Development of a new pressure drop correlation for open-cell foams based completely on theoretical grounds: Taking into account strut shape and geometric tortuosity

Abstract

Owing to their outstanding geometry and material related properties, open-cell foams as structured packing in reactors, columns or heat exchangers offer clear advantages over their traditional counterparts. For the reliable and efficient design of such systems, an accurate knowledge or prediction of the pressure drop is of fundamental importance, as it plays an important economic role in situations involving high space velocities (high Reynolds numbers). The state-of-the-art pressure drop correlations for open-cell foams are not predictive as they tend to show large deviations from measured data. These deviations are observed mainly due to empirical coefficients in the correlations that have been fitted to a particular data set, but without theoretical meaning these coefficients cannot be extrapolated to other foam structures. In addition, the incorporation of inappropriate definitions (originating from misinterpretation or negligence of important aspects) of the structural and geometrical parameters of foams can also induce a considerable error. In this paper, using the notion of geometric tortuosity and starting from the basic Hagen–Poiseuille equation, a generalized pressure drop correlation for open-cell foams is developed which is based completely on theoretical grounds. The correlation is validated for a wide range of open porosities, pore sizes and materials and its applicability for different working fluids is demonstrated. Furthermore, the definitions of structural and geometrical parameters (as mentioned, these are crucial for pressure drop correlations but are sometimes misinterpreted or neglected) of open-cell foams are reviewed. A consensus on these definitions and their incorporation into the pressure drop correlations is proposed. Consequently, with the proposed correlation, pressure drop in open-cell foams can be predicted by knowing only two structural parameters: the open porosity and the window diameter.