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Showing papers in "Journal of Heat Transfer-transactions of The Asme in 1999"


Journal ArticleDOI
TL;DR: In this paper, a transient hot-wire method was used to measure the thermal conductivity of a small amount of nanoparticles and the experimental results showed that these nanoparticles have substantially higher thermal conductivities than the same liquids without nanoparticles.
Abstract: Oxide nanofluids were produced and their thermal conductivities were measured by a transient hot-wire method. The experimental results show that these nanofluids, containing a small amount of nanoparticles, have substantially higher thermal conductivities than the same liquids without nanoparticles. Comparisons between experiments and the Hamilton and Crosser model show that the model can predict the thermal conductivity of nanofluids containing large agglomerated Al{sub 2}O{sub 3} particles. However, the model appears to be inadequate for nanofluids containing CuO particles. This suggests that not only particle shape but size is considered to be dominant in enhancing the thermal conductivity of nanofluids.

2,811 citations



Journal ArticleDOI
TL;DR: In this article, a complete numerical simulation of a growing and departing bubble on a horizontal surface has been performed, where a finite difference scheme is used to solve the equations governing conservation of mass, momentum, and energy in the vapor-liquid layers.
Abstract: In this study, a complete numerical simulation of a growing and departing bubble on a horizontal surface has been performed. A finite difference scheme is used to solve the equations governing conservation of mass, momentum, and energy in the vapor-liquid layers. The vapor-liquid interface is captured by a level set method which is modified to include the influence of phase change at the liquid-vapor interphase. The disjoining pressure effect is included in the numerical analysis to account for heat transfer through the liquid microlayer. From the numerical simulation, the location where the vapor-liquid interface contacts the wall is observed to expand and then retract as the bubble grows and departs. The effect of static contact angle and wall superheat on bubble dynamics has been quantified. The bubble growth predicted from numerical analysis has been found to compare well with the experimental data reported in the literature and that obtained in this work

430 citations


Journal ArticleDOI
TL;DR: In this article, a detailed and thorough parametric study of the Leidenfrost point (LFP) is presented, which serves as the temperature boundary between the transition and film boiling regimes.
Abstract: Recent demands for superior material properties and more efficient use of materials and production time are forcing manufacturers to develop intelligent processing techniques for enhanced process control in order to better dictate the end product. In the heat treatment and processing of metallic alloys, the desire to obtain parts of enhanced and uniform mechanical properties is requiring increased control over heat removal rates and enhanced temperature control. In particular, spray quenching has been shown to be an effective means to control and enhance the cooling rates of heat treatable aluminum alloys. This study presents a detailed and thorough parametric study of the Leidenfrost point (LFP), which serves as the temperature boundary between the transition and film boiling regimes. Sessile drop evaporation experiments were conducted with acetone, benzene, FC-72, and water on heated aluminum surfaces with either polished, particle blasted, or rough sanded finishes to observe the influential effects of fluid properties, surface roughness, and surface contamination on the LFP. A weak relationship between surface energies and the LFP was observed by performing droplet evaporation experiments with water on polished copper, nickel, and silver surfaces. Additional parameters which were investigated and found to have negligible influence on the LFP included liquidmore » subcooling, liquid degassing, surface roughness on the polished level, and the presence of polishing paste residues. The accumulated LFP data of this study were used to assess several existing models which attempt to identify the mechanisms which govern the LFP. The disagreement between the experimental LFP values and those predicted by the various models suggests that an accurate and robust theoretical model which effectively captures the LFP mechanisms is currently unavailable.« less

