Direct Pore-Scale Simulations of Fully Periodic Unit Cells of Different Regular Lattices

Open-cell metal foams are known for their superior heat dissipation capabilities. The morphological, pressure-drop and heat transfer characteristics of stochastic metal foams manufactured through traditional 'foaming' process are well established in the literature. Employment of stochastic metal foams in next generation heat exchangers, is however, challenged by the irregularity in the pore-and fiber-geometries, limited control on the pore-volume, and an inherent necessity of a bonding agent between foam and heat source. On the other hand, additive manufacturing is an emerging technology that is capable of printing complex user-defined unit cell topologies with customized fiber shapes directly on the heated substrates. Moreover, the user-defined regular lattices are capable of exhibiting better thermal and mechanical properties than stochastic metal foams. In this paper, we present a numerical investigation on fully periodic unit-cells of three different topologies, viz. Tetrakaidecahedron (TKD), Rhombic-dodecahedron (DDC), and Octet with air as the working fluid. Pressure gradient, interfacial heat transfer coefficient, friction factor, and Nusselt number are reported for each topology. Rhombic-dodecahedron yielded in the highest average interfacial heat transfer coefficient whereas Octet incurred the highest flow losses. Pore diameter, defined as the maximum diameter of a sphere passing through the polygonal openings of the structures, when used as the characteristics length scale for the presentation of Nusselt number and Reynolds number, resulted in a single trendline for all the three topologies.

Generalized model of Kapitza conductance across rough interfaces

This paper is devoted to the theoretical prediction of the interfacial heat transfer in nanostructured materials. The main task of this work is the analysis of interaction of elastic waves with the rough interface between two different solids. The presence of toughness leads to a significant increase in the resistance to heat transfer in nanostructures. This fundamental problem is discussed in relation to the commonly used method of wave scattering at rough surface: the Kirchhoff tangent plane method. The method assumes that at the point of the rough surface profile, the surface is regarded as locally smooth, and the reflection and transmission of the incident wave can be described by the scattering at the tangent plane of this point. Based on the elastic wave theory, we use the frequency-dependent continuity conditions to calculate the energy transmission coefficient at the interface. And then its effective value at the rough interface is estimated by using the Kirchhoff method. By substituting this effective value into the formula of Kapitza conductance, we can calculate the Kapitza conductance at the rough interface and analyze the effect of roughness on the interfacial heat transfer.