Guest Editorial

J. Thermal Sci. Eng. Appl. 2017;10(1):010301-010301-1. doi:10.1115/1.4037129.

Developing and applying renewable energies have moved to the forefront of scientific studies and industrial applications in recent years due to the rapid depletion of fossil fuel reserve. Likewise, in the last two decades, developing and applying novel materials have been pronounced as a core national strategy by many countries for both civil and military purposes. Heat transfer, a fundamental transport phenomenon, can be found in all processes of developing and applying renewable energies and novel materials. Though heat transfer problems in processes of developing and applying renewable energies and novel materials share some commons to those related to traditional energies and materials, the particularities of renewable energies and novel materials can bring several unique issues on heat transfer. For example, the heat transfer mechanisms of solar energy conversion through photovoltaic materials are very different from those in processes of developing and applying traditional energies and materials.

Topics: Heat transfer
Commentary by Dr. Valentin Fuster

Research Papers

J. Thermal Sci. Eng. Appl. 2017;10(1):011001-011001-8. doi:10.1115/1.4036767.

Internal Y-shaped bifurcation has been proved to be an advantageous way on improving thermal performance of microchannel heat sinks according to the previous research. Metal foams are known due to their predominate performance such as low-density, large surface area, and high thermal conductivity. In this paper, different parameters of metal foams in Y-shaped bifurcation microchannel heat sinks are designed and investigated numerically. The effects of Reynolds number, porosity of metal foam, and the pore density (PPI) of the metal foam on the microchannel heat sinks are analyzed in detail. It is found that the internal Y-shaped bifurcation microchannel heat sinks with metal foam exhibit better heat transfer enhancement and overall thermal performance. This research provides broad application prospects for heat sinks with metal foam in the thermal management of high power density electronic devices.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(1):011002-011002-6. doi:10.1115/1.4036768.

A simple model of homogeneous–heterogeneous process for Maxwell fluid flow in stagnation region past a stretched surface is constructed. It is assumed that the homogeneous process in the ambient fluid is governing by first-order kinetics and the heterogeneous process on the wall surface is given by isothermal cubic autocatalator kinetics. Flow by stretched surface with homogeneous–heterogeneous processes studied. Present problem is reduced to ordinary differential equations through appropriate transformation. Resulting problems have been solved for convergent solutions. Intervals of convergence for the obtained series solutions are explicitly determined. Behavior of important variables on the physical quantities is analyzed. Velocity is found decreasing function of Deborah number.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(1):011003-011003-8. doi:10.1115/1.4036769.

In this study, heat transfer and fluid flow of de-ionized water in two-dimensional parallel plates microchannel with and without micromixers have been investigated for various Reynolds numbers. The effects of heat transfer and fluid flow on height, diameter of micromixer, and also distance between the two micromixers are carried out in the study. Results showed that the diameter of the micromixer does not have much effect on heat transfer with a maximum enhancement of 9.5%. Whereas heat transfer gets enhanced by 85.57% when the height of the micromixer is increased from 100 μm to 400 μm, and also heat transfer gets improved by 11.45% when sb2 is increased from 4L to 5L. The separation and reattachment zone at the entry and exit of the micromixer cause the increase in heat transfer with the penalty of pressure drop. It is also found that increase of Reynolds number increases the intensity of the secondary flows leads to rapid increase in heat transfer and pressure drop. Finally, the optimized structure of micromixer is found out based on maximum heat transfer and minimum pressure drop.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(1):011004-011004-4. doi:10.1115/1.4036790.

We theoretically and numerically demonstrate optimal design of wavelength selective thermal emitter using one-dimensional (1D) and two-dimensional (2D) metal-dielectric gratings for thermophotovoltaic (TPV) applications. Proposed design consists of tungsten (W) and silicon dioxide (SiO2) gratings which can withstand high temperatures. Radiative properties of 1D grating were calculated using a numerical method, while effective medium approximation was used for 2D gratings. Optimal designs were obtained such that output power is maximum for GaSb photovoltaic (PV) cell at emitter temperature of 1500 K and radiated energy for longer wavelengths is limited to a low value. A constrained optimization was performed using genetic algorithm (GA) to arrive at optimal design.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(1):011005-011005-10. doi:10.1115/1.4036788.

