超临界CO2-DME混合物竖直圆管内传热特性数值研究

本文采用RNG k-ε湍流模型对超临界CO₂/DME(二甲醚)二元混合工质在竖直圆管内的传热特性进行了数值模拟研究。管径4 mm,管长为1000 mm;CO₂/DME浓度配比分别为97/3、95/5、92/8、90/10、85/15、以及70/30;质量流速为125~200 kg·m^-2.s^-1;热流密度为15~30 kW.m^-2,入口温度295~308 K,入口压力8~15 MPa。不同浓度配比的混合工质在各自临界压力下应用时,随着DME浓度的增加,换热系数的峰值逐渐减低,但在温度大于310 K时混合工质的换热系数会高于纯CO₂。压力相同时,随着DME浓度的增大,拟临界温度升高,换热系数峰值点也随之向温度升高的方向移动。混合工质的换热系数随质量流速的增大而增大。在拟临界点前,增大热流密度及降低压力对管内传热有利,而在拟临界点之后,换热系数随热流密度的升高以及压力的降低而降低。

Numerical Simulation of Heat Transfer Characteristics for Supercritical CO/DME Mixture in Vertical Tubes

The heat transfer characteristics of supercritical CO₂/DME(Dimethylether)binary mixture in a vertical circular tube were numerically simulated with RNG k-εturbulence model.The diameter and length of the tube is 4 mm and 1000 mm respectively;The CO₂/DME concentration ratio are97/3.95/5,92/8.90/10,85/15.and 70/30 respectively;Mass flux ranging from 125 to 200 kg·m^-2·s^-1;Heat flux ranging from 15-30 kW·m^-2:Inlet temperature ranging from 295 K to 308 K:Inlet pressure ranging from 8 to 15 MPa.When mixture with different concentration ratios under their respective critical pressures,the peak point of heat transfer coefficient gradually decreases with the increase of DME concentration.However.when the temperature is greater than 310 K,the heat transfer coefficient of mixture is higher than that of pure CO₂.Under the same pressure,the pseudocritical temperature increases with the increase of DME concentration,and the peak point of heat transfer coefficient also moves towards the direction of temperature increase.The heat transfer coefficient of mixture increases with the increase of mass flux.Before the pseudo critical point,the increasing of the heat flux and the decreasing of pressure is beneficial to the heat transfer in the tube.while after the pseudo critical point,the heat transfer coefficient decreases rapidly with the increase of the heat flux and the decrease of pressure.