Abstract:

The characteristics of heat transfer for CO₂ flow boiling in horizontal micro-tube were studied. Experimental conditions:saturated temperature -40-0℃, heat flux 5-35 kW·m^-2, mass flow rate 200-1500 kg·m^-2·s^-1, and inner diameter 1.5 mm. Experimental results show that:the increase of heat flux has significant effect on enhancing nucleate boiling heat transfer, meanwhile speeds up the dryout process and decreases the starting vapor quality of dryout; the effect of mass flow rate on heat transfer coefficient is slight, but as mass flow rate increasing, the starting vapor quality of dryout decreases, and the heat transfer coefficient increases after the dryout happens; the effect of saturation temperature on the physical properties of CO₂ is the main cause of the different heat transfer characteristics in different experimental conditions.With the increasing of saturation temperature, the starting vapor quality of dryout has a decline tendency and the heat transfer coefficient has a steeper fall after dryout happens. Compared with the prediction model, the Cheng's model has a higher prediction accuracy, it is presented with 77.1% of predicted points within ±30% and 20.5% mean absolute deviation before dryout phenomenon happened, while it has only 22.9% and 57.8% after dryout phenomenon happened.

Key words: carbon dioxide, flow boiling, heat transfer coefficient, dryout

摘要：

对CO₂在水平微细管内流动沸腾换热进行了实验研究。实验工况：饱和温度-40～0℃，热通量5～35 kW·m^-2，质量流率200～1500 kg·m^-2·s^-1，管径1.5 mm。实验结果表明：热通量增加对于强化核态沸腾换热具有显著影响，同时加快干涸发展进程，降低干涸起始干度；质量流率对于传热系数的影响较小，随着质量流率的增加干涸起始干度降低，干涸后的传热系数有所增加；饱和温度对CO₂物性的影响是造成其不同工况时换热特性差异的主要原因，饱和温度升高干涸起始干度具有降低的趋势，且干涸后传热系数下降更为剧烈。通过与理论预测模型的对比研究：Cheng模型对干涸前具有较高的预测精度，在30%误差带内预测精度达到77.1%，绝对平均偏差为20.5%，干涸后对应预测精度比与绝对平均偏差仅为22.9%与57.8%。

关键词: 二氧化碳, 流动沸腾, 传热系数, 干涸