Numerical Study On Convective Heat Transfer Characteristics Of Supercritical CO₂ Flowing In Heat Exchanger For Solar Thermal Power Generation

With the development of clean energy utilization technology,Supercritical CO2 Brayton Cycle has received increasing attention in the field of solar thermal power generation.At present,research on supercritical CO2 convective heat transfer characteristics has becom e a research hotspot in this field.In this paper,the numerical calculation method is used to study the convective heat transfer characteristics of supercritical CO2 in various tubular types such as horizontal round tube,vertical round tube and vertical spiral tube under various working conditions such as heating and cooling.The main work and conclusions are as follows:(1)The convective heat transfer characteristics of supercritical CO2 On horizontal circular tubes were studied.The effects of heat flux,mass flux,pressure,pipe diameter and other factors on the heat transfer process were analyzed,and the heat transfer differences between heating and cooling conditions were compared.The results show that the effect of the buoyancy on the heat transfer is significant,and the heat transfer of the fluid at the upper and lower walls of the same section of the horizontal tube is significant.Under heating conditions,the fluid heat exchange at the upper wall surface is prone to deterioration.Increasing the mass flux and system pressure can improve the deterioration phenomenon.The heat transfer effect under cooling conditions is better than the heating condition,and the heat transfer coefficient peaks near the quasi-critical temperature.The heat transfer is strengthened.(2)The convective heat transfer characteristics of supercritical CO2 in a vertical spiral tube were studied.In the vertical spiral tube,supercritical CO2 is affected by both centrifugal and buoyancy forces,and the flow field and temperature field have obvious non-uniform distribution characteristics.When the buoyancy force is strong,it is inclined,and when the centrifugal force is strong,it is distributed up and down.The buoyancy factor Gr/Re2.7 can be used to accurately judge the influence of floating lift force on heat transfer in a vertical spiral tube.(3)The strengthening effect of vertical helical tube on supercritical CO2 convective heat transfer is studied.Compared with the vertical round tube,helical tube can effectively enhance the supercritical CO2 convective heat transfer,and its circumferential average heat transfer coefficient is increased by 7.2%on average.Under the conditions of high heat flux,high pressure and low mass flux,the effect of heat transfer enhancement of spiral tube is more prominent,but the peak value of heat transfer coefficient is decreased.