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J. Thermal Sci. Eng. Appl. 2018;10(5):051001-051001-13. doi:10.1115/1.4039422.

In this paper, affecting parameters of porous medium to improve the rate of convective heat transfer in a two-dimensional porous gas heat exchanger (PGHE) for two arrangements (symmetric and asymmetric) of barriers are numerically investigated. Two barriers have been located on the top and bottom walls and one obstacle was placed in the central zone of the PGHE. In the present study, solving the momentum and energy equations has been done by Lattice–Boltzmann method with multiple-relaxation-time (LBM-MRT). The boundary conditions in both arrangements include the left and right walls which are kept at the cold constant temperature and both top and bottom walls are insulated. There is a volumetric heat source within the PGHE. The temperature of barriers and fixed obstacle are kept at hot temperature. In this study, impact of effective parameters in porous medium and heat transfer including dimensionless number of Darcy, porosity, and Rayleigh number on the flow and temperature fields has been investigated. According to the numerical results, it has been shown that the porous medium and barriers cause increase and improvement in the heat transfer within PGHE in both symmetrical and asymmetrical arrangements. The results also demonstrate that as dimensionless Darcy number increases, more convection occurs within the chamber. Examining arrangement of barriers shows that in asymmetrical arrangement, more space appears in chamber and convective heat transfer is done better.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051002-051002-14. doi:10.1115/1.4039544.

In the present framework, a model is constituted to explore the peristalsis of magnetohydrodynamics (MHD) viscoelastic (second grade) fluid with wall properties. The study is beneficial in understanding blood flow dynamics through microchannels. The mechanisms of heat and mass transfer are also modeled in the existence of viscous dissipation and Soret effects. The conducting second grade fluid is permeated by a vertical magnetic field. Perturbation technique is opted to present series solutions by assuming that the wavelength of the sinusoidal wave is small in comparison to the half-width of the channel. The solution profiles are computed and elucidated for a certain range of embedded parameters. Moreover, plots of heat transfer coefficient against the axial coordinate are also portrayed and deliberated. The main outcome of the current research is that both viscoelasticity and slip effect considerably alter the flow fields. Moreover, an increasing trend in solute concentration is anticipated for increasing the Soret effect strength.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051003-051003-9. doi:10.1115/1.4039355.

Present paper conducts an extensive numerical study on entropy analysis of mixed convective condensation inside a vertical parallel plate channel. A new approach is proposed to separate pump velocity component/Reynolds number from inlet mixed convection velocity. Influence of inlet governing parameters on condensation heat and mass transfer at different inlet pressure, velocity, channel length, and width are widely studied. The central focus of this paper is to study entropy generation under mixed convective condensation. Variation of local as well as overall entropy generation and second law efficiency for different geometric and environmental conditions are presented. For effective condenser design, present study provides two important correlations of overall volumetric entropy generation due to thermal transport and overall volumetric entropy generation due to mass transport.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051004-051004-12. doi:10.1115/1.4039460.

This paper establishes a multiscale design evaluation framework that integrates performance models for a thermal energy storage (TES) unit and a subsystem heat exchanger (HX). The modeling facilitates the analysis of transient input and extraction processes for the TES device which uses solid–liquid phase change to store thermal energy. We investigate sensible and latent heat transfer through the unit's matrix structure which contains phase change material (PCM) in the interstitial spacing. The heat transfer is driven by a temperature difference between fluid flow passages and the PCM matrix which experiences sensible heat transfer until it reaches the PCM fusion point; then it undergoes melting or solidification in order to receive, or reject, energy. To capture these physics, we establish a dimensionless framework to model heat transfer in the storage device much like effectiveness-number of transfer units (NTU) analysis methods for compact HX. Solution of the nondimensional governing equations is subsequently used to predict the effectiveness of the transient energy input and extraction processes. The TES is examined within the context of a larger subsystem to illustrate how a high efficiency design target can be established for specified operating conditions that correspond to a variety of applications. The general applicability of the model framework is discussed and example performance calculations are presented for the enhancement of a Rankine power plant via asynchronous cooling.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051010-051010-6. doi:10.1115/1.4039921.

A computational study of a metamaterial (MTM)-on-glass composite is presented for the purpose of increasing the energy efficiency of buildings in seasonal or cold climates. A full-spectrum analysis yields the ability to predict optical and thermal transmission properties from ultraviolet through far-infrared frequencies. An opportunity to increase efficiency beyond that of commercial low-emissivity glass is identified through a MTM implementation of Ag and dielectric thin-film structures. Three-dimensional finite difference time-domain (FDTD) simulations predict selective nonlinear absorption of near-infrared energy, providing the means to capture a substantial portion of solar energy during cold periods, while retaining high visible transmission and high reflectivity in far-infrared frequencies. The effect of various configuration parameters is quantified, with prediction of the net sustainability advantage. MTM window glass technology can be realized as a modification to commercial low-emissivity windows through the application of nanomanufactured films, creating the opportunity for both new and after-market sustainable construction.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051011-051011-10. doi:10.1115/1.4039925.

