Showing papers in "Journal of Heat Transfer-transactions of The Asme in 2014"

Performance of Near-Field Thermophotovoltaic Cells Enhanced With a Backside Reflector

Abstract: Thermophotovoltaic (TPV) systems are very promising for waste heat recovery. This work analyzes the performance of a near-field TPV device with a gold reflecting layer on the backside of the cell. The radiative transfer from a tungsten radiator, at a temperature ranging from 1250 K to 2000 K, to an In0.18Ga0.82Sb TPV cell at 300 K is calculated using fluctuational electrodynamics. The current generation by the absorbed photon energy is modeled by the minority carrier diffusion equations considering recombination. The energy conversion efficiency of the cell is determined from the generated electrical power and the net absorbed radiant power per unit area. A parametric study of the cell efficiency considering the gap spacing and other parameters is conducted. For an emitter at temperature 1250 K, the efficiency enhancement by adding a mirror, which reduces the sub-bandgap radiation, is shown to be as much as 35% relative to a semi-infinite TPV cell. In addition, the potential for further improvement by reducing surface recombination velocity from that of a perfect ohmic contact is examined. The cell performance is shown to increase with decreasing gap spacing below a critical surface recombination velocity.

Application Conditions of Effective Medium Theory in Near-Field Radiative Heat Transfer Between Multilayered Metamaterials

Abstract: This work addresses the validity of the local effective medium theory (EMT) in predicting the near-field radiative heat transfer between multilayered metamaterials, separated by a vacuum gap. Doped silicon and germanium are used to form the metallodielectric superlattice. Different configurations are considered by setting the layers adjacent to the vacuum spacer as metal–metal (MM), metal–dielectric (MD), or dielectric–dielectric (DD) (where M refers to metallic doped silicon and D refers to dielectric germanium). The calculation is based on fluctuational electrodynamics using the Green's function formulation. The cutoff wave vectors for surface plasmon polaritons (SPPs) and hyperbolic modes are evaluated. Combining the Bloch theory with the cutoff wave vector, the application condition of EMT in predicting near-field radiative heat transfer is presented quantitatively and is verified by exact calculations based on the multilayer formulation.

Observations of the Critical Heat Flux Process During Pool Boiling of FC-72

Abstract: Experimental work was undertaken to investigate the process by which pool-boiling critical heat flux (CHF) occurs using an IR camera to measure the local temperature and heat transfer coefficients on a heated silicon surface. The wetted area fraction (WF), the contact line length density (CLD), the frequency between dryout events, the lifetime of the dry patches, the speed of the advancing and receding contact lines, the dry patch size distribution on the surface, and the heat transfer from the liquid-covered areas were measured throughout the boiling curve. Quantitative analysis of this data at high heat flux and transition through CHF revealed that the boiling curve can simply be obtained by weighting the heat flux from the liquid-covered areas by WF. CHF mechanisms proposed in the literature were evaluated against the observations. [DOI: 10.1115/1.4025697]

Magnetohydrodynamics Flow and Heat Transfer Around a Solid Cylinder Wrapped With a Porous Ring

Abstract: The problem of the effect of an external magnetic field on fluid flow and heat transfer characteristics is relevant to several physical phenomena. In this paper, flow and heat transfer of an electrically-conductive fluid around a cylinder, wrapped with a porous ring and under the influence of a magnetic field, is studied numerically. The ranges of the Stuart (N), Reynolds (Re), and Darcy (Da) numbers are 0–7, 1–40, and 10 –10 , respectively. The Darcy–Brinkman–Forchheimer model was used for simulating flow in the porous layer. The governing equations provide a coupling between flow and magnetic fields. The governing equations, together with the relevant boundary conditions, are solved numerically using the finite-volume method (FVM). The effect of the Stuart, Reynolds, and Darcy numbers on the flow patterns and heat transfer rate are explored. Finally, two empirical equations for the average Nusselt number were suggested, in which the effect of a magnetic field and the Darcy numbers are taken into account. It was found that in the presence of a magnetic field, the drag coefficient and the critical radius of the insulation increases, while the wake length and Nusselt number decrease. [DOI: 10.1115/1.4026371]

Simple Mechanistically Consistent Formulation for Volume-of-Fluid Based Computations of Condensing Flows

