CO_2在立式螺旋管内流动沸腾换热的实验研究

Experimental Investigation on Flow Boiling Heat Transfer of CO_2 in a Helically Coiled Tube

在管内径9.0 mm、壁厚1.5 mm、螺旋管绕径283.0 mm的立式螺旋管内，对CO2流动沸腾换热特性进行实验研究。分析热流密度（q=1.4-48.0 kW/m2）、质量流速（G=54.0-400.0 kg/（m2？s））和运行压力（pin=5.6-7.0 MPa）对内壁温分布和换热特性的影响规律。结果表明：螺旋管内壁温周向分布不均匀，单相液体以及过热蒸汽区离心力的作用使内侧母线温度最高、外侧母线温度最低，在两相沸腾区蒸汽受到浮升力作用聚集在管上部而容易发生蒸干，因此上母线温度最高，温度最低值则由离心力和浮升力的相对大小共同决定。局部平均换热系数随热流密度以及进口压力的增加而显著增加，但增大质量流速对换热系数的影响不大，表明核态沸腾是 CO2在螺旋管内流动沸腾的主要传热模式而强制对流效应较弱；发现了随着热流密度增加所引起的核态沸腾强度变化以及干涸和再润湿使得换热系数随干度的变化可分成3个区域。并基于实验获得的2124个数据点拟合两相区沸腾换热关联式。

Within the ranges of pressure from 5.6 to 7.0 MPa, mass flux from 54.0 to 400.0 kg/（m2？s） and heat flux from 1.4 to 48.0 kW/m2, an experimental investigation was conducted on flow boiling heat transfer of CO2 through a helically coiled tube. The test helically coiled tube has an inner diameter of 9.0 mm, a wall thickness of 1.5 mm and a coil diameter of 283.0 mm. The effects of heat flux, mass flux and operating pressure on heat transfer coefficient and inner wall temperature distribution were discussed. The experimental results show that the inner wall temperature distributions along the circumference were non-uniform. For the sing-phase liquid flow and the superheated vapor flow, the inner wall temperature was highest in the inside and lowest in the outside, attributing to the centrifugal force. But in the two-phase area, the highest inner wall temperature appears at the top, which is because that the vapor phase gathers at the top due to the buoyancy force, tending to dry out. The location where the lowest temperature happened is determined by the combined effects of buoyancy force and centrifugal force. The local average heat transfer coefficient increases with increasing heat flux and inlet pressure, but the increment of mass flux has no effect on the heat transfer, suggesting that the nucleate boiling is the dominant mechanism while the forced convection effect is weak. The intensity of nucleate boiling changes with increasing heat flux and the variation of heat transfer coefficient with vapor quality can be divided into three different zones, which is induced by the alternative of wall dry-out and rewetting. A new correlation of local average heat transfer coefficient has been proposed based on the 2 124 data points.