Analysis of Simplifying Assumptions for the Numerical Modeling of the Heat and Mass Transfer in a Porous Desiccant Medium

ABSTRACT

The aim of this work is to assess the accuracy of different simplifying assumptions commonly adopted in the modeling of the thermodynamic behavior of porous desiccant media such as those composing the channel walls of compact heat and mass exchangers, such as desiccant wheels. The study is based on the one-dimensional numerical solution of the conservation equations for heat, water vapor, and adsorbed water inside the porous medium under the constraint of local equilibrium between the two phases, which is characterized by sorption isotherms without hysteresis. Systematic calculations are performed for both adsorption and desorption processes and particular air flow conditions. It is concluded that the surface diffusion is the most important mechanism of water transport within the porous medium and that the internal thermal resistance may be locally neglected, allowing a lumped heat capacitance model in the cross direction of the channel wall. Furthermore, it is also important to account for the local variation of some properties of the porous medium.