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Guest Editorial

J. Heat Transfer. 2017;139(5):050301-050301-1. doi:10.1115/1.4036016.
FREE TO VIEW

The 5th ASME Micro/Nanoscale Heat and Mass Transfer Conference (MNHMT 2016) was held in Singapore on Jan. 4–6, 2016. The Conference was organized by the School of Mechanical and Aerospace Engineering of Nanyang Technological University and sponsored by the ASME Heat Transfer Division, Xi'an Jiaotong University, Singapore Institute of Manufacturing Technology, Quantum Technology Group (Singapore) Pte. Ltd., and Nanjing Inout E-Business Co. Ltd. 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 our efforts in expanding the frontiers of micro/nanoscale heat and mass transfer. The previous four conferences were hosted by the National Cheng Kung University in Tainan (January 2008), Shanghai Jiao Tong University in Shanghai (December 2009), Georgia Institute of Technology in Atlanta (March 2012), and the University of Hong Kong in Hong Kong (December 2013).

Commentary by Dr. Valentin Fuster

Research Papers: Bio-Heat and Mass Transfer

J. Heat Transfer. 2017;139(5):051101-051101-7. doi:10.1115/1.4035246.

The objective is to validate a designed heating protocol in a previous study based on treatment efficacy of magnetic nanoparticle hyperthermia in prostate tumors. In vivo experiments have been performed to induce temperature elevations in implanted PC3 tumors injected with magnetic nanoparticles, following the same heating protocol designed in our previous microCT-based theoretical simulation. A tumor shrinkage study and histological analyses of tumor cell death are conducted after the heating. Tumor shrinkage is observed over a long period of 8 weeks. Histological analyses of the tumors after heating are used to evaluate whether irreversible thermal damage occurs in the entire tumor region. It has been shown that the designed 25 min heating (Arrhenius integral Ω ≥ 4 in the entire tumor) on tumor tissue is effective to cause irreversible thermal damage to PC3 tumors, while reducing the heating time to 12 min (Ω ≥ 1 in the entire tumor) results in an initial shrinkage, however, later tumor recurrence. The treated tumors with 25 min of heating disappear after only a few days. On the other hand, the tumors in the control group without heating show approximately an increase of more than 700% in volume over the 8-week observation period. In the undertreated group with 12 min of heating, its growth rate is smaller than that in the control group. In addition, results of the histological analysis suggest vast regions of apoptotic and necrotic cells, consistent with the regions of significant temperature elevations. In conclusion, this study demonstrates the importance of imaging-based design for individualized treatment planning. The success of the designed heating protocol for completely damaging PC3 tumors validates the theoretical models used in planning heating treatment in magnetic nanoparticle hyperthermia.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(5):051102-051102-9. doi:10.1115/1.4036015.

In this study, a three-layered skin tissue has been modeled to assess the heat transfer characteristics in laser skin tumor–tissue interaction. A finite-volume-based two-dimensional numerical bioheat transfer model has been put together to study the damage prediction of healthy tissues by considering both Fourier and non-Fourier laws. The combination of the bioheat transfer equation with Fourier law forms the parabolic equation (Pennes model) and with the non-Fourier equation forms the hyperbolic equation (thermal wave model). In this paper, the laser source is provided on the outer layer of the skin to dismantle the undesired tumor region exemplified as inhomogeneity (tumor) present in the intermediate layer. Heat input through the laser source is on until it reaches the tumor-killing criteria. The heat transport equation has been discretized by the finite volume method (FVM). The finite-volume-based numerical model is developed in such a way that the non-Fourier model predictions can be obtained through conventional Fourier-based solver. The central difference scheme is adopted for discretizing the spatial derivative terms. An implicit scheme is applied to treat the transient terms in the model. For few cases of the hyperbolic problems, certain limitation for a chosen implicit scheme has also been addressed in this paper. The results are validated with the existing literatures. The evaluated results are based on both the Fourier and the non-Fourier model, to investigate the temperature distribution and thermal damage by ensuring irreversible thermal damage in the whole tumor region placed in the dermis layer. Thermal damage of the healthy tissue is found to be more in the time scale of the thermal wave model.

