Heat and moisture transfer in energy wheels

Abstract

In this thesis, a numerical model for heat and moisture transfer in energy wheels is developed and validated. The numerical model is based on physics with only wheel and desiccant property data and inlet flow properties needed as input. The simultaneous heat and moisture transfer model includes moisture transfer due to sorption, condensation and frosting and considers a unique boundary condition for phase change at the interface between the air and matrix. The total uncertainty in the numerically predicted effectiveness (including the accuracy of the numerical algorithm and the uncertainty of the input properties for the model) is less than ±3% for sensible and ±5% for latent effectiveness; whereas, the uncertainty of reliable laboratory measurements are generally in the order of ±5% for sensible and ±6% to ±8% for latent effectiveness. Validation of the numerical model with laboratory and field measurements is presented for a significant range of operating conditions which include condensation and frosting. Effectiveness correlations and governing dimensionless groups for energy wheels are developed from physical principles. These new developments include an operating condition factor which is used to explain the sensitivity of effectiveness to operating conditions and the link between sensible, latent and total effectivenesses. The effectiveness correlations are accurate within ±2.5% of simulation data for a large range of operating conditions and wheel designs. The numerical model is also used to study several sensitivity issues such as: the importance of the energy of phase change, energy rate control, conduction in the matrix, extrapolation of experimental test data and the effect of operating conditions. Condensation and frosting in energy wheels are also studied.