Abstract: Numerous investigations have been conducted to extend adiabatic liquid-gas VOF flow solvers to include condensation phenomena by adding an energy equation and phase-change source terms. Some proposed phase-change models employ empirical rate parameters, or adapt heat transfer correlations, and thus must be tuned for specific applications. Generally applicable models have also been developed that rigorously resolve the phase-change process, but require interface reconstruction, significantly increasing computational cost and software complexity. In the present work, a simplified first-principles-based condensation model is developed, which forces interface-containing mesh cells to the equilibrium state. The operation on cells instead of complex interface surfaces enables the use of fast graph algorithms without reconstruction. The model is validated for horizontal film condensation, and converges to exact solutions with increasing mesh resolution. Agreement with established results is demonstrated for smooth and wavy falling-film condensation.Copyright © 2013 by ASME

"Entransy," and Its Lack of Content in Physics

Abstract: Here, I show that “entransy” has no meaning in physics, because, at bottom, it rests on the false claim that in order to transfer heat to a solid body of thermodynamic temperature T, the heat transfer must be proportional to T. Entransy “dissipation” is a number proportional to well known measures of irreversibility such as entropy generation and lost exergy (destroyed available work). Furthermore, the “principle of entransy dissipation minimization” adds nothing to existing work based on minimum entropy generation, minimum thermal resistance, and constructal law. The broader trend illustrated by the entransy hoax is that it is becoming easy to take an existing idea, change the keywords, and publish it as new.

Isoflux Nusselt Number and Slip Length Formulae for Superhydrophobic Microchannels

Abstract: We analytically and numerically consider the hydrodynamic and thermal transport behavior of fully developed laminar flow through a superhydrophobic (SH) parallel-plate channel. Hydrodynamic slip length, thermal slip length and heat flux are prescribed at each surface. We first develop a general expression for the Nusselt number valid for asymmetric velocity profiles. Next, we demonstrate that, in the limit of Stokes flow near the surface and an adiabatic and shear-free liquid–gas interface, both thermal and hydrodynamic slip lengths can be found by redefining existing solutions for conduction spreading resistances. Expressions for the thermal slip length for pillar and ridge surface topographies are determined. Comparison of fundamental half-space solutions for the Laplace and Stokes equations facilitate the development of expressions for hydrodynamic slip length over pillar-structured surfaces based on existing solutions for the conduction spreading resistance from an isothermal source. Numerical validation is performed and an analysis of the idealized thermal transport behavior suggests conditions under which superhydrophobic microchannels may enhance heat transfer.

Stagnation-Point Flow Toward a Stretching/Shrinking Sheet in a Nanofluid Containing Both Nanoparticles and Gyrotactic Microorganisms

Abstract: The stagnation-point flow and heat transfer toward a stretching/shrinking sheet in a nanofluid containing gyrotactic microorganisms with suction are investigated Using a similarity transformation, the nonlinear system of partial differential equations is converted into nonlinear ordinary differential equations These resulting equations are solved numerically using a shooting method The skin friction coefficient, local Nusselt number, local Sherwood number, and the local density of the motile microorganisms as well as the velocity, temperature, nanoparticle volume fraction and the density of motile microorganisms profiles are analyzed subject to several parameters of interest, namely suction parameter, thermophoresis parameter, Brownian motion parameter, Lewis number, Schmidt number, bioconvection Peclet number, and the stretching/shrinking parameter It is found that dual solutions exist for a certain range of the stretching/shrinking parameter for both shrinking and stretching cases The results indicate that the skin friction coefficient, local Nusselt number, local Sherwood number, and the local density of the motile microorganisms increase with suction effect It is also observed that suction widens the range of the stretching/shrinking parameter for which the solution exists

