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Showing papers on "Heat transfer coefficient published in 1998"


Journal ArticleDOI
TL;DR: In this article, an experimental study of heat transfer and flow regimes during condensation of refrigerants in horizontal tubes was conducted, where measurements were made in smooth, round tubes with diameters ranging from 3.14 mm to 7.04 mm.
Abstract: An experimental study of heat transfer and flow regimes during condensation of refrigerants in horizontal tubes was conducted. Measurements were made in smooth, round tubes with diameters ranging from 3.14 mm to 7.04 mm. The refrigerants tested were R-12, R-22, R-134a, and near-azeotropic blends of R-32/R-125 in 50 percent/50 percent and 60 percent/40 percent compositions. The study focused primarily on measurement and prediction of condensing heat transfer coefficients and the relationship between heat transfer coefficients and two-phase flow regimes. Flow regimes were observed visually at the inlet and outlet of the test condenser as the heat transfer data were collected. Stratified, wavy, wavy annular, annular, annular mist, and slug flows were observed. True mist flow without a stable wall film was not observed during condensation tests. The experimental results were compared with existing flow regime maps and some corrections are suggested. The heat transfer behavior was controlled by the prevailing flow regime. For the purpose of analyzing condensing heat transfer behavior, the various flow regimes were divided into two broad categories of gravity-dominated and shear-dominated flows. In the gravity dominated flow regime, the dominant heat transfer mode was laminar film condensation in the top of the tube. This regime was characterized by heat transfer coefficients that depended on the wall-to-refrigerant temperature difference but were nearly independent of mass flux. In the shear-dominated flow regime, forced-convective condensation was the dominant heat transfer mechanism. This regime was characterized by heat transfer coefficients that were independent of temperature difference but very dependent on mass flux and quality. Heat transfer correlations that were developed for each of these flow regimes successfully predicted data from the present study and from several other sources.

673 citations


Journal ArticleDOI
TL;DR: In this paper, an analog between convection and conduction with heat sources is made to have a further understanding of the mechanism of convective heat transfer, and three ways to raise the strength of heat sources/convection terms, and consequently to enhance the heat transfer are presented.

654 citations


Book
22 May 1998
TL;DR: In this paper, the analogy between heat and mass transfer is covered and applied in the analysis of heat transfer by conduction, convection and radiation, and the analysis is performed by using the handbook of numerical heat transfer.
Abstract: Handbook of Numerical Heat Transfer Free Full Download Links from Multiple Mirrors added by DL4W on 2015-04-10 02:13:35. Handbook of heat transfer / editors, W.M. Rohsenow, J.P. Hartnett. Y.I. Cho. m 3rd ed. p. cm. Includes bibliographical references and index. ISBN 0-07053555-8. Students investigate heat transfer by conduction, convection and radiation. The analogy between heat and mass transfer is covered and applied in the analysis.

564 citations


Journal ArticleDOI
TL;DR: In this paper, an analytical model is developed for model foams with simple cubic unit cells consisting of heated slender cylinders, based on existing heat transfer data on convective crossflow through cylinder banks.

558 citations


Journal ArticleDOI
TL;DR: In this article, the analysis of laminar mixed convection in boundary layers adjacent to a vertical, continuously stretching sheet has been presented, where the velocity and temperature of the sheet were assumed to vary in a power-law form.
Abstract: The analysis of laminar mixed convection in boundary layers adjacent to a vertical, continuously stretching sheet has been presented. The velocity and temperature of the sheet were assumed to vary in a power-law form, that is, u w (x)=Bx m and T w (x)−T ∞=Ax n . In the presence of buoyancy force effects, similarity solutions were reported for the following two cases: (a) n=0 and m=0.5, which corresponds to an isothermal sheet moving with a velocity of the form u w =Bx 0.5 and (b) n=1 and m=1, which corresponds to a continuous, linearly stretching sheet with a linear surface temperature distribution, i.e. T w −T ∞=Ax. Formulation of the present problem shows that the heat transfer characteristics depends on four governing parameters, namely, the velocity exponent parameter m, the temperature exponent parameter n, the buoyancy force parameter G *, and Prandtl number of the fluid. Numerical solutions were generated from a finite difference method. Results for the local Nusselt number, the local friction coefficient, and temperature profiles are presented for different governing parameters. Effects of buoyancy force and Prandtl number on the flow and heat transfer characteristics are thoroughly examined.

