J. Heat Transfer. 2013;135(11):110501-110501-1. doi:10.1115/1.4024626.

We take immense pleasure in taking out a special issue on recent works on high heat flux cooling of electronics. Research on high heat flux electronic cooling has gathered momentum and has been developed as a highly specialized study in the traditional as well as new and emerging areas in the field of thermal science and engineering. Papers on cutting edge research in the field of high heat flux electronic cooling have been submitted. In all, 27 papers were submitted. All the papers had undergone the stringent review process of ASME Journal of Heat Transfer standard. Finally, out of these 27 manuscripts, only 17 papers have been accepted for publication. Accepted papers exemplify the best ideas deserving archival publication in Journal of Heat Transfer.

Commentary by Dr. Valentin Fuster

Research Papers

J. Heat Transfer. 2013;135(11):111001-111001-9. doi:10.1115/1.4024594.

Thermal analysis of planar and near-square semiconductor device chips employing angular Fourier-series (AFS) expansion is presented for the first time. The determination of the device peak temperature using AFS requires only a single two-dimensional computation, while full three-dimensional temperature distribution can be obtained, if desired, by successively adding higher-order Fourier terms, each of which requires a separate 2D computation. The AFS method is used to compare the heat spreading characteristics of AlGaN/GaN high-electron-mobility transistors (HEMTs) fabricated on silicon, silicon carbide, and synthetic diamond. We show that AlGaN/GaN HEMTs built using GaN/diamond technology can offer better than half the thermal resistance of GaN/SiC HEMTs under worst-case cooling conditions. Furthermore, we show that, if left unmanaged, an inherent and non-negligible thermal boundary resistance due to the integration of semiconductor epilayers with non-native substrates will dampen the benefits of highly conductive substrates such as SiC and diamond.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(11):111002-111002-7. doi:10.1115/1.4024595.

The growing trend in miniaturization of electronics has generated a need for efficient thermal management of these devices. Boiling has the ability to dissipate a high heat flux while maintaining a small temperature difference. A vapor chamber with pool boiling offers an effective way to provide cooling and to maintain temperature uniformity. The objective of the current work is to investigate pool boiling performance of ethanol on enhanced microchannel surfaces. Ethanol is an attractive working fluid due to its better heat transfer performance and higher heat of vaporization compared to refrigerants, and lower normal boiling point compared to water. The saturation temperature of ethanol can be further reduced to temperatures suitable for electronics cooling by lowering the pressure. Experiments were performed at four different absolute pressures, 101.3 kPa, 66.7 kPa, 33.3 kPa, and 16.7 kPa using different microchannel surface configurations. Heat dissipation in excess of 900 kW/m2 was obtained while maintaining the wall surface below 85 °C at 33 kPa. Flammability, toxicity, and temperature overshoot issues need to be addressed before practical implementation of ethanol-based cooling systems can occur.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(11):111003-111003-7. doi:10.1115/1.4024596.

A new Stirling microrefrigeration system composed of arrays of silicon MEMS cooling elements has been designed and evaluated. The cooling elements are to be fabricated in a stacked array on a silicon wafer. A regenerator is placed between the compression (hot side) and expansion (cold side) diaphragms, which are driven electrostatically. Air at a pressure of 2 bar is the working fluid and is sealed in the system. Under operating conditions, the hot and cold diaphragms oscillate sinusoidally and out of phase such that heat is extracted to the expansion space and released from the compression space. Parametric study of the design shows the effects of phase lag between the hot space and cold space, swept volume ratio between the hot space and cold space, and dead volume ratio on the cooling power. Losses due to regenerator nonidealities are estimated and the effects of the operating frequency and the regenerator porosity on the cooler performance are explored. The optimal porosity for the best system coefficient of performance (COP) is identified.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(11):111004-111004-14. doi:10.1115/1.4024597.