Commentary by Dr. Valentin Fuster

Research Papers: Evaporation, Boiling, and Condensation

J. Heat Transfer. 2017;139(5):051501-051501-11. doi:10.1115/1.4035247.

Understanding heat transfer mechanisms is crucial in developing new enhancement techniques in pool boiling. In this paper, the available literature on fundamental mechanisms and their role in some of the outstanding enhancement techniques is critically evaluated. Such an understanding is essential in our quest to extend the critical heat flux (CHF) while maintaining low wall superheats. A new heat transfer mechanism related to macroconvection is introduced and its ability to simultaneously enhance both CHF and heat transfer coefficient (HTC) is presented. In the earlier works, increasing nucleation site density by coating a porous layer, providing hierarchical multiscale structures with different surface energies, and nanoscale surface modifications were some of the widely used techniques which relied on enhancing transient conduction, microconvection, microlayer evaporation, or contact line evaporation mechanisms. The microconvection around a bubble is related to convection currents in its immediate vicinity, referred to as the influence region (within one to two times the departing bubble diameter). Bubble-induced convection, which is active beyond the influence region on a heater surface, is introduced in this paper as a new macroconvection mechanism. It results from the macroconvection currents created by the motion of bubbles as they grow and depart from the nucleating sites along a specific trajectory. Directing these bubble-induced macroconvection currents so as to create separate vapor–liquid pathways provides a highly effective enhancement mechanism, improving both CHF and HTC. The incoming liquid as well as the departing bubbles in some cases play a major role in enhancing the heat transfer. Significant performance improvements have been reported in the literature based on enhanced macroconvection contribution. One such microstructure has yielded a CHF of 420 W/cm2 with a wall superheat of only 1.7 °C in pool boiling with water at atmospheric pressure. Further enhancements that can be expected through geometrical refinements and integration of different techniques with macroconvection enhancement mechanism are discussed here.

Commentary by Dr. Valentin Fuster

Research Papers: Forced Convection

J. Heat Transfer. 2017;139(5):051701-051701-6. doi:10.1115/1.4036004.

An incompressible electrically conducting viscous fluid flow influenced by a local external magnetic field may develop vortical structures and eventually instabilities similar to those observed in flows around bluff bodies (such as circular cylinder), denominated magnetic obstacle. The present investigation analyzes numerically the three-dimensional flow and heat transfer around row of magnetic obstacles. The vortex structures of magnetic obstacles, heat transfer behaviors in the wake of magnetic obstacles, and flow resistance are analyzed at different Reynolds numbers. It shows that the flow behind magnetic obstacles contains four different regimes: (1) one pair of magnetic vortices, (2) three pairs namely, magnetic, connecting, and attached vortices, (3) smaller vortex shedding from the in-between magnetic obstacles, i.e., quasi-static, and (4) regular vortex shedding from the row of magnetic obstacles. Furthermore, downstream cross-stream mixing induced by the unstable wakes can enhance wall-heat transfer, and the maximum value of percentage heat transfer increment (HI) is equal to about 35%. In this case, the thermal performance factor is more than one.

