On the validity of lumped capacitance approaches for the numerical prediction of heat and mass transfer in desiccant airflow systems

In the present work, a detailed model of simultaneous heat and mass transfer through a desiccant micro-porous medium of a channel wall of compact heat and mass exchangers, such as desiccant wheels, was developed. The relevant phenomena considered by this model within the porous medium are surface diffusion of the adsorbed water, Knudsen diffusion of water vapour, heat conduction and the sorption process. A one-dimensional formulation of this model is used to investigate the validity of two simplifying approaches based on the lumped-capacitance method. One consists of neglecting the transversal heat and mass transfer resistances within the porous medium, the other of cancelling only the thermal resistance. Results are presented for a wide range of values of the layer thickness of the porous medium. It is concluded that the hypothesis of null-resistances in the cross direction is valid only for layer thicknesses lower than 0.1 mm, approximately, while the approach based on the thermal lumped capacitance model is valid for layer thicknesses lower than 5 mm. Finally, a dimensional analysis of interfacial balances leads to the conclusion that the Biot number for surface diffusion is several orders of magnitude higher than the corresponding thermal Biot number. (C) 2007 Elsevier Masson SAS. All rights reserved.