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Showing papers on "Heat transfer coefficient published in 1993"


Journal ArticleDOI
TL;DR: In this article, an experimental investigation is described that characterizes heat transfer between a heated air stream and ceramic foam, and an apparatus is designed to determine the volumetric heat transfer coefficient between the foam and a stream of air using a single blow transient technique.

284 citations


Journal ArticleDOI
TL;DR: In this paper, the state of the art for the heat and flow characteristics of packed beds under conditions where radiant heat transfer may be neglected is presented for the prediction of convective heat transfer, pressure drop, effective thermal conductivity, and wall heat transfer.

278 citations


Journal ArticleDOI
TL;DR: In this article, the heat transfer characteristics in the laminar boundary layer of a viscous fluid over a linearly stretching, continuous surface with variable wall temperature subject to suction or blowing were investigated.

276 citations


Journal ArticleDOI
TL;DR: In this article, a study on boiling heat transfer of refrigerant R-113 in a small-diameter (2.92 mm) tube is reported, where local heat transfer coefficients are measured for a range of heat flux (8.8-90.75 kW/m[sup 2]), mass flux (50-300 kg/m [sup 2]s), and equilibrium mass quality (0-0.9).
Abstract: Results of a study on boiling heat transfer of refrigerant R-113 in a small-diameter (2.92 mm) tube are reported. Local heat transfer coefficients are measured for a range of heat flux (8.8-90.75 kW/m[sup 2]), mass flux (50-300 kg/m[sup 2]s), and equilibrium mass quality (0-0.9). The measured coefficients are used to evaluate 10 different heat transfer correlations, some of which have been developed specifically for refrigerants. High heat fluxes and low mass fluxes are inherent in small channels, and this combination results in high boiling numbers. In addition, based on a flow pattern map developed from adiabatic experiments with air-water mixtures, it has been shown that small-diameter channels produce a slug flow pattern over a large range of parameters when compared with larger-diameter channels. The effects of high boiling number and slug flow pattern lead to domination by a nucleation mechanism. As a result, the two-phase correlations that predicted this dominance also predicted the data the best when they properly modeled the physical parameters. The correlation of Lazarek and Black (1982) predicted the data very well. It is also shown that a simple form, suggested by Stephan and Abdelsalam (1980) for nucleate pool boiling, correlates the data equally well; both correlationsmore » are within a mean deviation of less than 13 percent. Results are applicable to boiling in compact heat exchangers. 28 refs., 11 figs., 3 tabs.« less

254 citations


Journal ArticleDOI
TL;DR: In this paper, the authors performed a numerical and experimental analysis to investigate the heat transfer and fluid flow behavior in a rectangular channel flow with streamwise-periodic ribs mounted on one of the principal walls.

180 citations


Journal ArticleDOI
TL;DR: In this article, the authors used real-time holographic interferometry to measure local and average heat transfer and friction in a channel with various-shaped ridges mounted on two opposite walls.

175 citations


Journal ArticleDOI
TL;DR: In this paper, a model of heat generation and transport in lithium/polymer-electrolyte batteries for electric vehicle applications has been conducted and the results demonstrate that thermal management may not be a serious problem for batteries under low discharge rates However, under high discharge rates, the temperature of a battery may increase remarkably if the thickness of a cell stack exceeds a certain value.
Abstract: Mathematical modeling of heat generation and transport in lithium/polymer‐electrolyte batteries for electric vehicle applications has been conducted The results demonstrate that thermal management may not be a serious problem for batteries under low discharge rates However, under high discharge rates, the temperature of a battery may increase remarkably if the thickness of a cell stack exceeds a certain value Also, due to the low thermal conductivity of the polymer, the improvement of cooling conditions is not an effective means of improving heat removal for large‐stack systems For a required operational temperature range and a given discharge rate, model predictions can be used to design appropriate battery structures and to choose a suitable cooling scheme

