Influence of Flow Coefficient and Flow Structure on Rotational Cavitation in Inducer

Abstract

Cavitation instability is a major vibration source in turbopump inducers, and its prevention is a critical design problem in rocket-engine development. As reported by Kang et al., (2009, “Cause of Cavitation Instabilities in Three Dimensional Inducer,” Int. J. Fluid Mach. Syst., 2(3), pp. 206–214), the flow coefficient plays an important role in the onset of cavitation instabilities such as rotating and asymmetric cavitation. At high flow rates, various cavitation instabilities occur; on the other hand, as the flow coefficient is reduced, these cavitation instabilities either become absent or may change in character. The purpose of the present study is to investigate the relationship between rotating cavitation and flow coefficient through numerical simulations using the Combustion Research Unstructured Navier-stokes solver with CHemistry (CRUNCH) computational fluid dynamics (CFD) code (Ahuja et al., 2001, “Simulations of Cavitating Flows Using Hybrid Unstructured Meshes,” J. Fluids Eng.Trans ASME, 123(2), pp. 331–340), and to investigate the internal flow. As a first step, the interaction between the tip vortex and inducer blade was investigated through steady-state simulations. The tip vortex was identified by a vortex detection variable, i.e., the Q-function, a second invariant of the velocity tensor, and the distance between the blade and Q-function peak was measured. For a better understanding of cavitation instabilities, unsteady simulations were also performed for two different flow coefficients. The internal flow was carefully investigated, and the relation between cavity collapse/growth and the change in angle of attack was evaluated. The tip-vortex interaction is not a primary cause of unsteady cavitation, but the negative flow divergence caused by cavity collapse has a great influence on the flow angle. Moreover, changes in flow angle also introduce backflow from the tip clearance; these two factors are primary causes of cavitation instability. When the flow coefficient is large, the backflow is weak, and the interaction with the cavity collapse is strong. In contrast, as the flow coefficient decreases, stronger backflow occurs, and the interaction between backflow, cavity collapse, and flow angle weakens.