)

Accary, G.; Raspo, I.; Bontoux, P. [Aix-Marseille-3 Univ. Paul Cezanne, CNRS, Lab. MSNM-GP UMR 6181, 13 - Marseille (France); Zappoli, B. [Centre National d' Etudes Spatiales (CNES), 31 - Toulouse (France); Polidori, G.; Fohanno, S. [Laboratoire de Thermomecanique, 51 - Reims (France); Hirata, S.C.; Goyeau, B.; Gobin, D. [Paris-6 et Paris-11 Univ., FAST-UMR CNRS 7608, 91 - Orsay (France); Cotta, R.M. [UFRJ/LTTC/PEM/EE/COPPE, Rio de Janeiro (Brazil); Perrin, L.; Reulet, P.; Micheli, F.; Millan, P. [Office National d' Etudes et de Recherches Aerospatiales (ONERA), 31 - Toulouse (France); Menard, V. [France Telecom R and D, 22 - Lannion (France); Benkhelifa, A.; Penot, F. [Ecole Nationale Superieure de Mecanique et d' Aerotechnique (ENSMA), Lab. d' Etudes Thermiques, UMR CNRS 6608, 86 - Poitiers (France); Ng Wing Tin, M.; Haquet, J.F.; Journeau, C. [CEA Cadarache (DEN/DTN/STRI/LMA), Lab. d' Essais pour la Maitrise des Accidents Graves, 13 - Saint-Paul-lez-Durance (France); Naffouti, T.; Hammani, M.; Ben Maad, R. [Faculte des Sciences de Tunis, Lab. d' Energetique et des Transferts Thermique et Massique, Dept. de Physique, Tunis (Tunisia); Zinoubi, J. [Institut Preparatoire aux Etudes d' Ingenieurs de Nabeul (Tunisia); Menard, V.; Le Masson, S.; Nortershauser, D. [France Telecom R and D, 22 - Lannion (France); Stitou, A.; Perrin, L.; Millan, P. [ONERA, 31 - Toulouse (France)

2005-07-01

365

Natural Convection Heat Transfer Experiment in a Hemispherical Pool

International Nuclear Information System (INIS)

Natural convection plays an important role in determining the thermal load from molten core accumulated in the reactor vessel lower head during a severe accident. Several numerical and experimental programs were conducted to study the heat transfer in the molten pool. Previous investigations were mostly related to the rectangular and semicircular pools. Except for COPO, UCLA, ACOPO, and BALI, previous investigations suffer from inadequate representation of high modified Rayleigh number (Ra') in the hemispherical pool that may be formed in the reactor core and lower plenum. Thus, experimental work is conducted utilizing SIGMA SP (Simulant Internal Gravitated Material Apparatus Spherical Pool) producing high Ra' turbulent natural convection in a hemispherical pool up to 5.3 x 1011. The heating method has already been tested in SIGMA CP (Circular Pool). Six thin cable-type heaters, each with a diameter of 6 mm, are employed to simulate internal heating in the pool. They are uniformly distributed in the hemispherical pool to supply a maximum of 7.8 kW power to the pool. SIGMA SP has the inner and outer diameters of 500 mm and 520 mm, respectively. The upper flat plate and the curved wall of pool, with a 10 mm thick stainless steel plate, are cooled by a regulated water loop. A water-cooling system is used to maintain the temperature of water surrounding the test section nearly constant with time. This study focuses on quantifying the directional heat losses, angular heat flux distribution, and temperature distribution inside the molten pool

366

Convective heat transfer analysis in aggregates rotary drum reactor

International Nuclear Information System (INIS)

Heat transport characterisation inside rotary drum dryer has a considerable importance linked to many industrial applications. The present paper deals with the heat transfer analysis from experimental apparatus installed in a large-scale rotary drum reactor applied to the asphalt materials production. The equipment including in-situ thermal probes and external visualization by mean of infrared thermography gives rise to the longitudinal evaluation of inner and external temperatures. The assessment of the heat transfer coefficients by an inverse methodology is resolved in order to accomplish a fin analysis of the convective mechanism inside baffled (or flights) rotary drum. The results are discussed and compared with major results of the literature. -- Highlights: ? A thermal and flow experimentation is performed on a large-scale rotary drum. ? Four working points is chosen in the frame of asphalt materials production. ? Evaluation of the convective transfer mechanisms is calculated by inverse method. ? The drying stage is performed in the combustion area. ? Wall/aggregates heat exchanges have a major contribution in the heating stage

