Research On Nouniform Heat Transfer Characteristics Of Supercritical Carbon Dioxide In Helical Tubes

The intercooling regenerative cycle can significantly improve the efficiency and power of the gas turbine cycle by reducing the compression work consumed by the high-pressure stage of the compressor,and it has been widely used in the aerospace field.Supercritical carbon dioxide is used as the refrigerant in the intercooler,and its thermal properties change with temperature changes,and the thermal properties change more rapidly near the quasi-critical temperature.The compact Helical tubes are often used in intercoolers.The coupling of pipe structure and thermophysical properties under the action of gravity leads to the problem of circumferential non-uniform heat transfer,which has attracted extensive attention.Based on the cooling heat transfer problem in an aero-engine intercooler,a numerical study on the heat transfer characteristics of supercritical pressure carbon dioxide in a vertical spiral tube is carried out.Five size physical models of spiral pipes were constructed,and a mathematical model was established according to the flow and heat transfer characteristics of supercritical fluid.Based on the thermophysical properties of CO₂ under different pressures,the grid independence was verified,and the optimal grid was obtained.The most accurate turbulence model is obtained through the comparison of three turbulence models.Basic heat transfer characteristics in the helical tube were explored by investigating heat transfer in the flow direction and the local heat transfer in the circumferential direction of the tube.The circumferential non-uniform heat transfer mechanism caused by the buoyancy force and centrifugal force was set forth by analyzing the distribution of the tube cross section temperature field,the density field,the flow field,and the turbulent energy,and the effects of secondary flow were analyzed quantitatively by secondary flow velocity and intensity.The results show that the non-uniform heat transfer upstream of the pipeline is from the comprehensive effect of buoyancy force and centrifugal force.The buoyancy force disappears in the pipeline downstream,and the non-uniform heat transfer is dominated by centrifugal.The impact mechanism of operating parameters on heat exchange was explored.The results show that the increase of operating pressure and mass flow rate and the decrease of heat flux density enhanced heat exchange and weaken the effects of buoyancy force and centrifugal force,and these weaken the circumferential non-uniform heat transfer.The effects of pipe structure parameters and inclination angle on the pipe heat transfer were explored.The research results show that the increase of the thermal conductivity of the pipe and the pitch of the helical pipe weakens the buoyancy force and centrifugal force,resulting in the weakening of the non-uniform heat transfer.However,the diameter of the helical tube is increased,which makes a singular change in the non-uniform heat transfer.Compared with the horizontal arrangement,the combined effect of the buoyancy force and the centrifugal force is stronger when the helical pipe is arranged vertically,the secondary flow is stronger,and the turbulent mixing is more complicated,which finally aggravates the phenomenon of non-uniform heat transfer in the circumferential direction of the pipe.The new buoyancy parameters and buoyancy criteria were proposed based on the pipeline structural characteristics,and a new heat transfer correlation was obtained.was obtained.The results show that when Bu≥1.6×10^-5 is satisfied,the buoyancy force plays a leading role in heat transfer.The new heat transfer correlation can be better applied to the prediction of heat transfer in the helical tube.