Showing papers on "Heat transfer coefficient published in 1991"
TL;DR: The authors showed that the eonvective term in the correlation should have a Prandtl number dependence and constructed an accurate predictive method with an explicit nucleate boiling term and without boiling number dependence.
803 citations
TL;DR: In this article, the heat transfer characteristics of microencapsulated phase change material slurry flow in circular ducts are presented by taking into consideration both the heat absorption (or release) due to the phase change process and the conductivity enhancement induced by the motion of the particles.
310 citations
TL;DR: In this article, the authors characterized local heat transfer coefficients for round, single-phase free liquid jets impinging normally against a flat uniform heat flux surface, and found that the local Nusselt number characteristics were dependent on nozzle diameter.
Abstract: The purpose of this investigation was to characterize local heat transfer coefficients for round, single-phase free liquid jets impinging normally against a flat uniform heat flux surface. The problems parameters investigated were jet Reynolds number Re, nozzle-to-plate spacing z, and jet diameter d. A region of near-constant Nusselt number was observed for the region bounded by 0 {le} r/d {le} 0.75, where is the radical distance from the impingement point. The local Nusselt number profiles exhibited a sharp drop for r/d > 0.75, followed by an inflection and a shower decrease thereafter. Increasing the nozzle-to-plate spacing generally decreased the heat transfer slightly. The local Nusselt number characteristics were found to be dependent on nozzle diameter. This was explained by the influence of the free-stream velocity gradient on local heat transfer, as predicted in the classical analysis of infinite jet stagnation flow and heat transfer. Correlations for local and average Nusselt numbers reveal an approximate Nusselt number dependence on Re{sup 1,3}.
219 citations
TL;DR: Triangular and rectangular longitudinal vortex generators were formed by punching small pieces out of flat plate fins so that they stuck out of the plates and formed an angle (angle of attack) with the main flow direction.
212 citations
12 Feb 1991
TL;DR: In this article, a complete two-dimensional flow/thermal model of the micro-channel cooler was developed, and the design parameters were optimized for the case of a 1 kW/cm/sup 2/ heat flux with the top surface at 25 degrees C.
Abstract: A complete two-dimensional flow/thermal model of the micro-channel cooler is developed. Optimization of the design parameters with this model is demonstrated for the case of a 1 kW/cm/sup 2/ heat flux with the top surface at 25 degrees C. For this case, pure water could be used as the coolant, or 92% water/8% methanol (-5 degrees C freezing point) if the heat is to be dumped to ice/water. The flow rate should be about 50 cc/s per cm/sup 2/ of surface area. The distribution manifold channel spacing (center-to-center) should be 333 mu m (30 channels/cm). The fin height should be about 167 mu m (H/sub F//L=1). The distribution manifold channel widths should be about 200 mu m (W/L=0.6). The micro-channels should be between 7 mu m and 14 mu m wide, while the ratio of fin thickness to micro-channel width should be from 0.5 to 1.0. With these design parameters, an effective heat transfer coefficient (surface heat flux divided by surface to coolant inlet temperature difference) on the order of 100 W/cm/sup 2/ K will be achieved with a total pressure drop of only about 2 bar. >
165 citations
149 citations
TL;DR: In this paper, a flow boiling map is developed to depict the relationships among the heat transfer coefficient, quality, heat flux, and mass flux for different fluids in the subcooled and the saturated flow boiling regions.
Abstract: The thermal behavior of a flow boiling system is represented by a flow boiling map to illustrate visually the relationships among various system parameter. An earlier flow boiling map by Collier (1981) does not include the effect of mass flux and is specific to water at low pressures. For other fluids, significant departures from the parametric trends displayed in Collier's map have been reported in the literature (e.g, Kandlikar). In the present paper, a new flow boiling map is developed to depict the relationships among the heat transfer coefficient, quality, heat flux, and mass flux for different fluids in the subcooled and the saturated flow boiling regions. The trends observed in the experimental data and correlations for water and refrigerants are used in deriving the present map. The particular areas where further investigation is needed to validate the trends are also indicated. In the subcooled investigation is needed to validate the trends are also indicated. In the subcooled boiling region, h{sub TP}/h{sub lo} is plotted against x with Bo as a parameter, while in the saturated boiling region, h{sub TP}/h{sub lo} is plotted against x with {rho}{sub l}/{rho}{sub g} and a modified boiling number Bo* as parameters. It is hopedmore » that the map would prove to be helpful in explaining the role of different heat transfer mechanisms in flow boiling of different fluids.« less
148 citations
TL;DR: Similarity solutions for buoyancy induced heat and mass transfer from a vertical plate embedded in a saturated porous medium are reported for constant wall temperature and concentration, constant wall heat and flux.
