Abstract:

An experimental investigation on the heat transfer characteristics of CO₂ during gas cooling process in a horizontal tube is conducted. The experimental data is obtained over a mass flux range of 79.6-358.1 kg·m^-2·s^-1, inlet pressure range of 7.5-9.0 MPa and mean bulk temperature from 25.0 to 50.0℃. The effects of mass flux, pressure and bulk temperature on the heat transfer efficiency are investigated. The combined parameter of Gr/Re² is used to quantify the buoyancy force effect on the heat transfer. The experimental results show that the heat transfer coefficient of the CO₂ increases with increasing mass flux. The peak value of the heat transfer coefficient shifts to a higher temperature region as the pressure increases. It appears that the peak value of the heat transfer coefficient occurs at bulk temperature slightly lower than the pesudo-critical temperature at low mass flux. The lower mass flux is reached, the greater influence of buoyancy force effect. The experimental heat transfer coefficients are compared with some existing correlations. The predicted results present obvious deviation compared to the experimental results at low mass flux. A new heat transfer correlation for the tube is proposed based on the experimental data. The maximum error between the predicted results of the new correlation and the experimental data is 20%.

Key words: heat transfer, supercritical CO₂, convection, buoyancy, pesudo-critical temperature

摘要：

开展了低质量流速下超临界CO₂在水平直管内冷却过程的换热特性的实验研究。实验压力为p=7.5~9.0 MPa,质量流速为G=79.6~358.1 kg·m^-2·s^-1,流体温度为25.0~50.0℃。分析了质量流速、压力、流体温度对换热的影响,并引入Richardson数阐述浮升力对超临界CO₂在水平直管内冷却换热影响。实验结果表明: 传热系数随着质量流速的增加而增大。传热系数峰值点随压力的升高向高温区偏移。当质量流速较小时,传热系数峰值点出现在准临界温度之前,且浮升力作用加大,流体处于混合对流状态。将传热系数的实验值和已有的换热关联式计算值作对比后发现在低质量流速下误差较大,拟合了低质量流速工况的超临界CO₂在水平直管内冷却换热的关联式,94%的实验值和拟合关联式误差在±20%范围内。

关键词: 传热, 超临界CO₂, 对流, 浮升力, 准临界温度