Measurements Of Transient Thermal Convection Of Supercritical Fluids Using Phase-shifting Interferometry

Supercritical fluid has extensive application prospects.In particular,the application of the supercritical CO2 Brayton cycle is of great significance for achieving the goal of "carbon neutrality".A comprehensive understanding of the heat and mass transfer characteristics of supercritical fluids is the basis for further application of supercritical fluids.And the research on the transport behavior of complex flow strongly depends on the development of high-efficiency measurement technology.Interferometry has been widely used in the visualization measurement of density fields and concentration fields.In recent years,phase-shifting interferometry has developed rapidly and widely used,which can improve spatial resolution effectively.And the application of phase-shifting interferometry has just begun,the experimental research of complex flow needs the development of high-efficiency interferometric technology and further exploration of its transport characteristics.This research focuses on measuring the transient thermal convection characteristics of supercritical CO2 based on phase-shifting interferometry.Firstly,this research will build a phase-shifting interferometer and develop related image processing technology such as phase-shifting technology and phase unwrapped technology.Then,experimental research of the slowly mass transfer and fastly mass transfer.Finally,experimental research on the transient thermal convection of the subcritical and supercritical CO2 in a square cavity.Firstly,this research summarizes the principle of phase-shifting interferometry,including phase shifting technology and phase unwrapping technology.A pixelatedarray masked phase-shifting interferometry based on Mach-Zehnder interferometry has been built.The phase resolution of phase-shifting interferometry is more than 100 higher than traditional interferometry.The phase-shifting interferometer can simultaneously acquire interference images with four phases.And the phase-shifting interferometry can more accurately measure flow fields with small density changes,small scales,or fast-changing.Precise measurement and visualization of the mass transfer process in diffusion system is of critical importance to the control of pollutants,drug diffusion,chemical reaction and crystallization processes,etc.The experiment first designed a constant temperature diffusion chamber and then measured the mass diffusion behavior of Ethanol-water solution system,including Liquor-water solution system.Furthermore,The experiment has calculated the mass diffusion coefficients of all solution systems.The phase-shifting data shows the movement of the diffusion interface and the detailed trends show transient dependence of mass transfer process on both initial concentration and time.Those results of real Liquor diffusion in the water phase show that the complex components inside the Liquor may affect a lot the mass diffusion process.In addition,the experiment measured the mass diffusion coefficient of NaCl,K2SO4,and NH4Cl solutions diffusion in the water.To further analyze the characteristics of the mass transport at the moment of contact of the binary solution.The experiment measured the transient concentration field of 50 mg/mL NaCl,glycerol and bovine serum albumin at the moment of contact with water using a high-speed phase shift interferometer.The equivalent mass diffusion coefficient to quantitatively compare the transient mass transfer speed has been proposed in this research.And the equivalent mass diffusion coefficient of NaCl,glycerol and bovine serum albumin is 1.49×10-5 m2/s,6.82×10-5 m2/s and 1.07×10-5 m2/s respectively.Furthermore,The experimental results show the different features of molecular diffusion,including the obvious center movement of sodium Chloride,expansion of mass diffusion front for glycerin and limited local mass transfer characteristics of BSA solution.The phase-shifting interferometer has been applied visualization research of supercritical fluid in this research,which has provided higher spatial resolution visualization results.This study has designed a visualization cell suitable for supercritical fluid interference experiments and proposed a method to measure transient thermal convection characteristics using phase-shifting interferometer.The higher spatial visualization results of the unwrapping phase field and density field have been obtained.The experiment results show that the instability of thermal convection increases with the increase of heat flux and pressure at the subcritical state.The experiment revealed a vaulted expanding heat flow under low pressure,a steadily rising thermal plume at higher pressure,and the thermal plume will become spiral rising with higher pressure.In the supercritical state,the instability of thermal convection will become stronger with the closer to the critical point and the increase of heat flux.The heat flow disturbance at the bottom develops in a small area when the input heat flux is small,and the thermal plume will fully develop at the bottom at the very early stages with the higher heat flux.The experiment has also observed several relatively independent development of thermal plumes,and several relatively separated bubblelike low-density clusters could be observed to rise at some moments.