Heat and mass transfer inside the rotary heat exchanger operating under high speed rotor conditions

Abstract

The paper presents an original mathematical model based on the modified ε-NTU method, developed for numerical simulations and analysis of coupled heat and mass transfer inside the rotary heat exchanger operating under frosting conditions. The proposed model was validated on the basis of experimental data. Positive validation results indicate that the developed model is capable to predict the behavior of the rotary heat exchanger at high rotor speed. Moreover, the nature of heat and mass transfer processes in the rotary heat exchanger under winter conditions is presented. Different variants of active heat and mass transfer zones in the regenerator channels are revealed. Schemes of safe (frost free operation – without a frost area accumulation) and unsafe operation (with a presence of frost area accumulation) of the device are discussed. Three methods of protecting the heat exchanger against frost are presented and analyzed. The influence of rotor speed on frost formation inside the matrix of rotary heat exchanger is discussed. For analyzed cases the increase in rotor speed by 65% results in an increase in heat exchanger effectiveness (from 0.70 to 0.87) with simultaneous decrease in the size of the frost area accumulation (ΔX¯1) from 0.22 to 0.07. The 68% decrease in the rotor speed was accompanied by a decrease in the heat exchanger effectiveness (up to 0.58) with complete elimination of the frost area accumulation. A similar effect was also observed when the outdoor air temperature slightly increased (from –15 °C to –13.5 °C) while maintaining constant efficiency (0.70) of the heat exchanger.