Generalized correlation for gas/solid mass-transfer coefficients in metallic and ceramic foams

A previous investigation of gas/solid mass transfer in metallic foams with high void fractions has been extended to ceramic foams with higher relative densities by means of CO combustion tests in the diffusion-limited regime over Pd/alpha-Al2O3 washcoated foams. A simple cubic-cell model consisting of a framework of connected struts was preferred to an analogy with packed beds to approximate the complex cellular structure of the foams. Accordingly, in the analysis of mass-transfer data, the strut diameter was adopted as the reference length in dimensionless parameters. Also, the interstitial velocity, u(max), was identified as a more adequate flow parameter than the superficial velocity, which emphasizes the analogy with heat transfer in flow across bundles of tubes. Following this approach, a generalized correlation was obtained that nicely fits the mass-transfer data of both ceramic and metallic foams, covering a range of void fractions from 0.84 to 0.95 and a range of Re-max values from 15 to 200.