关键词: 二氧化碳, 微通道, 两相流, 熵产数, 数值模拟

Abstract: Microchannels have become a hot issue in heat exchanger research. In this paper, the microchannel evaporator with CO₂ was taken as research subject. A new entropy mathematical model was proposed for microchannel evaporator with CO₂ flowing inside and air flowing outside. Phase transition existed on both sides. Entropy generation number N_s represented the losses due to temperature difference of heat transfer and pressure drop. A steady state distributed parameter model for the microchannel evaporator was established to solve entropy generation number. Comparisons of analytical and existing experimental data were made to verify and validate the model. The effects of mass flow rate of CO₂ and air, CO₂ evaporation temperature and inlet air temperature on system entropy generation number were analyzed. The results from the mathematic model showed that mass flow rate of CO₂ had little effect on system entropy generation number. System entropy was mostly caused by temperature difference of heat transfer between CO₂ and air sides. With rising inlet air temperature, entropy generation number became larger. While with rising CO₂ evaporation temperature, system entropy generation number decreased. With increasing air mass flow rate, system entropy generation number increased. The higher the evaporation temperature, the greater the impact of air mass flow rate on system entropy generation number.

Key words: carbon dioxide, microchannels, two-phase flow, entropy generation number, numerical simulation