378 citations


Journal ArticleDOI
TL;DR: In this paper, a heat transfer model for intube flow boiling in horizontal plain tubes is proposed that incorporates the effects of local two-phase flow patterns, flow stratification and partial dryout in annular flow.
Abstract: A new heat transfer model for intube flow boiling in horizontal plain tubes is proposed that incorporates the effects of local two-phase flow patterns, flow stratification and partial dryout in annular flow. Significantly, the local peak in the heat transfer coefficient versus vapor quality can now be determined from the prediction of the location of onset of partial dryout in annular flow. The new method accurately predicts a large, new database of flow boiling data, and is perticularly better than existing mehods at high vapor qualities (x > 85%) and for stratified types of flows.

376 citations


Journal ArticleDOI
TL;DR: In this paper, direct numerical simulations of the turbulent heat transfer for various Prandtl numbers ranging from 0.025 to 5 are performed to obtain statistical quantities such as turbulent heat flux, temperature variance and their budget terms.

347 citations


Journal ArticleDOI
TL;DR: In this paper, a complete numerical solution of the Navier-Stokes and energy equations, based on a modified SOLA-VOF method, was used to model droplet deformation and solidification and heat transfer in the substrate.

267 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of the injection and suction on heat transfer over a stretching surface with variable surface heat flux and uniform surface heat flow subject to injection was examined. And the boundary layer equations were transformed to ordinary differential equations, containing an injection parameter I, velocity exponent parameter M, heat flux exponent parameter and Prandtl number.
Abstract: Heat transfer over a stretching surface with variable surface heat flux and uniform surface heat flux subject to injection and suction is examined. The boundary layer equations are transformed to ordinary differential equations, containing an injection parameter I, velocity exponent parameter M, heat flux exponent parameter and Prandtl number . The effect of these parameters is studied. Suction increases the heat transfer from the surface, whereas injection causes a decrease in the heat transfer.

263 citations


Journal ArticleDOI
TL;DR: In this paper, the problem of cooling a heated plate by an axisymmetric isothermal fully developed turbulent jet has been studied numerically, and the normal-velocity relaxation turbulence model (V2F) was used to predict local heat transfer coefficient.

252 citations


Journal ArticleDOI
TL;DR: In this paper, the authors identified three locations in the subcooled flow boiling region: the onset of nucleate boiling, the point of net vapor generation, and the location where x = 0 is attained from enthalpy balance equations.
Abstract: Subcooled flow boiling covers the region beginning from the location where the wall temperature exceeds the local liquid saturation temperature to the location where the thermodynamic quality reaches zero, corresponding to the saturated liquid state. Three locations in the subcooled flow have been identified by earlier investigators as the onset of nucleate boiling, the point of net vapor generation, and the location where x = 0 is attained from enthalpy balance equations. The heat transfer regions are identified as the single-phase heat transfer prior to ONB, partial boiling (PB), and fully developed boiling (FDB). A new region is identified here as the significant void flow (SVF) region. Available models for predicting the heat transfer coefficient in different regions are evaluated and new models are developed based on our current understanding

Journal ArticleDOI
TL;DR: In this article, a visual study of vapor bubble growth and departure in vertical upflow and downflow forced convection boiling is presented, where high-speed digital images of flow boiling phenomena were obtained, which were used to measure bubble growth, departure diameters, and lift-off diameters.

Journal ArticleDOI
TL;DR: In this article, a new equivalent Reynolds number model, based on the heat-momentum analogy, is developed in order to predict the experimental Nusselt number of 1197 data points from 18 sources.
Abstract: In 1959, Akers et al. developed an in-tube condensation model, which defines the all-liquid flow rate that provides the same heat transfer coefficient as an annular condensing flow. This liquid flow rate was expressed by an equivalents Reynolds number and used in a single-phase, turbulent flow equation to predict the condensation coefficient. However, the assumptions on which the equivalent Reynolds number is based are shown in the present work to be faulty. This results in the underprediction of many researchers' data. A new equivalent Reynolds number model, based on the heat-momentum analogy, is developed in this study. This model is then shown to predict the experimental Nusselt number of 1197 data points from 18 sources with an average deviation of 13.64 percent. The data are for tube internal diameters between 3.14 and 20 mm.

