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Guest Editorial

J. Heat Transfer. 2011;133(6):060301-060301-1. doi:10.1115/1.4003447.
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This special issue of the Journal of Heat Transfer contains six papers selected out of the 53 presented at the Second International Conference on Thermal Issues in Emerging Technologies, Theory and Applications (ThETA2) held in Cairo, Egypt in December 17–20, 2008. Emerging technologies in various domains, including microelectronics, nanoscience and engineering, smart materials, microelectromechanical systems, biomedical engineering, and novel energy conversion approaches present many challenging and interesting thermal effects and interactions. In many applications, such effects become a dominant factor in determining the overall performance or response. ThETA2 theme topics included micro- and nanoscale heat transfer, modeling of multiscale heat transfer problems, thermal modeling of electronic systems, temperature aware computer systems design, cooling of electronic systems and data centers, compact thermal models, thermomechanical effects, MEMS—multiphysics problems, new and renewable energies, solid-state energy generation/cooling, fuel cells, energy conservation, energy—buildings—environment, multiphase flow with heat transfer, thermal issues in biomedical engineering, thermal issues in microfabrication technology, thermal issues in new materials, computational methods in heat transfer, and advanced experimental methods in heat transfer.

Commentary by Dr. Valentin Fuster

THERMAL ISSUES IN EMERGING TECHNOLOGIES

J. Heat Transfer. 2011;133(6):060901-060901-6. doi:10.1115/1.4003448.

To reduce particulate matters including soot, a diesel particulate filter (DPF) has been developed for the after-treatment of exhaust gas. Since the filter is plugged with particles that would cause an increase of filter back-pressure, filter regeneration process is needed. However, there is not enough data on the phenomena in DPF because there are many difficulties in measurements. In this study, the flow in DPF is simulated by the lattice Boltzmann method. To focus on a real filter, the inner structure of the filter is scanned by a 3D X-ray computed tomography technique. By conducting tomography-assisted simulation, the local velocity and pressure distributions in the filter can be visualized, which is hardly obtained by measurements. Results show that, even in cold flow, the complex flow pattern is observed due to the nonuniformity of pore structure inside the filter. Based on the flow characteristics in the range of 0.2–20 m/s, simulation results show a good agreement with the empirical equation of Ergun equation. In the combustion simulation, the time-dependent temperature field inside the filter is visualized. As the temperature of inflow gas is increased, the filter regeneration process is promoted.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2011;133(6):060902-060902-9. doi:10.1115/1.4003449.

Highly conductive porous media have recently been considered for enhanced cooling applications due to their large internal contact surface area, which promotes convection at the pore level. In this paper, graphite foams that possess high thermal conductivity but low permeability are investigated for convection heat transfer enhancement using air as coolant. Two novel heat sink structures are designed to reduce the fluid pressure drop. Both experimental and numerical approaches are adopted in the study. The experimental data show that the designed structures significantly reduce flow resistance in graphite foams while maintaining relatively good heat removal performance. The numerical results obtained based on the local thermal nonequilibrium model are validated by experimental data and show that the inlet air flow partially penetrates the structured foam walls, while the remaining air flows tortuously through slots in the structure. Flow mixing, which is absent in the block graphite foam, is observed in the freestream area inside the designed structure. It can be concluded that graphite foams with appropriately designed structures can be applied as air-cooled heat sinks in thermal management applications.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2011;133(6):060903-060903-8. doi:10.1115/1.4003450.

The aim of our study was to elucidate the fundamental cooling characteristics and to improve the cooling characteristics of a room-temperature magnetic refrigerator operated under an active magnetic regenerator (AMR) cycle. The AMR refrigeration cycle, which includes a thermal storage process and a regeneration process, is used to realize a practical magnetic refrigerator operating near room-temperature. The basic components of the target AMR system are a magnetic circuit, test section, fluid-displacing device, and associated instrumentation. Spherical gadolinium particles are packed in the test section as the magnetic working substance, and air and water are used as heat transfer fluids. The cooling characteristics of the target AMR system under various operating conditions are investigated. The results show that the AMR cycle is very effective in improving the cooling performance of the room-temperature magnetic refrigerator.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2011;133(6):060904-060904-9. doi:10.1115/1.4003451.