Numerical study of the effect of jet position (JP) on cooling process of an array of heated obstacles simulating electronic components has been investigated based on realizable k–ε model. Jet positions have been changed to impinge each row of obstacles consecutively. The experiments have been achieved at three different values of jet-to-channel Reynolds number ratio, Rej/Rec = 1, 2, and 4. In this study, a comparison between two different cooling processes, cross flow only (CF) and jet impingement with cross flow (JICF), has been achieved. The flow structure, heat transfer characteristics, and the pumping power have been investigated for different jet positions. The results show that the jet position affects significantly the flow structure, as well as the heat transfer characteristics. According to the results of average heat transfer coefficient and the pumping power, the more effective jet position for all values of jet-to-channel Reynolds number ratio (1, 2, and 4) is achieved when the jets impinge the third row of obstacles (JP3).

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(1):011006-011006-8. doi:10.1115/1.4036789.

Steam-assisted gravity drainage (SAGD) process has been an optimized method to explore heavy oil reservoirs in the world. The oil viscosity reduction and gravity force near the interface of steam–chamber are the main development mechanisms. In classical models, conductive heat transfer plays the only or dominant role in the heat transmission from high-temperature steam to low-temperature oil sands. Although some mathematical studies have paid attention to the convective heat transfer, the role of heat transfer by flowable oil normal to the steam–chamber interface has been given little attention. In SAGD, the viscosity of bitumen can be reduced by several orders of magnitude by the release of latent heat from injected steam. In this study, an analytical model is developed for the heat transfer process induced by flowable oil. Also, in order to accurately simulate the oil viscosity characteristics in steam–chamber, a correlation between oil viscosity and pressure is proposed. Results indicate that the oil mobility plays an important role on the flow normal to interface when the distance is smaller than 6 m. Even under the most extreme circumstances (μw = 0.1127 cp), the flowing of oil normal to steam–chamber interface also cannot be ignored. Comparing to Irani and Ghannadi model, it can be easy to draw the conclusion that the new model consists with the underground test facility (UTF) field data much better. This new analytical model will benefit to understanding the convective heat transfer mechanism in SAGD process.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(1):011007-011007-11. doi:10.1115/1.4036791.

With the rapid development of the supersonic aircraft technology, the aircraft Mach number continues increasing, but on the other hand, the working condition becomes progressively poor. The photonic crystals (PCs) material could reflect the energy of the thermal radiation effectively and prevent heat transferring into the substrate due to its low thermal conductivity. Consequently, the PCs material could be applied to thermal protection for the supersonic aircraft. In this paper, the aircraft state of Mach 5 is set as the target operating condition, and the PC thermal protection ability is simulated by the method of computational fluid dynamics. Based on the theory of the electromagnetics, the characteristics of the photonic band gaps for three-dimensional PCs are calculated and the effects of PCs' medium radius, refractive index, and lattice constant are fully taken into account. For the three-dimensional diamond PCs' structure, two major categories and totally five optimized design schemes are proposed, through combining the condition of supersonic aircraft aerodynamic heating. Results show that the temperature is reduced by 948.4 K when the heat passes through thermal protection layer and reduced by 930.4 K when the heat passes through PC layer. By the method of “coupled optimization strategy (COS),” the energy density which enters into substrate material would decrease by 7.99%. In conclusion, the thermal protection capacity for supersonic aircraft could be effectively improved by using the PCs.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(1):011008-011008-10. doi:10.1115/1.4036792.