141 citations
TL;DR: In this article, the Nusselt number and friction factor increase with increasing relative roughness height and decrease with increasing roughness pitch, but not in direct proportions, and design curves have been developed that give the optimal thermohydraulic performance combination of these parameters.
134 citations
TL;DR: In this paper, an experimental investigation is performed to characterize the convective heat transfer from a flat surface to a row of impinging, circular, submerged air jets formed by square-edged orifices having a length/diameter ratio of unity.
112 citations
TL;DR: In this article, a transient numerical model capable of predicting the thermal behavior of very small (100 jim) heat pipes during startup or variation in the evaporator thermal load was developed, which was used to identify, evaluate, and better understand the phenomena that govern the transient behavior of micro heat pipes as a function of the physical shape, the properties of the working fluid, and the principal dimensions.
Abstract: An analytical investigation was conducted to determine the potential advantages of incorporating very small (100 jim) "micro" heat pipes directly into semiconductor devices. As a result of this investigation, a transient numerical model capable of predicting the thermal behavior of these micro heat pipes during startup or variation in the evaporator thermal load was developed. This numerical model was used to identify, evaluate, and better understand the phenomena that govern the transient behavior of micro heat pipes as a function of the physical shape, the properties of the working fluid, and the principal dimensions. The modeling results were compared with the steady-state results from an earlier experimental investigation and were shown to accurately predict the steady state dry out limit for two different test pipes. Using the verified numerical model, the parameters that affect the axial heat transport capacity were evaluated. The results of this evaluation indicate that in micro heat pipes reverse liquid flow occurs in the liquid arteries during startup and/or rapid transients. In addition, the wetting angle was found to be one of the most important factors contributing to the transport capacity. Nomenclature A = cross-sectional area Cp = specific heat d = distance j = evaporation M = molecular weight m = mass flow rate P = pressure Q = heat transfer q = heat transfer rate R = universal gas constant r = radius of curvature T = temperature t = time V = velocity W = wetted perimeter x — length or distance y = distance
TL;DR: In this paper, an analytical study is conducted to determine the thermal response of a widely used ablative glass-filled polymer composite, which does not include the idealized assumption of local-thermal equilibrium existing between the solid matrix and decomposition gases within the tortuous pore network of the material.
TL;DR: In this article, the effect of wall heat flux ratio on the local heat transfer augmentation in a square channel with two opposite in-line ribbed walls was investigated for Reynolds numbers from 15,000 to 80,000.
Abstract: The effect of wall heat flux ratio on the local heat transfer augmentation in a square channel with two opposite in-line ribbed walls was investigated for Reynolds numbers from 15,000 to 80,000. The square channel composed of ten isolated copper sections has a length-to-hydraulic diameter ratio (L/D) of 20. The rib height-to-hydraulic diameter ratio (e/D) is 0.0625 and the rib pitch-to-height ratio (P/e) equals 10
TL;DR: In this article, experimental heat transfer coefficients were reported for HFC-134a and CFC-12 during in-tube single-phase flow, evaporation and condensation.