Because of their interesting heat transfer and mechanical properties, metal foams have been proposed for several different applications, thermal and structural. This paper aims at pointing out the effective thermal fluid dynamic behavior of these new enhanced surfaces, which present high heat transfer area per unit of volume at the expense of high pressure drop. The paper presents the experimental heat transfer and pressure drop measurements relative to air flowing in forced convection through four different aluminum foams, when electrically heated. The tested aluminum foams present 5, 10, 20 and 40 PPI (pores per inch), porosity around 0.92–0.93, and 0.02 m of foam core height. The experimental heat transfer coefficients and pressure drops have been obtained by varying the air mass flow rate and the electrical power, which has been set at 25.0kWm2, 32.5kWm2, and 40.0kWm2. The results have been compared against those measured for 40 mm high samples, in order to study the effects of the foam core height on the heat transfer. Moreover, predictions from two recent models are compared with heat transfer coefficient and pressure drop experimental data. The predictions are in good agreement with experimental data.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2011;133(6):060905-060905-9. doi:10.1115/1.4003452.

Large progress has been realized in modeling conduction heat transfer problems over the past decade by the introduction of high performance compact thermal models (CTMs) mainly developed for thermal design of complicated electronic systems. The objective of this paper is to generalize these advances to convective heat transfer. A new convective CTM is proposed, which offers many advantages over the traditional approach using the heat transfer coefficient (HTC). The latter is simply a zeroth order CTM. The HTC is quite handy and simple, but with unpredictable errors. It can be suitable for hand calculations of simple systems giving rather crude estimates. For a higher precision, users have no other option than time consuming 3D simulations. For large systems, in terms of number of components, 3D simulations can be rapidly impractical. The CTM bridges the gap between both approaches going gradually from “HTC” levels (low precision and calculations time) at the zeroth order, to 3D simulation precision and computing time levels at large orders. Fortunately, like for conduction, a CTM of order of few tens quickly approaches 3D simulation precision levels, while keeping computation time significantly lower than 3D simulation. Moreover, the CTM approach solves conjugate heat transfer problems in a quite elegant way. A “black box” model, developed for fluid domain alone, can be easily combined with classical CTM conduction models to generate good precision predictions for any combination of fluid/solid domains.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2011;133(6):060906-060906-14. doi:10.1115/1.4003512.

This paper reviews the microscale transport processes that arise in the fabrication of advanced materials. In many cases, the dimensions of the device being fabricated are in the micrometer length scale and, in others, underlying transformations that determine product quality and characteristics are at micro- or nanoscale levels. The basic considerations in these transport phenomena are outlined. A few important materials processing circumstances are considered in detail. These include the fabrication of multilayer and hollow optical fibers, as well as those where micro- and nanoscale dopants are added to achieve desired optical characteristics, thin film fabrication by chemical vapor deposition, and microscale coating of fibers and devices. It is shown that major challenges are posed by the simulation and experimentation, as compared with those for engineering or macroscale dimensions. These include accurate simulation to capture large gradients and variations over relatively small dimensions, simulating high pressures and viscous dissipation effects in microchannels, modeling effects such as surface tension that become dominant at microscale dimensions, and coupling micro- and nanoscale mechanisms with boundary conditions imposed at the macroscale. Similarly, measurements over microscale dimensions are much more involved than those over macro- or industrial scales because of difficult access to the regions of interest, relatively small effects such as tension, buoyancy effects, viscous rupture, bubble entrapment, and other mechanisms that are difficult to measure and that can make the process infeasible. It thus becomes difficult to achieve desired accuracy for validating the mathematical and numerical models. This paper reviews some of the approaches that have been adopted to overcome these difficulties. Comparisons between experimental and numerical results are included to show fairly good agreement, indicating the validity of the modeling of transport.

