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IN THIS ISSUE

### Guest Editorial

J. Heat Transfer. 2015;137(9):090301-090301-1. doi:10.1115/1.4030244.

The 4th ASME Micro/Nanoscale Heat and Mass Transfer International Conference (MNHMT-2013) was held at The University of Hong Kong, Hong Kong, during December 11–14, 2013 (http://www.asmeconferences.org/MNHMT2013/). It is part of a series of conferences and follows the first three conferences, which were held in Tainan (January 2008), Shanghai (December 2009), and Atlanta (March 2012). This conference series is dedicated to Dr. Chang-Lin Tien (1935–2002), a world renowned scholar and a leader in higher education, whose intellect and unique vision have continued to inspire efforts in expanding the frontiers of micro/nanoscale heat and mass transfer. The conference was co-sponsored by the ASME Heat Transfer Division and The University of Hong Kong and was organized by Yogesh Jaluria as the Conference Chair, Liqiu Wang as the Program Chair, and Chiu-On Ng as the Local Organizing Committee Chair. The conference also had financial support from NSF for participant travel from USA. Other committees included the International Advisory Committee and the Technical Program Committee, which included researchers in the field of micro/nanoscale heat and mass transfer from around the world. Dr. Chun-Ying Leung, the Chief Executive of the Hong Kong Special Administrative Region, wrote a special message for the conference.

Commentary by Dr. Valentin Fuster

### Review Article

J. Heat Transfer. 2015;137(9):090801-090801-12. doi:10.1115/1.4030239.

In this paper, numerical simulation approaches for multiscale process of heat transfer and fluid flow are briefly reviewed, and the existing coupling algorithms are summarized. These molecular dynamics simulation (MDS)–finite volume method (FVM), MD–lattice Boltzmann method (LBM), and direct simulation of Monte Carlo method (DSMC)–FVM. The available reconstruction operators for LBM–FVM coupling are introduced. Four multiscale examples for fluid flow and heat transfer are presented by using these coupled methods. It is shown that by coupled method different resolution requirements in the computational domain can be satisfied successfully while computational time can be significantly saved. Further research needs for the study of multiscale heat transfer and fluid flow problems are proposed.

Commentary by Dr. Valentin Fuster

### Research Papers

J. Heat Transfer. 2015;137(9):091001-091001-6. doi:10.1115/1.4030200.

The wetting kinetics of a water nanodroplet undergoing evaporation on a heated gold substrate were examined using molecular dynamics (MD) simulations. Various substrate and initial droplet temperatures were used to obtain different evaporation rates. The water molecule absorption–desorption behavior was analyzed in the vicinity of the contact line region to show the microscopic details of the spreading–evaporating droplet. Increasing substrate temperatures greatly affected the dynamic wetting process, while the initial water droplet temperature had very little effect. The effects of droplet size and substrate wettability on the droplet spreading–evaporating process were also examined. The radius versus time curves agree well with molecular kinetics theory (MKT) for spreading without evaporation but differ from MKT when the spreading induced evaporation. The enhancement of the wetting kinetics by the evaporation can be attributed to the reduction of the liquid–vapor surface tension and the increased water molecule motion in the contact line region and in the bulk droplet.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091002-091002-8. doi:10.1115/1.4030205.

A multiscale modeling of the anisotropic slip phenomenon for gas flows is presented in a tree-step approach: determination of the gas–wall potential, simulation and modeling of the gas–wall collisions, simulation and modeling of the anisotropic slip effects. The density functional theory (DFT) is used to examine the interaction between the Pt–Ar gas–wall couple. This potential is then passed into molecular dynamics (MD) simulations of beam scattering experiments in order to calculate accommodation coefficients. These coefficients enter in an effective gas–wall interaction model, which is the base of efficient MD simulations of gas flows between anisotropic surfaces. The slip effects are quantified numerically and compared with simplified theoretical models derived in this paper. The paper demonstrates that the DFT potential is in good agreement with empirical potentials and that an extension of the Maxwell model can describe anisotropic slip effects due to surface roughness, provided that two tangential accommodation parameters are introduced. MD data show excellent agreement with the tensorial slip theory, except at large Kundsen numbers (for example, Kn $≃$0.2) and with an analytical expression which predicts the ratio between transverse and longitudinal slip velocity components.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091003-091003-10. doi:10.1115/1.4030208.

