Numerical And Experimental Study On Flow And Heat Transfer Characteristics For Supercritical Carbon Dioxide In Vertical Tubes

As a fluid working fluid,supercritical CO2 is widely used in Brayton cycle power generation,printed circuit board heat exchangers and refrigeration systems,and the corresponding flow and heat transfer characteristics have important research significance.In this paper,supercritical CO2 is used as the research object,using experiments and numerical simulation methods to study the flow and heat transfer characteristics of supercritical CO2 when flowing vertically upwards in circular,square,and semicircular tubes.The influence of factors such as pipe diameter,flow direction,heat flux density,mass flow rate and buoyancy force on the flow and heat transfer characteristics of supercritical CO2 is numerically analyzed.By building a supercritical CO2 experimental platform,the experimental research of supercritical CO2 was carried out,and the flow and heat transfer characteristics under limited working conditions were obtained.The main work is as follows:1.In order to verify and analyze three turbulence models,the verification results show that the K-ε Realizable enhanced wall function turbulence model is in good agreement with the experimental values,and based on the verification results,the velocity field,temperature field and physical property distribution in the tube are analyzed.2.Analyzed the influence of inlet pressure,mass flow rate,heat flux density,inlet temperature,flow direction and pipe diameter on the heat transfer of supercritical CO2 in the vertical upward flow of the tube,and analyzed the reasons,using the least square method according to the numerical simulation.A new criterion correlation is obtained by data fitting.3.The effects of buoyancy and flow acceleration on the heat transfer of supercritical CO2 flow in a vertical tube are explored.When flowing vertically upwards,buoyancy restrains heat transfer,and when flowing vertically downwards,buoyancy enhances heat transfer.The applicability and threshold values of the four buoyancy parameters are analyzed.The results show that the BuL number describes the effect of buoyancy most accurately,and the threshold value of the BuL number is modified to make it more accurately describe the effect of buoyancy.4.The supercritical CO2 experimental platform was built,and the experimental investigation of the flow and heat transfer of supercritical CO2 in a vertical tube was carried out.Experimental results show that when supercritical CO2 flows vertically upwards,the buoyancy force enhances heat transfer,while when it flows vertically downwards,the buoyancy force inhibits heat transfer.The higher the heat flux density,the more obvious the effect is.