Commentary by Dr. Valentin Fuster

Research Papers: Evaporation, Boiling, and Condensation

J. Heat Transfer. 2011;133(6):061501-061501-11. doi:10.1115/1.4003433.

A semi-empirical model describing the heat transfer characteristics of the pseudo-dropwise condensation of binary vapor on a cooled vertical tube has been formulated. By ignoring the thin film always present on the condensation surface and the intensification of mass transfer caused by the Marangoni effect, the heat transfer characteristics of pseudo-dropwise condensation are tentatively formulated. The model involved an analysis of the diffusion process in the vapor boundary layer along with the heat transfer process through the condensate drops. This model was applied to the condensation of the saturated binary vapor of ethanol and water, and was examined using experimental data at vapor pressure values of 101.33 kPa (provided by Utaka and Wang, 2004, “Characteristic Curves and the Promotion Effect of Ethanol Addition on Steam Condensation Heat Transfer,” Int. J. Heat Mass Transfer, 47, pp. 4507–4516), 84.52 kPa and 47.36 kPa. Calculations using the model show a similar trend to the experimental measurements. With the change of the vapor-to-surface temperature difference, the heat transfer coefficients revealed nonlinear characteristics, with the peak values under all ethanol mass fractions of binary vapor. The heat transfer coefficients increased with decreasing ethanol mass fraction.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2011;133(6):061502-061502-5. doi:10.1115/1.4003546.

A visual and thermal experimental investigation of four oscillating heat pipes (OHPs) was conducted to observe fluid flow of liquid plugs and vapor bubbles in the OHP and its effect on the temperature distribution and heat transfer performance in an OHP. These four OHPs consist of an open loop water OHP, an open loop acetone OHP, a closed loop water OHP, and a closed loop acetone OHP. These copper OHPs were constructed identically with all six turns in the same plane. They were constructed out of 1.65 mm inner diameter copper tubing and copper heat spreading plates in the evaporator and condenser regions. The heat pipes were charged at a filling ratio of about 50%. The results show that the acetone OHP at low power performs better than the water OHP, while at high power the water OHP exceeds the acetone OHP. The experimental results show that both the acetone and water closed loop OHPs had reduced movement in the connecting turn between the two sides. However, in the water closed loop OHP, this prevented circulation altogether. Comparing the water closed loop OHP to the water open loop OHP, their flow patterns were similar. Therefore, improving the flow in this turn should increase the closed loop OHP’s performance.

Commentary by Dr. Valentin Fuster

Research Papers: Experimental Techniques

J. Heat Transfer. 2011;133(6):061601-061601-8. doi:10.1115/1.4003548.

Nanocomposites offer unique capabilities of controlling thermal transport through the manipulation of various structural aspects of the material. However, measurements of the thermal properties of these composites are often difficult, especially porous nanomaterials. Optical measurements of these properties, although ideal due to the noncontact nature, are challenging due to the large surface variability of nanoporous structures. In this work, we use a vector-based thermal algorithm to solve for the temperature change and heat transfer in which a thin film subjected to a modulated heat source is sandwiched between two thermally conductive pathways. We validate our solution with time domain thermoreflectance measurements on glass slides and extend the thermal conductivity measurements to SiO2-based nanostructured films.

Commentary by Dr. Valentin Fuster

Research Papers: Forced Convection

J. Heat Transfer. 2011;133(6):061701-061701-9. doi:10.1115/1.4003547.

A numerical and experimental investigation is undertaken for developing laminar flow in a duct with one opaque, uniformly heated wall and one transparent wall. In the numerical model, mixed convection, radiative exchange, as well as two-dimensional conduction in the substrate are considered. Experiments are conducted in a high-aspect-ratio rectangular channel using infrared thermography to validate the numerical model and visualize the temperature field on a heated surface. An extended parametric study using the validated model is also carried out to assess the impact of channel height, and thermal conductivity and thickness of the substrate. For a channel height of H=6mm and a heating power of qs=257W/m2, as Re increases from 150 to 940 the fraction of heat transfer by convection from the heated surface rises from 65% to 79%. At Re=150, as H increases from 6 mm to 25 mm, radiation from the heated surface increases from 35% to 70% of the total heating power. The influence of substrate conductivity and thickness on local flux distributions is limited to regions near the channel inlet and outlet. Over the entire parametric space considered, radiation loss from the interior duct surfaces to the inlet and outlet apertures is less than 2% of the total heat input and thus unimportant.

