Experimental Investigation Of Enhanced Heat Transfer Of CO₂ At Super-critical Pressures In Small Vertical Tubes

Recently,the supercritical pressure fluid is widely used in nuclear energy utilization,heat pump and cryogenic refrigeration systems,aerospace and other fields.At high heat flux density conditions or small mass flow rates,convective heat transfer of supercritical pressure fluids may cause heat transfer to deteriorate,causing significant effects on the system.Therefore,it is necessary to further reveal the laws of flow and heat transfer of supercritical pressure fluid in the vertical pipelines and then put forward structure to suppress the heat transfer deterioration and strengthen the heat transfer based on the existing research.In this paper,the turbulent convective heat transfer of supercritical pressure CO₂ in a vertical small tube with an inner diameter of 1 mm were experimentally studied.The experimental results showed that due to the drastic change of the properties of supercritical pressure fluid near the quasi-critical point,the formation of the floating force suppressed the turbulent mixing intensity at the wall surface when it flows upwards,resulting in the deterioration of heat transfer.For downward flow,floating force enhanced the turbulent blend strength and enhanced turbulent convective heat transfer.In order to further solve the problem of turbulent heat transfer deterioration of supercritical fluid flowing upward in a circular tube,this paper proposes the idea of reducing the turbulent flow field close to the wall by interpolating microstructures,and even by adjusting the structure of interpolation to enhance heat transfer.Therefore,this paper designed for the vertical diameter of 1mm diameter pipe straight spiral tube structure and carried out the supercritical pressure CO₂ turbulent heat transfer experimental study.Experiments were carried out on two different configurations of stainless steel spiral filaments:?1?stainless steel wire diameter 0.1mm,pitch 1.25mm;?2?stainless steel wire diameter 0.2mm,pitch 1.25mm.In these two kinds of interpolating spiral wire straight pipes,the temperature of the wall surface in the up-flow turbulent heat exchange increased monotonically along with the above supercritical pressure CO₂ turbulent heat transfer in a vertical smooth tube,no local peak,and the temperature of the upward and downward wall surfaces was almost the same.The experimental results showed that the turbulent heat transfer of upward flow of supercritical pressure fluid could be effectively suppressed and the heat transfer could be effectively enhanced by inserting the spiral structure in a vertical pipe.Compared with the experimental results of turbulent convective heat transfer of supercritical CO₂in smooth tube,the PEC of the 0.1mm diameter enhanced tube was between 1.8 and 5.6,while the PEC of the 0.2mm diameter enhanced tube was between 1 and 1.4,therefore the helical wire with a diameter of 0.1mm had a better overall enhanced heat transfer performance.