Superhydrophobic micro/nanostructured surfaces for dropwise condensation have recently received significant attention due to their potential to enhance heat transfer performance by shedding water droplets via coalescence-induced droplet jumping at length scales below the capillary length. However, achieving optimal surface designs for such behavior requires capturing the details of transport processes that is currently lacking. While comprehensive models have been developed for flat hydrophobic surfaces, they cannot be directly applied for condensation on micro/nanostructured surfaces due to the dynamic droplet-structure interactions. In this work, we developed a unified model for dropwise condensation on superhydrophobic structured surfaces by incorporating individual droplet heat transfer, size distribution, and wetting morphology. Two droplet size distributions were developed, which are valid for droplets undergoing coalescence-induced droplet jumping, and exhibiting either a constant or variable contact angle droplet growth. Distinct emergent droplet wetting morphologies, Cassie jumping, Cassie nonjumping, or Wenzel, were determined by coupling of the structure geometry with the nucleation density and considering local energy barriers to wetting. The model results suggest a specific range of geometries (0.5–2 μm) allowing for the formation of coalescence-induced jumping droplets with a 190% overall surface heat flux enhancement over conventional flat dropwise condensing surfaces. Subsequently, the effects of four typical self-assembled monolayer promoter coatings on overall heat flux were investigated. Surfaces exhibiting coalescence-induced droplet jumping were not sensitive (<5%) to the coating wetting characteristics (contact angle hysteresis), which was in contrast to surfaces relying on gravitational droplet removal. Furthermore, flat surfaces with low promoter coating contact angle hysteresis (<2 deg) outperformed structured superhydrophobic surfaces when the length scale of the structures was above a certain size (>2 μm). This work provides a unified model for dropwise condensation on micro/nanostructured superhydrophobic surfaces and offers guidelines for the design of structured surfaces to maximize heat transfer. Keywords: superhydrophobic condensation, jumping droplets, droplet coalescence, condensation optimization, environmental scanning electron microscopy; micro/nanoscale water condensation, condensation heat transfer.

Topics: Drops , Condensation
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(11):111005-111005-9. doi:10.1115/1.4024598.

The objective of this effort is to pursue artificial microscale surface roughness features in the form of dimples, on the walls of an air-cooled heat sink channel, as a passive option to energy-efficiently augment heat transfer in forced convection flows. High fidelity numerical simulations were employed for realizing an optimized dimple configuration and to comprehend the behavior of microsized dimples under high velocity (∼17 m/s) transitional flow conditions. Fully developed flow simulations were performed, and design of experiments with response surface methodology was employed for the numerical optimization. The results showed ∼30% heat transfer improvement and ∼15% pressure drop increase in the fully developed region compared to a smooth-walled channel. Practicability of manufacturing 200 μm deep dimples on a 600 μm thin aluminum fin was demonstrated. Experiments were also carried out to assess the performance of the aforementioned optimized configuration in a custom built setup in the laboratory, which showed up to 10.5% heat transfer improvement and ∼12% pressure drop increase over a corresponding smooth-walled channel. The above results indicate that the performance of dimples is allied with the flow development characteristics. In addition, experiments performed at Reynolds numbers other than one at which the dimples were optimized showed inferior performance showing that application-specific optimization of dimples is crucial. With further exploration of shape and design parameters, dimples might have the potential to improve thermal performance passively and form an attractive candidate to realize high-performance air-cooled heat sinks in the future.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(11):111006-111006-9. doi:10.1115/1.4024611.

The computational study of heat transfer and fluid flow in a porous media cold plate was investigated using lattice Boltzmann method. The study was carried out on a heat exchanger including two adiabatic inlet and outlet conduit and three hot fins with constant temperature. The porous medium is positioned between the fins to enhance heat transfer rate. The local thermal equilibrium assumption between the fluid and solid phases and the Brinkman–Forchheimer extended Darcy equation was used to simulate the porous domain. The effect of porosity on heat transfer from the fins surfaces was studied at different Reynolds and Prandtl numbers. Results show that by decreasing the porosity, the heat transfer rate increases and the fluid bulk temperature grows at less time for different Reynolds and Prandtl numbers.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(11):111007-111007-12. doi:10.1115/1.4024612.

A nonlinear spectral approach is proposed to simulate the post critical convective state for thermogravitational instability in a Newtonian fluid layer heated from below. The spectral methodology consists of expanding the flow and temperature fields periodically along the layer, and using orthonormal shape functions in the transverse direction. The Galerkin projection is then implemented to generate the equations for the expansion coefficients. Since most of the interesting bifurcation picture is close to criticality, a perturbation approach is developed to solve the nonlinear spectral system in the weakly post critical range. To leading order, the Lorenz model is recovered. The problem is also solved using amplitude equations for comparison. The similarity and difference among the three models are emphasized.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(11):111008-111008-13. doi:10.1115/1.4024613.

In this work, the effect of vapor chamber characteristics, the properties of its working fluid and the operating parameters on the vapor chamber performance are studied. Also, the effects of these parameters on the cooling process are considered. A three dimensional hydrodynamic model is used for solving the fluid flow through the liquid and vapor regions of the vapor chamber. The hydrodynamic model is coupled with a three dimensional thermal model to calculate the model temperature. The hydrodynamic model takes into consideration the circulation of liquid between the two wick regions. An implicit finite difference method is used to solve the numerical model and a validation of the numerical model is presented. The effect of porosity of the wick material, wick structure, solid wall material, working fluid, wick region thickness, vapor region thickness, power input, and heat transfer coefficient of the cooling fluid are taken in the study. Their effects on the heat pipe temperature, pressure difference of the heat pipe, liquid and vapor velocities and mass evaporated are studied. The results show that, to increase the cooling performance of the heat pipe, the porosity, wick thickness, power input, and vapor region thickness should be decreased and the heat transfer coefficient should be increased. To minimize the maximum pressure difference of the heat pipe, increase porosity, wick thickness, and vapor thickness and decrease heat transfer coefficient and power input. The study shows that the increase of wick thickness by a factor of four decreases the maximum pressure difference by about 75% and increases the maximum vapor chamber temperature 30%. It also shows that the vapor region thickness has an insignificant effect on the vapor chamber temperature and pressure. The increase of the heat transfer coefficient of the cooling liquid decreases its effect on heat pipe performance.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(11):111009-111009-8. doi:10.1115/1.4024614.

