Study On Density Field Distribution And Buoyancy Influence Of Supercritical CO₂ Binary Mixing Medium

Adding the second component propane to supercritical CO₂ can not only solve the problems of high operating pressure and difficult condensation,but also improve the system cycle efficiency.Therefore,supercritical CO₂-propane binary mixed working medium has important application value in air conditioning,heat pump and power circulation system.In this paper,the characteristics and influence rules of heat transfer deterioration of supercritical CO₂-propane binary mixing medium in a tube were studied,and the flow field structure information such as density field and velocity field was obtained.Based on the density field distribution,the influence mechanism of floating lift on heat transfer deterioration was analyzed.The main work contents are as follows:In this paper,the visualization experimental research platform of supercritical CO₂ binary mixing medium was built to verify the reliability of the experimental system and provide experimental data for the verification of mathematical models in the numerical simulation process of supercritical CO₂ binary mixing medium.Based on background Schlieren technique,the density field visual measurement experimental research system is constructed.The density field visual measurement experiment of gas-liquid two-phase mixed convection and natural convection heat transfer process in square cavity was carried out,and the density field distribution rule under different working conditions was obtained.The obtained experimental results were combined with the numerical simulation results to analyze the influence mechanism of floating lift on heat transfer deterioration.Numerical simulation of supercritical CO₂-propane working medium was carried out to analyze the influence of concentration ratio,heat flux,mass flow rate and other factors on heat transfer deterioration.The results show that with the increase of propane concentration and mass flow rate,the local wall temperature peak decreases gradually and the heat transfer deterioration degree weakens gradually.Increase heat flux,heat transfer deterioration degree increases.The phenomenon of double wall temperature peak is found and its action mechanism is analyzed.In the process of vertical upward flow,the first wall temperature peak is caused by the combined action of thermal conductivity and turbulent kinetic energy reduction in the transition zone.Under the combined action of both,the first wall temperature peak is very sharp.The second wall temperature peak is caused by the decrease of turbulent kinetic energy in the mainstream core area,and the second peak is relatively flat.Combined with the flow field distribution law measured by numerical simulation and experiment,the influence mechanism of buoyance effect on heat transfer deterioration of supercritical CO₂-propane binary mixing medium was analyzed.The results show that the buoyancy force caused by the uneven distribution of density field changes the velocity distribution of flow field,which inhibits turbulence and leads to the deterioration of heat transfer.The applicability of the two buoyancy parameters is analyzed.The results show that the two buoyancy parameters have good applicability in the identification of the influence of the supercritical CO₂-propane binary mixing medium on the buoyancy.