Yan Chenshuai, Xu Jinliang, Zhu Bingguo, Liu Guanglin
The Beijing Key Laboratory of Multiphase Flow and Heat Transfer, North China Electric Power University, Beijing 102206, China.
Key Laboratory of Power Station Energy Transfer Conversion and System, North China Electric Power University, Ministry of Education, Beijing 102206, China.
Materials (Basel). 2020 Feb 5;13(3):723. doi: 10.3390/ma13030723.
It is great significance to understand the mechanism of heat transfer deterioration of supercritical CO for heat exchanger design and safe operation in the supercritical CO Brayton cycle. Three-dimensional steady-state numerical simulation was performed to investigate the behavior of supercritical CO heat transfer in heated vertical up-flow tube with inner diameter d = 10 mm and heated length L = 2000 mm. Based on the characteristics of inverted-annular film boiling at subcritical pressure, the heat transfer model of supercritical CO flowing in the heated vertical tube was established in this paper. The mechanisms of heat transfer deterioration (HTD) and heat transfer recovery (HTR) for supercritical CO were discussed. Numerical results demonstrate that HTD is affected by multiple factors, such as the thickness and property of vapor-like film near the wall, the turbulence intensity near the interface between liquid-like and vapor-like, and in the liquid-like core region as well as the distribution of radial velocity vector. Among the above factors, the change of turbulent kinetic energy caused by the buoyancy effect seems to be a more important contributor to HTD and HTR. Furthermore, the influences of heat flux and mass flux on the distribution of wall temperature were analyzed, respectively. The reasons for the difference in wall temperature at different heat fluxes and mass fluxes were explained by capturing detailed thermal physical properties and turbulence fields. The present investigation can provide valuable information for the design optimization and safe operation of a supercritical CO heat exchanger.
对于超临界CO₂布雷顿循环中的换热器设计和安全运行而言，了解超临界CO₂传热恶化机理具有重要意义。本文进行了三维稳态数值模拟，以研究内径d = 10 mm、加热长度L = 2000 mm的垂直向上流动加热管内超临界CO₂的传热行为。基于亚临界压力下倒环状膜态沸腾的特性，建立了超临界CO₂在垂直加热管内流动的传热模型。讨论了超临界CO₂传热恶化（HTD）和传热恢复（HTR）的机理。数值结果表明，传热恶化受多种因素影响，如壁面附近类汽态膜的厚度和性质、类液态与类汽态界面附近以及类液态核心区域的湍流强度，以及径向速度矢量的分布。在上述因素中，浮力效应引起的湍动能变化似乎是传热恶化和传热恢复的更重要贡献因素。此外，分别分析了热流密度和质量流率对壁温分布的影响。通过获取详细的热物理性质和湍流场，解释了不同热流密度和质量流率下壁温差异的原因。本研究可为超临界CO₂换热器的设计优化和安全运行提供有价值的信息。