Commentary by Dr. Valentin Fuster

Research Papers: Heat Exchangers

J. Heat Transfer. 2017;139(5):051801-051801-10. doi:10.1115/1.4035997.

A formulation of the unit cell model and the corresponding thermal performance analysis for the cross-flow heat exchanger are carried out, with the design goal of dissipating 175 W from a high-power electronic chip in a compact space. A liquid to liquid heat exchanger in the cross-flow arrangement is preferred due to its compact size and high effectiveness. The unit cell model is formulated based on the volume-averaging method to determine the heat transfer coefficient involving two heat exchanging fluids and a solid. The various factors such as channel shape, channel edge length, channel size, and heat exchanger material can be examined based on the unit cell model. The obtained heat transfer coefficients are used for the estimation of the heat exchanger thermal performance based on the effectiveness–number of transfer units (NTU) correlation. To verify the model formulation, the heat and fluid flow over the cross-flow heat exchangers are investigated through the full-field numerical computation. The amount of heat exchanged from the numerical computation is extracted and compared with the predicted results from the unit cell model. A fairly good agreement is obtained between the two approaches. Based on the unit cell model, an aluminum cross heat exchanger with eight channel layers for the hot and cold fluids, 15 channels in each layer with a channel diameter of 2 mm, is able to meet the design target.

Commentary by Dr. Valentin Fuster

Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2017;139(5):052001-052001-6. doi:10.1115/1.4035248.

Ab initio simulation is one of the most effective theoretical tools to study the electrons evolved heat transfer process. Here, we report the use of finite-temperature density functional theory (DFT) to investigate the electron thermal excitation, electron–phonon coupled heat transfer, and the corresponding thermal response induced by energy deposition of femtosecond laser pulse in gold. The calculated results for cases with different scales of electron excitations demonstrate significant electron temperature dependence of electron heat capacity and electron–phonon coupling factor. Bond hardening of laser-irradiated gold and structural variation from solid to liquid are observed. The obtained results shed light upon the ultrafast microscopic processes of thermal energy transport from electron subsystem to lattice subsystem and serve for an improved interpretation of femtosecond laser–metal interaction.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(5):052002-052002-11. doi:10.1115/1.4036036.

In the present study, the interfacial dynamics of displacement of three-dimensional spherical droplet on a rectangular microchannel wall considering wetting effects are studied. The two-phase lattice Boltzmann Shan−Chen model is used to explore the physics. The main focus of this study is to analyze the effect of wettability, low viscosity ratio, and capillary number on the displacement of spherical droplet subjected to gravitational force on flat as well as grooved surface of the channel wall. The hydrophobic and hydrophilic natures of wettabilities on wall surface are considered to study for viscosity ratio, M1. The results are presented in the form of temporal evolution of wetted length and wetted area for combined viscosity ratios and wettability scenario. In the present study, it is observed that in dynamic droplet displacement, the viscosity ratio and the capillary number play a significant role. It is found that as the viscosity ratio increases, both the wetted area and the wetted length increase and decrease in the case of hydrophilic and hydrophobic wettable walls, respectively. The groove area on the vertical wall tries to entrap fraction of droplet fluid in case of hydrophilic surface of the vertical wall, whereas in hydrophobic case, droplet moves past the groove without entrapment.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(5):052003-052003-8. doi:10.1115/1.4036001.

Outlet boundary conditions (OBCs) and their numerical descriptions are critical to computational fluid dynamics (CFD) since they have significant influence on the numerical accuracy and stability. They present significant challenges to the two-phase lattice Boltzmann (LB) method, especially in the limit of large density ratio. In this study, three commonly used OBCs: convection boundary condition (CBC), Neumann boundary condition (NBC), and extrapolation boundary condition (EBC), are investigated and improved on basis of two LB models for large density ratios (single and double distribution function models). The existing numerical schemes for the OBCs are not directly applicable to the LB models because of the deviation of the momentum balance at the outlet boundary. The deviation becomes substantial at a large density ratio. Thus, in this work, modified OBC schemes are proposed to make the OBCs suitable for the two-phase LB models by adding an independent equation to obtain the outlet velocity. Numerical tests on droplet flowing in a channel are performed to evaluate the performance of the modified OBC schemes. Results indicate that the modified OBC schemes may be extended to tackle large density ratio situations. The modified NBC and EBC schemes are only suitable for the LB model with single distribution function. Three modified CBC schemes exhibit optimum performance for both single and double distribution function LB models which can be implemented for large density ratios.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(5):052004-052004-9. doi:10.1115/1.4036002.

