CO_2-空气微通道蒸发器两相区熵产分析

Entropy analysis of CO_2-air microchannel evaporator in two-phase flow region

微通道已成为换热器研究领域的热点,以CO2微通道蒸发器为研究对象,建立了CO2微通道蒸发器两相区内、外侧均有相变的熵产模型,通过建立的CO2微通道蒸发器二维分布参数模型求解系统熵产数。分析CO2与空气侧质量流率、空气入口温度及CO2蒸发温度对系统熵产数的影响。结果表明:CO2质量流率对系统熵产数影响很小;系统熵产数主要由CO2与空气两侧温差传热引起;系统熵产数随空气入口温度的增大而增大,随CO2的蒸发温度的增大而减小;随着空气质量流率的增大,系统熵产数增大,且蒸发温度越高,空气质量流率对系统熵产数的影响越大。

Microchannels have become a hot issue in heat exchanger research. In this paper, the microchannel evaporator with CO_2 was taken as research subject. A new entropy mathematical model was proposed for microchannel evaporator with CO_2 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 CO_2 and air, CO_2 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 CO_2 had little effect on system entropy generation number. System entropy was mostly caused by temperature difference of heat transfer between CO_2 and air sides. With rising inlet air temperature, entropy generation number became larger. While with rising CO_2 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.