Analyses of microchannel and microgap cooling show that galinstan, a recently developed nontoxic liquid metal that melts at −19 °C, may be more effective than water for direct liquid cooling of electronics. The thermal conductivity of galinstan is nearly 28 times that of water. However, since the volumetric specific heat of galinstan is about half that of water and its viscosity is 2.5 times that of water, caloric, rather than convective, resistance is dominant. We analytically investigate the effect of using structured surfaces (SSs) to reduce the overall thermal resistance of galinstan-based microgap cooling in the laminar flow regime. Significantly, the high surface tension of galinstan, i.e., 7 times that of water, implies that it can be stable in the nonwetting Cassie state at the requisite pressure differences for driving flow through microgaps. The flow over the SS encounters a limited liquid–solid contact area and a low viscosity gas layer interposed between the channel walls and galinstan. Consequent reductions in friction factor result in decreased caloric resistance, but accompanying reductions in Nusselt number increase convective resistance. These are accounted for by expressions in the literature for apparent hydrodynamic and thermal slip. We develop a dimensionless expression to evaluate the tradeoff between the pressure stability of the liquid–solid–gas system and hydrodynamic slip. We also consider secondary effects including entrance effects and temperature dependence of thermophysical properties. Results show that the addition of SSs enhances heat transfer.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091004-091004-9. doi:10.1115/1.4030211.

The necessity of conserving both scattered energy and asymmetry factor for ballistic incidence after finite volume method (FVM) or discrete-ordinates method (DOM) discretization is shown. A phase-function normalization technique introduced previously by the present authors is applied to scattering of ballistic incidence in 3D FVM/DOM to improve treatment of anisotropic scattering through reduction of angular false scattering errors. Ultrafast radiative transfer predictions generated using FVM and DOM are compared to benchmark Monte Carlo to illustrate the necessity of ballistic phase-function normalization. Proper ballistic phase-function treatment greatly improves predicted heat fluxes and energy deposition for anisotropic scattering and for situations where accurate numerical modeling is crucial.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091005-091005-8. doi:10.1115/1.4030213.

Bone has very different thermal and electrical properties with the surrounding tissues. Misjustification of the heating dosage during an electromagnetic (EM) hyperthermia may lead to the failure of the treatment. Here aiming to disclose such clinically important issue, the present study presented a theoretical evaluation on the heating effects of magnetic-nanoparticles (MNPs) enhanced hyperthermia on the liver tumor underneath the ribs with bone features particularly addressed. The results revealed the following factors: (1) The existence of bone structure, i.e., ribs has an inevitable effect on the distribution of EM field; specifically, due to its lower dielectric property, the bone structure served as a barrier to attenuate the transport of EM energy and conversion of heat into the tissues, especially the tumor in the deep body. (2) Applying higher dosage or larger size MNPs would significantly enhance the temperature elevation at the target tumor tissues and thereby guarantee the performance of the hyperthermia. (3) Further parametric studies indicated that a higher frequency EM field would result in a worse heating effect; while stronger EM field will evidently enhance the heating effects of the hyperthermia process. This study promoted the better understanding of the EM heating on the bone structured tissues, and the findings are expected to provide valuable reference for planning an accurate surgery in future clinical liver tumor EM ablation.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091006-091006-9. doi:10.1115/1.4030217.