As a relatively new type of functional material, porous graphite foam exhibits unique thermophysical properties. It possesses the advantages of low density, high specific surface area, and high bulk thermal conductivity and could be used as the core component of compact, lightweight, and efficient heat exchangers. Effective thermal conductivity serves one of the key thermophysical properties of foam-based heat exchangers. The complex three-dimensional topology and interstitial fluids significantly affect the heat conduction in the porous structure, reflecting a topologically based effective thermal conductivity. This paper presents a novel geometric model for representing the microstructure of graphite foams with simplifications and modifications made on the realistic pore structure, where the complex surfaces and tortuous ligaments were converted into a simplified geometry with cylindrical ligaments connected between cuboid nodes. The multiple-layer method was used to divide the proposed geometry into solvable areas, and the series–parallel relation was used to derive the analytical model for the effective thermal conductivity. To explore heat conduction mechanisms at the pore scale, direct numerical simulation was also conducted on the realistic geometric model. Achieving good agreement with experimental data, the simplified geometric model was validated. The numerically simulated conductivity followed the simplified model prediction that the two geometries are equivalent from thermal aspect. It validates further that the simplified model is capable of reflecting the internal microstructure of graphite foam, which would benefit the understandings of the thermophysical mechanisms of pore-scaled heat conduction and microstructures of graphite foam.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(5):052005-052005-8. doi:10.1115/1.4035999.

The present study presents a concept of biporous metal foam heat sink applicable to electronic cooling. This heat sink has two metal foam layers arranged in parallel along the primary flow direction, with different metal foam thickness, porosity, and pore density for each layer. The forced convective heat transfer in biporous metal foam heat sink is numerically investigated by employing the Forchheimer–Brinkman extended Darcy momentum equation and local thermal nonequilibrium energy equation. The effects of geometrical and morphological parameters on thermal and hydraulic performance are discussed in detail, and the heat transfer enhancement mechanism of biporous metal foam is analyzed. The thermal performance of biporous metal foam heat sink is compared with that of uniform metal foam heat sink. The results show that the thermal resistance of the biporous metal foam heat sink decreases with decrease of top layer metal foam porosity at a fixed bottom metal foam porosity of 0.9. It is seen that the biporous metal foam heat sink can outperform the uniform metal foam heat sink with a proper selection of foam geometrical and morphological parameters, which is attributed to the presence of high velocity gradient at the boundary layer that can enhance the convective heat transfer. The best observed thermal performance of biporous metal foam heat sink is achieved by employing 30 pores per inch (PPI) metal foam at the bottom layer, with a fixed 50 PPI metal foam at the top layer for the porosities of both layers equal to 0.9, and the optimal thickness of the bottom foam layer is about 1 mm.

Commentary by Dr. Valentin Fuster

Research Papers: Jets, Wakes, and Impingment Cooling

J. Heat Transfer. 2017;139(5):052201-052201-8. doi:10.1115/1.4035661.

An orderly droplet splashing is established when a water droplet train impinges onto a superheated copper surface. The droplets continuously impinge onto the surface with a rate of 40,000 Hz, a diameter of 96 μm or 120 μm, and a velocity of 8.4 m/s or 14.5 m/s. The heat transfers under different wall temperatures are measured, and the corresponding droplet splashing is recorded and analyzed. The effects of wall temperature, droplet Weber number, and surface roughness on the transition of the droplet splashing are investigated. The results suggest that the transferred energy is kept a constant in the transition regime, but a sudden drop of around 25% is observed when it steps into post-transition regime, indicating that the Leidenfrost point is reached. A higher Weber number of droplet train results in a more stable splashing angle and a wider range of splashed droplet diameter. The surface roughness plays no significant role in influencing the splashing angle in the high Weber number case, but the rougher surface elevates the fluctuation of the splashing angle in the low Weber number case. On the rougher surface, the temporary accumulation of the impact droplets is observed, a “huge” secondary droplet can be formed and released. The continuous generation of the huge droplets is observed at a higher wall temperature. Based on the result of droplet tracking of the splashed secondary droplets, the diameter and velocity are correlated.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(5):052202-052202-8. doi:10.1115/1.4036035.

