Simulation And Experimental Study On Evaporation Phase Transition Of Cryogenic Liquid ZBO Storage System

With the wide use of cryogenic liquids in energy,space and other fields,cryogenic technology has developed rapidly and penetrated into various industries of the national economy,cryogenic liquid storage and transportation technology has also been widely concerned.Zero Boil-Off(ZBO)storage technology proposed by NASA can realize long-term on-orbit ZBO storage of cryogenic liquids through the comprehensive application of refrigeration technolog y and adiabatic technology.In order to study the change rule of flow field in ZBO storage tank deeply,a ZBO storage experimental platform for liquid nitrogen was built,in which a vertical storage tank with high vacuum Variable-Density Multilayer Insulation(VD-MLI)was taken as a main research object.The ZBO storage experiment of liquid nitrogen storage tank with a filling rate of 0.7 was carried out,and the theoretical analysis,simulation calculation and experimental verification of the flow field variation in the storage tank were carried out when the refrigerator was turned off and the power of refrigerator was 5W@77K respectively.The liquid nitrogen tank was simplified based on the thermal stratification model,and the Grashof number was used as the basis for judging the flow states of nitrogen and liquid nitrogen in the storage tank.A two-dimensional axisymmetric model of gas-liquid two-phase fluid in the tank was established.The simulation calculation was carried out by CFD numerical simulation method,and the heat and mass transfer model of gas-liquid phase in the process of evaporation and condensation was defined by the user-defined functions(UDF),and the transient change regulation of temperature and velocity fields of the fluid in the storage tank were obtained.The experiment was designed to get the change regulation of pressure and temperature in the storage tank,and qualitative analyses were made on it.The results of theoretical calculation,simulation study and experimental test are in good agreement with each other,the following conclusions were drawn based on the contrastive analysis results.(1)When the refrigerator was turned off,the calculation results of Grashof number showed that the natural convection state of the gas phase was laminar flow under the effect of heat leakage on the side wall of storage tank,and the efficiency of heat transfer was low.The simulation results showed that the temperature of nitrogen changed smoothly and decreased gradually from the boundary to the center of the tank and the gas flow rate was relatively slow,which was conducive to slowing down the evaporation of cryogenic liquid nitrogen.The experimental results showed that the pressure and temperature of nitrogen in the tank continued to rise,and the rising rate gradually slowed down.The gas temperature at the top of the tank was higher than that of other areas,the efficiency of heat exchange between the nitrogen gases was low.(2)When the refrigerator was turned off,the calculation results of Grashof number showed that the natural convection state of liquid phase was turbulent flow under the effect of heat leakage on the side wall of storage tank,the flow field fluctuated greatly and the efficiency of heat transfer was high.The simulation results showed that liquid nitrogen formed a thermal boundary layer on the side wall of the tank,and the fluid of the thermal boundary layer flowed upward,reached the gas-liquid interface,flowed to the center of the tank,and finally formed a radial convection.After evaporation and cooling,hot fluid passed through the gas-liquid interface and formed vortices under the gas-liquid interface.A big vortex was developed in the liquid nitrogen under the effcet of heat leakage at the bottom of the tank,which has caused large disturbances in the fluid.(3)When the power of refrigerator was 5W@77K,the calculation results of Grashof number showed that the natural convection states of gas and liqu id phases were both turbulent flow due to the cold energy provided by the refrigerator,and the efficiency of heat transfer was high.The simulation results showed that the transfer rate of cold energy in the heat conducting strap was higher than that betw een the heat conducting strap and the surrounding nitrogen,and the velocity of gas was high and the temperature droped fast in the local area near the heat conducting strap of the refrigerator.The experimental results showed that the pressure and tempera ture in the tank continued to decrease in a higher speed,which proved that the refrigerator had a good control effect on the pressure and temperature in the tank.(4)When the power of refrigeration was 6W@77K,comparing with the working condition where the power of refrigeration was 5W@77K,the transfer rate of cold energy along the heat conducting strap was faster,the temperature of nitrogen around the heat conducting strap fluctuated earlier and the temperature of nitrogen decreased faster.The flow field fluctuation of the liquid phase in the tank was more voilent and the phenomenon of vortex cell flow was more obvious.The vortex at the bottom of the tank absorbed energy from the liquid phase,caused turbulent fluctuation and mixing in the liquid phase,avoided stratification and rollover of liquid nitrogen fluid,which indicated that the increase of refrigeration power was beneficial to the safe storage of cryogenic liquid.(5)The large vortex of liquid nitrogen at the bottom of the tank was explained according to the principle of thermodynamic "dissipation".