Boiling heat transfer enhancement via compound effect of the electrohydrodynamic (EHD) effect and microstructured surfaces has been experimentally and analytically investigated. A fluorinated dielectric liquid (Asahi Glass Co., Ltd., AE-3000) was selected as the working fluid. Pool boiling heat transfer in the saturated liquid was measured at atmospheric pressure. Microstructured surfaces, which are mainly used for cutting tools, were developed with diamond particles using electrodeposition technique. Four different particle diameters were prepared: 5, 10, 15, and a mixture of 5 and 1.5 μm. The critical heat flux (CHF) for diamond particle surfaces showed 27–30 W/cm2 which was 26–40% increase for comparing with a noncoated surface (21.5 W/cm2). Upon application of a −5 kV/mm electric field to the microstructured surface (a mixture of 5 and 1.5 μm particles), a CHF of 70.2 W/cm2 at a superheat of 21.7 K was obtained. The previous theoretical equation of pool boiling predicted the CHF with electric field and without the electrode within 10%. Also, the CHF enhanced by the diamond coated surfaces was correlated well with the contact angle.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091007-091007-8. doi:10.1115/1.4030218.

In this work, an experimental heat transfer investigation was carried out to investigate the combined influence of both amorphous carbon (a-C) layer thickness and carbon nanofibers (CNFs) on the convective heat transfer behavior. Synthesis of these carbon nanostructures was achieved using catalytic chemical vapor deposition process on a 50 μm nickel wire at 650 °C. Due to their extremely high thermal conductivity, CNFs are used to augment/modify heat transfer surface. However, the inevitable layer of a-C that occurs during the synthesis of the CNFs layer exhibits low thermal conductivity which may result in insulating the surface. In contrast, the amorphous layer helps in supporting and mechanically stabilizing the CNFs layer attachment to the polycrystalline nickel (Ni270) substrate material. To better understand the influences of these two layers on heat transfer, the growth mechanism of the CNFs layer and the layer of carbon is investigated and growth model is proposed. The combined impact of both a-C and CNFs layers on heat transfer performance is studied on three different samples which were synthesized by varying the deposition period (16 min, 23 min, and 30 min). The microwire samples covered with CNF layers were subjected to a uniform flow from a nozzle. Heat transfer measurement was achieved by a controlled heat dissipation through the microwire to attain a constant temperature during the flow. This measurement technique is adopted from hot wire anemometry calibration method. Maximum heat transfer enhancement of 18% was achieved. This enhancement is mainly attributed to the surface roughness and surface area increase of the samples with moderate CNFs surface area coverage on the sample.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091008-091008-6. doi:10.1115/1.4030219.

Enhancement in carbon dioxide absorption in water has been studied using SiO2 and TiO2 nanoparticles using the capillary tube apparatus for which previous results on Fe3O4 nanoparticles were reported earlier. Enhancements of up to 165% in the mass transfer coefficients were observed at fairly low volume fractions of the particles. A model which accounts for the effect of particles in terms of a superimposed convection has been proposed to explain the observed effects of particle size, hold-up, and material density. The model provides a good fit to the data from wetted wall column and capillary tube experiment for Fe3O4 from the previous literature, as well as for the data from this work.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091009-091009-9. doi:10.1115/1.4030222.

During the past decade, research on carbon nanotubes has revealed potential advances in thermal engineering applications. The present study investigates the radiative absorption and reflection of vertically aligned carbon nanotubes (VACNTs) in the broad spectrum from the near-infrared to far-infrared regions. The optical constants of VACNT are modeled based on the dielectric function of graphite and an effective medium approach that treats the CNT film as a homogenized medium. Calculated radiative properties show characteristics of near-unity index matching and high absorptance up to around 20 μm wavelength. The packing density and degree of alignment are shown to affect the predicted radiative properties. The Brewster angle and penetration depth of VACNTs are examined in the infrared spectrum. The radiative properties for VACNT thin films are also evaluated, showing some reduction of absorptance in the near-infrared due to transmission for film thicknesses less than 50 μm. This study provides a better understanding of the infrared behavior of VACNT and may guide the design for its applications in energy harvesting, space-borne detectors, and stealth technology.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091010-091010-6. doi:10.1115/1.4030223.

