Effectiveness Parameters for the Heat and Mass Transfer in a Desiccant Wheel
The desiccant wheel is the key component in a solid-desiccant system for air dehumidification. The heat and mass transfer phenomena occurring within the porous channel walls of the wheel and with the airflow are strongly coupled, and some properties of the airflow and of the desiccant medium exhibit important changes during the sorption/desorption processes. The dynamic analysis of such devices integrated in non-conventional HVAC&R systems can be easily done by a project designer using the NTU-effectiveness method, provided that appropriate correlations for two independent effectiveness parameters are available. In this work, the performance of a desiccant wheel was evaluated by numerical modelling the cyclic behaviour of a representative channel of the hygroscopic matrix. The physical model adopted takes into account the gas-side and solid-side resistances, as well as the simultaneous heat and mass transfer coupled with the water adsorption/desorption process in the channel wall domain. Two phases co-exist in equilibrium inside the desiccant porous medium, the equilibrium being characterized by sorption isotherms. The desiccant medium considered is silica gel RD. In the numerical model, the airflow is treated as a bulk flow, and its interaction with the wall channel matrix is represented by appropriate convective heat and mass transfer coefficients. Two independent effectiveness parameters were defined. A set of cases was numerically simulated and the results were analysed to assess the dependence of those effectiveness parameters on the process and regeneration airflow rates and on the channel length. As a conclusion, novel empirical correlations are here purposed.
Andreas Öchsner, Graeme E. Murch and João M.P.Q. Delgado
C.R. Ruivo et al., "Effectiveness Parameters for the Heat and Mass Transfer in a Desiccant Wheel", Defect and Diffusion Forum, Vols. 312-315, pp. 205-210, 2011