Investigation on Suppression Mechanism of Rotating Cavitation in Inducers

LE-7发动机液氧泵诱导轮和包含该泵的流体系统分别发生了旋转空蚀和空蚀喘振。通过对诱导轮上游壳体的简单改造，这两种现象几乎完全被抑制。然而，抑制的机制不能完全解释。为了研究旋转空化的抑制机理以及旋转空化与空化振荡的共同特征，首先，对LE-7泵的三种不同尺寸的诱导轮壳体进行了实验研究，得到了非定常空化流动的模态图，其中包括一种改型的诱导轮壳体和一种与LE-7泵的诱导轮尺寸基本相同的诱导轮壳体。结果表明，改进后的诱导轮壳体将旋转空化发生的区域转移到了更高的流速。其次，利用高速摄影和粒子图像测速仪（PIV）对诱导轮上游的流动进行了观察。结果表明，改进后的导流罩增加了导流罩进口端部附近的回流，回流增加了空腔体积，从而导致空化柔度增加。空化顺度的增加也被认为是抑制旋转空化的一个主要原因。第三，从实验和理论分析两个方面阐明了旋转空化和空化振荡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，可以解释相当高频率的空泡振荡，而质量流量增益因子是旋转空泡和空泡振荡的主要原因。第四，确认了包含空泡效应的低温两相流分析可以应用于诱导轮非定常内部流动。

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.