Cooling technique in a miniscale heat sink is essential with the development of high-power electronics, such as electronic chip. As heat transfer techniques, jet impingement cooling and convective cooling by roughened surface are commonly adopted. To obtain a good cooling efficiency, the cooling structure within the heat sink should be carefully designed. In the present study, the miniscale heat sink with a feature size of 1–100 mm is setup. Arrangement of the jet impingement and dimple/protrusion surface is designed as heat transfer augmentation approaches. The effect of dimple/protrusion configuration and depth to diameter ratio is discussed. From the result, the heat transfer coefficient h distribution of heat sink surface is demonstrated for each case. The pressure penalty due to the arrangement of roughened structure is evaluated. Also, thermal performance (TP) and performance evaluation plot are adopted as evaluations of cooling performance for each configuration. Comparing all the cases, optimal cooling structure considering the energy-saving performance is obtained for the miniscale heat sink. Referencing the statistics, a new insight has been provided for the design of cooling structure inside the miniscale heat sink.

Commentary by Dr. Valentin Fuster

Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2017;139(5):052401-052401-8. doi:10.1115/1.4034845.

The present study focuses on nanowire-based metamaterials selective solar absorbers. Finite-difference time-domain (FDTD) simulation is employed for numerically designing a broadband solar absorber made of lossy tungsten nanowires which exhibit spectral selectivity due to the excitation of magnetic polariton (MP). An inductor–capacitor circuit model of the nanowire array is developed in order to predict the resonance wavelengths of the MP harmonic modes. The effects of geometric parameters such as nanowire diameter, height, and array period are investigated and understood by the sweep of geometric parameters, which tunes the MP resonance and the resulting optical and radiative properties. In addition, the optical properties and conversion efficiency of this nanowire-based absorber are both demonstrated to be insensitive on incidence angles, which illustrates the potential applicability of the proposed nanowire-based metamaterial as a high-efficiency wide-angle selective solar absorber. The results show that the nanowire-based selective solar absorber with base geometric parameters can reach 83.6% of conversion efficiency with low independence of incident angle. The results will facilitate the design of novel low-cost and high-efficiency materials for enhancing solar thermal energy harvesting and conversion.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(5):052402-052402-7. doi:10.1115/1.4035015.

Silicene, the silicon-based two-dimensional structure with honeycomb lattice, has been discovered and expected to have tremendous application potential in fundamental industries. However, its thermal transport mechanism and thermal properties of silicene have not been fully explained. We report a possible way to control the thermal transport and thermal rectification in silicene nanosheets by distributing triangular cavities, which are arranged in a staggered way. The nonequilibrium molecular dynamics (NEMD) simulation method is used. The influences of the size, number, and distribution of cavities are investigated. The simulation results show that reflections of phonon at the vertex and the base of the triangular cavities are quite different. The heat flux is higher when heat flow is from the vertex to the base of cavities, resulting in thermal rectification effect. The thermal rectification effect is strengthened with increasing cavity size and number. A maximum of thermal rectification with varying distance between columns of cavities is observed.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(5):052403-052403-7. doi:10.1115/1.4035589.

This paper attempts to experimentally investigate the influence of channel length on the flow behavior and heat transfer characteristics in circular microchannels. The diameters of the channels were 0.4 mm and the length of them were 5 mm, 10 mm, 15 mm, and 20 mm, respectively. All experiments were performed with air and completed with Reynolds number in the range of 300–2700. Results of the experiments show that the length of microchannels has remarkable effects on the performance of flow behavior and heat transfer characteristics. Both the friction factor and Poiseuille number drop with the increase of channel length, and the experimental values are higher than the theoretical ones. Moreover, the channel length does not influence the value of critical Reynolds number. Nusselt number decrease as the increase of channel length. Larger Nusselt numbers are obtained in shorter channels. The results also indicate that in all cases, the friction factor decreases and the Poiseuille number increases with the increase of the Reynolds number. It is also observed that the value of critical Reynolds number is between 1500 and 1700 in this paper, which is lower than the value of theoretical critical Reynolds number of 2300.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(5):052404-052404-7. doi:10.1115/1.4035588.