Constructal Design of Convective Y-Shaped Cavities by Means of Genetic Algorithm

Abstract: In the present work constructal design is employed to optimize the geometry of a convective, Y-shaped cavity that intrudes into a solid conducting wall. The main purpose is to investigate the influence of the dimensionless heat transfer parameter a over the optimal geometries of the cavity, i.e., the ones that minimize the maximum excess of temperature (or reduce the thermal resistance of the solid domain). The search for the best geometry has been performed with the help of a genetic algorithm (GA). For square solids (H/L = 1.0) the results obtained with an exhaustive search (which is based on solution of all possible geometries) were adopted to validate the GA method, while for H/L ≠ 1.0 GA is used to find the best geometry for all degrees of freedom investigated here: H/L, t1/t0, L1/L0, and α (four times optimized). The results demonstrate that there is no universal optimal shape that minimizes the thermal field for all values of a investigated. Moreover, the temperature distribution along the solid domain becomes more homogeneous with an increase of a, until a limit where the configuration of “optimal distribution of imperfections” is achieved and the shape tends to remain fixed. Finally, it has been highlighted that the GA method proved to be very effective in the search for the best shapes with the number of required simulations much lower (8 times for the most difficult situation) than that necessary for exhaustive search.

Apparent Temperature Jump and Thermal Transport in Channels With Streamwise Rib and Cavity Featured Superhydrophobic Walls at Constant Heat Flux

Abstract: This paper presents an analytical investigation of constant property, steady, fully developed, laminar thermal transport in a parallel-plate channel comprised of metal superhydrophobic (SH) walls. The superhydrophobic walls considered here exhibit microribs and cavities aligned in the streamwise direction. The cavities are assumed to be nonwetting and contain air, such that the Cassie–Baxter state is the interfacial state considered. The scenario considered is that of constant heat flux through the rib surfaces with negligible thermal transport through the air cavity interface. Closed form solutions for the local Nusselt number and local wall temperature are presented and are in the form of infinite series expansions. The analysis show the relative size of the cavity regions compared to the total rib and cavity width (cavity fraction) exercises significant influence on the aggregate thermal transport behavior. Further, the relative size of the rib and cavity module width compared to the channel hydraulic diameter (relative module width) also influences the Nusselt number. The spatially varying Nusselt number and wall temperature are presented as a function of the cavity fraction and the relative module width over the ranges 0–0.99 and 0.01–1.0, respectively. From these results, the rib/cavity module averaged Nusselt number was determined as a function of the governing parameters. The results reveal that increases in either the cavity fraction or relative module width lead to decreases in the average Nusselt number and results are presented over a wide range of conditions from which the average Nusselt number can be determined for heat transfer analysis. Further, analogous to the hydrodynamic slip length, a temperature jump length describing the apparent temperature jump at the wall is determined in terms of the cavity fraction. Remarkably, it is nearly identical to the hydrodynamic slip length for the scenario considered here and allows straightforward determination of the average Nusselt number for any cavity fraction and relative rib/cavity module width.

Free Convection in Shallow and Slender Porous Cavities Filled by a Nanofluid Using Buongiorno's Model

Abstract: A numerical study of the steady free convection flow in shallow and slender porous cavities filled by a nanofluid is presented The nanofluid model takes into account the Brownian diffusion and the thermophoresis effects The governing dimensional partial differential equations are transformed into a dimensionless form before being solved numerically using a finite difference method Effort has been focused on the effects of four types of influential factors such as the aspect ratio, the Rayleigh and Lewis numbers, and the buoyancy-ratio parameter on the fluid flow and heat transfer characteristics

Mixing and Heat Transfer Enhancement in Microchannels Containing Converging-Diverging Passages

Abstract: Fully-developed flow and heat transfer in periodic converging-diverging channels with rectangular cross sections are studied using computational fluid dynamics (CFD) simulations for Reynolds numbers ranging from 50 to 200. Experimental laser sheet flow visualizations have also been utilized with the aid of an enlarged transparent Perspex model, which serves as a form of secondary verification of the CFD results. The CFD investigations focus on two principal configurations of converging-diverging channels, namely the constant curvature and sinusoidal converging-diverging channel. Heat transfer simulations have been carried out under constant wall temperature conditions using liquid water as the coolant. It is found that due to the fluid mixing arising from a pair of recirculating vortices in the converging-diverging channels, the heat transfer performance is always significantly more superior to that of straight channels with the same average cross sections; at the same time the pressure drop penalty of the converging-diverging channels can be much smaller than the heat transfer enhancement. The effects of channel aspect ratio and amplitude of the converging-diverging profiles have been systematically investigated. The results show that for a steady flow, the flow pattern is generally characterized by the formation of a pair of symmetrical recirculating vortices in the two furrows of the converging-diverging channel. Both the optimal aspect ratio and channel amplitude are being presented with the support of CFD analyses. Experimental flow visualizations have also been utilized and it was found that the experimental results agrees favorably with the CFD results. The present study shows that these converging-diverging channels have prominent advantages over straight channels. The most superior configuration considered in this paper has been found to yield an improvement of up to 60% in terms of the overall thermal-hydraulic performance compared to microchannels with straight walls, thus serving as promising candidates for incorporation into efficient heat transfer devices.