Commentary by Dr. Valentin Fuster

Research Papers: Heat Transfer Enhancement

J. Heat Transfer. 2011;133(6):061901-061901-11. doi:10.1115/1.4003555.

The transient thermal response of a sandwich panel with pin-fin core subjected to nonuniform impinging jet heating was investigated theoretically and experimentally. Forced convection flow passing through the sandwich channel was employed to remove heat imposed nonuniformly on the pin-fin sandwich. A semi-empirical model was developed to predict the transient thermal fields in the front and back facesheets of the sandwich, in the pin-fins, and in the forced convective flow. Transient heat transfer measurements were conducted to validate the model with hot air impinging jet heating. The temperature history of the sandwich was predicted under two different boundary conditions: (1) continuous and (2) cyclic heating from a flame impinging jet. Heating by the flame impinging jet was modeled by prescribing heat flux distribution expressed with an exponential function. For continuous heating, systematic parametrical studies were carried out to examine the effects of convection Reynolds number, fin pitch, fin thickness, and facesheet thickness on the maximum facesheet temperature. For cyclic heating, the thermal performance of the sandwich as a function of heat flux intensity was quantified. It was demonstrated that pin-fin sandwiches are capable of thermally managing nonuniformly distributed heat fluxes having high intensities, either continuously or cyclically imposed.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2011;133(6):061902-061902-9. doi:10.1115/1.4003558.

The blade tip region encounters high thermal loads because of the hot gas leakage flows, and it must therefore be cooled to ensure a long durability and safe operation. A common way to cool a blade tip is to design serpentine passages with a 180 deg turns under the blade tip-cap inside the turbine blade. Improved internal convective cooling is therefore required to increase blade tip lifetime. Pins, dimples, and protrusions are well recognized as effective devices to augment heat transfer in various applications. In this paper, enhanced heat transfer of an internal blade tip-wall has been predicted numerically. The computational models consist of a two-pass channel with 180 deg turn and arrays of circular pins, hemispherical dimples, or protrusions internally mounted on the tip-wall. Inlet Reynolds numbers are ranging from 100,000 to 600,000. The overall performance of the two-pass channels is evaluated. Numerical results show that the heat transfer enhancement of the pinned-tip is up to a factor of 3.0 higher than that of a smooth tip while the dimpled-tip and protruded-tip provide about 2.0 times higher heat transfer. These augmentations are achieved at the cost of an increase of pressure drop by less than 10%. By comparing the present cooling concepts with pins, dimples, and protrusions, it is shown that the pinned-tip exhibits best performance to improve the blade tip cooling. However, when disregarding the added active area and considering the added mechanical stress, it is suggested that the usage of dimples is more suitable to enhance blade tip cooling, especially at low Reynolds numbers.

Commentary by Dr. Valentin Fuster

Research Papers: Melting and Solidification

J. Heat Transfer. 2011;133(6):062301-062301-7. doi:10.1115/1.4003542.

The heat conduction problem, in the presence of a change of state, was solved for the case of an indefinitely long cylindrical layer cavity. As boundary conditions, it is imposed that the internal surface of the cavity is maintained below the fusion temperature of the infilling substance and the external surface is kept above it. The solution, obtained in nondimensional variables, consists in two closed form heat conduction equation solutions for the solidified and liquid regions, which formally depend of the, at first, unknown position of the phase change front. The energy balance through the phase change front furnishes the equation for time dependence of the front position, which is numerically solved. Substitution of the front position for a particular instant in the heat conduction equation solutions gives the temperature distribution inside the cavity at that moment. The solution is illustrated with numerical examples.

Commentary by Dr. Valentin Fuster

Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2011;133(6):062401-062401-11. doi:10.1115/1.4003549.