Journal ArticleDOI
TL;DR: In this paper, a series of full-scale measurements in the real environment were made to assess the magnitude and variability of wind-induced convective heat transfer from a raised heated surface mounted directly on to the pitched roof of a domestic size building.

Journal ArticleDOI
TL;DR: In this paper, the authors present a review of the state-of-the-art of the science and technology of vacuum glazing, and their method of manufacturing in the laboratory.

Journal ArticleDOI
TL;DR: In this article, a comprehensive experimental study on flow boiling heat transfer is presented for five refrigerants (R134a, R123, R402A, R404A and R502) evaporating inside plain horizontal, copper tube test sections.
Abstract: A summary of a comprehensive experimental study on flow boiling heat transfer is presented for five refrigerants (R134a, R123, R402A, R404A and R502) evaporating inside plain horizontal, copper tube test sections. The test data were obtained for both 12.00 mm and 10.92 mm diameters using hot water as the heating source. Besides confirming known trends in flow boiling heat transfer data as a function of test variables, it was also proven that the heat flux level at the dryout point at the top of the tube in annular flow has a very significant downstream effect on heat tranfer coefficients in the annular flow regime with partial dryout.

Journal ArticleDOI
TL;DR: In this paper, a comparison of small diameter mechanically expanded round-tube heat exchangers and brazed microchannel-type units for CO2 gas coolers has been conducted.
Abstract: Compact and lightweight heat exchangers are needed for motor vehicle air-conditioning systems and for several types of unitary equipment. The high-pressure natural refrigerant CO2 is now being evaluated for use in such applications, and efficient heat exchangers are being developed and investigated. Carbon dioxide heat exchangers are designed for high refrigerant mass flux and use small-diameter tubes or extruded flat microchannel tubes. Refrigerant-side heat transfer coefficients are higher than with fluorocarbons, and reduced internal surface areas can therefore be tolerated. Both small-diameter mechanically expanded round-tube heat exchangers and brazed microchannel-type units have been built and tested successfully. Results show that compact heat exchangers optimized for CO2 are very competitive with baseline HFC/HCFC units in terms of physical dimensions, exchanger mass and thermal performance. Smaller tube and manifold dimensions can give reduced size compared with HFC-134a equipment. The temperature approach between air inlet and refrigerant outlet is much lower in CO2 gas coolers than in baseline system condensers of equal size and capacity, and the reduced refrigerant exit temperature has a marked influence on the coefficient of performance, Microchannel heat exchangers give the best overall efficiency. Refrigerant distribution in multiport manifolds and heat transfer tubes does not seem to be a problem.

Journal ArticleDOI
TL;DR: In this paper, an L-A-S computer aided language is used to describe variations in the plant thermal performance, the specific heat transfer area, and the specific flow rate of cooling water in terms of the top brine temperature and the number of effects.
Abstract: desalting system. The algorithm consists of 10 calculation blocks and 6 logical blocks. The algorithm is implemented using L-A-S computer aided language. Results show that the heat transfer coefficients increase with the boiling temperature. Also, the heat transfer coefficient in the evaporator is always higher than that in the feed preheater at the same boiling temperature. The plant thermal performance ratio is nearly independent of the top brine temperature and strongly related to the number of effects. The specific heat transfer area increases by raising the number of effects and reducing the top brine temperature. The effect of the top brine temperature on the specific heat transfer area is more pronounced with a larger number of effects. The required specific heat transfer areas at a top brine temperature of 100 ∞C are 30.3% and 26% of that required at 60 ∞C when the number of effects are 6 and 12, respectively. The specific flow rate of cooling water is nearly constant at different values of top brine temperature and tapers off at a high rate as the number of effects is increased. Two correlations are developed to relate the heat transfer coefficients in the preheater and the evaporator to the boiling temperature. Design correlations are also developed to describe variations in the plant thermal performance, the specific heat transfer area, and the specific flow rate of cooling water in terms of the top brine temperature and the number of effects.