174 citations


Journal ArticleDOI
TL;DR: In this paper, a chemical heat pump reactor with its porous reactive medium is used to fill the vessel with a mixture of graphite binder and an inert graphite mixture to obtain a given apparent density.
Abstract: The method used to fill a chemical heat pump reactor with its porous reactive medium governs the heat and mass transfer in the reactor and hence its performance in terms of heat, power and capacity. The filling operation consists of mixing the salt, which takes place in the solid-gas reaction, and an inert graphite binder, and in confining this mixture to obtain a given apparent density. Depending on the binder properties and the mixing technique, the values (experimentally obtained) of the effective conductivities, λe, of the heat transfer coefficients, hsw, at the exchanger wall and of the permeabilities, k, cover a very large range: 0.2 W m −1 K −1 ⩽λ e ⩽40 W m −1 K −1 15 W m −2 K −1 K −1 ⩽h sw ⩽15,000 R m −2 K −1 , 0.03 × 10 −12 m 2 ⩽k⩽30 × 10 −12 m 2 . For some simple cases in which one limitation predominates over all the others, a simple relation has been set up between the reactor's power output per unit volume and a single filling parameter.

173 citations


Journal ArticleDOI
TL;DR: In this article, it is shown numerically that the buoyancy induced circulation resonates to a certain (single) frequency of the pulsating heat input, characterized by maximum fluctuations in the total heat transfer rate through the vertical midplane of the cavity.

170 citations


Journal ArticleDOI
TL;DR: In this paper, the authors derived an effective condensation thermal conductivity by expressing the driving potential for mass transfer as a difference in saturation temperatures and using appropriate thermodynamic relationships, and demonstrated that condensation obeys the heat and mass transfer analogy, when condensation and sensible heat tranfer are considered simultaneously.
Abstract: In turbulent condensation with noncondensable gas, a thin noncondensable layer accumulated and generates a diffusional resistance to condensation and sensible heat transfer. By expressing the driving potential for mass transfer as a difference in saturation temperatures and using appropriate thermodynamic relationships, here an effective condensation thermal conductivity is derived. With this formulation, experimental results for vertical tubes and plates demonstrate that condensation obeys the heat and mass transfer analogy, when condensation and sensible heat tranfer are considered simultaneously

170 citations


Book ChapterDOI
01 Jan 1993
TL;DR: The need for intensification of process heat exchangers has led to the development of several types of compact heat exchanger suitable for evaporation heat transfer as mentioned in this paper. But, the data available in the literature is limited.
Abstract: The need for intensification of process heat exchangers has led to the development of several types of compact heat exchangers suitable for evaporation heat transfer. These heat exchangers are characterised by small multi-channel passages operating in parallel. The various regimes of two-phase flow which occur within these passages determine the heat transfer and hence the heat exchanger effectiveness. Currently there are limited data available in the literature of use in the design and selection of compact heat exchangers for evaporation.

Journal ArticleDOI
TL;DR: In this paper, the influence of uneven wall temperature on the local heat transfer coefficient in a rotating, two-pass, square channel with smooth walls is investigated for rotation numbers from 0.0352 to 0.352 by varying Reynolds numbers from 25,000 to 2500.
Abstract: The influence of uneven wall temperature on the local heat transfer coefficient in a rotating, two-pass, square channel with smooth walls is investigated for rotation numbers from 0.0352 to 0.352 by varying Reynolds numbers from 25,000 to 2500. The two-pass square channel, composed of 12 isolated copper sections, has a length-to-hydraulic diameter ratio of 12. The mean rotating radius to the channel hydraulic diameter ratio is kept at a constant value of 30. Three cases of thermal boundary conditions are studied: (A) four walls at the same temperature, (B) four walls at the same heat flux, and (C) trailing wall hotter than leading with side walls unheated and insulated. The results for case A of four walls at the same temperature show that the first channel (radial outward flow) heat transfer coefficients on the leading surface are much lower than that of the trailing surface due to the combined effect of Coriolis and buoyancy forces. The second channel (radial inward flow) heat transfer coefficients on the leading surface are higher than that of the trailing surface. The difference between the heat transfer coefficients for the leading and trailing surface in the second channel is smaller than that in the first channel due to the opposite effect of Coriolis and buoyancy forces in the second channel. However, the heat transfer coefficients on each wall in each channel for cases B and C are higher than case A because of interactions between rotation-induced secondary flows and uneven wall temperatures in cases B and C. The results suggest that the effect of uneven wall temperatures on local heat transfer coefficients in the second channel is greater than that in the first channel.