Plasma is a host of numerous analytes such as proteins, metabolites, circulating nucleic acids (CNAs), and pathogens, and it contains massive information about the functioning of the whole body, which is of great importance for the clinical diagnosis. Plasma needs to be completely cell-free for effective detection of these analytes. The key process of plasma extraction is to eliminate the contamination from blood cells. Centrifugation, a golden standard method for blood separation, is generally lab-intensive, time consuming, and even dangerous to some extent, and needs to be operated by well-trained staffs. Membrane filtration can filter cells very effectively according to its pore size, but it is prone to clogging by dense particle concentration and suffers from limited capacity of filtration. Frequent rinse is lab-intensive and undesirable. In this work, we proposed and fabricated an integrated microfluidic device that combined particle inertial focusing and membrane filter for high efficient blood plasma separation. The integrated microfluidic device was evaluated by the diluted (×1/10, ×1/20) whole blood, and the quality of the extracted blood plasma was measured and compared with that from the standard centrifugation. We found that the quality of the extracted blood plasma from the proposed device can be equivalent to that from the standard centrifugation. This study demonstrates a significant progress toward the practical application of inertial microfluidics with membrane filter for high-throughput and highly efficient blood plasma extraction.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(5):052405-052405-7. doi:10.1115/1.4035854.

This paper presents a theoretical and numerical study on the heat conduction of gas confined in a cuboid nanopore, in which there exists a temperature difference between the top and bottom walls and the side walls are adiabatic. A modified gas mean free path in confined space is proposed by considering the impact of collisions between molecules and solid surfaces, with which an effective thermal conductivity model of gas in the transition regime is derived. A direct simulation Monte Carlo (DSMC) study on the heat conduction of argon and helium in a cuboid nanopore is carried out to validate the present model. The influences of the Knudsen number and the treatments of boundary conditions on the heat conduction and effective thermal conductivity of gas in nanopores are studied. The temperature jumps and the reduction of heat flux near side walls are analyzed.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(5):052406-052406-9. doi:10.1115/1.4036017.

The present work aims at investigating a new challenging use of aluminum oxide (Al2O3) nanoparticles to enhance the thermal properties (thermal conductivity, specific heat, and latent heat) of pure paraffin waxes to obtain a new class of phase change materials (PCMs), the so-called nano-PCMs. The nano-PCMs were obtained by seeding 0.5 and 1.0 wt  % of Al2O3 nanoparticles in two paraffin waxes having melting temperatures of 45 and 55 °C, respectively. The thermophysical properties such as specific heat, latent heat, and thermal conductivity were then measured to understand the effects of the nanoparticles on the thermal properties of both the solid and liquid PCMs. Furthermore, a numerical comparison between the use of the pure paraffin waxes and the nano-PCMs obtained in a typical electronics passive cooling device was developed and implemented. A numerical model is accomplished to simulate the heat transfer inside the cavity either with PCM or nano-PCM. Numerical simulations were carried out using the ansys-fluent 15.0 code. Results in terms of solid and liquid phase fractions and temperatures and melting time were reported and discussed. They showed that the nano-PCMs determine a delay in the melting process with respect to the pure PCMs.

Commentary by Dr. Valentin Fuster

Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2017;139(5):052701-052701-9. doi:10.1115/1.4034839.