The particle-continuum hybrid Laplacian method is extended as a framework for modeling all transport phenomena in fluids at the micro and nanoscale including multicomponent mass transfer and chemical reactions. The method is explained, and the micro-to-macro and macro-to-micro coupling steps are discussed. Two techniques for noise reduction (namely, the bonsai box (BB) and the seamless strategy) are discussed. Comparisons with benchmark full-molecular dynamics (MD) cases for micro and nano thermal and reacting flows show excellent agreement and good computational efficiency.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091011-091011-7. doi:10.1115/1.4030224.

A simulation work aiming to study heat transfer coefficient between argon fluid flow and copper plate is carried out based on atomistic-continuum hybrid method. Navier–Stokes equations for continuum domain are solved through the pressure implicit with splitting of operators (PISO) algorithm, and the atom evolution in molecular domain is solved through the Verlet algorithm. The solver is validated by solving Couette flow and heat conduction problems. With both momentum and energy coupling method applied, simulations on convection of argon flows between two parallel plates are performed. The top plate is kept as a constant velocity and has higher temperature, while the lower one, which is modeled with FCC copper lattices, is also fixed but has lower temperature. It is found that the heat transfer between argon fluid flow and copper plate in this situation is much higher than that at macroscopic when the flow is fully developed.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091012-091012-6. doi:10.1115/1.4030225.

Traditional fossil fuel power generation process typically has low efficiency. Large amount of the energy loss in Rankine cycle steam turbines (ST) is due to the temperature difference between the combustion flame temperature ∼2250 K (adiabatic) and the high pressure steam temperature up to 900 K. However, some of this energy can be harvested using solid-state thermoelectric (TE) power generators which are placed into the gap between the flame temperature and the steam temperature that produce additional electrical power. This study investigates the potential placement of TE on water tube wall inside a boiler at a coal-fired power plant. Three-dimensional (3D) numerical model of a simplified TE module is developed, and hot gas temperature and steam temperature from the boiler are used as boundary conditions at the hot side and cold side of the TE. The numerical results are compared with analytical calculations. The 3D effects of the thermal spreading in the TE module are investigated. Parameters such as TE leg cross section area and TE fill factor are examined in order to maximize the electrical power production of the TE without sacrificing the boiler efficiency (i.e., reducing the steam temperature). The study also looks into the various locations inside the boiler that have good potential for TE installation.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091013-091013-7. doi:10.1115/1.4030226.

This study aims to investigate the specific heat capacity of a carbonate salt eutectic-based multiwalled carbon nanomaterial (or high temperature nanofluids). The specific heat capacity of the nanomaterials was measured both in solid and liquid phase using a differential scanning calorimetry (DSC). The effect of the carbon nanotube (CNT) concentrations on the specific heat capacity was examined in this study. The carbonate molten salt eutectic with a high melting point around 490 °C, which consists of lithium carbonate of 62% and potassium carbonate of 38% by the molar ratio, was used as a base material. Multiwalled CNTs were dispersed in the carbonate salt eutectic. A surfactant, sodium dodecyl sulfate (SDS) was utilized to obtain homogeneous dispersion of CNT into the eutectic. Four different concentrations (0.1, 0.5, 1, and 5 wt.%) of CNT were employed to explore the specific heat capacity enhancement of the nanomaterials as the concentrations of the nanotubes varies. In result, it was observed that the specific heat capacity was enhanced by doping with the nanotubes in both solid and liquid phase. Additionally, the enhancements in the specific heat capacity were increased with increase of the CNT concentration. In order to check the uniformity of dispersion of the nanotubes in the salt, scanning electron microscopy (SEM) images were obtained for pre-DSC and post-DSC samples. Finally, the specific heat capacity results measured in present study were compared with the theoretical prediction.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091014-091014-6. doi:10.1115/1.4030227.