Journal ArticleDOI
TL;DR: In this paper, the authors present results for natural convection heat transfer coefficients of a heated wall, a heated floor and a heated ceiling which have been calculated using CFD Two turbulence models have been used to calculate these coefficients: a standard k − ǫ model using ‘wall functions’ and a low Reynolds number k - � model.

Journal ArticleDOI
TL;DR: In this article, the authors compared the heat transfer and pressure drop coefficients for a plain, microfin, and twisted-tape insert-tubes in a single shell-and-tube heat exchanger.

Journal ArticleDOI
TL;DR: In this article, a transient liquid crystal technique was used to measure the detailed heat transfer coefficients on the target surface for an array of in-line jets impinging on a target plate with different crossflow orientations.
Abstract: Detailed heat transfer distributions are presented for an array of in-line jets impinging orthogonally on a target plate with different crossflow orientations. A transient liquid crystal technique was used to measure the detailed heat transfer coefficients on the target surface. Different crossflow directions are created by changing the test section open ends. For each exit orientation, measurements are made at four flow Reynolds numbers between 4.8 x 10 3 and 1.83 X 10 4 . Results show that the now exit crossflow direction significantly affects the flow and heat transfer coefficient distributions on the target plate. Local heat transfer coefficient increases with an Increase in average jet Reynolds number over the entire impingement target surface. Highest heat transfer coefficients are obtained for a crossflow orientation where flow exits in both directions. Nusselt number results are correlated for the various exit flow orientations

Journal ArticleDOI
TL;DR: In this article, the volumetric heat transfer coefficients between cellular ceramics and a stream of air were measured using the single-blow transient experimental technique in conjunction with an inverse analysis.

Proceedings ArticleDOI
TL;DR: In this article, a mass transfer analogy based on the naphthalene sublimation technique was used to reveal the individual heat transfer contributions from pins and endwalls with the entire wetted surface thermally active.
Abstract: Short pin-fin arrays are often used for cooling turbine airfoils, particularly near the trailing edge. An accurate heat transfer estimation from a pin-fin array should account for the total heat transfer over the entire wetted surface, which includes the pin surfaces and uncovered endwalls. One design question frequently raised is the actual magnitudes of heat transfer coefficients on both pins and endwalls. Results from earlier studies have led to different and often contradicting conclusions. This variation, in part, is caused by imperfect or unrealistic thermal boundary conditions prescribed in the individual test models. Either pins or endwalls, but generally not both, were heated in those previous studies. Using a mass transfer analogy based on the naphthalene sublimation technique, the present experiment is capable of revealing the individual heat transfer contributions from pins and endwalls with the entire wetted surface thermally active. The particular pin-fin geometry investigated, S/D = X/D = 2.5 and H/D = 1.0, is considered to be one of the optimal array arrangement for turbine airfoil cooling. Both inline and staggered arrays with the identical geometric parameters are studied for 5000 < Re < 25,000. The present results reveal that the general trends of the row-resolved heat transfer coefficients on either pins or endwalls are somewhat insensitive to the nature of thermal boundary conditions prescribed on the test surface. However, the actual magnitudes of heat transfer coefficients can be substantially different, due to variations in the flow bulk temperature. The present study also concludes that the pins have consistently 10 to 20 percent higher heat transfer coefficient than the endwalls. However, such a difference in heat transfer coefficient imposes very insignificant influence on the overall array-averagea heat transfer, since the wetted area of the uncovered endwalls is nearly four times greater than that of the pins.

Journal ArticleDOI
TL;DR: In this article, the effect of the transverse magnetic field on flow field patterns and heat transfer processes in a tilted square cavity was numerically investigated, where the horizontal walls of the enclosure are assumed to be insulated while the vertical walls are kept isothermal.
Abstract: This paper numerically investigates the effect of the transverse magnetic field on flow field patterns and heat transfer processes in a tilted square cavity. The horizontal walls of the enclosure are assumed to be insulated while the vertical walls are kept isothermal. The power law control volume approach is developed to solve the conservation equations at Prandtl number of 0.71. Validation tests with existing data demonstrate the ability of the present scheme to produce accurate results. The effects of Grashof number, enclosure inclination angle, and Hartmann number are also investigated. The study covers the range of the Hartmann number from 0 to 100, the enclosure inclination angle from 0° to ‐90° with Grashof number of 104 and 106. The effect of the magnetic field is found to suppress the convection currents and heat transfer inside the cavity. This effect is significant for low inclination angles and high Grashof numbers. Additionally, it is noted that there is no variation of average Nusselt number with respect to inclination angle for high Hartmann number.

