热流密度与质量流速之比对超临界CO₂竖直圆管内传热影响

Effect of Ratio of Heat Flux to Mass Flow Rate on Heat Transfer of Supercritical CO₂ in a Vertical Tube

超临界二氧化碳(SCO2)布雷顿循环动力系统具有效率高、结构紧凑等特点,虽然SCO2作为循环工质具有诸多优势,但会伴随着传热恶化现象,影响设备性能。本文在压力为8.7~9.0 MPa、质量流速为80~380 kg∙m−2∙s−1、热流密度为3~120 kW∙m−2的参数范围内,对内径为10 mm的垂直向上流动光滑圆管中SCO2的传热特性进行了实验与数值研究,观察到明显传热恶化现象。分析了热流密度与质量流速之比(qw/G)对SCO2传热特性的影响,并进一步通过将数值模拟结果与实验结果进行对比分析,对数值模拟结果进行评估。结果表明:在同一工作压力下,qw/G决定了SCO2的传热状态,随着qw/G增加,圆管内CO2传热出现恶化,并且随着qw/G增加壁温峰值向加热段起点移动;模拟结果表明,当qw/G大于某一临界值时,在轴向壁温分布中出现两处壁温峰值,且在相应位置传热系数出现下降;数值模拟得到相应测温点处壁温与实验测得壁温相对误差为±8%,对于qw/G较小的工况,误差为±3%。

With the advantages of high efficiency and compact equipment, supercritical carbon dioxide (SCO2) is extremely promising, at the same time, supercritical carbon dioxide as the working fluid will be accompanied by the deterioration of heat transfer and affect the performance of equipment. In this paper, both experimental and numerical simulation methods are used to study the heat transfer characteristics of SCO2 in vertical upward tube under the pressure range of 8.7~9.0 MPa, mass flow rate range of 80~380 kg∙m−2∙s−1 and heat flux range of 3~120 kW∙m−2. The effect of the ratio of heat flux to mass flow rate on heat transfer of SCO2 was analyzed, and the numerical simulation results were compared with the experimental results. The results show that under the same working pressure, qw/G determines the heat transfer state of SCO2. With the increase of qw/G, heat transfer of SCO2 deteriorates in the vertical tube, and the peak of wall temperature moves to starting point of heating section. The simulation results show that when qw/G is greater than a certain critical value, there are two peak values of wall temperature in the axial wall temperature distribution, and the heat transfer coefficient decreases in the corresponding position. In addition, at the same position, the relative error between the wall temperature calculated and measured in the experiment is ±8%, while for the working condition with small G, the error is ±3%.