367

Liquid oil painting: Free and forced convection in an enclosure with mechanical and thermal forcing

A fluid dynamics video is linked to this article, which have been submitted to the Gallery of Fluid Motion as part of the 65th American Physical Society meeting of the Division of Fluid Dynamics, held in San Diego, California, USA, over 17-20 November 2012. The video serves to visualize flows generated in a rectangular enclosure that are subjected to both mechanical and thermal forcing through a common horizontal boundary. This system exhibits features consistent with either horizontal convection or lid-driven cavity flows depending on the ratio between thermal and mechanical stirring, and three different cases are visualized in the linked videos.

Sheard, Gregory J; King, Martin P

2012-01-01

368

Natural convective boundary layer flow of a nano-fluid past a convectively heated vertical plate

Energy Technology Data Exchange (ETDEWEB)

Natural convective flow of a nano-fluid over a convectively heated vertical plate is investigated using a similarity analysis of the transport equations followed by their numerical computations. The transport model employed includes the effect of Brownian motion and thermophoresis. The analysis shows that velocity, temperature and solid volume fraction of the nano-fluid profiles in the respective boundary layers depend, besides the Prandtl and Lewis numbers, on four additional dimensionless parameters, namely a Brownian motion parameter Nb, a thermophoresis parameter Nt, a buoyancy-ratio parameter Nr and convective parameter Nc. In addition to the study of these parameters on the boundary layer flow characteristics (velocity, temperature, solid volume fraction of the nano-fluid, skin friction, and heat transfer), correlations for the Nusselt and Sherwood numbers have been developed based on a regression analysis of the data. These linear regression models provide a highly accurate (with a maximum standard error of 0.004) representation of the numerical data and can be conveniently used in engineering practice. (authors)

Aziz, A. [Department of Mechanical Engineering, School of Engineering and Applied Science, Gonzaga University, Spokane, WA 99258 (United States); Khan, W.A. [Department of Engineering Sciences, PN Engineering College, National University of Sciences and Technology, Karachi 75350 (Pakistan)

2012-03-15

369

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 prween 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.

370

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

371

International Nuclear Information System (INIS)

The natural convection heat transfer coefficients on horizontal cylinders were derived numerically from the fundamental equations for laminar natural convection heat transfer by the finite difference method for a wide range of Rayleigh numbers. A new correlation for natural convection heat transfer from a horizontal cylinder, which can describe rigorous numerical solutions for uniform heat fluxes for Prandtl numbers ranging from 0.005 to 10 within ±4%, was derived. (author)

372

Directory of Open Access Journals (Sweden)

Full Text Available The effects of magnetic force, acting vertically downward on natural convection within a nanofluid filled tilted trapezoidal enclosure saturated with an electrically conducting fluid have been investigated numerically. The bottom wall of the enclosure is subjected to a constant cold temperature and the top wall experiences a heat source whereas the remaining sidewalls are kept adiabatic. The physical problems are represented mathematically by different sets of governing equations along with the corresponding boundary conditions. By using approximations of finite difference method, the non-dimensional governing equations are discritized. For natural convection the influential parameters are Rayleigh number Ra, the rotational angle of the enclosure? and the Hartmann number Ha, through which different thermo-fluid characteristics inside the enclosure are obtained. In the present study, the obtained results are presented in terms of streamlines, isotherms and average Nusselt number along the heat source. The result shows that with increasing Ha, the diffusive heat transfer become prominent even though Rayleigh number increases. Optimum heat transfer rate is obtained at higher values of Ra in the absence of magnetic force.

