Contribution of vortex structures and flow separation to local and overall pressure and heat transfer characteristics in an ultralightweight lattice material

Abstract

Ultralightweight lattice-frame materials (LFMs) with open, periodic microstructures are attractive multifunctional systems that can perform structural, thermal, actuation, power storage and other functions [A.G. Evans, J.W. Hutchinson, M.F. Ashby, Multifunctionality of cellular metal systems, Prog. Mater. Sci. 43 (1999) 171–221]. This paper presents experimental and numerical studies of local fluid flow behaviour and its contribution to local and overall pressure and heat transfer characteristics of such a lattice material with tetrahedral unit cells. A single layer of the LFM with porosity of 0.938 is sandwiched between impermeable endwalls that receive uniform heat flux and the heat transfer is subjected to forced air convection. Experimental measurements with particle image velocity (PIV) and thermochromic liquid crystal (TLC), backed by computational fluid mechanics (CFD) simulations, revealed two dominant local flow features in the LFM. Distinctive vortex structures near the vertices where the LFM meets the endwalls and flow separation on the surface of LFM struts were observed. The vortex structures formed around the vertices include horseshoe vortices and arch-shaped vortices. The horseshoe vortex increases local heat transfer on the endwall region up to 180% more than that in regions where the least influence of the horseshoe vortex is present. The arch-shaped vortex behind the vertices creates regions of flow recirculation and reattachment, leading to relatively high heat transfer. The location of flow separation along the struts varies with the spanwise position due to the presence of vertices (or endwalls). The regions on the strut surface before flow separation contribute approximately 40% of the total heat transfer in the LFM. The delay of the flow separation leads to an increase in the overall heat transfer. Comparisons with foams and other heat dissipation media such as packed beds, louvered fins and microtruss materials suggest that the LFMs compete favourably with the best available heat dissipation media.