Investigation on Suppression Mechanism of Rotating Cavitation in Inducers

在LE-7发动机液氧泵诱导体和含液氧泵的流体系统中分别发生了旋转空化现象和空化喘振现象，通过对诱导体上游壳体的简单改造，这两种现象几乎完全被抑制。然而，抑制的机制还不能完全解释。研究了旋转空化的抑制机理以及旋转空化与空化喘振的共同特征。首先，采用与LE-7泵诱导体尺寸相近的诱导体和改进型诱导体进行了实验，得到了非定常空化流动的模态图。结果表明，改进后的诱导腔将发生旋转空化的区域转移到更高的流量。其次，利用高速薄膜和粒子图像测速仪（PIV）进行实验，观察诱导体上游的流动情况。结果表明，改进后的诱导壳增加了诱导入口尖端附近的回流，回流增加了腔体体积，导致空化顺应性增加。空化顺应度的增加也被认为是抑制旋转空化的主要原因。第三，从实验和理论分析两方面阐明了旋转空化和空化气涌是一种非常相似的现象。此外，考虑到质量流量增益因子的相位延迟是导致旋转空化和空化喘振的主要原因，可以解释空化喘振频率较高。第四，证实了考虑空化气泡影响的低温两相流分析可以应用于诱导器的非定常内部流动。

Rotating cavitation and cavitation surge occurred in a liquid oxygen pump inducer of the LE-7 engine and in a fluid system including the pump, respectively, Both the phenomena were almost completely suppressed by a simple modification of the inducer upstream housing. However, the mechanism of the suppression could not be fully explained. In this study suppression mechanism of the rotating cavitation and the common characteristics between rotating cavitation and cavitation surge were investigated.First, an experiment was performed using three inducer housings including a modified one and an inducer which had almost similar dimensions of the LE-7 pump inducer to obtain a map of modes of unsteady cavitating flows. It was clarified that the modified inducer housing shifted the region in which the rotating cavitation occurred to higher flow rates.Second, another experiment was conducted using a high speed filming and a particle image velocimeter(PIV) to observe the flow upstream of the inducer. It was clarified that the modified inducer housing increased back flow near the tip of the inducer inlet and that the back flow increased the cavity volume, which resulted in the increase of cavitation compliance. The increase of cavitation compliance is also considered to be a main cause of suppressing the rotating cavitation.Third, it was also clarified that rotating cavitation and cavitation surge was a very similar phenomena from both the experimental and theoretical analyses. Furthermore, fairly high frequency of the cavitation surge could be explained by taking into consideration the phase delay of the mass flow gain factor which is a major cause of both the rotating cavitation and cavitation surge.Fourth, it was confirmed that an analysis of a cryogenic two-phase flow which includes the effect of cavitating bubbles could be applied to the unsteady internal flow of an inducer.