Continued reduction in characteristic dimensions in nanosystems has given rise to increasing importance of material interfaces on the overall system performance. With regard to thermal transport, this increases the need for a better fundamental understanding of the processes affecting interfacial thermal transport, as characterized by the thermal boundary conductance. When thermal boundary conductance is driven by phononic scattering events, accurate predictions of interfacial transport must account for anharmonic phononic coupling as this affects the thermal transmission. In this paper, a new model for phononic thermal boundary conductance is developed that takes into account anharmonic coupling, or inelastic scattering events, at the interface between two materials. Previous models for thermal boundary conductance are first reviewed, including the diffuse mismatch model, which only considers elastic phonon scattering events, and earlier attempts to account for inelastic phonon scattering, namely, the maximum transmission model and the higher harmonic inelastic model. A new model is derived, the anharmonic inelastic model, which provides a more physical consideration of the effects of inelastic scattering on thermal boundary conductance. This is accomplished by considering specific ranges of phonon frequency interactions and phonon number density conservation. Thus, this model considers the contributions of anharmonic, inelastically scattered phonons to thermal boundary conductance. This new anharmonic inelastic model shows improved agreement between the thermal boundary conductance predictions and experimental data at the Pb/diamond and Au/diamond interfaces due to its ability to account for the temperature dependent changing phonon population in diamond, which can couple anharmonically with multiple phonons in Pb and Au. We conclude by discussing phonon scattering selection rules at interfaces and the probability of occurrence of these higher order anharmonic interfacial phonon processes quantified in this work.

Commentary by Dr. Valentin Fuster

Research Papers: Natural and Mixed Convection

J. Heat Transfer. 2011;133(6):062501-062501-9. doi:10.1115/1.4003534.

Lattice Boltzmann method (LBM) is employed to investigate natural convection inside porous medium enclosures at high Rayleigh numbers. Volume averaged porous medium model is coupled with the lattice Boltzmann formulation of the momentum and energy equations for fluid flow. A parallel implementation of the single relaxation time LBM is used, which allows the porous medium modified Rayleigh number Ram to be as high as 108. Heat transfer results in the form of the enclosure averaged Nusselt number Nu are obtained for higher modified Rayleigh numbers 104Ram108. The Nu values are compared with values in the absence of the form drag term. The form drag due to the porous medium is found to influence Nu considerably. The effect of the form drag on Nu is studied by using a form drag modified Rayleigh number RaC (extended from Ram). Utilizing the results for Nu in the high Ram range, a correlation is proposed between Nu and RaC for Darcy numbers 106Da102, encompassing the non-Darcy flow regime.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2011;133(6):062502-062502-8. doi:10.1115/1.4003535.

Microwave heating of a porous medium with a nonuniform porosity is numerically investigated based on a proposed numerical model. A two-dimensional variation of porosity of the medium is considered. The generalized non-Darcian model developed takes into account the presence of a solid drag and the inertial effect. The transient Maxwell’s equations are solved by using the finite difference time domain method to describe the electromagnetic field in the waveguide and medium. The temperature profile and velocity field within a medium are determined by solution of the momentum, energy, and Maxwell’s equations. The coupled nonlinear set of these equations is solved using the SIMPLE algorithm. In this work, a detailed parametric study is conducted on heat transport inside a rectangular enclosure filled with a saturated porous medium of constant or variable porosity. The numerical results agree well with the experimental data. Variations in porosity significantly affect the microwave heating process as well as the convective flow pattern driven by microwave energy.

Commentary by Dr. Valentin Fuster

Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2011;133(6):062701-062701-10. doi:10.1115/1.4003552.