M. A. Mansour

2013-09-01

373

Characterization of fluid flow patterns and heat transfer in horizontal channel mixed convection

Energy Technology Data Exchange (ETDEWEB)

Two mechanisms of roll initiation are highlighted in a horizontal channel flow, uniformly heated from below, at constant heat flux ({gamma}=10, Pr=7, 50{<=}Re{<=}100, 0{<=}Ra{<=}10{sup 6}). The first mechanism is the classical one, it occurs for low Rayleigh numbers and is initiated by the lateral wall effect. The second occurs for higher Rayleigh numbers and combines the previous effect with a supercritical vertical temperature gradient in the lower boundary layer, which simultaneously triggers pairs of rolls in the whole zone in between the two lateral rolls. We have found that in the present configuration, the transition between the two roll initiation mechanisms occurs for Ra/Re{sup 2} {approx}18. Consequently, the heat transfer is significantly enhanced compared to the pure forced convection case owing to the flow pattern responsible of the continuous flooding the heated wall with cold fluid. (orig.)

Benderradji, A. [University of Batna, Department of Mechanical Engineering, Batna (Algeria); Haddad, A.; Taher, R.; Medale, M.; Abid, C.; Papini, F. [Technopole de Chateau-Gombert, IUSTI-CNRS UMR 6595, Polytech' Marseille, Marseille (France)

2008-10-15

374

DEFF Research Database (Denmark)

The present investigation is an extension of the authors’ previous work on ducts with different cross sections. It concerns application of turbulence models for forced convective heat transfer in three-dimensional corrugated or wavy ducts. Different wavy ducts with fully developed flow and temperature fields are considered. The numerical approach is based on the finite volume technique with a non-staggered grid arrangement. For handling the pressure-velocity coupling the SIMPLEC-algorithm is used. Cyclic boundary conditions are imposed in the main flow direction to achieve fully developed conditions. The non-linear k-e model of Speziale with wall functions is used to calculate the turbulent stresses. The simple eddy diffusivity concept is applied to calculate the heat fluxes, but the GGDH and the WET methods are also used in some cases. The influence of the geometry parameters and comparison between different ducts are presented in terms of the friction factor and average Nusselt number. In particular the secondary velocity field and the cross sectional temperature distributions are investigated. Keyword: Convection, Corrugated ducts, Duct flow, Turbulence

Rokni, Masoud; Sundén, Bengt

1998-01-01

375

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

376

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.)

377

International Nuclear Information System (INIS)

Obtained is a set of equations describing nonstationary convection heat transfer in the channels with incompressible liquid flow. Heat flux on the channel wall is defined according to the average geometric and mass medium temperatures. Found is the application region of the obtained set and the Newton cooling law for calculating nonstationary convection heat transfer

378

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

379

Energy Technology Data Exchange (ETDEWEB)

A mathematical model for simulating the behaviour of a solar crop dryer in forced convection is presented. Then, this model is validated by comparing the obtained results with the experimental ones. Also, we present the influence of some parameters over the behaviour. Finally, we give the optimal values for the parameters to give the best possible behaviour. (authors) 12 refs.

Belghit, A.; Belahmidi, M.; Bennis, A.; Boutaleb, B.C. [Faculte des Sciences de Semlalia, Laboratoire de mecanique des fluides et energetique (Morocco); Benet, S. [Perpignan Univ., 66 (France)

1997-12-01

380

Convective heat transfer for viscoelastic fluid in a curved pipe

Energy Technology Data Exchange (ETDEWEB)

In this paper, fully developed convective heat transfer of viscoelastic flow in a curved pipe under the constant heat flux at the wall is investigated analytically using a perturbation method. Here, the curvature ratio is used as the perturbation parameter and the Oldroyd-B model is applied as the constitutive equation. In the previous studies, the Dirichlet boundary condition for the temperature at the wall has been used to simplify the solution, but here exactly the non-homogenous Neumann boundary condition is considered to solve the problem. Based on this solution, the non-axisymmetric temperature distribution of Dean flow is obtained analytically and the effect of flow parameters on the flow field is investigated in detail. The current analytical results indicate that increasing the Weissenberg number, viscosity ratio, curvature ratio, and Prandtl number lead to the increase of the heat transfer in the Oldroyd-B fluid flow. (orig.)