The ultrasonic effect on the heat transfer performance in oscillating heat pipes (OHPs) was investigated experimentally. Ultrasonic sound was applied to the evaporating section of the OHP by using electrically controlled piezoelectric ceramics. The heat pipes were tested with or without the ultrasonic effect. The effects of heat input, filling ratio, orientation, operating temperature, and working fluids (water and acetone) were investigated. The experimental results showed that ultrasonic sound can affect the oscillating motions and enhance the heat transfer performance of an OHP. However, the heat transfer enhancement mainly occurs at low heat input. In addition, it was found that heat transfer enhancement of the ultrasonic effect depends on the working fluid and operating temperature. At an operating temperature of 20 °C, the enhancement percentage of the water OHP is higher than acetone OHP. However, when the operating temperature was increased to 40 °C, the enhancement percentage of the water OHP was lower than the acetone OHP.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091015-091015-8. doi:10.1115/1.4030228.

Solar energy can be harvested via thermal, photovoltaic, and photovoltaic/thermal (PV/T) hybrid technologies. PV/T systems are advantageous because they utilize more of the solar spectrum and achieve a higher combined efficiency. One approach to PV/T design is to keep the operating temperature of the PV low while achieving a high temperature for the thermal absorber. Various designs of PV/T hybrids (both flat plate and concentrated) have already been proposed which utilize air or water to remove the heat from PV cells in order to enhance the overall efficiency of PV/T hybrid collector. We propose that a nanofluid can be used instead, doubling as both the heat transfer medium and an optical filter, which allows for thermal isolation of the PV and thermal receiver. Thus, unwanted IR and UV light is filtered before it hits the PV cells, which allows for higher overall efficiencies. In this study, a new design of a PV/T hybrid collector was proposed and two nanofluid filters (based on gold and silver nanoparticles) were tested with a silicon (Si) PV cell. The corresponding stagnation temperatures of PV/T hybrid collector were measured and compared with a theoretical model. The experimental measurements validate the theoretical model, giving similar results over the range of parameters tested. The silver nanofluid design achieved the highest thermal, PV and overall efficiency and both nanofluid configurations out-performed an analogous surface absorber PV/T design under similar conditions. Overall, this study shows that nanofluids represent a feasible and viable multifunctional (optical filter and heat transfer) media in PV/T solar systems.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091016-091016-11. doi:10.1115/1.4030229.

Microwater droplet splitting and merging in a parallel-plate electrowetting-on-dielectric (EWOD) device have been studied numerically. The transient governing equations for the microfluidic flow are solved by a finite volume scheme with a two-step projection method on a fixed computational domain. The interface between liquid and gas is tracked by a coupled level set (LS) and volume-of-fluid (CLSVOF) method. A continuum surface force (CSF) model is employed to model the surface tension at the interface. Contact angle hysteresis which is an essential component in EWOD modeling is implemented together with a simplified model for the viscous stresses exerted by the two plates at the solid–liquid interface. The results of the numerical model have been validated with published experimental data and the physics of droplet motion within the EWOD device has been examined. A parametric study has been performed in which the effects of channel height and several other parameters on the fluid motion have been studied.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091017-091017-9. doi:10.1115/1.4030233.