Control of Laminar Pulsating Flow and Heat Transfer in Backward-Facing Step by Using a Square Obstacle

Abstract: In the present study, laminar pulsating flow over a backward-facing step in the presence of a square obstacle placed behind the step is numerically studied to control the heat transfer and fluid flow. The working fluid is air with a Prandtl number of 0.71 and the Reynolds number is varied from 10 and 200. The study is performed for three different vertical positions of the square obstacle and different forcing frequencies at the inlet position. Navier–Stokes and energy equation for a 2D laminar flow are solved using a finite-volume-based commercial code. It is observed that by properly locating the square obstacle the length and intensity of the recirculation zone behind the step are considerably affected, and hence, it can be used as a passive control element for heat transfer augmentation. Enhancements in the maximum values of the Nusselt number of 228% and 197% are obtained for two different vertical locations of the obstacle. On the other hand, in the pulsating flow case at Reynolds number of 200, two locations of the square obstacle are effective for heat transfer enhancement with pulsation compared to the case without obstacle.

Thermophoretic and Nonlinear Convection in Non-Darcy Porous Medium

Abstract: In this paper, we study the effects of nonlinear convection and thermophoresis in steady boundary layer flow over a vertical impermeable wall in a non-Darcy porous medium. Both the fluid temperature and the solute concentration are assumed to be nonlinear while at the wall, both the temperature and concentration are maintained at a constant value. A similarity transformation was used to obtain a system of nonlinear ordinary differential equations, which were then solved numerically using the Matlab bvp4c solver. A comparison of the numerical results with previously published results for special cases shows a good agreement. The effects of the nonlinear temperature and concentration parameters on the velocity and heat and mass transfer are shown graphically. A representative sample of the results is presented showing the effects of thermophoresis on the fluid velocity and heat and mass transfer rates. It is found among other results, that the concentration profiles decreased with increasing values of the thermophoretic parameter.

Convective Condensation Inside Horizontal Smooth and Microfin Tubes

Abstract: An experimental investigation was performed for convective condensation of R410A inside one smooth tube (3.78 mm, inner diameter) and six microfin tubes (4.54, 4.6 and 8.98 mm, fin root diameter) of different geometries for mass fluxes ranging from 99 to 603 kg m-2s-1. The experimental data were analyzed with updated flow pattern maps and evaluated with existing correlations. The heat transfer coefficient in the microfin tubes decreases at first and then increases or flattens out gradually as mass flux decreases. This obvious non-monotonic heat transfer coefficient-mass flux relation may be explained by the complex interactions between the microfins and the fluid, mainly by surface tension effects. The heat transfer enhancement mechanism in microfin tubes is mainly due to the surface area increase at large mass fluxes, while liquid drainage by surface tension and interfacial turbulence enhance heat transfer greatly at low mass fluxes. (Less)

Electrohydrodynamic Microfabricated Ionic Wind Pumps for Thermal Management Applications

Abstract: This work demonstrates an innovative microfabricated air-cooling technology that employs an electrohydrodynamic (EHD) corona discharge (i.e., ionic wind pump) for electronics cooling applications. A single, micro fabricated ionic wind pump element consists of two parallel collecting electrodes between which a single emitting tip is positioned. A grid structure on the collector electrodes can enhance the overall heat-transfer coefficient and facilitate an IC compatible batch process. The optimized devices studied exhibit an overall device area of 5.4 mm × 3.6 mm, an emitter-to-collector gap of ~0.5 mm, and an emitter curvature radius of ~12.5 μm. The manufacturing process developed for the device uses glass wafers, a single mask-based photolithography process, and a low-cost copper-based electroplating process. Various design configurations were explored and modeled computationally to investigate their influence on the cooling phenomenon. The single devices provide a high heat-transfer coefficient of up to ~3200 W/m 2 K and a coefficient of performance (COP) of up to ~47. The COP was obtained by dividing the heat removal enhancement, ΔQ by the power consumed by the ionic wind pump device. A maximum applied voltage of 1.9 kV, which is equivalent to approximately 38 mW of power input, is required for operation, which is significantly lower than the power required for the previously reported devices. Furthermore, the microfabricated single device exhibits a flexible and small form factor, no noise generation, high efficiency, large heat removal over a small dimension and at low power, and high reliability (no moving parts); these are characteristics required by the semiconductor industry for next generation thermal management solutions.