361 citations


Journal ArticleDOI
TL;DR: In this paper, experimental heat transfer and isothermal pressure drop data for single-phase water flows in a plate heat exchanger (PHE) with chevron plates are presented.
Abstract: Experimental heat transfer and isothermal pressure drop data for single-phase water flows in a plate heat exchanger (PHE) with chevron plates are presented. In a single-pass U-type counterflow PHE, three different chevron plate arrangements are considered: two symmetric plate arrangements with {beta} = 30 deg/30 deg and 60 deg/60 deg, and one mixed-plate arrangement with {beta} = 30 deg/60 deg. For water (2 < Pr < 6) flow rates in the 600 < Re < 10{sup 4} regime, data for Nu and f are presented. The results show significant effects of both the chevron angle {beta} and surface area enlargement factor {phi}. As {beta} increases, and compared to a flat-plate pack, up to two to five times higher Nu are obtained; the concomitant f, however, are 13 to 44 times higher. Increasing {phi} also has a similar, though smaller effect. Based on experimental data for Re {ge} 1000 and 30 deg {le} {beta} {le} 60 deg, predictive correlations of the form Nu = C{sub 1}({beta}) D{sub 1}({phi}) Re{sup p1({beta})} Pr{sup 1/3} ({mu}/{mu}{sub w}){sup 0.14} and f = C{sub 2}({beta}) D{sub 2}({phi}) Re{sup p2({beta})} are devised. Finally, at constant pumping power, and depending upon Re, {beta}, and {phi}, the heat transfermore » is found to be enhanced by up to 2.8 times that in an equivalent flat-plate channel.« less

310 citations


Journal ArticleDOI
Sung Jin Kim1, Duckjong Kim1
TL;DR: In this article, the EPRC (Electronic Packaging Research and Development Center) at KAIST and KOSEF (Korea Science and Engineering Foundation) were jointly investigated.
Abstract: This work was sUIψorted by EPRC (Electronic Packaging Research Center) at KAIST, and KOSEF (Korea Science and Engineering Foundation).

238 citations


Journal ArticleDOI
TL;DR: In this article, a detailed experimental and theoretical analysis on maximum heat transfer capabilities of two copper-water FMHP's with diagonal trapezoidal micro capillary grooves and one copper water FMHP with axial rectangular micro-capillary grasps is presented.
Abstract: Flat miniature heat pipes (FMHP's) are shown to be very promising in the cooling of electronic component systems. This investigation presents a detailed experimental and theoretical analysis on maximum heat transfer capabilities of two copper-water FMHP's with diagonal trapezoidal micro capillary grooves and one copper-water FMHP with axial rectangular micro capillary grooves. Maximum heat flux on the evaporator wall of the 120-mm long axial grooved heat pipe, with a vapor channel cross-sectional area of approximately 1.5 x 12 mm 2 and rectangular grooves of dimensions 0.20 mm wide by 0.42 mm deep, exceeded 90 W/cm 2 in the horizontal orientation and 150 W/cm 2 in the vertical orientation. Theoretical prediction of the capillary limitation in the horizontal orientation agreed reasonably well with the experimental data.

213 citations


Journal ArticleDOI
TL;DR: In this article, the evaporation heat transfer coefficient and pressure drop for refrigerant R-134a flowing in a plate heat exchanger were investigated experimentally in a two vertical counterflow channels were formed in the exchanger by three plates of commercial geometry with a corrugated sine shape of a chevron angle of 60 deg.
Abstract: Plate heat exchangers (PHE) have been widely used in food processing, chemical reaction processes, and other industrial applications for many years. Particularly, in the last 20 years plate heat exchangers have been introduced to the refrigeration and air conditioning systems as evaporators or condensers for their high efficiency and compactness. Here, the evaporation heat transfer coefficient and pressure drop for refrigerant R-134a flowing in a plate heat exchanger were investigated experimentally in this study. Two vertical counterflow channels were formed in the exchanger by three plates of commercial geometry with a corrugated sine shape of a chevron angle of 60 deg. Upflow boiling of refrigerant R-134a in one channel receives heat from the hot downflow of water in the other channel. The effects of the mean vapor quality, mass flux, heat flux, and pressure of R-134a on the evaporation heat transfer and pressure drop were explored. The quality change of R-134a between the inlet and outlet of the refrigerant channel ranges from 0.09 to 0.18. Even at a very low Reynolds number, the present flow visualization of evaporation in a plate heat exchanger remains turbulent. It is found that the evaporation heat transfer coefficient of R-134a in the plates ismore » much higher than that in circular pipes and shows a very different variation with the vapor quality from that in circular pipes, particularly in the convective evaporation dominated regime at high vapor quality. Relatively intense evaporation on the corrugated surface was seen from the flow visualization. Moreover, the present data showed that both the evaporation heat transfer coefficient and pressure drop increase with the vapor quality. At a higher mass flux the pressure drop is higher for the entire range of the vapor quality but the evaporation heat transfer is clearly better only at the high quality. Raising the imposed wall heat flux was found to slightly improve the heat transfer, while at a higher refrigerant pressure, both the heat transfer and pressure drop are slightly lower. Based on the present data, empirical correlations for the evaporation heat transfer coefficient and friction factor were proposed.« less