Lattice Boltzmann method (LBM) is performed to study numerically combined natural convection and surface radiation inside an inclined two-dimensional open square cavity. The cavity is heated by a constant temperature at the wall facing the opening. The walls normal to the heated surface are assumed to be adiabatic, diffuse, gray, and opaque while the open boundary is assumed to be black at ambient temperature. A Bathnagar, Gross and Krook (BGK) collision model with double distribution function (D2Q9-D2Q4) is adopted. Effects of surface radiation, inclination angle, and Rayleigh number on the heat transfer are analyzed and discussed. Results are presented in terms of isotherms, streamlines, and Nusselt number. It was found that the presence of surface radiation enhances the heat transfer. The convective Nusselt number decreases with increasing surface emissivity as well as with Rayleigh number, while the total Nusselt number increases with increasing surface emissivity and Rayleigh number. The inclination angle has also a significant effect on flow and heat transfer inside the cavity. However, the magnitude of total heat transfer decreases considerably when open cavity is tilted downward.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051012-051012-11. doi:10.1115/1.4039924.

In this study, forced convective heat transfer inside a circular tube automobile radiator is experimentally and numerically investigated. The investigation is carried out using Al2O3 and CuO nanofluids with water as their base fluid. A single radiator circular tube with the same dimensions is numerically modeled. Numerical model is validated using the experimental study results. In the experimental study, Al2O3 and CuO nanofluids of 0.05% volume concentrations (ϕ) were recirculated through the radiator for the Reynolds number (Re) between 260 and 1560. The numerical investigation is conducted for the nanoparticle volume concentration from 0% to 6.0% and 260 < Re < 1560. The investigation shows an enhancement of convective heat transfer coefficient (h) with the increase in nanoparticle volume concentration and with the Reynolds number. A maximum enhancement of 38% and 33% were found for Al2O3 and CuO nanofluids of ϕ = 1% and Re = 1560. For the same cooling load of the radiator, the pumping power can be reduced by 8% and 10%, when Al2O3 and CuO nanofluids (ϕ = 0.8%) were used. Enhancement in convective heat transfer can be utilized to reduce the radiator surface area required. However, the addition of nanofluid results in an enhancement of density (ρ) and viscosity (μ) along with a reduction in specific heat capacity (Cp). Hence, the selection of nanoparticle volume concentration should consider its effect on the thermophysical properties mentioned earlier. It is found that the preferred concentration is between 0.4% and 0.8% for both Al2O3 and CuO nanofluids. In our investigations, it is observed that the convective heat transfer performance of Al2O3 nanofluid is better than the CuO nanofluid.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051013-051013-15. doi:10.1115/1.4039966.

In this paper, a Ranque–Hilsch vortex tube (RHVT) has been optimized utilizing convergent (φ), straight, and divergent (θ) axial angles for hot-tube. Effects of divergent (θ) and convergent (φ) angles on the flow behavior have been investigated by computational fluid dynamic (CFD) techniques. By using a renormalization group (RNG) k–ε turbulence model based on finite volume method, all the computations have been carried out. The isentropic efficiency (ηis) and coefficient of performance (COP) of machine was studied under five different divergent angles (θ), namely 1 deg, 2 deg, 3 deg, 4 deg, and 6 deg, two different convergent (φ) angles (φ) namely 1 deg and 2 deg adjusted to the hot-tube. Furthermore, some geometrical and operational parameters including cold outlet diameter, hot-tube length, and different inlet pressures and mass flow rates have been analyzed in detail (spanwisely) in order to optimize the cooling efficiency of vortex tube (straight). The results show that utilizing the divergent hot-tubes increases the isentropic efficiency (ηis) and COP of device for most values of inlet pressures, and helps to become more efficient than the other shape of vortex tubes (straight and convergent). Finally, some results of the CFD models have been validated by the available experimental and numerical data, which show reasonable agreement, and others are compared qualitatively.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2018;10(5):051014-051014-13. doi:10.1115/1.4040034.

This paper reports the local multifaceted and area-averaged convective heat transfer coefficients (CHTCs) of longitudinal and transverse bricks arranged in lattice brick setting in tunnel kilns, using a three-dimensional (3D) computational fluid dynamics (CFD) model. A mesh sensitivity analysis was performed and the model was validated against reported experimental data in tunnel kilns. Three turbulence models were tested: the standard k–ε, re-normalization group (RNG) k–ε, and k–ω. The k–ω model provided the closest results to the experimental data. The CHTCs from the front, back, left, and right faces of the longitudinal and transverse bricks were calculated under various conditions. Area-averaged CHTCs for bricks were determined from the multifaceted CHTCs. Effects of rows, layers, and walls on faces and area-averaged CHTCs were investigated. A sensitivity analysis was performed to explore the effect of flow channels on the CHTCs. The numerical results showed that the CHTCs are enhanced by 17% for the longitudinal bricks and 27% for the transverse bricks when a uniform flow is reached in the tunnel kilns. Also, similar area-averaged CHTCs for the longitudinal and transverse bricks were obtained as a result of the uniform flow. Therefore, the specific energy consumption, quality, and quantity of brick production could be enhanced.

Commentary by Dr. Valentin Fuster

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