Abstract:

An experiment was conducted to study the heat transfer characteristics of supercritical pure R41, mixtures of CO₂/R41 (20.5/79.5), and CO₂/R41 (51.4/48.6) cooled in a horizontal smooth mini-channel tube with inner-diameter of 2 mm. Mass flow rate was in the range of 400 to 800 kg·m^-2·s^-1, pressure changed from 6.0 to 8.0 MPa,heat flux ranged from 12 to 48 kW·m^-2 and bulk temperature varied from 20℃ to 80℃. The maximum heat transfer coefficient (α) of the three fluids increased with the CO₂ mass fraction. Additionally, α of pure R41 is smaller than that of the mixtures. However, α variation tendency of the mixtures is the same as that of pure R41. α of the three working fluids are ranging from 2 kW·m^-2·K^-1 to 25 kW·m^-2·K^-1 under the present test condition. The influence of pressure is significant. The closer of the pressure approaches the critical pressure, the higher of the maximum α is. The heat flux has little influence on α when the bulk temperature is far away from the critical temperature. However for the bulk temperature near the critical temperature, α increases with the decrease of heat flux. The experimental results are compared with prediction values calculated by twelve correlations. It is concluded that four correlations predicting with good precisions, and the errors are within ±30%. Nevertheless, the prediction errors increase with CO₂ mass fraction.

Key words: supercritical CO₂, R41, mixture, convection, heat transfer, mini-channel

摘要：

对R41和混合工质CO₂/R41 (20.5/79.5)、CO₂/R41(51.4/48.6)在直径为2 mm的水平光滑圆管中的超临界冷却流动换热特性进行了实验研究。质量流速范围为400～800 kg·m^-2·s^-1,压力为6.0～8.0 MPa,热通量为12～48 kW·m^-2,流体温度为20～80℃。3种工质的对流传热系数的极值随CO₂含量的增加而增大,在相同条件下R41的传热系数小于CO₂/R41的传热系数。混合物的超临界传热系数变化规律与纯R41相同。实验条件下,3种流体的传热系数在2～25 kW·m^-2·K^-1之间,压力的影响显著,越接近临界压力对应压力条件下的传热系数极值越高。在远离准临界点的区域传热系数随热通量变化不明显,而在准临界点附近对流传热系数的极值随热通量的增加而小幅减小。将实验结果与经验关联式计算结果进行了比较,有4个关联式的预测效果较好,误差均在±30%以内,预测误差随CO₂含量的增加而下降。

关键词: 超临界CO₂, R41, 混合物, 对流, 换热, 小通道