194 citations


Journal ArticleDOI
TL;DR: In this article, a comprehensive model is developed to study the heating, melting, evaporation, and resolidification of powder particles in plasma flames, where the particle is assumed to be a spherical and one-dimensional heat conduction equation with phase change within the particle and is solved numerically using an appropriate coordinate transformation and finite difference method.
Abstract: A comprehensive model is developed to study the heating, melting, evaporation, and resolidification of powder particles in plasma flames. The well-established LAVA code for plasma flame simulation is used to predict the plasma gas field under given power plasma flame simulation is used to predict the plasma gas field under given power conditions, and provide inputs to the particle model. The particle is assumed to be a spherical and one-dimensional heat conduction equation with phase change within the particle is solved numerically using an appropriate coordinate transformation and finite difference method. Melting, vaporization, and resolidification interfaces are tracked and the particle vaporization is accounted for by the mass diffusion of vapor through the boundary layer around the particle. The effect of mass transfer on convective heat transfer is also included. Calculations have been carried out for a single particle injected into an Ar-H{sub 2} plasma jet. Zirconia and nickel are selected as solid particles because of their widespread industrial applications as well as significant differences in their thermal properties. Numerical results show strong nonisothermal effect of heating, especially for materials with low thermal conductivity, such as zirconia. The model also predicts strong evaporation of the material at high temperatures.

124 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the local convective heat transfer on a wall-mounted cube placed in a developing turbulent channel flow for Reynolds numbers between 2750 < ReH < 4970.
Abstract: This paper presents some results of the experimental investigation of the local convective heat transfer on a wall-mounted cube placed in a developing turbulent channel flow for Reynolds numbers between 2750 < ReH < 4970 Experiments were conducted using a specially designed cubic assembly made of heated copper core and a thin epoxy layer on its surface The distribution of the local heat transfer coefficient was obtained from the surface heat flux evaluated from the heat input and computed temperature field in the epoxy layer, and from the surface temperature distribution acquired by infrared thermography In parallel, the flow field was studied using laser doppler anemometer and flow visualizations, aimed at correlating the local heat transfer with the flow pattern and turbulence field The complex vortex structure around the cube, in particular at the top and the side faces, caused large variation in the local convective heat transfer The largest gradients in the distributions of the surface heat transfer were found at locations of flow separation and reattachment Areas of flow recirculation are typically accompanied by a minimum in the heat transfer coefficient It is argued that the local temperature rise of the air in the recirculation zone is caused by the trapped vortex, which acts as an insulation layer preventing the removal of heat from the surface of the cubes In contrast, the intermittent reattachment of the low-temperature shear flow was found to produce large heat transfer coefficients

122 citations


Journal ArticleDOI
TL;DR: In this article, a multiple regression technique was used to correlate 47 sets of heat exchanger data to develop the heat transfer and friction correlation, which is applicable to heat exchangers having small diameter tubes (or large tube pitch to tube diameter ratio).
Abstract: This paper deals with heat exchangers having plain fins on a staggered array of circular tubes. Correlations are developed to predict the air-side heat transfer coefficient and friction factor as a function of the Reynolds number and geometric variables of the heat exchanger such as tube diameter, tube pitch, fin spacing, etc. A multiple regression technique was used to correlate 47 sets of heat exchanger data to develop the heat transfer and friction correlation. The correlations are applicable to heat exchangers having small diameter tubes (or large tube pitch to tube diameter ratio), whose performance previous correlations failed to predict adequately. The heat transfer correlation applicable to three or more row configuration predicts 94% of the data within {+-}20%, and the heat transfer correlation applicable to one- or two-row configuration predicts 94% of the data within {+-}20%. The friction correlation predicts 90% of the data within {+-}20%.

