Study on Vibration Instability Induced by Cavitating Flow in Densified Subcooled Cryogenic Fluids

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Experiments on cavitating flow in a converging-diverging nozzle were conducted at several temperatures (70-77 K) in order to clarify the temperature dependence of cavitation characteristics in subcooled cryogenic fluids (liquid nitrogen). Cavitation behavior was found to change as the temperature decreased, and various pressure fluctuations were observed. Cavitation was generated continuously above 76 K, but not maintained stably below 76 K. Moreover, strong pressure fluctuations occurred upstream of the nozzle between 73 K and 76 K. A large vibration of the piping system in the experimental apparatus also occurred with the pressure fluctuation below 76 K, whereas a similar vibration was rarely observed above 76 K. These behaviors of cavitating flows are caused by choking at the nozzle throat due to a reduction in the speed of sound in bubbly flow. In particular, the strong pressure fluctuations are considered to be induced by the water hammer phenomenon resulting from the fluctuation of the flow velocity at the nozzle throat. In addition, further experiments on cavitating flow were conducted, in which liquid nitrogen dissolved many bubble nuclei, resulting in the reduction of the fluctuations of the flow velocity before and after the occurrence of cavitation. This result showed that the amplitude of pressure fluctuations was reduced as the fluctuation of the flow velocity decreased. From this result, it was concluded that the pressure fluctuations with cavitation in subcooled liquid nitrogen are affected by a reduction in the speed of sound in bubbly flows. Furthermore, the acceleration fluctuations of the piping system in the radius direction was found to be correlated strongly with the pressure fluctuations induced by cavitating flow in subcooed liquid nitrogen.