Abstract: Experimental heat transfer coefficients are reported for HFC-134a and CFC-12 during in-tube single-phase flow, evaporation and condensation. These heat transfer coefficients were measured in a horizontal, smooth tube with an inner diameter of 8.0 mm and a length of 3.67 m. The refrigerant in the test-tube was heated or cooled by using water flowing through an annulus surrounding the tube. Evaporation tests were performed for a refrigerant temperature range of 5–15°C with inlet and exit qualities of 10 and 90%, respectively. For condensation tests, the refrigerant temperature ranged from 30 to 50°C, with et and exit qualities of 90 and 10%, respectively. The mass flux was varied from 125 to 400 kg m−2 s−1 for all tests. For similar mass fluxes, the evaporation and condensation heat transfer coefficients for HFC-134a were significantly higher than those of CFC-12. Specifically, HFC-134a showed a 35–45% increase over CFC-12 for evaporation and a 25–35% increase over CFC-12 for condensation.
TL;DR: In this article, hydrodynamic resonance and its effect on heat transfer in laminar flows through ducts with periodically spaced transverse grooves cut into one wall are presented.
TL;DR: In this article, an empirical relation developed for glass cover temperature is based on an approximate solution of the heat balance equation, which enables prediction of the performance of a solar still through simple calculations.
Abstract: The present work enables prediction of the performance of a solar still through simple calculations. Estimation of the temperature of the glass cover by an empirical relation developed in this work permits calculation of the heat-transfer coefficients, the upward heat flow, and evaporation. Since some of the heat-transfer coefficients vary substantially and nonlinearly with temperature, the empirical relation developed for glass cover temperature is based on an approximate solution of the heat balance equation. Hence, the overall upward heat flow factor is obtained with a maximum absolute error of three percent compared to the value obtained through a numerical solution of the heat balance equation along with the relations for vapor pressure and latent heat. The fraction of upward heat flow utilized for evaporation is determined with a maximum absolute error of 0.5 percent. The range of variables covered is 30{degrees}C to 80{degrees}C in water temperature, 5W/m{sup 2}K to 40W/M{sup 2}K in wind heat-transfer coefficient, and 5{degrees}C to 40{degrees}C in ambient temperature.
TL;DR: In this paper, the authors investigated the heat transfer in a discretely heated enclosure for single and multiple heater configurations and found that the heat source location corresponding to maximum heat transfer is a function of Grashof number.
TL;DR: In this article, the heat transfer characteristics in a viscoelastic fluid over a stretching sheet with frictional heating and internal heat generation or absorption were analyzed in terms of Kummer's and parabolic cylinder functions.
TL;DR: In this article, surface temperature and heat flux distributions have been measured on a flat, upward facing, constant heat flux surface cooled by a planar, impinging water jet, and the results for the stagnation and boundary layer flows are correlated by expressions of the form Nu = CRenPrm.
TL;DR: In this paper, the Darcy-Brinkman-Forchheimer (DBF) equations of motion were used to predict the porosity and thermal conductivity in a horizontal porous cavity of aspect ratio A = 5.
Abstract: Experimental results for natural convection in a horizontal porous cavity of aspect ratio A = 5 and heated from below are reported. A wide range of governing parameters are covered by careful selection of bead size, solid material, and fluid. These results fully support the effects of fluid-flow parameters (Rayleigh and Prandtl numbers), porous matrix-structure parameters (Darcy and Forchheimer numbers), and the conductivity ratio as predicted by the formulation based on the Darcy-Brinkman-Forchheimer (DBF) equations of motion. The DBF flow model, with variable porosity and variable thermal conductivity in the wall regions, predicts reasonably well in comparison with the experimental data. However, the difference between the predictions and the measurements increases as the ratio of solid-to-fluid thermal conductivity becomes very large. 32 refs.
TL;DR: In this paper, a numerical analysis and experimental verification of the effects of heat load distribution on the vapor temperature, wall temperature, and the heat transport capacity for heat pipes with multiple heat sources is presented.
Abstract: A numerical analysis and experimental verification of the effects of heat load distribution on the vapor temperature, wall temperature, and the heat transport capacity for heat pipes with multiple heat sources is presented. A numerical solution of the elliptic conjugate mass, momentum and energy equations in conjunction with the thermodynamic equilibrium relations and appropriate boundary conditions for the vapor region, wick structure, and the heat pipe wall are given. The experimental testing of a copper-water heat pipe with multiple heat sources was also made showing excellent agreement with the numerical results. An optimization of the heat distribution for such heat pipes was performed and it was concluded that by redistribution of the heat load, the heat capacity can be increased.