Journal ArticleDOI
TL;DR: In this article, the thermal conductivity and wall heat transfer coefficient of solid sorbents were measured and the results showed that the thermal transfer quality of these materials can be improved by combining metallic foam and zeolite.
Abstract: The low thermal conductivity of the solid sorbents is one of the major drawbacks of the sorption heat pumps development. A better thermal transfer in the adsorber fixed beds is required to obtain a decreased time of the processing cycles and thus a reduced adsorber size per unit of power. Small improvements in the parameters of thermal transfer were obtained with unconsolidated porous mixtures such as bimodal mixtures and metallic foams. New consolidated materials made of metallic foam and zeolite were developed. The measurements of the thermal conductivity and of the wall heat transfer coefficient show a great improvement in the thermal transfer quality. Resistances to mass transfer appear but they are consistent with the adsorption heat pump process. With this type of composite material it seems possible to reduce the adsorber size by a factor from 5 to 10.

Journal ArticleDOI
TL;DR: In this article, the authors have shown that the heat transfer behavior for low gas flow rates is largely determined by heat conduction in the longitudinal direction within the wall and fin materials.
Abstract: By the method of mechanical manufacture of microstructures jointly developed by the Karlsruhe Nuclear Research Center (KfK) and Messerschmitt-Bolkow-Blohm (MBB) very compact cross-flow micro heat exchangers have been fabricated whose active volumes are 1 × 1 × 1 cm3 and whose typical channel cross-sections are 75 × 90 μm2. Measurements with nitrogen, argon and helium as heat transfer fluids were performed on two copper and stainless steel finned plate heat exchangers. With the stainless steel heat exchanger overall heat transfer coefficients up to 1700 W m−2 K−1 have been attained in the range investigated using helium as the fluid. For low flow rates greatly reduced overall heat transfer coefficients have been measured compared with the values to be anticipated for laminar flow. In some ranges higher heat transfer rates have been achieved with the stainless steel heat exchanger compared with the system made from copper. This leads to the conclusion that the heat transfer behavior for low gas flow rates is largely determined by heat conduction in the longitudinal direction within the wall and fin materials. With a homogeneous model the influence of longitudinal heat conduction in the stationary walls can be explained and is in fairly good agreement with the experimental data.

Journal ArticleDOI
TL;DR: In this article, a series of computations were conducted in which three principal parameters governing the heat transfer in this geometry (i.e., channel expansion ratio ER, Reynolds number Re and Prandtl number Pr) were systematically changed.

Journal ArticleDOI
TL;DR: In this article, an analytical study of the fluid flow and heat transfer in the stagnation zone of an unsubmerged liquid jet is presented, where the role of surface tension is emphasized.
Abstract: The stagnation-zone characteristics of an impinging liquid jet are of great interest because the maximum heat transfer coefficient occurs in that region. This is an analytical study of the fluid flow and heat transfer in the stagnation zone of an unsubmerged liquid jet. The role of surface tension is emphasized. Stagnation-zone transport is strongly dependent on the potential flow above the boundary layer. Numerical solutions for a laminar unsubmerged jet are obtained, using a simulation method for steady, inviscid, incompressible flow with surface tension

Journal ArticleDOI
TL;DR: In this article, 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 46mm-i.d. vertical tube.
Abstract: 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 containmentmore » cooling system (PCCS).« less

Journal ArticleDOI
TL;DR: In this article, the effects of free convection and the presence of heat generation/absorption on the flow and heat transfer characteristics are considered, and the equations of conservation of momentum, mass, and energy are solved numerically by using a variable order, variable step size finite-difference method.
Abstract: Analysis of convection flow and heat transfer of a viscous heat-generating fluid near an infinite vertical stretching surface is carried out. The effects of free convection and the presence of heat generation/absorption on the flow and heat transfer characteristics are considered. The equations of conservation of momentum, mass, and energy, which govern the flow and heat transfer problem, are solved numerically by using a variable order, variable step size finite-difference method. The numerical results obtained for the flow and heat transfer characteristics reveal many interesting behaviors. These behaviors warrant further study of the effects of free convection on the flow and heat transfer characteristics.

