Heat Transfer Characterization of Metallic Foams

Abstract

Gas−solid heat transfer coefficients were determined in open-celled metal foams as part of a study aimed at evaluating the application of metal foams as catalyst supports in gas−solid catalytic processes with short contact times and high reaction rates, typically controlled by diffusional mass transport. Examples of such processes are found in the field of environmental catalysis, including, for example, catalytic combustion, selective catalytic reduction of NOx by NH3 (SCR-DeNOx), automotive exhaust gas after treatment, and also in the catalytic partial oxidation of hydrocarbons for syngas or H2 generation processes. In this work, foam samples made of FeCrAlloy and Cu with nominal porosities of 10 and 20 pores per inch (ppi) were characterized by performing non-steady-state cooling measurements. Convective gas−solid heat transfer coefficients were determined by applying a one-dimensional, heterogeneous model of the cooling structure. The correlation Nu = 1.2Re0.43Pr1/3 well described the dependence of the dimensionless heat transfer coefficients on Re and Pr numbers for all the tests made in a range of flow superficial velocities from 1.2 to 5.7 m/s, independently from the foam cell size (20 < Re < 240). Such a correlation was derived assuming a prismatic idealization of the unit cell and selecting the equivalent strut diameter as the characteristic size of the foams. This expression satisfies the Colburn analogy with the correlation for mass transfer coefficients derived in a previous investigation and resembles semitheoretical literature correlations for heat transfer in flow across banks of tubes at low Reynolds numbers.