Journal ArticleDOI
TL;DR: In this paper, the Kattan-Thome-Favrat flow boiling model accurately perdicted the local heat transfer coefficients measured in all these flow regimes with only two small modifications to their flow map (to extend its application to G < 100 kg/m2s).
Abstract: Experimental test results for flow boiling of pure ammonia inside horizontal tubes were obtained for a plain stainless steel tube. Tests were run at a nominal saturation temperature of 4°C, nine mass velocities from 20- 140 kg/m2s, vapor qualities from 1-99% and heat fluxes from 5-58 kW/m2. Two-phase flow observations showed that the current test data covered the following regimes: fully stratified, stratified-wavy, intermittent, annular and annular with partial dryout. The Kattan-Thome-Favrat flow boiling model accurately perdicted the local heat transfer coefficients measured in all these flow regimes with only two small modifications to their flow map (to extend its application to G < 100 kg/m2s). Their flow boiling model was also successfully compared to the earlier ammonia flow boiling data of Chaddock and Buzzard (1986). The Gungor-Winterton (1987) correlation instead gave very poor accuracy for ammonia.

Journal ArticleDOI
TL;DR: In this article, the exact narrow-band averaged radiative transfer equation was solved using a ray-tracing method using a three-dimensional rectangular enclosure containing (i) an isothermal pure water vapor at 1000 K and I atm, (ii) an inhomogeneous H 2 O/N 2 mixture at 1000 k and I k, and (iii) a nonisothermal and homogeneous mixture of CO 2 /H 2 O /N 2 2 at 1 atm.
Abstract: Three-dimensional non-grey gas radiation analyses were conducted using the statistical narrow-band model along with updated band parameters. The exact narrow-band averaged radiative transfer equation was solved using a ray-tracing method. Accurate numerical results were presented for non-grey real gas radiative transfer in a three-dimensional rectangular enclosure containing (i) an isothermal pure water vapor at 1000 K and I atm, (ii) an isothermal and inhomogeneous H 2 O/N 2 mixture at 1000 K and I atm, and (iii) a nonisothermal and homogeneous mixture of CO 2 /H 2 O/N 2 at 1 atm

Journal ArticleDOI
TL;DR: In this article, the effects of phonon interference and tunneling on the heat conduction and the thermal conductivity of thin films and superlattices were studied based on the consideration of the acoustic wave propagation in thin film structures and neglecting the internal scattering.
Abstract: Heat conduction in thin films and superlattices is important for many engineering applications such as thin-film based microelectronic, photonic, thermoelectric, and thermionic divides. Past modeling efforts on the thermal conductivity of thin films were based on solving the Boltzmann transport equation that treats phonons as particles. The effects of phonon interference and tunneling on the heat conduction and the thermal conductivity of thin films and superlattices remain to be explored. In this work, the wave effects on the heat conduction in thin films and superlattices are studied based on the consideration of the acoustic wave propagation in thin film structures and neglecting the internal scattering. A transfer matrix method is used to calculate the phonon transmission and heat conduction through these structures. The effects considered in this work include the phonon interference, tunneling, and confinement. The phonon dispersion is considered by introducing frequency-dependent Lamb constants. A ray-tracing method that treats phonons as particles is also developed for comparison. Sample calculations are performed on double heterojunction structures resembling Ge/Si/Ge and n-period superlattices similar to Ge/Si/n(Si/Ge)/Ge. It is found that phonon confinements caused by the phonon spectra mismatch and by the total internal reflection create a dramatic decrease of the overall thermalmore » conductance of thin films. The phonon interference in a single layer does not have a strong effect on its thermal conductance but for superlattice structures, the stop bands created by the interface effects can further reduce the thermal conductance. Tunneling of phonon waves occurs when the constituent layers are 1--3 monolayer thick and causes a slight recovery in the thermal conductance when compared to thicker layers. The thermal conductance obtained from the ray tracing and the wave methods approaches the same results for a single layer. For superlattices, however, the wave method leads to a finite thermal conductance even for infinitely thick superlattices while the ray tracing method gives a thermal conductance that decreases with increasing number of layers. Implications of these results on explaining the recent thermal conductivity data of superlattices are explored.« less