Norouzi, M.; Kayhani, M.H. [Shahrood University of Technology, Mechanical Engineering Department, Shahrood (Iran); Nobari, M.R.H. [Amirkabir University of Technology, Mechanical Engineering Department, Tehran (Iran); Joneidi, A.A. [Eindhoven University of Technology, Mechanical-Polymer Technology Group, Eindhoven (Netherlands)

2010-10-15

381

Directory of Open Access Journals (Sweden)

Full Text Available The present work investigates the laminar forced convection flow of a radiating gas over an inclined backward facing step (BFS in a horizontal duct. The momentum and energy equations are solved numerically by the CFD techniques to obtain the velocity and temperature fields. Since, the twodimensional Cartesian coordinate system is used to solve the governing equations; the flow over inclined surface is simulated by considering the blocked-off region in regular grid. Discretized forms of the governing equations in the (x,y plane are obtained by the control volume method and solved using the SIMPLE algorithm. The fluid is treated as a gray, absorbing, emitting and scattering medium. Therefore, all of the convection, conduction and radiation heat transfer mechanisms take place simultaneously in the gas flow. For computation of the radiative term in the gas energy equation, the radiative transfer equation (RTE is solved numerically by the discrete ordinates method (DOM to find the radiative heat flux distribution inside the radiating medium. In the numerical results, effects of inclination angle, optical thickness, scattering albedo and the radiation-conduction parameter on the heat transfer behavior of the convection flow are investigated. This research work is a new one in which a combined convection-radiation thermal system with a complex flow geometry is simulate by efficient numerical techniques.

Ansaria Amir Babak

2013-01-01

382

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

383

Heat flux scaling in turbulent Rayleigh-B\\'enard convection with an imposed longitudinal wind

We present a numerical study of Rayleigh-B\\'enard 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 plumes-sweeping, and then increases again when turbulent forced convection dominates over the buoyancy. As a result, the Nusselt number is a non-monotonic function of the shear Reynolds number. We provide a simple model that captures with good accuracy all the dynamical regimes observed. We expect that our findings can lead the way to a more fundamental understanding of the of the complex interplay between mean-wind and plumes ejection in the Rayleigh-B\\'enard phenomenology.

Scagliarini, Andrea; Toschi, Federico

2013-01-01

384

Free convection on heated horizontal circular cylinder in presence of heat generation

This study investigates free convection about a heated infinite horizontal circular cylinder in a Newtonian fluid in the presence of heat generation. The cylinder is fixed and immersed in a stationary fluid, in which the temperature is uniformly heated about the temperature of the surrounding fluid. The governing equation is non-dimensionlized and solved numerically by employing a finite difference method. The effects of heat generation parameter,? and the Prandtl number, Pr, of the temperature and velocity profile are shown graphically. It is found that the presence of heat generation significantly increases the temperature and velocity distribution thus as Pr increases, the temperature and velocity distribution decrease too.

Zainuddin, Najahulfazliah; Hashim, Ishak; Ismoen, Muhaimin; Roslan, Rozaini

2014-06-01

385

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

386

An experimental study of natural convection heat transfer through a doorway in a two room passive solar heated building is described. Similitude modeling was the method used to measure natural convection heat transfer coefficients (h/sub NC/) in a model geometrically scaled down by a factor of 5. Freon gas was used as the working fluid to obtain dynamic similarity within the model. A temperature difference was maintained between the two rooms by a heated vertical wall which simulated a Trombe wall in one room, and by a cooled vertical wall which simulated a thermal storage wall in the other room. Heat transfer through the doorway was measured as a function of a characteristic temperature differential and the geometry of the doorway.

Weber, D. D.

1980-06-01

387

Energy Technology Data Exchange (ETDEWEB)

