Abstract
The influence of complex pore structure, their characteristic length, and the contrast between compressed and uncompressed foam structures in forced-convective behavior are explored experimentally. Data for air flows (Pr - 0.71; 300 ≤ Re ≤ 10,000) are presented to highlight the relative performance behavior of uncompressed foams vis-a-vis compressed metal foams. Moreover, a critical issue of the appropriate definition of the hydraulic diameter, especially with foams of different ppi and different compressions, is addressed. Instead of the usual characterization as the square root of permeability, fin theory is applied to consider the void volume of the foam structure, which is mapped with precision using micro-CT scans of the foam cores. The veracity of defining the hydraulic diameter as (4*Vv/Aw) is supported by results for different uncompressed and compressed foam cores made of aluminum that further underscore the influence of foam structure. Although local heat transfer coefficients deteriorate somewhat with smaller pore size and compression, the increased surface area of high PPI compressed foam nevertheless more than compensates and results in a net benefit in thermal performance – 40 PPI 3x compressed foams have 2.5x the effective heat transfer compared to 10 PPI uncompressed foam, and 80% better than 40 fins per inch plane fins and the latter also incur much higher pressure drop penalty. The influence of foam structure and their degree of compression on the thermal-hydrodynamic performance are also delineated, and these results provide uniquely nuanced insights in characterizing enhanced heat transfer by applying fin theory to metal foams.
