| Due to the unique thermal sensitivity of cryogenic fluid,the temperature decrease in cavitation zone caused by the latent heat of vaporization cannot be ignored when cavitation occurs.Therefore,the cavitation characteristics are mainly determined by the coupling of thermal effect,cavitation and vorticity.Cryogenic fluid is widely used in various fields such as aerospace.However,due to its very complex cavitation flow mechanism,this paper mainly studies the cavitation flow mechanism of cryogenic fluid by means of numerical simulation.The main contents and conclusions are as follows:(1)The existing cavitation model and turbulence model were modified,and the accuracy of the modification was verified by the hydrofoil test results of Hord.Based on this,the influence of cavitation number on the unsteady cavitation characteristics of liquid hydrogen flowing around the NACA 0018 airfoil was explored.It is found that different cavitation numbers affect the cavitation characteristics of liquid hydrogen mainly by changing the size of the evaporation region,but have little effect on the mass transfer rate between phases.With the decrease of cavitation number,the pressure pulsation frequency composition of liquid hydrogen becomes more complex.When the far field cavitation number of liquid hydrogen is lower than 2.05,the obvious cavitation shedding of liquid hydrogen will occur.(2)Using NACA 0018 airfoil with different angles of attack as the model,the unsteady numerical simulation of liquid hydrogen cavitation flow was carried out.It is found that the change of attack Angle can not only affect the area of liquid hydrogen evaporation,but also change the evaporation rate.Although the larger the Angle of attack is,the more complex the vortex structure is,the larger the Angle of attack is,the vortex structure will be distributed over the suction surface of the airfoil rather than close to the airfoil,which is not conducive to the occurrence of falling off.The stable existence of the main cavitation zone at different angles of attack indicates that the stable existence of the main cavitation zone is a general cavitation characteristic of the NACA 0018 airfoil around liquid hydrogen flow.(3)Unsteady calculation of liquid hydrogen,liquid oxygen and liquid nitrogen flow around the NACA 0018 airfoil with an Angle of attack of 20° was carried out under the same working condition.The results show that the cavitation characteristics of liquid oxygen and liquid nitrogen are similar,and the cavitation area is much larger than that of liquid hydrogen.The energy loss caused by cavitation shedding of liquid oxygen and liquid nitrogen plays a dominant role in the overall energy loss.However,the energy loss caused by cavitation shedding of liquid hydrogen has little influence on the overall energy loss due to its small cavitation zone and small unsteady shedding influence area.The change of medium affects the development of cavitation zone not only by changing the mass transfer rate between phases,but also by affecting the evaporation area.The evaporation rate and area of liquid oxygen and liquid nitrogen are much larger than that of liquid hydrogen.The cylindrical vortex structure formed by liquid nitrogen and liquid oxygen after the main cavitation zone causes the liquid nitrogen and oxygen to fall off the cloud cluster and quickly enter the main cavitation zone.There are cavitation cores in the detachment cloud images of different lowtemperature fluid media,and the consumption rate of the cavitation cores of liquid nitrogen is the lowest,which is mainly determined by a dimensional number only related to temperature and medium.The larger the value,the smaller the consumption rate of the cavitation cores.The thermal diffusivity of different low-temperature fluids can better predict the variation trend of temperature field in the cavitation zone of different low-temperature media.The larger the thermal diffusivity,the smaller the maximum temperature difference in the cavitation zone of low-temperature fluids. |
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