Study On Heat Transfer Performance And Dynamic Characteristics Of Carbon Dioxide Cryogenic Heat Pipes

With the rapid development of cryogenic refrigeration technology,choosing an efficient and fast method to transfer cooling capacity has a very important impact on the field of cryogenic refrigeration.As a high-performance heat-conducting element,heat pipe has been widely used in heat transfer in medium and high temperature fileds,but its application in low temperature field is still insufficient.As an efficient refrigerant,carbon dioxide is an ideal cryogenic heat pipe working fluid.Therefore,the study of carbon dioxide cryogenic heat pipe can effectively broaden the application field of heat pipe.In this paper,carbon dioxide thermosiphon and its experimental system were designed and manufactured independently,and the cryogenic thermosiphon with R134 a as working fluid was the compared group.The heat pipe was heated by constant heat flow heating and alcohol bath heating.Among them,constant heat flow heating is indirect heating,and alcohol bath heating is at the bottom of the Dewar bottle.The experiment mainly measures the axial temperature of the heat pipe,and calculates the thermal resistance and the equivalent thermal conductivity of the heat pipe.In this paper,the steady-state heat transfer performance of carbon dioxide cryogenic thermosiphon under different working conditions is studied experimentally,including heating rate,liquid filling rate,length and position of condensation section,working temperature,non-condensable gas and inclination angle.The results show that:(1)Carbon dioxide thermosiphon has great heat transfer performance in the range of-56.56-0℃,and its performance is much better than that of heat pipe with R134 a as working fluid.Under the same experimental conditions,the thermal resistance of carbon dioxide heat pipe is about 25% of that of R134 a heat pipe,and the thermal resistance is range from 0.1 to 0.4K/W;(2)With the increase of heating rate and filling rate,carbon dioxide cryogenic thermosiphon has better heat transfer performance.,and thermal resistance decreases gradually;(3)Non-condensable gas has a great influence on heat transfer performance of heat pipe;heat transfer performance of heat pipe mixed with non-condensable gas decreases obviously,and the influence of non-condensable gas on R134 a heat pipe is much stronger than that of carbon dioxide heat pipe.Appropriate increase of the length of condensation section or downward movement of condensation section can effectively reduce the influence of non-condensable gas;(4)The angle of inclination has little effect on the heat transfer performance of carbon dioxide heat pipesIn order to study the stability of carbon dioxide heat pipe under variable working conditions,the dynamic performance of carbon dioxide cryogenic heat pipe was studied experimentally,including the influence of heating rate,filling rate and working temperature on start-up time and start-up temperature.The heat transfer performance of carbon dioxide heat pipe under variable working conditions and extreme working conditions was analyzed.The results show that:(1)there are two basic conditions for the start-up of carbon dioxide cryogenic gravity heat pipes:(1)the condensation temperature is lower than the evaporation temperature;and(2)there is an optimum temperature range between the evaporation temperature and the condensation temperature.When the condensation temperature or the evaporation temperature is lower than the three-phase point of carbon dioxide,the carbon dioxide cryogenic gravity heat pipes can not work normally;(2)the carbon dioxide cry The start-up time of carbon dioxide cryogenic gravity heat pipe increases with the decrease of condensation temperature,and decreases with the increase of heating power and filling rate;(3)Within the normal working temperature range,carbon dioxide cryogenic gravity heat pipe has a relatively fast response to temperature change,which can quickly respond to the temperature change in the condensation section and transfer the cooling capacity downward;(4)carbon dioxide cryogenic gravity heat pipe is extremely large.The axial temperature distribution fluctuates obviously under heating power,and there is a large temperature difference between the condensation section temperature and the condensation alcohol bath temperature.