高温吸热管内超临界CO2传热特性的数值模拟

研究超临界CO2在高温吸热管内的传热特性是将其应用于聚光太阳能热发电技术中的基础.本文对此进行了数值模拟研究,分析了流体温度、流动方向、系统压力、质量流率和热流密度对对流传热系数和Nu数的影响.结果表明:高温区(800—1050 K)的对流传热系数和Nu数受流动方向和系统压力的影响均很小,但都随着质量流率的增大以及热流...

Numerical investigation on heat transfer of supercritical CO in solar receiver tube in high temperature region

Supercritical CO2 can be used as a heat transfer fluid in a solar receiver,especially for a concentrating solar thermal power tower system.Such applications require better understanding of the heat transfer characteristics of supercritical CO2 in the solar receiver tube in a high temperature region.However,most of the existing experimental and numerical studies of the heat transfer characteristics of supercritical CO2 in tubes near the critical temperature region,and the corresponding heat transfer characteristics in the high temperature region are conducted.In this paper,a three-dimensional steady-state numerical simulation with the standard k-εturbulent model is established by using ANSYS FLUENT for the flow and heat transfer of supercritical CO2 in a heated circular tube with an inner diameter of 6 mm and a length of 500 mm in the high temperature region.The effects of the fluid temperature(823-1023 K),the flow direction(horizontal,downward and upward),the pressure(7.5-9 MPa),the mass flux(200-500 kg-m-2·s-1)and the heat flux(100-800 kW·m-2)on the convection heat transfer coefficient and Nusselt number are discussed.The results show that the convection heat transfer coefficient increases while Nusselt number decreases nearly linearly with fluid temperature increasing.Both fluid direction and pressure have negligible effects on the convection heat transfer coefficient and Nusselt number.Moreover,the convective heat transfer coefficient and Nusselt number are enhanced greatly with the increasing of mass flux and the decreasing of heat flux,which is more obvious at a higher heat flux.The influences of buoyancy and flow acceleration on the heat transfer characteristics are also investigated.The buoyancy effect can be ignored within the present parameter range.However,the flow acceleration induced by the high heat flux significantly deteriorates the heat transfer preformation.Moreover,eight heat transfer correlations of supercritical fluid in tubes are evaluated and compared with the present numerical data.The comparison indicates that the correlations based on the thermal property modification show better performance in the heat transfer prediction in the high temperature region than those based on the dimensionless number modification.And Nusselt number predicted by the best correlation has a mean absolute relative deviation of 8.1%compared with the present numerical results,with all predicted data points located in the deviation bandwidth of±20%.The present work can provide a theoretical guidance for the optimal design and safe operation of concentrating solar receivers where supercritical CO2 is used as a heat transfer fluid.