The effects of tube layout on the heat losses of solar cavity receiver were numerically investigated. Two typical tube layouts were analyzed. For the first tube layout, only the active surfaces of cavity were covered with tubes. For the second tube layout, both the active cavity walls and the passive cavity walls were covered with tubes. Besides, the effects of water–steam circulation mode on the heat losses were further studied for the second tube layout. The absorber tubes on passive surfaces were considered as the boiling section for one water–steam circulation mode and as the preheating section for the other one, respectively. The thermal performance of the cavity receiver with each tube layout was evaluated according to the previous calculation model. The results show that the passive surfaces appear to have much lower heat flux than the active ones. However, the temperature of those surfaces can reach a quite high value of about 520 °C in the first tube layout, which causes a large amount of radiative and convective heat losses. By contrast, the temperature of passive surfaces decreases by about 200–300 °C in the second tube layout, which leads to a 38.2–70.3% drop in convective heat loss and a 67.7–87.7% drop in radiative heat loss of the passive surfaces. The thermal efficiency of the receiver can be raised from 82.9% to 87.7% in the present work.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(1):011009-011009-13. doi:10.1115/1.4036795.

Nanoparticle dispersions or more popularly “nanofluids” have been extensively researched for their candidature as working fluid in direct-volumetric-absorption solar thermal systems. Flexibility in carving out desired thermophysical and optical properties has lend the nanofluids to be engineered for solar thermal and photovoltaic applications. The key feature which delineates nanofluid-based direct absorption volumetric systems from their surface absorption counterparts is that here the working fluid actively (directly) interacts with the solar irradiation and hence enhances the overall heat transfer of the system. In this work, a host of nanoparticle materials have been evaluated for their solar-weighted absorptivity and heat transfer enhancements relative to the basefluid. It has been found that solar-weighted absorptivity is the key feature that makes nanoparticle dispersions suitable for solar thermal applications (maximum enhancement being for the case of amorphous carbon nanoparticles). Subsequently, thermal conductivity measurements reveal that enhancements on the order of 1–5% could only be achieved through addition of nanoparticles into the basefluid. Furthermore, dynamic light scattering (DLS) and optical measurements (carried out for as prepared, 5 h old and 24 h old samples) reveal that nanoclustering and hence soft agglomeration does happen but it does not have significant impact on optical properties of the nanoparticles. Finally, as a proof-of-concept experiment, a parabolic trough collector employing the amorphous carbon-based nanofluid and distilled water has been tested under the sun. These experiments have been carried out at no flow condition so that appreciable temperatures could be reached in less time. It was found that for the same exposure time, increase in the temperature of amorphous carbon based nanofluid is approximately three times higher as compared to that in the case of distilled water.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(1):011010-011010-4. doi:10.1115/1.4036794.

In order to really assess the thermal performance of a wall incorporating phase change material (PCM), a reduced scale cavity has been monitored during two heating cycles. For each cycle, the heat source inside the test cell is switched “on” for 5 h and its setpoint is 38 °C and then switched off for 4 h. The outdoor air temperature is kept constant at a low temperature of 20 °C. Two walls are equipped with a PCM layer at different depths in order to study the optimal PCM location. The two other walls are wooden and glass to model a real building. The comparison between the four walls is made based on the absorbed heat fluxes and outside surface temperatures. The results show that the location of the PCM close to the heat source reaches its melting temperature and then reduces the surface temperature. At this location, the PCM layer stores the major part of the inlet heat flux. It takes 10 h to release the absorbed heat flux. However, the PCM layer, practically, does not have an effect on the surface temperatures and absorbed heat fluxes, when it is placed far from the heat source.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(1):011011-011011-5. doi:10.1115/1.4036796.

Graphene nanoplatelets (GNPs) have excellent thermal conductivity. It can significantly improve the heat-conducting property of epoxy resin (EP) matrix. In this paper, the GNPs/EP composites were successfully prepared by using ultrasonication and the cast molding method. The prepared GNPs/EP composites were systematically characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermal conductivity analyzer. Some factors affecting the thermal transfer performance of the composites were discussed. The defoamation has great influence on the thermal conductivity of composite. There is a nearly linear relationship between the mass fraction and the thermal conductivity of composite when the mass fraction of GNPs is below 4.3%. The results show that when the mass fraction of GNPs is 4.3% with crushing time of 2 s, the thermal conductivity of GNPs/EP composite is up to 0.99 W/m K. The thermal conductivity is increased by 9.0% compared with that without pulverization treatment (0.91 W/m K). When it is ground three times, the thermal conductivity of composite reaches the maximum (1.06 W/m K) and it is increased by 307.7% compared with that of epoxy resin matrix.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(1):011012-011012-5. doi:10.1115/1.4036798.

