Study on Instability of Subcooled Cryogenic Cavitating Flows

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In this study, the experimental research was carried out about the cavitating flow of liquid nitrogen both at normal boiling point (NBP) and subcooled densified condition, and some knowledge about subcooled cryogenic fluid has been obtained.The experiments were performed at cryogenic two phase flow test equipment. A cylindrical converging-diverging nozzle is installed in the test section, which has 2.2mm throat diameter and a 1.21/25 contraction area ratio. During the experiment, pressures and temperatures at both upstream and downstream of the throat were measured, as well as volume flow rate and kinetic pressure.In the case of subcooled condition, from the results of experimental measurements and observations, the fluid of higher flow rate was allowed to be passed through the throat without cavitation, compared with NBP conditions. The flow rate of subcooled condition at 68K reaches three times as much as that of NBP condition. Also, the different types of cavitating flow were observed in subcooled conditions. The cavitation occurrence was intermittently, that is, cavitating flow could not stay long time period and it shifted back to the single phase liquid flow. This is explained by choked flow at the throat caused by reduced acoustic velocity. This intermittent occurrence of cavitation was not only interesting phenomena, but also important in engineering application. While the cavitation occurred intermittently, the large amplitude of oscillations was induced at the same time. The pressure fluctuation amplitude reaches a maximum 35% of the base pressure, and seems to be a dangerous phenomenon in engineering applications.These phenomena about subcooled cavitating flow are governed by acoustic velocity and cavitation number which change with temperature conditions. In this research, the relationship between the flow appearance and cavitation number or acoustic velocity was shown for predicting the cavitating flow phenomena of subcooled cryogenic fluids.