Study On The Flow And Heat Transfer Characteristics Of Supercritical Nitrogen Within Vertically Downward Round Tube

In the 21st century,the fields of energy,aerospace,life sciences,medical treatment,new materials and other application projects have developed rapidly,and power of the equipment has gradually increased with the corresponding heat load increasing at the same time.As a basic discipline,heat transfer is also facing unprecedented challenges.It is urgent to find efficient and reliable cooling methods to deal with the challenge of heat load.Supercritical fluids,due to their different flow and heat transfer characteristics from other state fluids,make supercritical fluids own a wide range of applications or potential application value in chemical engineering,aerospace technology,power engineering and so on.More and more researchers turn the research content to supercritical fluid heat transfer.At present,there are many researches who focus on supercritical CO₂ and water at home and abroad,but there is few basic research on supercritical nitrogen.Therefore,the vertical downward flow heat transfer of supercritical nitrogen in circular tube is studied in this paper.The influence of system parameters such as mass flow rate,heat flux and system pressure on the heat transfer characteristics of supercritical nitrogen is analyzed.The characteristics of heat transfer are analyzed from the buoyancy and thermal acceleration,and the mechanism of heat transfer is deeply studied.At the same time,the influence of pipe diameter on supercritical nitrogen heat transfer is also analyzed.The characteristics of supercritical nitrogen heat transfer are analyzed from the aspects of heat transfer coefficient,buoyancy force,thermal acceleration,boundary layer effect and so on by simulating and comparing the system parameters such as mass flow rate,heat flux density,system pressure and so on.The results show that when the system pressure is different,the heat transfer coefficient reaches the peak value and then decreases with the increase of the mainstream temperature.Given higher system pressure,the peak value of the heat transfer coefficient moves backward and the heat transfer coefficient decreases as a whole.It is found that a higher mass flow rate can make the heat transfer effect better and the enhancement effect of heat transfer more obvious.In the process of falling flow,the buoyancy force generated will inhibit heat transfer and deteriorate the heat transfer of fluid.Therefore,the higher the heat flux is,the lower the heat transfer coefficient is.At the same time,when the mass flow rate is high,the buoyancy criterion number Bo^*is less than the threshold value,and the effect on heat transfer is ignored,which can effectively restrain the deterioration of heat transfer.The number of thermal acceleration criterion Kv is lower than the threshold value,and the effect of thermal acceleration on heat transfer is not obvious and neglected in the vertical downward flow process.