Book ChapterDOI
01 Jan 1993
TL;DR: In this article, a simulation of the fully developed thermal field in a two-dimensional turbulent channel flow was carried out at a molecular Prandtl number of Pr = 0.025 to investigate low PrandTL number effects on the transport mechanism in wall turbulence.
Abstract: A direct numerical simulation of the fully developed thermal field in a two-dimensional turbulent channel flow was carried out. The simulation was made at a molecular Prandtl number of Pr = 0.025 to investigate low Prandtl number effects on the transport mechanism in wall turbulence. The computation was executed on about 1.6 × 106 grid points by using a spectral method. Mean field parameters as well as various thermal turbulence statistics including rms temperature fluctuations, turbulent heat fluxes and turbulent Prandtl numbers were obtained. Each term in the budget equations of temperature variance, its dissipation rate and turbulent heat fluxes was also calculated in order to establish a data base of convective heat transfer for thermal turbulence modeling. The results were compared mainly with those at Pr = 0.71 obtained by Kasagi et al. (1992). In addition, the computed thermal fields were visualized to investigate the role of the near-wall quasi-coherent structures in the turbulent transport mechanism.

Journal ArticleDOI
TL;DR: In this paper, an investigation on joint rock-water heat transfer was carried out using a test set-up for hydro-thermo-mechanical experiments on rock samples from the Carnmenellis granite from an analysis of steady state data collected under the present range of experimental conditions.

Journal ArticleDOI
TL;DR: In this paper, local heat transfer rates have been estimated from the outputs of three-wire differential thermocouple heat flux sensors, and the effective area for a nozzle has been defined when a non-dimensional heat transfer rate, H, has decreased from unity to e −1, Q = 0.54 is a good average value.

Journal ArticleDOI
TL;DR: In this paper, a well-posed mathematical model is obtained if the finiteness of the adjustment time of the turbulence field to the variations of temperature gradient is taken into account.
Abstract: It is commonly assumed that heat flux and temperature diffusivity coefficients obtained in steady-state measurements can be used in the derivation of the heat conduction equation for fluid flows. Meanwhile it is also known that the steady-state heat flux as a function of temperature gradient in stably stratified turbulent shear flow is not monotone: at small values of temperature gradient the flux is increasing, whereas it is decreasing after a certain critical value of the temperature gradient. Therefore the problem of heat conduction for large values of temperature gradient becomes mathematically ill-posed, so that its solution (if it exists) is unstable.In the present paper it is shown that a well-posed mathematical model is obtained if the finiteness of the adjustment time of the turbulence field to the variations of temperature gradient is taken into account. An evolution-type equation is obtained for the temperature distribution (a similar equation can be derived for the concentration if the stratification is due to salinity or suspended particles). The characteristic property which is obtained from a rigorous mathematical investigation is the formation of stepwise distributions of temperature and/or concentration from continuous initial distributions.

Journal ArticleDOI
TL;DR: In this paper, the authors describe the flow structure over repeated two-dimensional square ribs of side length D, placed at a pitch S on a ground plane, and calculate the value of S/D which most augments the turbulence of the free stream and hence the heat transfer is calculated.
Abstract: This paper describes the flow structure over the repeated two-dimensional square ribs of side length D, placed at a pitch S on a ground plane. The value of S/D which most augments the turbulence of the free stream and, hence the heat transfer is calculated. The region of interest in this investigation is far downstream where the velocity and temperature distributions follow similarity rules. The time-mean velocity, static pressure, and the velocity vectors were measured by Pitot- and static pressure tubes and a three hole cylindrical yawmeter. The turbulence intensities and integral scale were obtained using a hot wire anemometer. The mean temperature distribution was measured by thermocouples and the local heat transfer coefficient was then calculated. It is found that at S/D = 9 the turbulence intensity is maximized. As a result of this effect and the fact that for S/D = 9 the flow reattaches within a groove, the heat transfer is also maximized. The measurements show how the location of reattachment depends on S/D and that high local heat transfer coefficient coincides with the reattachment point. The average heat transfer coefficient and the pressure drop correlation is quantified.

