An appraisal of the heat transfer properties of metallic open-cell foams for strongly exo-/endo-thermic catalytic processes in tubular reactors

摘要

We assess the heat transfer properties of open-cell foams, i.e. cellular materials made of interconnected struts, in view of their use as catalyst carriers. Metallic foams made of FeCrAlY and aluminum with 89-95% porosities and 10-40 PP1 (pores per linear in.) pore densities were investigated. Geometrical measurements of all samples were performed using optical microscopy. X-ray micro-computed tomography techniques were applied for a detailed morphological characterization of selected samples. Heat transfer data were collected in heating and cooling flow experiments, under operating conditions expected to involve conductive as well as both convective and radiative heat exchange, using N2 flow rates from 15 to 35 Nl min~(-1) in the 400-800 K T-range. The effective radial and axial conductivities and the wall heat transfer coefficient were estimated from the measured steady-state temperature profiles. The estimates of the effective radial conductivity were between 0.3 W m~(-1) K~(-1) and 0.9 W m~(-1) K~(-1) for FeCrAlY samples with ～95% porosity, while they increased up to 7.7 W m~(-1) K~(-1) for aluminum samples with ～89% porosity. The latter value is comparable with or higher than typical effective radial thermal conductivities in technical packed bed reactors. The role of heat conduction in the foam solid matrix was also analyzed by solving the Laplace heat diffusion equation in a 3D mesh of foam samples virtually reconstructed by micro-computed tomography. It was found that the conductive mechanism is the dominant contribution to the effective radial conductivity for both FeCrAlY and Al foams. In the case of highly conductive materials, such as Al, the convective and radiative contributions are essentially negligible, and the wall coefficient (h_w) becomes the controlling heat transfer parameter. The estimates of the wall heat transfer coefficient exhibit an inverse dependency on the foam cell size, but are unaffected by changes in the porosity and in the fabrication material of the foams.