Journal ArticleDOI
TL;DR: In this article, the effects of chevron angle β, corrugation aspect ratio γ, and flow conditions (Re, Pr, μ/μw on Nu and f) on the performance of a single-pass U-type counterflow plate heat exchanger with Chevron plates were investigated.
Abstract: Steady-state heat transfer and pressure drop data for single-phase viscous fluid flows (2 ≤ Re ≤ 400) in a single-pass U-type counterflow plate heat exchanger (PHE) with chevron plates are presented. With vegetable oil as test fluid (130 < Pr < 290), three different plate arrangements are employed: two symmetric (β = 30 deg/30 deg and 60 deg/60 deg) and one mixed (β = 30 deg/60 deg). The effects of chevron angle β, corrugation aspect ratio γ, and flow conditions (Re, Pr, μ/μw on Nu and f characteristics of the PHE are delineated. The results show a rather complex influence of plate surface corrugations on the enhanced thermal-hydraulic behavior. Relative to the performance of equivalent flat-plate packs, chevron plates sustain up to 2.9 times higher heat transfer rates on a fixed geometry and constant pumping power basis, and require up to 48 percent less surface area for the fixed heat load and pressure drop constraint.


Journal ArticleDOI
TL;DR: In this paper, the main emphasis is on high-volume production of silica-based optical fibers by using a large preform and a high-speed drawing, and the governing equations at each section were formulated to describe the process mechanics and to identify the key control parameters for drawing and coating.
Abstract: The paper is primarily to highlight the current issues concerning fiber drawing and coating. The main emphasis is on high-volume production of silica-based optical fibers by using a large preform and a high-speed drawing. The commercial application of these processes has led to increasing the productivity in fiber manufacturing and resulted in a low cost of produced fibers. In order to systematically address the problems associated with the fiber manufacturing process, the fiber drawing system was divided into three major functional sections: heating, cooling, and coating zones. The governing equations at each section were formulated to describe the process mechanics and to identify the key control parameters for drawing and coating. These process parameters are the basic elements of implementing a streamline production system of optical fibers.



Journal ArticleDOI
TL;DR: In this paper, an experimental investigation of the forced convective heat transfer of individual and arrays of multiple two-dimensional obstacles is reported, and the effects upon the Nusselt numbers and obstacle temperature differences of parametric changes in the Reynolds number, channel height, array configuration, and input heat flux are established.
Abstract: An experimental investigation of the forced convective heat transfer of individual and arrays of multiple two-dimensional obstacles is reported. The airflow rate was varied from 800 < ReDh =s 13000. The effects upon the Nusselt numbers and obstacle temperature differences of parametric changes in the Reynolds number, channel height, array configuration, and input heat flux are established. The input heat fluxes to the obstacles ranged from 950 < q" == 20200 W/m2, which significantly extends beyond that seen in the open literature for forced convective air cooling of simulated electronic components. Comparisons of the obstacle mean Nusselt numbers are made with a two-dimensional laminar numerical model employing the Navier-Stokes equa­ tions. A set of correlations characterizing the heat transfer from the protruding heat sources within the channel is obtained. It was found that the obstacle temperature, the critical measure for electronic device failure, must be shown along with the corresponding Nusselt number to fully characterize the thermal state of the heated obstacle as the ratio definition of the Nusselt number can obscure large temperature increases. The results find that the proper placement of geometrically dissimilar obstacles, such as a taller obstacle, can be used to passively enhance the heat transfer in its vicinity. This effect would be dependent upon the flow rate and geometries in order to control the reattachment zones and their subsequent convective augmen­ tation. The experimental results are found to be in good agreement with the results from the numerical simulation. Finally, a set of pertinent correlations for the arrays of channel mounted obstacles is given.