Journal ArticleDOI
TL;DR: In this article, the effects of jet velocity and nozzle-to-plate spacing were studied in the range of Re = 5.0 × 103 3.6 × 104, where heat transfer rates at the stagnation point and local heat transfer distributions were correlated.
Abstract: An experimental study was performed to investigate the local characteristics of heat transfer from vertical simulated micro-chips to impinging submerged circular water jets. The effects of jet velocity, and nozzle-to-plate spacing were studied in the range of Re = 5.0 × 103 3.6 × 104 . Heat transfer rates at the stagnation point and local heat transfer distributions were correlated. Transition from laminar to turbulent flow was observed.

Journal ArticleDOI
TL;DR: In this paper, a real-time hue conversion process on a complex curved surface is adopted for a transient heat transfer technique with high spatial resolution, which is different from existing steady-state hue capturing studies.
Abstract: Accurate determination of convective heat transfer coefficients on complex curved surfaces is essential in the aerothermal design and analysis of propulsion system components. The heat transfer surfaces are geometrically very complex in most of the propulsion applications. This study focuses on the evaluation of a hue capturing technique for the heat transfer interpretation of liquid crystal images from a complex curved heat transfer surface. Impulsively starting heat transfer experiments in a square to rectangular transition duct are reported. The present technique is different from existing steady-state hue capturing studies. A real-time hue conversion process on a complex curved surface is adopted for a transient heat transfer technique with high spatial resolution. The study also focuses on the use of encapsulated liquid crystals with narrow color band in contrast to previous steady-state hue based techniques using wide band liquid crystals. Using a narrow band crystal improves the accuracy of the heat transfer technique. Estimated uncertainty for the heat transfer coefficient from the technique is about 5.9 percent. A complete heat transfer map of the bottom surface was possible using only seven liquid crystal image frames out of the 97 available frames during the transient experiment. Significant variations of heat transfer coefficientsmore » are quantitatively visualized on the curved surfaces of the transition duct. 33 refs.« less

Journal ArticleDOI
TL;DR: In this paper, the effect of unsteady wake on surface heat transfer coefficients of a gas turbine blade was experimentally determined using a spoked wheel type wake generator using a five-airfoil linear cascade in a low-speed with tunnel facility.
Abstract: The effect of unsteady wake on surface heat transfer coefficients of a gas turbine blade was experimentally determined using a spoked wheel type wake generator. The experiments were deformed with a five-airfoil linear cascade in a low-speed with tunnel facility. The cascade inlet Reynolds number based on the blade chord was varied from 1 to 3×010 5 . The wake Strouhal number was varied between 0 and 1.6 by changing the rotating wake frequency (rod speed and rot number), rod diameter, and cascade inlet velocity. A hot-wire anemometer system was located at the cascade inlet to detect the instantaneous velocity, phase-averaged mean velocity, and turbulence intensity induced by the passing wake

Journal ArticleDOI
TL;DR: Within the linearized theory of heat conduction with fading memory, some restrictions on the constitutive equations are found as a direct consequence of thermodynamic principles as mentioned in this paper, which allow us to obtain existence, uniqueness, and stability results for the solution to the heat flux equation.
Abstract: Within the linearized theory of heat conduction with fading memory, some restrictions on the constitutive equations are found as a direct consequence of thermodynamic principles. Such restrictions allow us to obtain existence, uniqueness, and stability results for the solution to the heat flux equation. Both problems, which respectively occur when the instantaneous conductivity kQ is positive or vanishes, are considered.

Journal ArticleDOI
TL;DR: In this paper, the effects of outersurface geometry on the performance of flat plain fins and round tube crossflow heat exchangers are considered, with the finning parameter varying from 11 to 23.

Journal ArticleDOI
TL;DR: In this paper, a regular model for turbulent pipe flow with penetration theory was used to account for the disturbance of the thermal boundary layer by the scraper action, and the model showed that the heat transfer coefficient depends more strongly on the rotational frequency of the scrapers than the model predicts.

Journal ArticleDOI
TL;DR: In this paper, an extended two-dimensional model was used to compute the heat transfer and the experimental verification revealed that convection heat transfer in the Trombe channel depends upon the distance from the bottom vents in a similar way as convention heat flow in the entrance region in air ducts.