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

为了研究过冷低温液体（液氮）中的空化特性与温度的关系，在不同温度（70-77 K）下对缩放喷管中的空化流动进行了实验研究。空化行为被发现的变化，随着温度的降低，并观察到各种压力波动。在76 K以上，空化持续产生，但在76 K以下不能稳定维持.此外，在73 K ~ 76 K温度范围内，喷嘴上游出现了强烈的压力波动.实验装置中的管道系统在压力波动低于76 K时也会发生大的振动，而在76 K以上很少观察到类似的振动。空化流的这些行为是由于在喷嘴喉部由于在气泡流中的声速的降低而阻塞引起的。特别地，强烈的压力波动被认为是由喷嘴喉部处的流速波动引起的水锤现象引起的。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。从这一结果，可以得出结论，过冷液氮中的空化压力波动受到泡状流中声速降低的影响。此外，管道系统在径向方向上的加速度波动被发现是强烈相关的压力波动所引起的空化流在subcooed液氮。

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.