Vertically aligned carbon nanotube (VACNT) films with high thermal conductance and mechanical compliance offer an attractive combination of properties for thermal interface applications. In current work, VACNT films synthesized by the chemical vapor deposition method were used as thermal interface material (TIM) and investigated experimentally. The liquid metal alloy (LMA) with melting point of 59 °C was used as bonding material to attach VACNT films onto copper plates. In order to enhance the contact area of LMA with the contact surface, the wettability of the contact surface was modified by plasma treatment. The thermal diffusivity, thermal conductivity, and thermal resistance of the synthesized samples were measured and calculated by the laser flash analysis (LFA) method. Results showed that: (1) VACNT films can be used as TIM to enhance the heat transfer performance of the contact surface; (2) the LMA can be used as bonding material, and its performance is dependent on the LMA wettability on the contact surface. (3) When applying VACNT film as the TIM, LMA is used as the bonding material. After plasma treatment, comparison of VACNT films with the dry contact between copper and silicon showed that thermal diffusivity can be increased by about 160%, the thermal conductivity can be increased by about 100%, and the thermal resistance can be decreased by about 31%. This study shows the advantages of using VACNT films as TIMs in microelectronic packaging.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091018-091018-6. doi:10.1115/1.4030234.

Polyalphaolefins (PAOs) are widely implemented for electronics cooling, but suffer from a low thermal conductivity of about 0.14 W/mK. However, adding thermally conductive, phase-change-material (PCM) particles to a PAO can significantly improve the fluid thermal properties. In this paper, PCM microcapsules and silver-coated PCM microcapsules were synthesized using the emulsion polymerization method and the thermal performance of PCM fluids was studied in a microchannel heat sink and compared with that of the pure PAO. A test loop was designed and fabricated to evaluate the synthesized PCM fluids and it was found that fluid with uncoated PCM microcapsules has a 36% higher heat transfer coefficient than that of the pure PAO. Additionally, the heat transfer coefficient of PCM fluids with silver-coated PCM microcapsules was also 27% higher than that of pure PAO, but lower than that of fluids with uncoated PCM microcapsules. The thermal resistance of the uncoated PCM fluid was about 20% lower than that of the pure PAO fluid at the same pumping power, despite the PCM fluid's higher viscosity. Pumping tests were run for several hours and showed no evidence of particle accumulation or settling within the heat transfer loop.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091019-091019-5. doi:10.1115/1.4030235.

In order to obtain stable latent functionally thermal fluids for heat transfer and heat storage, microencapsulated phase change material (MPCM) suspensions with binary propanol–water mixtures of different proportions as base fluid were formulated. The stability study finds the binary propanol–water mixtures, after having stood for 48 hr, with a density of 941 kg/m3 exhibit the best stability. The morphology and thermophysical properties of the 10–40 wt.% MPCM suspensions, such as diameter distribution, latent heat and heat capacity, rheology and viscosity, thermal conductivity, and thermal expansion coefficients, were studied experimentally. The influence of MPCM concentration and temperature on the thermophysical properties was analyzed as well.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091020-091020-5. doi:10.1115/1.4030236.

Recently, several significant progresses have been made on the studies of extracellular and intracellular ice formation based on high-speed camera and cryomicroscope. This experimental methodology could accurately capture the rapid formation process of ice crystals at microscale. However, quantitative interpretation on such phase change behavior still reserved a tough issue. Here, in this paper, we quantitatively studied the ice crystals growth in three kinds of cryoprotectants like dimethyl sulfoxide (DMSO), sucrose, and trehalose via high-speed camera, cryomicroscope as well as the proposed data processing method. Several critical impact factors such as the concentration of cryoprotectants and the cooling rate have been investigated. Particularly, an efficient image processing technology has been developed to quantify the growth rate and morphology of the ice crystals. The results indicate that the species and concentration of cryoprotectants and the cooling rate could significantly affect the growth rate and morphology of ice crystals. DMSO is better than trehalose and sucrose as cryoprotectant because of the molecular structure. This work established a new methodology to quantify the ice crystals growth and would enhance current understanding of the factors for ice crystals formation. It is also expected to help optimize the cryopreservation process in the near future.

Topics: Crystals , Ice , Cooling , Cryonics
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091021-091021-12. doi:10.1115/1.4030237.