The effects of channel gap size on mixed forced and free convective heat transfer characteristics were experimentally investigated for water flowing near atmospheric pressure in a 750 mm long and 50 mm wide channel heated from both sides. The channel gap sizes investigated were 2.5, 6, 18 and 50 mm. Experiments were carried out for both aiding and opposing forced convective flows with a Reynolds number Re{sub x} of 4x10 to 6x10{sup 6} and a Grashof number Gr{sub x} of 2x10{sup 4} to 6x10{sup 11}, where the distance x from the inlet of the channel is adopted as the characteristic length in Re{sub x} and Gr{sub x}. As for the results, the following were revealed for the parameters ranges investigated in this study. (1) When the dimensionless parameter, Gr{sub x}/Re{sub x}{sup 21/8}Pr{sup 1/2} is less than 10{sup -4}, the flow shows the nature of forced convective heat transfer for a channel with any channel gap size in both aiding and opposing flows. (2) When Gr{sub x}/Re{sub x}{sup 21/8}Pr{sup 1/2} is larger than 10{sup -2}, the flow shows the nature of free convective heat transfer for a channel with any channel gap size in both aiding and opposing flows. (3) When Gr{sub x}/Re{sub x}{sup 21/8}Pr{sup 1/2} is between 10{sup -4} and 10{sup -2} for the channel with a channel gap size equal to or larger than 6 mm, the heat transfer coefficients in both aiding and opposing flows become, on the average, higher than those predicted by the previous correlations for either the pure turbulent forced convection or the pure free convection, and can be expressed in simple forms with a combination of Gr{sub x}/Re{sub x}{sup 21/8}Pr{sup 1/2} and the previous correlation for either the pure turbulent forced convection or the free convection along a flat plate. (Abstract Truncated)

Sudo, Y.; Kaminaga, M. (Japan Atomic Energy Research Inst., Tokai, Ibaraki (Japan)); Minazoe, K. (NKK Corp., Kawasaki, Kanagawa (Japan))

1990-06-01

388

Droplet combustion experiments in varying forced convection using microgravity environment

International Nuclear Information System (INIS)

A new microscopic model of the interaction between droplet flames and fine vortex tubes which compose a coherent structure of turbulence was developed. Three non-dimensional numbers were introduced to extend the length scale and time scale so as to be suitable for microgravity experiments using droplets of combustion of about 1 mm in diameter. An experimental apparatus for combustion of a single droplet and that of an array of two droplets in varying airflow was developed, and experiments were performed in microgravity and normal gravity at pressures up to 2.0 MPa for n-nonane and ethanol as fuels. Variations of the instantaneous burning rate constant, K i, in response to the varying flow velocity was successfully observed. At high pressure, the effects of droplet Reynolds number Re on K i was clearly seen, while the effects of natural convection, which increases K i with Re, was seen in normal gravity even in the forced airflows. As for the experiments on combustion of an array of two droplets, K i reduction of the downstream droplet became weak when the flow direction was varied. However, the K i reduction of the downstream droplet for flow direction variations was clearly seen for n-nonane droplets but almost not for ethanol droplets. The interaction mechanism between upstream and downstream droplets is considered to result from the elimination of oxidizer supply to the downstream droplet, indicating strong interaction effects of n-nonane droplets for a stoichiometric oxygen-fuel ratio of n-nonane (i.e., 14.0) greater than that of ethanol (i.e., 3.0)

389

Drying modelling of defrosted pork meat under forced convection conditions.

Drying is the lengthiest and the most energy consuming step during the production of dry-cured ham, affecting also the curing process and consequently product quality. In order to manage the drying process, it is quite interesting to establish the complexity of model needed. For that purpose, pork meat cylinders (Biceps femoris and Semimembranosus muscles) were dehydrated under forced convection conditions (25°C and air velocity 0.6±0.1, 2.0±0.1 and 2.8±0.1 m/s). Experimental drying kinetics were modelled by means of 4 diffusion models: model 1 (not considering shrinkage and no external resistance), model 2 (considering shrinkage and no external resistance), model 3 (not considering shrinkage and considering external resistance) and model 4 (considering both shrinkage and external resistance). From the effective diffusivity values identified, it was concluded that when external resistance was negligible (air velocity 2.0±0.1 and 2.8±0.1 m/s), the results obtained for D(e) with the four models were the same. Nevertheless, when external resistance was not negligible (0.6±0.1 m/s) the D(e) identified was influenced by the model due to the fact that models 1 and 2 neglect that resistance and for that reason they do not describe experimental conditions properly. The effect of shrinkage did not influence the identified D(e) values for the drying conditions considered. In order to model water losses in meat curing chambers, external resistance must be considered. PMID:21334143

