Numerical Study of the Cyclic Behavior of a Desiccant Layer of a Hygroscopic Rotor

Abstract

Enthalpy and desiccant wheels are often used in commercial and industrial applications for energy recovery and air dehumidification operations, respectively. The modeling of these hygroscopic rotors is of great relevance either for manufacturer product optimization or for hourly simulations, when integrated in air treatment installations. Some simplified numerical methods have been proposed to predict the behavior of hygroscopic rotors, most of them assuming negligible internal resistances to heat and mass transfer and/or constant properties of the desiccant wall. In this article, a one-dimensional physical model is used to numerically investigate the behavior of an element of a porous desiccant wall that is assumed to belong to a hygroscopic wheel, and is submitted to an adsorption/desorption cyclic operation. The mathematical model is based on the solution of the differential equations of mass and energy conservation inside the porous medium. Moreover, the model was developed to treat the coupled heat and mass transfer phenomena in a detailed way, taking into account specified convective boundary conditions and considering the local and time changes of the medium properties during the sorption processes. The corresponding numerical model is used to perform simulations considering two distinct values of the wall thickness and different durations of the adsorption/desorption cycle. The results lead to a good understanding of the relationship between the characteristics of the sorption processes and the behavior of hygroscopic wheels, and provide guidelines for the wheel optimization, namely of the adsorption/desorption partition of the wheel frontal area.