Numerical study of frost formation in a membrane for HVAC applications

Abstract

In this thesis, a numerical model for heat and mass transfer in a membrane is developed to identify the onset of saturation (frosting) in the membrane and verified. The numerical model is a porous media model based on the theory of local volume averaging and local thermal equilibrium and determines the temperature and relative humidity profiles inside the membrane in order to show the location and time of saturation. Warm and humid air flows above the membrane and cold liquid desiccant flows below the membrane. The goal of this research is to determine how to avoid saturation conditions through the membrane because saturation is essential for frosting. The numerical model is validated with experimental data and shows that frost formation can be prevented or delayed by controlling the moisture transfer rate through the membrane which is a new idea and thus a contribution to the research literature. The results of the numerical model show that the temperature and humidity profiles inside the membrane are linear at steady-state conditions. Therefore, an analytical model based on thermal and mass resistances is used to accurately predict the temperature and relative humidity at the top and bottom surfaces of the membrane under steady-state conditions. The analytical model is verified with experimental and numerical data at steady-state conditions. With the analytical model, the conditions that result in saturation conditions can be determined by directly solving two algebraic equations. The numerical and analytical models are also used to determine the sensitivity of several parameters on the time and location of saturation, including: the vapor diffusion coefficient, the heat and mass transfer coefficients, the thickness of the membrane, the liquid desiccant concentration, and the thermal conductivity of the membrane.