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Research Papers: Forced Convection

J. Heat Transfer. 2018;140(5):051701-051701-2. doi:10.1115/1.4038553.

The heat function concept introduced by Kimura and Bejan (1983, “The Heatline Visualization of Convective Heat Transfer,” ASME J. Heat Transfer, 105(4), pp. 916–919) for two-dimensional (2D) heat transfer is being extended in this note to three dimensions. It is shown that a heat flux vector potential exists and can be used in three-dimensional (3D) heat convection problems. It is further shown that this heat flux vector potential degenerates to the heat function introduced by Kimura and Bejan (1983, “The Heatline Visualization of Convective Heat Transfer,” ASME J. Heat Transfer, 105(4), pp. 916–919) when the heat convection is two-dimensional.

Topics: Convection , Heat flux , Heat
Commentary by Dr. Valentin Fuster

Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2018;140(5):052001-052001-4. doi:10.1115/1.4038449.

A general expression has been obtained to estimate thermal conductivities of both stochastic and periodic structures with high-solid thermal conductivity. An air layer partially occupied by slanted circular rods of high-thermal conductivity was considered to derive the general expression. The thermal conductivity based on this general expression was compared against that obtained from detailed three-dimensional numerical calculations. A good agreement between two sets of results substantiates the validity of the general expression for evaluating the stagnant thermal conductivity of the periodic structures. Subsequently, this expression was averaged over a hemispherical solid angle to estimate the stagnant thermal conductivity for stochastic structures such as a metal foam. The resulting expression was found identical to the one obtained by Hsu et al., Krishnan et al., and Yang and Nakayama. Thus, the general expression can be used for both stochastic and periodic structures.

Commentary by Dr. Valentin Fuster

Research Papers: Melting and Solidification

J. Heat Transfer. 2018;140(5):052301-052301-11. doi:10.1115/1.4038423.

Selective laser melting (SLM) is a widely used powder-based additive manufacturing process. However, it can be difficult to predict how process inputs affect the quality of parts produced. Computational modeling has been used to address some of these difficulties, but a challenge has been accurately capturing the behavior of the powder in a large, bed-scale model. In this work, a multiscale melting model is implemented to simulate the melting of powder particles for SLM. The approach employs a particle-scale model for powder melting to develop a melt fraction–temperature relationship for use in bed-scale simulations of SLM. Additionally, uncertainties from the particle-scale are propagated through the relationship to the bed scale, thus allowing particle-scale uncertainties to be included in the bed-scale uncertainty estimation. Relations, with uncertainty, are developed for the average melt fraction of the powder as a function of the average temperature of the powder. The utility of these melt fraction–temperature relations is established by using them to model phase change using a continuum bed-scale model of the SLM process. It is shown that the use of the developed relations captures partial melt behavior of the powder that a simple melting model cannot. Furthermore, the model accounts for both uncertainty in material properties and packing structure in the final melt fraction–temperature relationship, unlike simple melting models. The developed melt fraction–temperature relations may be used for bed-scale SLM simulations with uncertainty due to particle effects.

Commentary by Dr. Valentin Fuster

Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2018;140(5):052401-052401-7. doi:10.1115/1.4038359.

The basic theme of this investigation is to analyze heat and mass transport for three-dimensional (3D) stagnation-point flow of nanofluid caused by an exponentially stretched surface when water is treated as base fluid. In this study, we invoked the boundary layer phenomena and suitable similarity transformation of exponential character; as a result, our 3D nonlinear equations of momentum and energy are transmuted into nonlinear and nonhomogeneous differential equations involving ordinary derivatives. Final equations are then puzzled out by applying homotopy analysis technique. Interesting outcomes of aggressing parameters involved in this study, and effecting profiles of temperature field and velocity are explained in detail. Graphical results of involved parameters appearing in considered nanofluid are presented separately. Different aspects of skin friction coefficient as well as Nusselt number are calculated. It is worth mentioning that skin friction (as we go) along x and y-direction is maximal for Cu-water nanofluid and minimal for AL2O3-water nanofluid. Also, the resulting quantity of local Nusselt number came out maximum for Cu-water nanofluid whereas minimum for TiO2-water nanofluid.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Heat Transfer. 2018;140(5):054501-054501-7. doi:10.1115/1.4038477.

Nonequilibrium molecular dynamics (MD) simulations have been performed to understand the evaporation of a liquid droplet in the presence of a solid nanoparticle. The influence of solid–liquid interaction strength (εsl) on the evaporation properties was addressed. The system consists of a solid nanoparticle (platinum) engulfed in a droplet (argon) in Argon vapor environment. After the equilibration of this nanoparticle embedded droplet with its vapor, the boundary of this system is heated continuously to evaporate the droplet. It is observed that the addition of a nanoparticle to the droplet resulted in a slower evaporation rate when compared to that of a pure droplet. It was found that the evaporation rate of the droplet is decreased with increasing solid–liquid interaction strength (εsl) and those liquid atoms around the solid nanoparticle with higher εsl are able to delay evaporation even at higher temperature owing to its decreased interfacial resistance. In order to analyze further on the vibrational coupling of the solid and liquid atoms, the vibrational density of states (VDOS) of the solid atoms is studied. It is observed that the DOS of the solid atoms exhibited a higher population in the lower frequency range with the highest peak observed for a lower value of εsl. For low values of εsl, we observe a decrease in the overlap between the VDOS of the solid atom and the interfacial liquid atoms. It is observed that for higher values of εsl, the particle is able to retain a structured layer of liquid even at high temperature and also a higher heat input is necessitated to break the interaction strength of the liquid molecules around the solid nanoparticle, which makes it possible in delaying the complete evaporation of the droplet.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(5):054502-054502-4. doi:10.1115/1.4038425.

This paper presents a novel adaptive thermal management technique to improve the efficiency of solid-state power amplifier (SSPA) for geo-synchronous satellites. The thermal management for space segment is very important as it determines the reliability of the satellite. The microwave power amplifiers (MPAs), either traveling wave tube amplifiers (TWTAs) or SSPAs, are the maximum power consuming and heat dissipative elements in the satellite and their power efficiency determines weight, volume, cost, and reliability of the satellite. So, it is necessary to improve the efficiency of the SSPA. A novel technique is presented, which improves the efficiency of the SSPA and hence, saving of costly DC power generation on-board and reduction of the heat dissipation.

Commentary by Dr. Valentin Fuster

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