Ultrafast Spectroscopy of Electron-Phonon Coupling in Gold

Abstract: Transient reflectance of gold was measured using ultrafast spectroscopy by varying the wavelength of the probe laser beam in the visible range. Based on the band structure of gold, the influence of the probe beam wavelength on the signal trend is analyzed in terms of sensitivity, effect of nonthermalized electrons, and relaxation rate. It is found that probing around 490 nm renders the best sensitivity and a simple linear relation between the transient reflectance and the electron temperature. The two-temperature model (TTM) is applied to calculate the electron-phonon coupling factor by fitting the transient reflectance signal. This work clarifies the ultrafast energy transfer dynamics in gold and the importance of using proper probe laser wavelength for modeling the transient heat transfer process in metal. [DOI: 10.1115/1.4028543]

Effect of Temperature Dependent Fluid Properties on Heat Transfer in Turbulent Mixed Convection

Abstract: The effect of the uniform fluid properties approximation (Oberbeck-Boussinesq (OB)) in turbulent mixed convection is investigated via direct numerical simulation (DNS) of water flows with viscosity (l) and thermal expansion coefficient (b) both independently and simultaneously varying with temperature (non-Oberbeck-Boussinesq conditions (NOB)). Mixed convection is analyzed for the prototypical case of Poiseuille-RayleighB enard (PRB) turbulent channel flow. In PRB flows, the combination of buoyancy driven (Rayleigh-B enard) with pressure driven (Poiseuille) effects produce a complex flow structure, which depends on the relative intensity of the flow parameters (i.e., the Grashof number, Gr, and the shear Reynolds number, Res). In liquids, however, temperature variations induce local changes of fluid properties which influence the macroscopic flow field. We present results for different absolute values of the shear Richardson numbers (Ris 1⁄4 Gr=Re2s ) under constant temperature boundary conditions. As Ris is increased buoyant thermal plumes are generated, which induce large scale thermal convection that increases momentum and heat transport efficiency. Analysis of friction factor (Cf) and Nusselt number (Nu) for NOB conditions shows that the effect of viscosity is negligible, whereas the effect of thermal expansion coefficient is significant. Statistics of mixing show that (i) mixing increases for increasing Ris (and decreases for increasing Res) and (ii) the effect of thermal expansion coefficient on mixing increases for increasing Ris (and decreases for increasing Res). A simplified phenomenological model to predict heat transfer rates in PRB flows has also been developed. [DOI: 10.1115/1.4025135]

Do We Really Need “Entransy”? A Critical Assessment of a New Quantity in Heat Transfer Analysis

Abstract: Recently, a group of scientists introduced a new quantity for the analysis of heat transfer problems. They called it entransy since according to their understanding it is both, an indication of the nature of energy as well as that of the heat transfer ability. This concept is critically assessed on the background of two questions: Is entransy as an extension of the well established theory of heat transfer consistent with this classical approach? And: Is there a real need for the extension of the classical theory by introducing entransy as a quantity that was missing in the past?

Non-Darcy Natural Convection From a Vertical Cylinder Embedded in a Thermally Stratified and Nanofluid-Saturated Porous Media