Radiation exchanges must be taken into account to improve large eddy simulation (LES) prediction of turbulent combustion, in particular, for wall heat fluxes. Because of its interaction with turbulence and its impact on the formation of polluting species, unsteady coupled calculations are required. This work constitutes a first step toward coupled LES-radiation simulations, selecting the optimal methodology based on systematic comparisons of accuracy and CPU cost. Radiation is solved with the discrete ordinate method (DOM) and different spectral models. To reach the best compromise between accuracy and CPU time, the performance of various spectral models and discretizations (angular, temporal, and spatial) is studied. It is shown that the use of a global spectral model combined with a mesh coarsening (compared with the LES mesh) and a minimal coupling frequency Nit allows to compute one radiative solution faster than Nit LES iterations while keeping a good accuracy. It also appears that the impact on accuracy of the angular discretization in the DOM is very small compared with the impact of the spectral model. The determined optimal methodology may be used to perform unsteady coupled calculations of turbulent combustion with radiation.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Heat Transfer. 2011;133(6):064501-064501-7. doi:10.1115/1.4003532.

The necessity for an efficient thermal management system covering large areas is growing rapidly with the push toward more electric systems. A significant amount of research over the past 2 decades has conclusively proved the suitability of jet, droplet, or spray impingement for high heat flux cooling. However, all these research consider small heat source areas, typically about a few cm2. Can a large array of impingement pattern, covering a much wider area, achieve similar heat flux levels? This article presents liquid microjet array impingement cooling of a heat source that is about two orders of magnitude larger than studied in the previous works. Experiments are carried out with 441 jets of de-ionized water and a dielectric liquid HFE7200, each 200μm diameter. The jets impinge on a 189cm2 area surface, in free surface and confined jet configurations. The average heat transfer coefficient values of the present experiment are compared with correlations from the literature. While some correlations show excellent agreement, others deviate significantly. The ensuing discussion suggests that the post-impingement liquid dynamics, particularly the collision between the liquid fronts on the surface created from surrounding jets, is the most important criterion dictating the average heat transfer coefficient. Thus, similar thermal performance can be achieved, irrespective of the length scale, as long as the flow dynamics are similar. These results prove the scalability of the liquid microjet array impingement technique for cooling a few cm2 area to a few hundred cm2 area.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2011;133(6):064502-064502-10. doi:10.1115/1.4003155.

Fundamental problem of heat transfer within a half-space due to a moving heat source of hyperelliptical geometry is studied in this work. The considered hyperelliptical geometry family covers a wide range of heat source shapes, including star-shaped, rhombic, elliptical, rectangular with round corners, rectangular, circular, and square. The effects of the heat source speed, aspect ratio, corners, and orientation are investigated using the general solution of a moving point source on a half-space and superposition. Selecting the square root of the heat source area as the characteristics length scale, it is shown that the maximum temperature within the half-space is a function of the heat source speed (Peclet number) and its aspect ratio. It is observed that the details of the exact heat source shape have negligible effect on the maximum temperature within the half-space. New general compact relationships are introduced that can predict the maximum temperature within the half-space with reasonable accuracy. The validity of the suggested relationships is examined by available experimental and numerical data for the grinding process, for medium Peclet numbers. For ultrafast heat sources, an independent experimental study is performed using a commercial CO2 laser system. The measured depth of the engraved grooves is successfully predicted by the proposed relationships.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2011;133(6):064503-064503-5. doi:10.1115/1.4003434.

In this work a three-dimensional fluid flow and heat transfer model was developed to predict the flow pattern and superheat dissipation in funnel shaped mold of a thin slab continuous caster with a novel tetrafurcated design for the submerged entry nozzle. Low Reynolds kε turbulent model was adopted to account for the turbulent effect. The transport equations were solved numerically using finite volume method. The results were compared with a full scale water model of the caster. Good agreement between mathematical and physical models was obtained. Parametric studies were carried out to evaluate the effect of casting speed, nozzle submergence depth, and inlet temperature on the superheat dissipation, flow pattern, and surface turbulence in the mold region. The results indicate a special flow pattern and heat distribution in the caster while using a tetrafurcated nozzle. Aiming to achieve more product capacity, in the case of casting with lower superheat temperature, a higher casting speed, together with higher submergence depth, is recommended in order to avoid surface turbulence and high heat flux across the narrow face.

Commentary by Dr. Valentin Fuster

Errata

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