The thermal transport in metallic thin films can be reduced by the electron scattering and there are very little available knowledge that can be used to explain the mechanism. In this work, we characterized the thermal and electron transport of 3.2 nm thin gold films coated on alginate fiber by the transient electrothermal (TET) technique. The results reveal that the thermal and electrical conductivities are reduced significantly from the respective values of bulk material by 76.2% and 93.9%. At the same time, the Lorenz number is calculated as 8.66 × 10−8 W Ω K−2 and it is almost three times increased from the value of bulk material. The intrinsic thermal diffusivity of alginate fiber is 3.25 × 10−7 m2 s−1 and the thermal conductivity is 0.51 W m−1 K−1.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(1):011013-011013-14. doi:10.1115/1.4036966.

One important safety issue in automotive industry is the efficient cooling of brake system. This research work aims to introduce an optimized cooling vane geometry to enhance heat removal performance of ventilated brake disks. The novel idea of using airfoil vanes is followed as the basis of this investigation (Nejat et al., 2011, “Heat Transfer Enhancement in Ventilated Brake Disk Using Double Airfoil Vanes,” ASME J. Therm. Sci. Eng. Appl., 3(4), p. 045001). In order to perform the optimization technique efficiently, an integrated shape optimization process is designed. According to the aerodynamic and heat transfer considerations, first an appropriate airfoil is selected as the base profile to be optimized. For the shape modification purpose, a curve parameterization method named class shape transformation (CST) is utilized. The control parameters defined in CST method are then established as the geometrical design variables of an improved territorial particle swarm optimization (TPSO) algorithm. In order to overcome the potential bottleneck of high computational cost associated with the required computational fluid dynamics (CFD)-based function evaluations, TPSO algorithm is coupled with a predictive artificial neural networks (ANN), well trained with an input dataset designed based on the Taguchi method. The obtained profile shows an evident convective heat dissipation improvement accomplished mainly via airflow acceleration over the vanes, avoiding early flow detachment and adjusting the flow separation region at the rear part of the suction sides. The results also reveal the approaches by which such a superior performance is achieved by means of the modified surface curvatures.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(1):011014-011014-5. doi:10.1115/1.4036797.

A simple approach is developed to obtain a multiscale network of heat conducting by filling spherical alumina (S-Al2O3) and graphene nanoplatelets (GnPs) into silicone rubber (SR). This unique structure effectively minimizes the thermal contact resistance between fillers and matrix. The physical properties of the composites are characterized by thermal conductivity, density, and tensile strength. A high thermal conductivity of 3.37 Wm−1 K−1 has been achieved, which is 47.1% higher than the single filler at the same loading. A strong and obvious synergistic effect has been observed as S-Al2O3 and GnPs filled into silicone rubber matrix. It is interesting that the composites with GnPs have the lower density (2.62 g/cm3, reduced by 6%) and the superior tensile performance, compared to silicone rubber composite with neat S-Al2O3. The composites have the potential applications in heat dissipation of light-emitting diode.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2017;10(1):011015-011015-5. doi:10.1115/1.4036800.

Near-spherical gold nanoparticles were synthesized using a facile chemical reduction method. The optical properties, size, and morphology of nanofluids were characterized using ultraviolet–visible–near-infrared (UV–Vis–NIR) spectroscopy and transmission electron microscope (TEM). All the gold nanofluids showed better photothermal conversion characteristics than H2O due to the strong localized surface plasmon resonance (LSPR) effect. The increase in gold nanoparticles diameters resulted in lower photothermal conversion properties, so the appropriate reducing agents have great influence on the optical properties of gold nanofluids in our experimental system. Trisodium citrate is the optimum reducing agents compared with NaBH4 and ascorbic acid (AA).

Commentary by Dr. Valentin Fuster

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