Study On The Flow And Heat Transfer Characteristics Of Supercritical CO₂ Within Vertically Annual Upward Channel

In recent years,with the intensification of energy consumption,China has faced serious problems of energy resource shortage and environmental pollution.It is urgent to find ways to improve energy utilization efficiency and reduce pollutant emission.Due to its special heat transfer characteristics,easy access and high safety,supercritical CO₂ has attracted extensive attention.Therefore,It is of great significance to study the flow heat transfer of supercritical CO₂.In this paper,by using experimental methods and numerical simulation methods,the visualization experimental study and corresponding numerical simulation study of supercritical CO₂ in a square annular cavity channel were carried out,and the heat transfer mechanism of supercritical CO₂ was analyzed from the macroscopic level and the microscopic boundary layer level.In the experiment,it is observed that the blurred shadow phenomenon occurs near the heating wall surface under multiple heat transfer deterioration conditions.The analysis shows that the area is a fluid with lower density and higher flow velocity,which is the same as the mainstream state.This phenomenon well confirms the single-phase convection theory,and the blurred shadow can be the direct evidence of heat transfer deterioration.Then the effects of system pressure,mass flow rate,inlet temperature and heat load on the heat transfer characteristics of supercritical CO₂ flow are analyzed.The result shows that the influence of pressure on heat transfer can be ignored in this experiment.Under the condition of low mass flow,the influence of buoyancy on heat transfer is weak.Under the condition of high mass flow,the local buoyancy effect has a great influence on heat transfer.With the increase of inlet temperature and heat flux,the peak value of wall temperature will gradually increase,and the axial position corresponding to the peak value of wall temperature will be closer to the inlet position of the experimental section.Then,the applicability of the floating lift parameters was verified,and the experimental results were compared with the classical heat transfer correlation.Finally,the heat transfer correlation with higher accuracy is fittedIn the numerical simulation study,it is found that under the condition of heat transfer deterioration,the density of the fluid in the boundary layer decreases and the velocity increases.The turbulent kinetic energy is greatly reduced,the buoyancy effect is enhanced,the turbulence intensity is weakened,and the deterioration process is further intensified.When the deterioration degree is the most serious,a special velocity distribution similar to the"M"shaped velocity distribution in the circular pipe is formed in the boundary layer,the radial shear stress increases in reverse direction,which weakens the heat transfer deterioration process,the turbulence intensity begins to recover until the heat transfer deterioration process finally ends.Under the condition of heat transfer enhancement,the peak value of the heat transfer coefficient of supercritical CO₂ corresponds to the quasi-critical temperature of the fluid in the buffer layer at the axial position,and the quasi-critical temperature in the bottom layer of laminar flow is a sign that the heat transfer coefficient begins to rise sharply.