XinGang Liang

Molecular Dynamics Study of Solid Thin-Film Thermal Conductivity

This study uses the molecular dynamics computational technique to investigate the thermal conductivity of solid thin films in the direction perpendicular to the film plane. In order to establish a benchmark reference, the computations are based on the widely used Lennard-Jones argon model due to its agreement with experimental liquid-phase data, its physically meaningful parameters, and its simple two-body form. Thermal conductivity increases with film thickness, as expected from thin-film experimental data and theoretical predictions. The calculated values are roughly 30 percent higher than anticipated. Varying the boundary conditions, heat flux, and lateral dimensions of the films causes no observable change in the thermal conductivity values. The present study also delineates the conditions necessary for meaningful thermal conductivity calculations and offers recommendations for efficient simulations. This work shows that molecular dynamics, applied under the correct conditions, is a viable tool for calculating the thermal conductivity of solid thin films. More generally, it demonstrates the potential of molecular dynamics for ascertaining microscale thermophysical properties in complex structures.

Effective thermal conductivity of gas-solid composite materials and the temperature difference effect at high temperature

Abstract At high temperature the effective thermal conductivity of a bulk material with a large temperature difference may differ from the local equivalent thermal conductivity of a unit cell at the mean temperature due to radiation. This paper analyzed the local equivalent thermal conductivity and the effective thermal conductivity for porous materials with cylindrical and spherical air cavities. Prediction for a spherical case agrees well with experimental data. Investigation shows that the equivalent thermal conductivity at mean temperature can well represent the effective thermal conductivity for a cavity diameter of

Thermal conductance of randomly oriented composites of thin layers

Abstract This work studied the thermal conductance of thin layers with randomly oriented composites by the percolation theory, and developed an effective-medium approximation (EMA) model for triple bond percolation systems. The results showed that the thin layer is anisotropic in conductivities when its thickness is lower than the correlation length. The conductivity in normal direction increases with decreasing thickness while the in-plane conductivity declines. The significance of this thickness effect is a function of the concentration of good conductor and the thermal conductivity ratio of the phases in the composites. A threshold concentration exists for the conductivity of bulk composites, beyond which the effective thermal conductivity increases significantly from that of the poor conductor with increasing concentration. The developed EMA model agrees quite well with the numerical simulation and is expected being applicable to predict thermal conductivities of composites using the coordination number as a fitting parameter

Closure to "Discussion of 'Do We Really Need "Entransy"?'"

The following is our response to Herwig’s paper [1] entitled, “Do we really need “entransy”? A critical assessment of a new quantity in heat transfer”. In Herwig’s paper, he questioned the consistency of G (entransy) in Sec. 4.1 and the necessity of G in Sec. 4.2. Before responding to these two questions, we would like to state once again that the entransy approach is consistent with first and second law of thermodynamics, and is needed for optimizing heat transfer process not involving in a thermodynamic cycle.

Thermal effect on the recirculation zone in sudden-expansion gas flows

A systematic study has been carried out numerically and compared with some experimental data to examine the heating effect on the corner recirculation zone (CRZ) in sudden-expansion gas flows. The heat addition to such flows will lead to the reduction of the CRZ length, and the CRZ can even disappear if the heating intensity is sufficiently large. The concept of thermal drag (heating induced pressure drop in duct flows) has been used to clarify the underlying mechanism of the heating effect on CRZ. The heating induced reduction of the adverse pressure gradient in sudden-expansion flows should be largely responsible for the shrinkage of the CRZ due to heating. Computational results also show that heating the CRZ and the upstream part of expansion flow are more efficient for changes of the CRZ.

Application of Entransy Optimization to One-Stream Series-Wound and Parallel Heat Exchanger Networks

Entransy theory has developed in recent years to describe heat transfer behavior from another point of view. Entransy dissipation is proposed to evaluate the irreversibility of heat transfer and is used to optimize heat transfer processes. We apply this theory to the one-stream series-wound and parallel heat exchanger (HE) networks. The heat transfer optimization for the HE networks includes the distribution of the heat transfer area or heat load. The distribution of the cold fluid flow rate in parallel HE networks can also be optimized. For these problems, physical and mathematical models are set up, analyzed, and discussed with the entransy theory. It is found that the optimization objectives of these problems and the optimization directions of the extremum entransy dissipation principle are consistent. The optimized results for the distribution optimization problem with given heat load are in agreement with the uniformity principle of temperature difference field. Examples of simple one-stream HE networks are given; the distributions of the heat transfer area or heat load are optimized by the extremum entransy dissipation method.