Clemente, G; Bon, J; Sanjuán, N; Mulet, A

2011-07-01

390

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, whic period was calculated as 0.89 years, which was also very small considering the life of the system (20 years). (author)

391

International Nuclear Information System (INIS)

An investigation of the hydromagnetic free and forced convection in a parallel plate channel formed by two finitely conducting parallel porous walls taking Hall effects into account, the liquid being permeated by a transverse magnetic field, is made. There is uniform axial temperature variation along the walls. An exact solution of the governing equations is obtained. The flow phenomenon has been characterized by the non-dimensional numbers like M2 (Hartmann number), G(Grashof number), ?sub(o)(suction parameter), thetasub(1), thetasub(2) (conductance parameters) and m (Hall parameter). The induced magnetic field and the heat transfer characteristcs in the flow are also determined. Expressions for the shearing stress components have also been sought. The effect of Hall parameter on the velocity, the induced magnetic field and shearing stress is interpreted with the aid of graphs and a table. (author)

392

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

393

Study of turbulent natural-circulation flow and low-Prandtl-number forced-convection flow

International Nuclear Information System (INIS)

Calculational methods and results are discussed for the coupled energy and momentum equations of turbulent natural circulation flow and low Prandtl number forced convection flow. The objective of this paper is to develop a calculational method for the study of the thermal-hydraulic behavior of coolant flowing in a liquid metal fast breeder reactor channel under natural circulation conditions. The two-equation turbulence model is used to evaluate the turbulent momentum transport property. Because the analogy between momentum transfer and heat transfer does not generally hold for low Prandtl number fluid and natural circulation flow conditions, the turbulent thermal conductivity is calculated independently using equations similar to the two-equation turbulence model. The numerical technique used in the calculation is the finite element method

394

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

395

Energy Technology Data Exchange (ETDEWEB)

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 S{sub 4} 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. (author)

Ben Nejma, F.; Mazgar, A.; Abdallah, N.; Charrada, K. [Unite de recherche: Etudes des Milieux Ionises et Reactifs (EMIR), Institut Preparatoire aux Etudes d' Ingenieur de Monastir, Avenue Ibn Eljazar, Monastir 5019 (Tunisia)

2008-07-15

396

We study the finite-Péclet number forced convective heat transfer from a uniform temperature sphere placed in otherwise uniform fluid stream within a porous medium. A numerical study is undertaken to determine how the lack of local thermal equilibrium between the phases affects temperature fields of the two phases and the respective rates of heat transfer from the sphere. On the upstream side of the sphere the temperature field extends further from the sphere in the solid phase than it does for the fluid phase, but the opposite is true on the downstream side.

Kwan, Hilda H. Y.; Rees, D. Andrew S.; Pop, Ioan

2008-09-01

397

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

398

In vivo measurement of swine endocardial convective heat transfer coefficient.

We measured the endocardial convective heat transfer coefficient h at 22 locations in the cardiac chambers of 15 pigs in vivo. A thin-film Pt catheter tip sensor in a Wheatstone-bridge circuit, similar to a hot wire/film anemometer, measured h. Using fluoroscopy, we could precisely locate the steerable catheter sensor tip and sensor orientation in pigs' cardiac chambers. With flows, h varies from 2500 to 9500 W/m2 x K. With zero flow, h is approximately 2400 W/m2 x K. These values of h can be used for the finite element method modeling of radiofrequency cardiac catheter ablation. PMID:15311835

Tangwongsan, Chanchana; Will, James A; Webster, John G; Meredith, Kenneth L; Mahvi, David M

2004-08-01

399

Nonstationary convective heat transfer in rectangular cross section channels

International Nuclear Information System (INIS)

Non-stationary problems of convection heat exchange in channels with a rectangular cross section in the case of boundary conditions of the third kind, as well as in the case of variable heat exhange over the perimeter of a heat current on the wall, are solved. The edge problems of a non-stationary heat exchange during the liquid flow in a channel of a rectangular cross section is drawn to the integration of the energy equation. As results have shown, Nu number values differ insignificantly from corresponding values, obtained when solving analogous problems i when the wall temperature is given (boundary conditions of the first kind). The above approach to the solution of problems permits to obtain results for channels of other forms of cross section, as well; to investigate the effect of non-stationarity, geometry, boundary conditions and other parameters on the process of heat exchange, to solve problems when accounting energy dissipations in a current, with the presence of heat sources

