Study on Instability of Subcooled Cryogenic Cavitating Flows

本研究对液氮在正常沸点和过冷致密状态下的空化流动进行了实验研究，获得了一些关于过冷低温流体的知识。实验在低温两相流试验装置上进行。试验段安装圆柱形聚散喷嘴，喉道直径为2.2mm，收缩面积比为1.21/25。在实验中，测量了喉部上下游的压力和温度，以及体积流量和动压。在过冷工况下，从实验测量和观察结果来看，与NBP工况相比，允许更高流量的流体通过喉部而不产生空化。68K过冷工况的流量是NBP工况的3倍。在过冷条件下，观察到不同类型的空化流动。空化现象的发生是间歇性的，即空化流动不能长时间停留，而向单相液体流动转变。这可以用声速降低引起的喉部呛流来解释。这种间歇性空化现象不仅是一个有趣的现象，而且在工程应用中具有重要意义。空化现象间歇发生的同时，也引起了大振幅的振荡。压力波动幅度最大可达基压的35%，在工程应用中似乎是一种危险现象。这些现象受声速和空化数的影响，声速和空化数随温度的变化而变化。在本研究中，展示了流动形态与空化数或声速之间的关系，用于预测过冷低温流体的空化流动现象。

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.