The Scaling Effect on the Thermal Processes at Mini/Microscale

The present work discusses the scaling effect on steady and unsteady thermal processes at mini/microscale that behave differently from the bulk case. Fast transient thermal processes could not be realized at bulk scale because of large thermal inertia and seldom have applications for practical use. However, due to the development of state-of-the-art technology, the miniaturization of device size makes it possible to utilize thermal responses due to the significantly reduced thermal inertia. For the steady case, the thermal effect varies with the boundary conditions and flow parameters because the relative importance of different forces changes with size at mini/microscale. Thus, the thermal phenomena are different from the conventional cases and vary with size.

Molecular dynamics study on thermal conductivity of nano-scale thin films

A simple and effective model of heat conduction across thin films is set up and molecular dynamics simulations are implemented to explore the thermal conductivity of nanoscale thin dielectric films in the direction perpendicular to the film plane. Solid argon is selected as the model system due to its reliable experimental data and potential function. Size effects of the thermal conductivity across thin films are found by computer simulations: in a film thickness range of 2–10 nm, the conductivity values are remarkably lower than the corresponding bulk experimental data and increase as the thickness increases. The consistency between the approximate solution of the phonon Boltzmann transport equation and the simulation results ascribes the thermal conductivity size effect to the phonon scattering at film boundaries.

Entransy - a Novel Theory in Heat Transfer Analysis and Optimization

It has been estimated that, of all the worldwide energy utilization, more than 80% involves the heat transfer process, and the thermal engineering has for a long time recognized the huge potential for conserving energy and decreasing CO2 release so as to reduce the global warming effect through heat transfer efficiency techniques (Bergles, 1988, 1997; Webb, 1994; Zimparov, 2002). In addition, since the birth of electronic technology, electricity-generated heat in electronic devices has frequently posed as a serious problem (Arden, 2002; Chein & Huang, 2004), and effective cooling techniques are hence needed for reliable electronic device operation and an increased device lifespan. In general, approaches for heat transfer enhancement have been explored and employed over the full scope of energy generation, conversion, consumption and conservation. Design considerations to optimize heat transfer have often been taken as the key for better energy utilization and have been evolving into a well-developed knowledge branch in both physics and engineering. During the last several decades and promoted by the worldwide energy shortage, a large number of heat transfer enhancement technologies have been developed, and they have successfully cut down not only the energy consumption, but also the cost of equipment itself. However, comparing with other scientific issues, engineering heat transfer is still considered to be an experimental problem and most approaches developed are empirical or semi-empirical with no adequate theoretical base (Gu et al., 1990). For instance, for a given set of constraints, it is nearly impossible to design a heat-exchanger rig with the optimal heat transfer performance so as to minimize the energy consumption. Therefore, scientists developed several different theories and methods to optimize heat transfer, such as the constructal theory (Bejan, 1997) and the minimum entropy generation (Bejan, 1982). Then heat transfer processes were optimized with the objective of minimum entropy generation. Based on this method, several researchers (Nag & Mukherjee, 1987; Sahin, 1996; Sekulic et al., 1997; Demirel, 2000; Sara et al., 2001; Ko, 2006) analyzed the influences of geometrical, thermal and flow boundary conditions on the entropy generation in various convective heat transfer processes, and then optimized them based on the premise that the minimum entropy generation will lead to the most efficient heat transfer performance. However, there are some scholars (Hesselgreaves, 2000; Shah & Skiepko, 2004; Bertola & Cafaro, 2008) who questioned whether the entropy generation is the universal irreversibility measurement for heat transfer, or the minimum entropy generation is the general optimization criterion for all heat transfer processes, regardless of the nature of the

Thermal radiation characteristics of plane-parallel SiC wafer

The spectral and directional absorptivity of plane-parallel SiC wafer is investigated in the IR region. The result demonstrates that interference takes place for thermal radiation emitted by plane-parallel SiC layers of the thickness from several tens to 100 microns. Owing to particular optical property of SiC, the spectral absorptivity of 10-micron radiant wave is 0.98, close to 1, the absorptivity of black body. Nevertheless, the absorptivity approaches 0 in the range from 10.5 to 12.4 microns wavelength. Our calculation also shows that total hemispherical emissivity relates to wafer’s temperature. It is between 300 and 500K where higher total hemispherical emissivity exists.