Journal ArticleDOI
TL;DR: In this article, the authors derived a macroscopic two-equation system which requires the closure modeling of new unknown terms due to interfacial transport, namely, the tortuosity term and the interfacial heat transfer term.
Abstract: Equations governing the transient heat conduction in porous materials consisting of solids and fluids of different thermal properties were derived with a volumetric average scheme under the assumption of nonthermal equilibrium. The derivation leads to a macroscopic two-equation system which requires the closure modeling of new unknown terms due to interfacial transport, namely, the tortuosity term and the interfacial heat transfer term. Closure relations were obtained from the microscopic equations for temperature fluctuation under quasi-steady assumption. The closure coefficients appeared in the closure relations then depend on the media geometry as well as thermal properties. To demonstrate these dependencies, the closure coefficient for the thermal tortuosity is evaluated based on the effective stagnant thermal conductivity model proposed by Hsu et al. (1995) for periodically packed cubes, and the coefficient for interfacial heat transfer based on a quasi-steady heat conduction of dispersed spheres immersed in fluids. The salient features as well as the applicability and limitation of the newly proposed transient heat conduction model were discussed.

Journal ArticleDOI
TL;DR: In this article, the effect of sliding vapor bubbles on forced convection boiling heat transfer was investigated for both vertical upflow and downflow configurations using FC-87 and showed that sliding bubbles enhance the bulk liquid turbulence at the wall, which contributes significantly to the macroscale heat transfer.
Abstract: We describe experimental efforts aimed at examining the effect of vapor bubble sliding on forced convection boiling heat transfer. Flow boiling experiments using FC-87 were conducted for vertical upflow and downflow configurations. Both slightly subcooled single-phase and saturated annular flow boiling were considered. Significantly higher heat transfer rates were measured for vertical upflow than for downflow with the same wall superheat, and slightly subcooled single-phase inlet conditions. This increase in heat transfer is directly attributable to sliding vapor bubbles, which remain attached to the wall during upflow and lift off the wall during downflow. Differences in the measured upflow and downflow heat transfer rates are not as significant for annular flow boiling, which is clue in part to the similar vapor bubble dynamics which have been observed for upflow and downflow. Heat transfer experiments in single-phase subcooled upflow with air bubble injection at the heating surface suggest that sliding bubbles enhance the bulk liquid turbulence at the wall, which contributes significantly to the macroscale heat transfer

Journal ArticleDOI
TL;DR: In this paper, a detailed mathematical model for predicting the heat transport capability and temperature gradients that contribute to the overall axial temperature drop as a function of heat transfer in a micro heat pipe has been developed.
Abstract: A detailed mathematical model for predicting the heat transport capability and temperature gradients that contribute to the overall axial temperature drop as a function of heat transfer in a micro heat pipe has been developed. The model utilizes a third-order ordinary differential equation, which governs the fluid flow and heat transfer in the evaporating thin film region; an analytical solution for the two-dimension heat conduction equation, which governs the macro evaporating film region in the triangular corners; the effects of the vapor flow on the liquid flow in the micro heat pipe; the flow and condensation of the thin film caused by the surface tension in the condenser; and the capillary flow along the axial direction of the micro heat pipe. With this model, the temperature distribution along the axial direction of the heat pipe and the effect on the heat transfer can be predicted. In order to verify the model presented here, an experimental investigation was also conducted and a comparison with experimental data made

Journal ArticleDOI
TL;DR: In this article, natural convection in a square enclosure with heated vertical walls and temperature-dependent conductivity and viscosity was simulated for Prandtl numbers ranging from 0.01 to 1.0 and Ra ≤ 10 6.
Abstract: Natural convection in a square enclosure with heated vertical walls and temperature-dependent conductivity and viscosity was simulated for Prandtl numbers ranging from 0.01 to 1.0 and Ra ≤ 10 6 . Although the variable properties produced observable changes in the the temperatures and velocities, the overall heat transfer, as represented by the Nusselt number, was found to be unaffected

Journal ArticleDOI
TL;DR: In this article, the effects of the most influential geometric parameters on heat transfer including chip number, channel height, and Reynolds number are tested. And the experimental results indicate that the heat transfer coefficient is affected strongly by the number of chips and the Reynolds number and weakly by the channel height.
Abstract: Experiments have been performed using water to determine the single-phase forced convection heat transfer from in-line four simulated electronic chips, which are flush-mounted to one wall of a vertical rectangular channel. The effects of the most influential geometric parameters on heat transfer including chip number, and channel height are tested. The channel height is varied over values of 0.5, 0.7, and 1.0 times the heat source length. The heat flux is set at the three values of 5 W/cm2 , 10 W/cm2 , and 20 W/cm2 , and the Reynolds number based on the heat source length ranges from 6 × 102 to 8 × 104 . Transition Reynolds numbers are deduced from the heat transfer data. The experimental results indicate that the heat transfer coefficient is affected strongly by the number of chips and the Reynolds number and weakly by the channel height. Finally, the present results from liquid-cooling are compared with other results from air-cooling, and Prandtl number scaling between air and water is investigated.