400

Mixed convection in a channel provided with heated porous blocks of various shapes

Energy Technology Data Exchange (ETDEWEB)

The present work is a numerical simulation of laminar mixed convective in a two-dimensional parallel-plate channel provided with porous blocks of various shapes. The upper plate is thermally insulated while the blocks, heated from below, are attached on the lower one. The Brinkman-Forchheimer extended Darcy model with the Boussinesq approximation is adopted for the flow in the porous regions. The governing equations with the appropriate boundary conditions are solved by the control volume method. The influence of the buoyancy force intensity, the porous blocks shape going from the rectangular shape to the triangular shape, their height, the porous medium permeability, the Reynolds number and the thermal conductivity ratio is analyzed. The results reveal essentially, that the shape of the blocks can alter substantially the flow and heat transfer characteristics. In addition, it is shown that judicious choices of these parameters can lead to high heat transfer rates with a moderate increase of pressure drop.

Guerroudj, N. [Faculty of Mechanical and Process Engineering, Houari Boumediene University of Sciences and Technology, B.P. 32, El Alia, Bab Ezzouar 16111, Algiers (Algeria); Kahalerras, H., E-mail: kahalerrashenda@yahoo.f [Faculty of Mechanical and Process Engineering, Houari Boumediene University of Sciences and Technology, B.P. 32, El Alia, Bab Ezzouar 16111, Algiers (Algeria)

2010-03-15

401

Mixed convection in a channel provided with heated porous blocks of various shapes

International Nuclear Information System (INIS)

The present work is a numerical simulation of laminar mixed convective in a two-dimensional parallel-plate channel provided with porous blocks of various shapes. The upper plate is thermally insulated while the blocks, heated from below, are attached on the lower one. The Brinkman-Forchheimer extended Darcy model with the Boussinesq approximation is adopted for the flow in the porous regions. The governing equations with the appropriate boundary conditions are solved by the control volume method. The influence of the buoyancy force intensity, the porous blocks shape going from the rectangular shape to the triangular shape, their height, the porous medium permeability, the Reynolds number and the thermal conductivity ratio is analyzed. The results reveal essentially, that the shape of the blocks can alter substantially the flow and heat transfer characteristics. In addition, it is shown that judicious choices of these parameters can lead to high heat transfer rates with a moderate increase of pressure drop.

402

International Nuclear Information System (INIS)

The paper deals with the investigation of thermo-magnetic convection in a paramagnetic liquid subjected to a non-uniform magnetic field in weightlessness conditions. Indeed, in zero-g space conditions such as realized in International Space Station (ISS), or in artificial satellite, or in free-flight space vessels, the classical thermo-gravitational convection in fluid disappears. In any cases, it may be useful to restore the convective thermal exchange inside fluids such as liquid oxygen. In this paper, the restoration of heat exchange by the way of creation of magnetic convection is numerically studied.

403

Natural convection heat transfer in a rectangular pool with volumetric 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. The heat transfer within the molten core material can be characterized by buoyancy-induced flows resulting from internal heating due to decay of fission products. The thermo-fluid dynamic characteristics of the molten pool 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, natural convection involving internal heat generation is delineated in terms of the modified Rayleigh number, Ra', which quantifies the internal heat source and hence the strength of buoyancy. The test section is of rectangular cavity whose length, width, and height are 500 mm, 80 mm, and 250 mm, respectively. A total of twenty-four T-type thermocouples were installed in the test loop to measure temperature distribution. Four T-type thermocouples were utilized to measure temperatures on the boundary. A direct heating method was adopted in this test to simulate the uniform heat generation. The experiments covered a range of Rayleigh number, Ra, between 4.87x107 and 2.32x1014 and Prandtl number, Pr, between 0.7 and 3.98. Tests were conducted with water and air as simulant. The upper and lower boundary conditions were maintained at a uniform temperature of 10degC. (author)

404

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 princip