This paper focuses on two forced convection methods—steady jet flow and pulsating flow by synthetic jets—that can be used in applications requiring significant amounts of heat removal from electronics components. Given the dearth of available data, we have experimentally investigated steady jets and piezoelectrically driven synthetic jets that provide pulsating flow of air at a high coefficient of performance. To mimic a typical electronics component, a 25.4-mm × 25.4-mm vertical heated surface was used for heat removal. The impingement heat transfer, in the form of Nusselt number, is reported for both steady and unsteady jets over Reynolds numbers from 100 to 3000. The effect of jet-to-plate surface distance on the impingement heat transfer is also investigated. Our results show that synthetic jets can provide significantly higher cooling than steady jets in the Reynolds number range of 100 to 3000. We attribute the superior performance of synthetic jets to vortex shedding associated with the unsteady flow.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(11):111010-111010-10. doi:10.1115/1.4024615.

Conductive heat transfer is of importance in the cooling of electronic equipment. However, in order for conductive cooling to become effective, the use of high-conducting materials and the correct distribution thereof is essential, especially when the volume which needs to be cooled has a low thermal conductivity. An emerging method of designing internal solid-state conductive systems by means of topology optimization is considered in this paper. In this two-dimensional study, the optimum distribution of high conductive material within a square-shaped heat-generating medium is investigated by making use of the “method or moving asymptotes” (MMA) optimization algorithm coupled with a numerical model. The use of such a method is considered for a number of cost (driving) functions and different control methods to improve the definiteness of the boundaries between the heat-generating and high-conduction regions. It is found that the cost function used may have a significant influence on the optimized material distribution. Also of interest in this paper are the influences of thermal conductivity and the proportion of the volume occupied by the high-conducting solid on the resulting internal cooling structure distribution and its thermal conduction performance. For a square domain with a small exposed isothermal boundary centered on one edge, a primary V-shaped structure was found to be predominantly the most effective layout to reduce the peak operating temperature and to allow for an increase in the internal heat flux levels.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(11):111011-111011-10. doi:10.1115/1.4024616.

The paper presents the enhancement in the operational limits (boiling, entrainment, sonic, viscous and capillary limits) of heat pipes using silver nanoparticles dispersed in de-ionized (DI) water. The tested nanoparticles concentration ranged from 0.003 vol. % to 0.009 vol. % with particle diameter of <100 nm. The nanofluid as working fluid enhances the effective thermal conductivity of heat pipe by 40%, 58%, and 70%, respectively, for volume concentrations of 0.003%, 0.006%, and 0.009%. For an input heat load of 60 W, the adiabatic vapor temperatures of nanofluid based heat pipes are reduced by 9 °C, 18 °C, and 20 °C, when compared with DI water. This reduction in the operating temperature enhances the thermophysical properties of working fluid and gives a change in the various operational limits of heat pipes. The use of silver nanoparticles with 0.009 vol. % concentration increases the capillary limit value of heat pipe by 54% when compared with DI water. This in turn improves the performance and operating range of the heat pipe.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(11):111012-111012-10. doi:10.1115/1.4024620.

Impinging jets are widely used to achieve a high local convective heat flux, with applications in high power density electronics and various other industrial fields. The heat transfer to steady impinging jets has been extensively researched, yet the understanding of pulsating impinging jets remains incomplete. Although some studies have shown a significant enhancement compared to steady jets, others have shown reductions in heat transfer rate, without consensus on the heat transfer mechanisms that determine this behavior. This study investigates the local convective heat transfer to a pulsating air jet from a long straight circular pipe nozzle impinging onto a smooth planar surface (nozzle-to-surface spacing 1 ≤ H/D ≤ 6, Reynolds numbers 6000 ≤ Re ≤ 14,000, pulsation frequency 9 Hz ≤ f ≤ 55Hz, Strouhal number 0.007 ≤ Sr = fD/Um ≤ 0.1). A different behavior is observed for the heat transfer enhancement in (i) the stagnation zone, (ii) the wall jet region and overall area average. Two different modified Strouhal numbers have been identified to scale the heat transfer enhancement in both regions: (i) Sr(H/D) and (ii) SrRe0.5. The average heat transfer rate increases by up to 75–85% for SrRe0.5 ≅ 8 (Sr = 0.1, Re = 6000), independent of nozzle-to-surface spacing. The stagnation heat transfer rate increases with nozzle-to-surface distance H/D. For H/D = 1 and low pulsation frequency (Sr < 0.025), a reduction in stagnation point heat transfer rate by 13% is observed, increasing to positive enhancements for Sr(H/D) > 0.1 up to a maximum enhancement of 48% at Sr(H/D) = 0.6.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(11):111013-111013-2. doi:10.1115/1.4024621.

