Cavitation Instabilities in Turbopump Inducers

- Analyses in 1-3 dimensions -

抄録

Stability analyses of 1-3 dimensional cavitating flow through turbopump inducers are reviewed with a special focus on the cause of cavitation instabilities. In one-dimensional analysis, cavitation is modeled with the cavitation compliance, defined as the decrease of cavity volume due to the increase of inlet pressure, and the mass flow gain factor, defined as the decrease of cavity volume due to the increase of flow rate. It was shown that the positive mass flow gain factor is the cause of cavitation surge and rotating cavitation. In two-dimensional stability analysis, the blade surface cavity is modeled by a free streamline with a constant pressure. It is shown that various modes of cavitation instabilities start to occur when the cavity length becomes about 65% of the blade spacing. It was found that there is a region near the cavity trailing edge in which the incidence angle to the next blade is decreased. This flow occurs to satisfy the continuity equation near the cavity closure. The cavitation instabilities start to occur when this region starts to interact with the leading edge of the next blade. In three-dimensional real flows, cavitation occurs mostly near the tip. Cavitation instabilities are simulated by three dimensional unsteady cavitating CFD. By separating out the disturbance caused by cavitation, it was found that there exists a flow component towards the trailing edge of tip cavities to fill up the volume of collapsing bubbles. This disturbance flow has an effect to reduce the incidence angle to the next blade. It was found that cavitation instabilities start to occur when this disturbance flow starts to interact with the leading edge of the next blade. So, it was found that the steady cavity length at the tip is the most important parameter in three dimensional real flow. Thus, it was found that the continuity equation plays the most important role in the mechanism of cavitation instabilities in 1-3 dimensional flows.