Study on Instability of Subcooled Cryogenic Cavitating Flows

本文在低温两相流实验装置上，对液氮在正常沸点和过冷致密化条件下的空化流动进行了实验研究，获得了有关过冷低温流体的一些认识。试验段安装了一个喉部直径为2.2mm、收缩面积比为1.21/25的圆柱形缩放喷管。在实验过程中，测量了喉部上游和下游的压力和温度，以及体积流量和动压。在过冷条件下，从实验测量和观察的结果来看，与NBP条件相比，允许更高流量的流体通过喉部而没有空化。68 K过冷条件下的流量是NBP条件下流量的3倍。此外，在过冷条件下观察到不同类型的空化流动。空化的发生是间歇性的，即空化流不能持续很长时间，又会转变为单相液流。这是由声速降低引起的喉部阻塞流所解释的。空泡的间歇性发生不仅是一种有趣的现象，而且具有重要的工程应用价值。当空化间歇发生时，同时诱发了大振幅的振荡。过冷空化流动中的压力脉动幅度最大可达基础压力的35%，是一种危险的工程现象。过冷空化流动中的这些现象是由声速和空化数决定的，声速和空化数随温度变化。本研究揭示了过冷低温流体空化流动现象与空化数或声速之间的关系，为预测过冷低温流体的空化流动现象提供了理论依据。

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.