Enhanced Air-Flow Forced Convection Through Metal Foams: Contrasting Compressed and Uncompressed Foams

The influence of complex pore architecture, its characteristic length, and the contrast between compressed and uncompressed open-cell foam structures in forced convection are explored experimentally. Air-flow (Pr similar to 0.71; 300 < Re < 10,000) data is presented and a critical issue of the appropriate definition of the hydraulic diameter, especially with foams of different pores/inch (ppi) and compressions, is addressed. Instead of the usual characterization, fin theory is applied and for this, the foam void volume structure is precision mapped by micro-CT scans. The veracity of defining the hydraulic diameter as 4x void volume divided by wetted area is supported by forced convection heat transfer results for different uncompressed and compressed metallic (aluminum) foam cores. All foams promote higher heat transfer coefficients, albeit accompanied with greater pressure drop. While the latter increases with foam ppi and compression (decreasing porosity), the former has a more complex interplay with these factors along with surface area changes and ligament fin effects. This scales with thermally effective surface area density beta(e) (product of area density and overall fin effectiveness), and the overall convective-conductance of the foam (product of empty-duct-based heat transfer coefficient h o and beta(e)) increases. The consequent enhancement, when evaluated by a modified volume goodness factor figure of merit, shows that the 40 ppi compressed foam (x3-in-x) performance is the highest (similar to 15-45 times an empty duct for the same fan power) with significant reduction in the volume of a heat exchanger.