TL;DR: In this paper, a numerical analysis is carried out to investigate the evaporative cooling of liquid falling film through interfacial heat and mass transfer in natural convection channel flows, and the results indicate that the heat transfer from the interface to the gas stream is predominated by the transport of the latent heat in conjunction with the liquid film evaporation.
TL;DR: In this paper, a numerical study of the heat transfer characteristics of porous radiant burners was performed using a one-dimensional conduction, convection, and radiation model, where the combustion phenomenon was modeled as spatially dependent heat generation and the spherical harmonics approximation was used to solve the radiative transfer equation.
Abstract: This paper reports a numerical study of the heat transfer characteristics of porous radiant burners, which have significant advantages over conventional burners. The heat transfer characteristics are investigated using a one-dimensional conduction, convection, and radiation model. The combustion phenomenon is modeled as spatially dependent heat generation. Nonlocal thermal equilibrium between the gas and solid phases is accounted for by using separate energy equations for the two phases. The solid matrix is assumed to emit, absorb, and scatter radiant energy. The spherical harmonics approximation is used to solve the radiative transfer equation. The coupled energy equations and the radiative transfer equations are solved using a numerical iterative procedure. The effects of the various factors on the performance of porous radiant burners rae determined. It is revealed that for a given rate of heat generation, large optical thicknesses and high heat transfer coefficients between the solid and gas phases are desirable for maximizing radiant output. Also, low solid thermal conductivities, scattering albedos and flow velocities, and high inlet environment reflectivities produced high radiant output.
TL;DR: The estimation of the heat transfer coefficient at the direct contact condensation of cold water and steam is a very hard task since the phenoma are essentially undsteady and the interface motion is so complicated that an exact estimation of its area is almost impossible as mentioned in this paper.
TL;DR: In this paper, an analytical study was performed to examine the mass transfer characteristics on the free-convection flow of an incompressible viscous fluid past an exponentially accelerated infinite vertical plate under the action of constant heat flux.
Abstract: An analytical study is performed to examine the mass-transfer characteristics on the free-convection flow of an incompressible viscous fluid past an exponentially accelerated infinite vertical plate under the action of constant heat flux. The effects of various parameters occurring into the problem, is extensively discussed.
TL;DR: In this article, a mechanistic model for heat transfer with the effect of active particle motion is proposed, and the average heat transfer coefficients calculated from the model compare well with experimental data in the literature.
TL;DR: In this paper, an experimental study was performed to investigate the effects of two arrays (in-line and staggered) of cylinders on the mass transfer from a flat surface on which the cylinders are situated.
TL;DR: In this paper, a 9.3 m tall, 152 mm i.d. transparent cold model circulating fluidized bed for 171 μm Ottawa sand was used to study local heat transfer mechanisms and hydrodynamics.
TL;DR: In this article, a two-zone model of heat transfer between a fluidized bed and an immersed surface is used to correctly take into account the effect of the fluidized gas pressure and of the surface and bed temperatures on the overall heat transfer coefficient considered as the sum of conductive (h cond ), convective ( h conv ), and radiative (h r ) components.
TL;DR: In this paper, a new method using either an analytical or a boundary element inverse technique, is developed for measurement of local heat transfer coefficients, which is submitted to a given heat transfer coefficient angular profile on its outer radius and on an uniform temperature on its inner radius.
Abstract: A new method using either an analytical or a boundary element inverse technique, is developed for measurement of local heat transfer coefficients. The direct model calculates the temperature field inside a cylindrical pipe. This is submitted to a given heat transfer coefficient angular profile on its outer radius and on an uniform temperature on its inner radius. Experimental temperature measurements inside the cylinder are processed by two techniques. Their results are very close and coherent with those of other authors. Variation of the cylinder conductivity with temperature, implemented by the boundary element technique, seems to show that the averaging of its value yields a regularization effect.