The photon transport and energy conversion of a near-field thermophotovoltaic (TPV) system with a selective emitter composed of alternate tungsten and alumina layers and a photovoltaic cell sandwiched by electrical contacts are theoretically investigated in this paper. Fluctuational electrodynamics along with the dyadic Green's function for a multilayered structure is applied to calculate the spectral heat flux, and the photocurrent generation and electrical power output are solved from the photon-coupled charge transport equations. The tungsten and alumina layer thicknesses are optimized to obtain maximum electrical power output for bare TPV cell. The spectral heat flux is much enhanced when plain tungsten emitter is replaced with the multilayer emitter due to the effective medium intrinsic lossy property and additional surface plasmon polariton coupling in the tungsten thin film, for which the invalidity of effective medium theory to predict photon transport in the near field with multilayer emitters is discussed. Effects of a gold back reflector and indium tin oxide front coating with nanometer thickness, which could practically act as the electrodes to collect the photon-generated charges on the TPV cell, are explored. The conversion efficiency of 23.7% and electrical power output of 0.31 MW/m2 can be achieved at a vacuum gap distance of 100 nm when the emitter and receiver temperature are, respectively, set as 2000 K and 300 K.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(5):052702-052702-9. doi:10.1115/1.4035249.

The artificial opals are three-dimensional photonic crystals (PCs) whose microspheres are arranged periodically in a face-centered-cubic (FCC) lattice. In this work, we investigated the reflective properties of artificial opals composed of submicron silica spheres. The finite-difference time-domain (FDTD) method for electromagnetics was used to calculate the directional–hemispherical reflectance spectra of artificial opals. Factors including structural parameters, filling dielectrics, and incident light were considered to study their effect on the reflectance. It is found that the shape, value, and position of peak of the reflectance spectra can be affected by these factors. Furthermore, by analyzing the distribution and propagation of the Poynting vectors at normal incidence of P-polarization, the high reflectance of artificial opals can be attributed to the fact that reflected light from parallel crystal face generates constructive interference to strengthen the total reflected beam. As to the engineering applications, we performed a detailed analysis of the detection sensitivity of artificial opals acting as a chemical sensor. It is found that the diameter of the spheres of artificial opals has a prominent influence on the detection sensitivity which is improved with the increase in the diameter of the spheres. This work will facilitate the design, manufacture, and application of artificial opals.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(5):052703-052703-8. doi:10.1115/1.4035251.

This work describes a novel technique for simultaneously determining local temperature and thickness of a heavily doped Si heater having a submicron width by using two-wavelength thermoreflectance microscopy. The doped silicon line heater, whose thickness and width are, respectively, 480 nm and 900 nm, is fabricated by conventional microfabrication techniques on a fused silica wafer. The full width at half maximum (FWHM) of the focused laser beam is measured to be 2.00 μm and 2.28 μm for green (λ = 516 nm) and red (λ = 640 nm) lasers, respectively. Because the heater width is narrower than the focused laser beam size, the reflected beam contains background information (i.e., reflection from the fused silica substrate) in addition to the thermoreflectance signal from the doped silicon heater. With precise knowledge of the laser beam size, heater width, and exact location of the laser beam spot on the heater, one can quantitatively model the reflectance. In reality, however, due to the difficulty of aligning the laser beam with respect to the submicron-wide Si heater, precise determination of local temperature from thermoreflectance signal is not easily attained. In the present study, instead of aligning the laser beam to the center of the submicron silicon heater, the probe laser horizontally scans over a region of the heater. By taking into account the size of the focused laser beam and the width of the doped silicon heater, it is possible to determine the absolute temperature of a local region of the heater from the measured reflectance during the scanning, even though the width of the heater line is only 39% of the size of the laser beam.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(5):052704-052704-8. doi:10.1115/1.4035356.

