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

为了阐明过冷低温液体(液氮)中空化特性的温度依赖性，在不同温度(70~77K)下对收敛-扩张喷嘴内的空化流动进行了实验研究。随着温度的降低，空化行为发生变化，并观察到各种压力波动。在76K以上持续产生空化，但在76K以下不能稳定地维持在76K以下，并且在73~76K之间喷嘴上游出现了强烈的压力波动，实验装置中的管道系统也发生了很大的振动，压力波动在76K以下，而在76K以上很少观察到类似的振动。空化流动的行为是由于气泡流动中声速的降低而在喷嘴喉部发生堵塞。特别是喷嘴喉部流速波动引起的水击现象，导致了较强的压力波动。此外，还进行了进一步的空化流动实验，液氮溶解了大量的气泡核，减小了空化前后的流速波动。结果表明，随着流速波动的减小，压力波动的幅度减小。由此得出结论，过冷液氮中空化引起的压力波动受泡状流中声速降低的影响。此外，还发现管道系统径向的加速度波动与液氮中空化流动引起的压力波动密切相关。

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.