Due to excessive droplet feeding, a period of flooding occurs as part of a typical droplet based thermal management cycle. The conventional superhydrophilic surface, which is designed for thin film evaporation because of its highly wettable character, has a limited improvement on the thermal performance during the flooded condition. This paper investigates microstructures which combine micropillars and four engineered wettability patterns to improve the heat dissipation rate during flooding. Using the transient, 3D volume-of-fluid (VOF) model, the bubble behaviors of growth, coalescence, and departure are analyzed within different microstructures and the effects of pillar height and wettability patterns on the thermal performance are discussed. The wettability gradient patched on the pillar's side is demonstrated to promote the bubble's upward movement due to the contact angle difference between the upper and lower interfaces. However, insufficient pulling force results in large bubbles being pinned at the pillar tops, which forms a vapor blanket, and consequently decreases the heat transfer coefficient. When only a patch of hydrophobic material is present on the pillar top, effective pulling forces can be developed to help bubbles in the lower level depart from the pillar forest, since bubble merging between them generates most of the power required to pull the bubbles to the surface. The simulation results, including heat source temperatures and heat transfer coefficients, indicate that a patch of hydrophobic material on the pillar top works best out of all of the cases studied.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091022-091022-6. doi:10.1115/1.4030238.

We present in this paper numerical simulation results of the thermal radiative properties of a two-dimensional (2D) rectangular SiC grating atop a photonic crystal (PC). The results show that surface phonon polaritons (SPhPs) can be excited by both TE and TM waves when they are scattered by the 2D grating. Excitation of SPhPs, PC modes, and magnetic polaritons (MPs), and interactions between them give rise to great enhancement of the emissivity. Distinct effects of the grating geometry on the resonance of SPhPs, PC modes, and MPs were revealed, which suggest a way to effectively manipulate the emissivity by tuning the structure's geometry. Furthermore, the results indicate that quasi-diffuse emissivity of the structure can be obtained for both TE and TM waves.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091023-091023-10. doi:10.1115/1.4030240.

Electro-osmotic flow (EOF) is widely used in microfluidic systems. Here, we report an analysis of the thermal effect on EOF under an imposed temperature difference. Our model not only considers the temperature-dependent thermophysical and electrical properties but also includes ion thermodiffusion. The inclusion of ion thermodiffusion affects ionic distribution, local electrical potential, as well as free charge density, and thus has effect on EOF. In particular, we formulate an analytical model for the thermal effect on a steady, fully developed EOF in slit microchannel. Using the regular perturbation method, we solve the model analytically to allow for decoupling several physical mechanisms contributing to the thermal effect on EOF. The parametric studies show that the presence of imposed temperature difference/gradient causes a deviation of the ionic concentration, electrical potential, and electro-osmotic velocity profiles from their isothermal counterparts, thereby giving rise to faster EOF. It is the thermodiffusion induced free charge density that plays a key role in the thermodiffusion induced electro-osmotic velocity.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(9):091024-091024-12. doi:10.1115/1.4030243.

In this paper, the radiative properties of electron beam physical vapor deposition (EB-PVD) and air plasma sprayed (APS) partially yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) have been comparatively studied for the first time by measuring the spectral diffuse reflectance and transmittance in a broad spectral band ranging from 250 nm to 15 μm. The radiation transfer mechanisms inside the coatings are explored based on the experimental data and theoretical model. The results indicate that the distinctive micronanostructures of APS and EB-PVD coatings have an important effect on the radiative heat transfer. In particular, the larger grain boundary and the total porosity strongly affect the volume scattering properties of the coatings, and the scattering coefficient is closely related to the arrangement of grain boundary as well as the pore architecture (i.e., its size, morphology, and its distribution). Compared to the laminar microstructure of APS TBCs, the columnar microstructure of EB-PVD freestanding coatings exhibits a higher transmittance, a lower reflectance, and a larger absorption in the spectral region from 400 nm to 10 μm, which leads to an increase of the total heat flux. By modifying the microstructure of TBCs properly, the radiative heat flux can be reduced and thereby providing a better thermal protection for the metallic substrate.

Commentary by Dr. Valentin Fuster