Journal ArticleDOI
TL;DR: In this article, flow field and heat transfer in sine-wave crossed-corrugated ducts have been investigated by numerical solution of the Navier-Stokes and energy equations in the laminar and transitional flow regime between Re = 170 and 2000.
Abstract: Regenerative and recuperative plate heat exchangers consist of ten of stacked corrugated plates. The orientation of the corrugation between neighboring plates is nonaligned. Multitude contact points between the plates lead to high compressive strength of the heat exchangers. The corrugations give structural stability even at low material thickness of the plates and produce highly complex flow structure which determines the thermal performance and the flow loss of the heat exchanger. Here, flow field and heat transfer in sine-wave crossed-corrugated ducts have been investigated by numerical solution of the Navier-Stokes and energy equations in the laminar and transitional flow regime between Re = 170 and 2000. The ratio of the corrugation wave length {lambda}{sup *} to amplitude a{sup *} has been varied between 7 and 10. The angle of the corrugation of the neighboring plates has been kept fixed at 45 deg. Results show that the critical Reynolds number for self-sustained flow oscillations is about 240. For Reynolds numbers larger than 1000, the Nusselt number and the friction factor are nearly independent of the dimensionless wavelength. Computational results compare well with available experimental results.


Journal ArticleDOI
TL;DR: In this article, wind tunnel tests were performed on three different fin geometries (wavy, lanced, and louver) under wet and dry conditions, and the effect of the fin press oil on wet pressure drop was also studied.
Abstract: Limited previous work has shown that use of special hydrophilic coatings will provide lower air pressure drop in finned tube heat exchangers operated under dehumidifying conditions. However, no detailed work has been reported on the effect of different coating types, or different fin surface geometries on the wet pressure drop. In this study, wind tunnel tests were performed on three different fin geometries (wavy, lanced, and louver) under wet and dry conditions. All dehumidification tests were done for fully wet surface conditions. For each geometry, the tests were performed on uncoated and coated heat exchangers. For all three fin geometries, the wet-to-dry pressure drop ratio was 1.2 at 2.5 m/s frontal air velocity. The coatings have no influence on the wet or dry heat transfer coefficient. However, the wet surface heat transfer coefficient was 10 to 30% less than the dry heat transfer coefficient, depending on the particular fin geometry. The effect of the fin press oil on wet pressure drop was also studied. If the oil contains a surfactant, good temporary wetting can be obtained on an uncoated surface; however, this effect is quickly degraded as the oil is washed from the surface during wet operation. This work alsomore » provides a critical assessment of data reduction methods for wet surface operation, including calculation of the fin efficiency.« less

Journal ArticleDOI
TL;DR: In this paper, a laminar mixed convection in a parallel-plate vertical channel is investigated in the case of non-negligible viscous heating, where the channel walls are subjected to asymmetric boundary conditions: one wall experiences a constant and uniform heat flux, while the other is kept at a uniform and constant temperature.
Abstract: In the last years, several analyses of combined forced and free convection in vertical channels have appeared in the literature. Most of the interest in this subject is due to its applications, for instance, in the design of cooling systems for electronic devices and in the field of solar energy collection. Fully developed and laminar mixed convection in a parallel-plate vertical channel is investigated in the case of non-negligible viscous heating. The channel walls are subjected to asymmetric boundary conditions: One wall experiences a constant and uniform heat flux, while the other is kept at a uniform and constant temperature. The velocity field and the temperature field are evaluated analytically by means of perturbation expansions with respect to a buoyancy parameter, i.e., the ratio between the Grashof number and the Reynolds number. The Nusselt numbers and the friction factors are obtained as functions of the buoyancy parameter.