超临界压力CO2在水平圆管内流动传热数值分析

采用SST k-w低雷诺数湍流模型对加热条件下超临界压力CO2在内径di=22.14 mm,加热长度Lh=2440 mm水平圆管内三维稳态流动与传热特性进行了数值计算.通过超临界CO2在水平圆管内的流动传热实验数据验证了数值模型的可靠性和准确性.首先,研究了超临界压力CO2在水平圆管内的流动传热特点,基于超临界CO2在类临界温度Tpc处发生类液-类气“相变”的假设,揭示了水平圆管顶母线和底母线区域不同的流动传热行为.然后,分析了热流密度qw和质量流速G对水平圆管内超临界压力CO2流动换热的影响,通过获取流体域内的物性分布、速度分布和湍流分布等详细信息,重点解释了不同热流密度qw和质量流速G下顶母线内壁温度Tw,i分布产生差异的传热机理,分析结果确定了类气膜厚度d、类气膜性质、轴向速度u和湍动能k是影响顶母线壁温分布差异的主要因素.研究结果可以为超临界压力CO2换热装置的优化设计和安全运行提供理论指导.

Numerical analysis on flow and heat transfer of supercritical CO2 in horizontal tube

In the present study,the three-dimensional steady-state numerical simulation has been performed by using ANSYS Fluent15.0 with SST k-w low Reynolds turbulence model to study flow and heat transfer characteristics for supercritical CO2 in the horizontal straight tube with inner diameter di=22.14 mm and heating length Lh=2440 mm under heating condition.The reliability and accuracy of the numerical model was verified by the experimental data of flow and heat transfer of supercritical CO2 in horizontal tube.Firstly,flow and heat transfer characteristics of supercritical CO2 was studied in horizontal tube.Based on the assumption that the supercritical CO2 will undergoes“phase transition”between liquid-like and vapor-like at pseudocritical temperature Tpc,the differences between top generatrix and bottom generatrix of horizontal tube at flow and heat transfer behaviors were revealed.The results show flow and heat transfer characteristics of supercritical CO2 in horizontal tube are similar to those under subcritical pressure.Then,the influences of heat flux qw and mass flux G on flow and heat transfer of supercritical CO2 were analyzed.The higher heat flux qw is or the smaller mass flux G is,the higher inner wall temperature Tw,i at top generatrix is.The reasons for difference in the distribution of inner wall temperature Tw,i at top generatrix under different heat flux qw and mass flux G were explained by capturing detailed information about thermophysical properties distribution including specific heat at constant pressure cp and thermal conductivity l,axial velocity distribution and turbulent kinetic energy distribution in the fluid domain.It is observed that vapor-like film thickness d,vapor-like film property characterized by specific heat at constant pressure cp and thermal conductivity l,axial velocity u and turbulent kinetic energy k are the main factors affecting the difference in inner wall temperature distribution at top generatrix.The present work can provide a theoretical guidance for design and safe operation of heat exchanger for supercritical CO2 Brayton cycle.