Numerical Study On The Thermal Performance Of U-Tube Evacuated Solar Collector Tube

The utility of solar energy which is a renewable energy is an effective way to reduce the consumption of fossil fuel and improve the environment and air quality. The thermal performance of solar collector as one of utilization mode of solar energy is widely studied. The U-tube solar collector is widely used in our country for its high pressure-bearing and strong freezing tolerance. At present, the method to study the performance of U-tube solar collector is mainly one-dimensional theory analysis and there is rarely the three-dimensional research on the coupled heat transfer of U-tube solar collector. The study of heat transfer mechanism of U-tube solar collector is significant to improve the solar-thermal conversion efficiency and the comprehensive utilization efficiency of solar energy. In this paper, the heat transfer mechanism and heat transfer mode of single U-tube solar collector tube are researched further in three-dimensional numerical method. The main content of this paper is as below.Firstly, the physical model and mathematical model of U-tube, U-type vacuum solar collector tube with copper fin and filled-type evacuated solar collector tube with U-tube are established. The conduction, convection and radiation in the solar collector tube are considered and the non-uniform boundary condition is more close to the practical situation.Secondly, the heat transfer characteristic of U-tube and parameters affecting its thermal performance are studied by numerical method. Results show that the natural convection in the cross section of U-tube can strengthen the heat transfer in the U-tube. The coefficient of heat transfer of U-tube in mixed convection is double that of U-tube in pure forced convection when the heat flux density on the wall is1000W/m2.Thirdly, the thermal performance of single U-type vacuum solar collector tube with copper fin is researched. The effects of air layer between the absorber tube and copper fin and natural convection in the U-tube on the heat transfer of U-type vacuum solar collector tube with copper fin are discussed. The elements influencing the heat transfer of U-type vacuum solar collector tube with copper fin such as fluid inlet velocity, fluid inlet temperature and solar radiation intensity are analyzed. It turns out that the air layer between the absorber tube and copper fin reduced the thermal efficiency of U-type vacuum solar collector tube with copper fin by6%.Finally, the thermal performance of filled-type evacuated solar collector tube with U-tube is studied. The influence of boundary condition, filler conductivity and natural convection in the cross section of U-tube on the heat transfer characteristic are analyzed. The temperature field and the flow field in different fluid inlet temperature, different inlet velocity and different radiation are researched. The thermal performance of U-type vacuum solar collector tube with copper fin and filled-type evacuated solar collector tube with U-tube are compared. The results indicate that the thermal efficiency of filled-type evacuated solar collector tube with U-tube is6%higher than that of U-type vacuum solar collector tube with copper fin, which exactly eliminates the influence of air layer between the absorber tube and copper fin on the thermal efficiency of U-type vacuum solar collector tube with copper fin. And the filler conductivity has great influence on the temperature field, however the total heat gain of filled-type evacuated solar collector tube with U-tube change little in different filler conductivity.In this paper, the study of single the U-tube solar collector can lay the theoretical foundation for further study of it and promote its utility in expansive areas.