Study On Flow And Heat Transfer Characteristics Of Supercritical CO₂-propane Binary Mixture

Propane（R290）is a natural environmentally friendly refrigerant（ODP=0,GWP=3）.After mixing with CO₂,it can effectively solve the problems of high operating pressure and difficult condensation of the system,while ensuring high system cycle efficiency,CO₂-propane binary mixed working fluid has broad outlook for applications in air conditioners,heat pumps,refrigeration systems,and S-ORC power cycle systems.In this paper,the heat transfer characteristics of a two-component mixture of supercritical CO₂-propane in a horizontal circular tube were studied by experiments and numerical simulations.The physical field parameter distribution characteristics of the mixed refrigerant in the pipe are obtained,and it is analyzed that the effects of dramatic changes in physical parameters and buoyancy affect the laws of flow and heat transfer.In this paper,an experimental system for supercritical CO₂ binary mixed working fluid flow and heat transfer in a horizontal circular tube is built.Compared with the traditional D-B heat transfer correlation,the reliability of the experimental system for flow heat transfer of supercritical CO₂ mixed medium is verified,which provides a data basis for numerical simulation of turbulence model verification of supercritical CO₂ mixed medium.In this paper,a numerical simulation study of supercritical CO₂-propane mixed working fluid in a horizontal circular tube is carried out,and analyze the effects of mixed working fluid concentration ratios,heat flux,mass flow,inlet pressures,and other factors on heat transfer.The results show that:The CO₂-propane mixed working fluid can reduce the critical pressure of the system and increase the critical temperature.After the quasi-critical temperature,the heat transfer coefficient of the mixed working fluid is higher than that of pure CO₂.Increase the heat flux,weaken the heat transfer in the tube;Heat transfer can be improved by increasing mass flow rate.The physical field distribution characteristics of the binary mixture of supercritical CO₂-propane in a horizontal circular tube have been carried out,and the research shows that:As the concentration of propane increases,the peak specific heat capacity of the mixed working fluid gradually decreases,and the density,specific heat capacity and thermal conductivity are greater than pure CO₂.The buoyancy effect caused by the density difference between the fluids causes the temperature and velocity distribution along the radial distribution and the tube length direction to produce stratification.The temperature at the top of the pipe is higher than that at the bottom,and the velocity at the top of the pipe is lower than that at the bottom.The increase in mass flow rate makes the streamline changes on both sides relatively regular,the vortex on both sides is more obvious,the turbulent kinetic energy and speed are significantly enhanced,and the heat exchange is enhanced.Develop the influence of buoyancy effect on heat transfer,and calculate three buoyancy factors,all of which are greater than their thresholds,it shows that buoyancy plays an important role in the flow and heat transfer in the supercritical CO₂-propane mixed working fluid tube.The buoyancy factor increases first and then decreases with increasing liquid temperature.With increasing mass flow,the buoyancy effect weakens and the heat transfer increases.With increasing heat flux,the buoyancy increases and the heat transfer in the pipe weakens.