Near-field thermophotovoltaic (NFTPV) devices have received much attention lately as an alternative energy harvesting system, whereby a heated emitter exchanges super-Planckian thermal radiation with a photovoltaic (PV) cell to generate electricity. This work describes the use of a grating structure to enhance the power throughput of NFTPV devices, while increasing the energy conversion efficiency by ensuring that a large portion of the radiation entering the PV cell is above the band gap. The device contains a high-temperature tungsten grating that radiates photons to a room-temperature In0.18Ga0.82Sb PV cell through a vacuum gap of several tens of nanometers. Scattering theory is used along with the rigorous coupled-wave analysis (RCWA) to calculate the radiation energy exchange between the grating emitter and the TPV cell. A parametric study is performed by varying the grating depth, period, and ridge width in the range that can be fabricated using available fabrication technologies. It is found that the power output can be increased by 40% while improving the efficiency from 29.9% to 32.0% with a selected grating emitter as compared to the case of a flat tungsten emitter. Reasons for the enhancement are found to be due to the enhanced energy transmission coefficient close to the band gap. This work shows a possible way of improving NFTPV and sheds light on how grating structures interact with thermal radiation at the nanoscale.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Heat Transfer. 2017;139(5):054501-054501-5. doi:10.1115/1.4035357.

In this study, entropy analysis was introduced to characterize the thermodynamic properties of a two-dimensional (2D) thermal cloak consisting of multiple layers. The local entropy generation rate distribution was obtained, and the total entropy generation of different models was calculated. The irreversible extent of the heat transfer increased in the even layers with larger thermal conductivities. A better thermal cloak not only enhances thermal protection but also concentrates the energy fluctuations on the plate. The augmentation entropy generation number is used to identify the best cloaking scheme by varying the cloaking layer number from 1 to 20. This work shows that the fitting equation derived by analysis of variance (ANOVA) can be used to optimize the number of layers of the cloaking structure.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(5):054502-054502-4. doi:10.1115/1.4035250.

An annulus-shaped nanostructure having a three-layer structure consisting of an annulus aluminum pattern, Al2O3 spacer layer, and Al substrate is presented. High absorption peaks can be easily obtained between 2.5 μm and 12.5 μm in the case of a uniform plane wave at normal incidence. A unilateral annulus structure is designed to obtain an absorption peak of almost 100% by optimal geometry parameters. Moreover, a double annulus consisting of two unilateral annuli is proposed to compare their performances. The results indicate that the double annulus inherit the single one's absorption characteristics, and that the outer loop coupling with the inner one enhances the absorption peak of the inner loop. This structure provides a great potential for application in designing selective thermal emitters, biosensing, etc.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(5):054503-054503-5. doi:10.1115/1.4035590.

The phenomenon of steam condensation occurring on one surface in a rectangular horizontal channel was experimentally studied. The experiment was conducted using a visualization method with a steam quality of 0.1–0.9 and mass flux of 20–50 kg/m2 s. Four flow patterns (annular, wave, slug, and plug) were observed, and the effects of quality and mass flux on the condensing heat transfer were analyzed. The mass flux and steam quality primarily affect the condensing heat transfer coefficient in the shear-dominated flow regime. The condensing heat transfer coefficients are nearly constant only in a certain range of steam quality. This result is disparate from what has been reported in previous literatures. It was also observed that the condensing heat transfer coefficient rises with an increase in the quality. Two flow regime maps were employed to predict the flow regimes observed in this study. The result reveals that the Tandon flow regime map agrees quite well with the experimental results.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(5):054504-054504-4. doi:10.1115/1.4035998.

A new way was proposed to enhance the interfacial thermal conductance (ITC) of silicon carbide (SiC) composite through the overlapped carbon nanotubes (CNTs) and intertube atoms. By nonequilibrium molecular dynamics (NEMD) simulations, the dependence of ITC on both the number of intertube atoms and the temperature was studied. It is indicated that the ITC can be significantly enhanced by adding intertube atoms and finally becomes saturated with the increase of the number of intertube atoms. And the mechanism is discussed by analyzing the probability distributions of atomic forces and vibrational density of states (VDOS). This work may provide some guidance on enhancing the ITC of CNT-based composites.

Commentary by Dr. Valentin Fuster

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