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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

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: 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

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

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

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

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