Journal ArticleDOI
TL;DR: In this article, a model using the population balance concept is developed to predict the drop size distribution for small drops that grow by direct condensation, and the resistances to heat transfer due to the drop (conduction through the drop, vapor-liquid interfacial resistance, drop curvature) and due to promoter layer and the sweeping effect of falling drops are incorporated into the model and are also included in calculating the heat transfer rate through a single drop.

Journal ArticleDOI
TL;DR: In this paper, two basic separator configurations, open flow separators and screen separators, were studied and the experimental results showed that as the permeate flux increases, the temperature and recirculation rate increase.

Journal ArticleDOI
TL;DR: In this paper, an analysis of visco-elastic fluid flow and heat transfer characteristics in a saturated porous medium over an impermeable stretching surface with frictional heating and internal heat generation or absorption was carried out.
Abstract: An analysis is carried out to study the visco-elastic fluid flow and heat transfer characteristics in a saturated porous medium over an impermeable stretching surface with frictional heating and internal heat generation or absorption. Two cases are considered; namely (i) the sheet with prescribed surface temperature (PST-Case) and (ii) the wall heat flux (PHF-Case). Exact solution for the velocity field and the skin friction are obtained. Also, the solutions for the temperature and the heat transfer characteristics are obtained in terms of Kummer's function.

Journal ArticleDOI
TL;DR: In this article, the authors analyzed the flow and heat transfer characteristics of buoyancy-driven air convection behind photovoltaic panels, where both convection and radiation heat exchanges were considered as the heat transfer mechanisms by which the thermal energy is transferred into the air.
Abstract: The impetus of this paper is to analyse numerically and experimentally the flow and heat transfer characteristics of buoyancy-driven air convection behind photovoltaic panels. Both convection and radiation heat exchanges are considered as the heat transfer mechanisms by which the thermal energy is transferred into the air. Numerical and experimental results are obtained for a channel of 7.0 m in height and the channel walls are separated by a distance of 0.23 m. In the experiment heat is supplied to the air gap from heating foil attached to one of the vertical walls. Different input heat fluxes and emissivity of the bounding surfaces are considered in order to show their effect on the heat transfer across the air layer. Detailed studies of the flow and thermal fields in the air are presented in order to explore the thermal behaviour of the air in the channel. Velocity and temperature profiles of the outlet air and the surface temperature of the heated and insulated wall is presented. The numerical results agreed well with the experimental measurements.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the local heat transfer and pressure drop on the shell side of shell-and-tube heat exchangers with segmental baffles for different baffle spacings.

Book
28 Dec 1998
TL;DR: In this paper, the authors present a model for predicting the heat transfer coefficient of liquid fluidized beds in three phases of three-phase fluidization, including two phases and three phases.
Abstract: Introduction Fluidlike Behavior Fluidization State Advantages of Fluidized Bed Disadvantages of Fluidized Bed Properties of Particles and the Granular Bed Particles Granular Bed Grouping of Gas Fluidization Hydrodynamics Based Groups Hydrodynamics and Thermal Properties Based Groups Variables Affecting Fluidization Varieties of Fluidization Hydrodynamics of Two Phase Fluidization Minimum Fluidization Velocity Terminal Velocity Flow Phenomena Particulate and Aggregative Fluidization Regimes of Fluidization Three Phase Fluidization Introduction Classification Hydrodynamics Turbulent Contact Absorber (TCA) Heat Transfer Introduction Groups Models Predictions of Heat Transfer Coefficient Heat Transfer to Immersed Surfaces Effects of Operating Variables Heat Transfer in Liquid Fluidized Beds Heat Transfer in Three Phase Fluidized Beds Mass Transfer Introduction Mass Transfer Steps Mass Transfer in Three Phase Fluidized Beds End Zones Grid Zone Elutriation References