NUMERICAL INVESTIGATION OF FLUID FLOW AND HEAT TRANSFER IN PERIODIC POROUS LATTICE-FRAME MATERIALS

Structured porous media possess multifunctional capabilities, such as load bearing in conjunction with effective heat dissipation, that are tunable. A numerical study of fluid flow and heat transfer through periodic, open-celled, structured porous lattice-frame materials (LFM) consisting of cylindrical aluminum struts of constant crosssection is presented. Three different lattice configurations are considered, namely, tetrahedral, kagome, and pyramidal. Simulations of flow through representative LFM unit cells with constant wall heat flux boundary conditions are performed to assess their utility as heat exchange media in terms of Nusselt number and friction factor. The unit cells investigated are structurally anisotropic; the thermal performance is studied for two mutually orthogonal orientations. The porosity of the unit cells is also varied. Lowering the porosity typically improves heat transfer, but increases the pressure drop, due to an increase in the specific surface area. A thermal efficiency index incorporating both heat transfer and pressure drop is employed for comparing the performance of various unit cells as a function of Reynolds number. Computed LFM characteristics are compared against extant data on other open-celled heat dissipation media, such as stochastic metal foams, and copper textiles, and the enhancement in performance demonstrated. Furthermore, effective thermal conductivity of LFMs in the high-conductance direction transverse to fluid flow is also calculated numerically, and is found to be approximately twice that of stochastic metal foams manufactured at similar porosities. This improvement is attributed to the constant, cylindrical cross-sections of LFM struts, as compared to the non-uniform ligament thickness of foams. To illustrate potential device-level heat exchange performance gains, a comparison is also made between LFMs and stochastic metal foams in simplified channel geometries representative of typical heat exchangers, and the observed pressure drops and thermal resistances compared. With appropriate design, lattice-frame materials may be employed as suitable alternatives to such foam structures with improved heat dissipation capabilities.