Abstract: In recent years, nanofluids have attracted attention as a new generation of heat transfer fluids in building heating, heat exchangers, plants, and automotive cooling applications because of their excellent thermal performance. Various benefits of the application of nanofluids include improved heat transfer, heat transfer system size reduction, minimal clogging, microchannel cooling, and miniaturization of systems. In this paper, a study of steady, laminar, natural convection boundary-layer flow adjacent to a vertical cylinder embedded in a thermally stratified nanofluid-saturated non-Darcy porous medium is investigated. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis, and a generalized porous media model, which includes inertia and boundary effects, is employed. The cylinder surface is maintained at a constant nanoparticles volume fraction, and the wall temperature is assumed to vary with the vertical distance according to the power law form. The resulting governing equations are nondimensionalized and transformed into a nonsimilar form and then solved by Keller box method. A comparison is made with the available results in the literature, and our results are in very good agreement with the known results. A parametric study of the physical parameters is made, and a representative set of numerical results for the velocity, temperature, and volume fraction, as well as local shear stress and local Nusselt and Sherwood numbers, are presented graphically. The salient features of the results are analyzed and discussed. The results indicate that, when the buoyancy ratio or modified Grashof number increases, all of the local shear stress, local Nusselt number, and the local Sherwood number enhance while the opposite behaviors are predicted when the thermophoresis parameter increases. Moreover, increasing the value of the surface curvature parameter leads to increases in all of the local shear stress and the local Nusselt and Sherwood numbers while the opposite behaviors are obtained when either of the thermal stratification parameter or the boundary effect parameter increases. [DOI: 10.1115/1.4025559]

Performance Augmentation and Optimization of Aluminum Oxide-Water Nanofluid Flow in a Two-Fluid Microchannel Heat Exchanger

Abstract: In this paper, laminar forced convection and entropy generation in a counter flow microchannel heat exchanger (CFMCHE) with two different working fluids in hot and cold channels, ie, pure water and Al2O3–water nanofluid are investigated numerically using a three-dimensional conjugate heat transfer model The temperature distribution, effectiveness, pumping power and performance index for various volume fractions between 001–004, three nanoparticles diameters, ie, 29, 384, and 47 nm and a range of Reynolds number from 120 to 480 are given and discussed According to second law of thermodynamics and entropy generation rate in the CFMCHE, the analysis of optimal volume fraction, particles size, Reynolds number as well as optimal placement of using nanoparticles in hot/cold channels is carried out It is found that decreasing particles size and increasing nanoparticles concentration lead to higher effectiveness and pumping power as well as lower temperature in the solid phase of CFMCHE Furthermore, the frictional contribution of entropy increases with decreasing particles size and increasing volume fractions while the trends for heat transfer contribution of entropy are reverse Total entropy decreases as particles size decreases and volume fraction increases hence the maximum performance occurred at lower particles sizes and higher volume fractions The Reynolds number has significant effect on performance of system and with decreasing it the effectiveness increases and heat transfer contribution of entropy decreases while the pumping power and frictional contribution of entropy decrease Finally, it is seen that the capability of heat transfer of Al2O3–water nanofluids is higher when they are under heating conditions because the effectiveness of CFMCHE is higher when nanoparticles are used in cold channels

Evaluation of Pressure Drop Performance During Enhanced Flow Boiling in Open Microchannels With Tapered Manifolds

Abstract: Boiling can provide several orders of magnitude higher performance than a traditional air cooled system in electronics cooling applications. It can dissipate large quantities of heat while maintaining a low surface temperature difference. Flow boiling with microchannels has shown a great potential with its high surface area to volume ratio and latent heat removal. However, flow instabilities and low critical heat flux (CHF) have prevented its successful implementation. A novel flow boiling design is experimentally investigated to overcome the above-mentioned disadvantages while presenting a very low pressure drop. The design uses open microchannels with a tapered manifold (OMM) to provide stable and efficient operation. The effect of tapered manifold block with varied dimension is investigated with distilled, degassed water at atmospheric pressure. Heat transfer coefficient and pressure drop results for uniform and tapered manifolds with plain and microchannel chips are presented. The OMM configuration yielded a CHF of over 500 W/cm in our earlier work. In the current work, a heat transfer coefficient of 277.8 kW/m C was obtained using an OMM design with an inlet gap of 127 lm and an exit gap of 727 lm over a 10 mm flow length. The OMM geometry also resulted in a dramatic reduction in pressure drop from 158.4 kPa for a plain chip and 62.1 kPa for a microchannel chip with a uniform manifold, to less than 10 kPa with the tapered OMM design. A tapered manifold (inlet and exit manifold heights of 127 and 727 lm, respectively) with microchannel provided the lowest pressure drop of 3.3 kPa. [DOI: 10.1115/1.4026306]