In a discussion initiated by the German Research Foundation (DFG) about cooling of electronics, two aspects turned out to be important: The need for a paradigm shift from an “add on” to an “integrated multidisciplinary” solution and the definition of generic demonstrators for cooling strategies.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(11):111014-111014-10. doi:10.1115/1.4024622.

This study characterizes the pool boiling performance of HFO-1234yf (hydrofluoroolefin 2,3,3,3-tetrafluoropropene). HFO-1234yf is a new, environmentally friendly refrigerant likely to replace HFC-134a in automotive air-conditioning systems. Pool boiling experiments were conducted at system pressures ranging from 0.7 to 1.7 MPa using horizontally oriented 1-cm2 heated surfaces. Test results for pure (oil-free) HFO-1234yf and HFC-134a were compared. The results showed that the boiling heat transfer coefficients of HFO-1234yf and HFC-134a were nearly identical at lower heat fluxes. HFO-1234yf yielded lower heat transfer coefficients at higher heat fluxes and lower critical heat flux (CHF) values as compared with HFC-134a. To enhance boiling heat transfer, a copper microporous coating was applied to the test surfaces. The coating enhanced both the boiling heat transfer coefficients and CHF for both refrigerants at all tested pressures. Increasing pressure decreased the level of heat transfer coefficient enhancements and increased the level of CHF enhancements. The experimental data were then used to develop a correlation for predicting the CHF for a smooth/plain heated surface.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(11):111015-111015-9. doi:10.1115/1.4024623.

The semiconductor industry, following Moore's law, has consistently followed a trajectory of miniaturization that enables design engineers to achieve greater levels of innovation in the same or smaller die footprints. According to Samsung technologists, the next generation of semiconductor technology will cost about $10 billion to create. Alternatively, improved performance through lowering of signal delays can also be achieved using stacked or 3D packaging. With this architectural achievement come cooling challenges as it is difficult to utilize conventional cooling technology and especially when stacking logic and memory processors for high end applications. The accumulation of excessive heat within the stack is a challenge that has caused thermal engineers to focus on the issue of extracting this heat from the system. Thus, one important aspect of design is the ability to obtain an accurate analytical temperature solution of the multilayer stack packages beforehand in order to sustain the reliability of the 3D stack packages albeit for a more simplified configuration. This study addresses the analytical solution of temperature distribution in multilayer bodies by using the Mathematica code developed in this study. The numerical approach using ansys Workbench is discussed, and the results are compared with the one obtained analytically.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(11):111016-111016-13. doi:10.1115/1.4024624.

Two-phase flow instabilities in microchannels exhibit pressure and temperature fluctuations with different frequencies and amplitudes. An active way to suppress the dynamic instabilities in the boiling microchannels is to introduce synthetic jets into the channel fluid. Thus, the bubbles can be condensed before they clog the channel and expand upstream causing flow reversal. The present work experimentally investigated the effects of synthetic jets on microchannel flow boiling. An array of synthetic jets was introduced into the microchannel flow. The strength and frequency of the jets were controlled by changing the driving signals of the piezoelectric driven jet actuator. It is found that the bubbles were effectively condensed inside the jet cavity. The boiling flow reversals were notably delayed by the synthetic jets. Meanwhile, the pressure fluctuation amplitudes were suppressed to some extent. It was also observed that synthetic jets can help to uniformize the heat sink temperature distribution.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(11):111017-111017-13. doi:10.1115/1.4024625.

This work presents the single- and two-phase results of a parametric study investigating the performance of oblique jet arrays impinging at 45 deg on a 3.63 cm2 square copper heater surface using R-245fa. It was found that the parameters that most impact heat transfer changed as the system progressed from single- to two-phase flow behavior. The single-phase performance was governed by the jet geometry and the volumetric flow rate, while in the two-phase region, heat transfer performance was primarily affected by the fluid conditions and the heat flux applied. A single-phase correlation was developed to capture the low heat flux response, and the two-phase results were well-correlated by a pool boiling correlation. A new general correlation for jet impingement heat transfer with phase change is presented combining these correlations. Critical heat flux (CHF) data were compared with literature correlations and a new correlation was developed for arrays of boiling jets.

Commentary by Dr. Valentin Fuster

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