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.

Characterization of Rotating Cavitation in a Four Bladed Inducer

“…In this paper we hypothesize that there are two routes that can lead to breakdown of ABC and onset of RC. At low cavitation numbers, σ<0.035 for the inducer investigated here, the onset of RC is triggered by the cavity sheet breakoff of a cavitating blade that leads to an incidence swing for the adjacent blade, as indicated by Tani et al [11]. At higher cavitation numbers (σ>0.035) RC is triggered by the tip vortex cavity of one of the non-cavitating blades interacting with the adjacent blade.…”

“…Cavitation -adapted from [11] The mechanism of formation and propagation of RC proposed by Tani et al [11] and Iga et al [3] cannot explain entirely the dynamic behavior observed in experiments of four bladed inducers. The dynamic behavior of the inducer under consideration in this study is presented in Fig.…”

“…The cavity region under certain conditions is synchronous and remains locked to one blade, as described in Tani et al [11]. In inducers with odd number of blades (three or five), a large cavity can form on one or two blades, and the corresponding mode of operation is often referred to as Asymmetric Cavitation (AC).…”

“…This interaction is inhibited at low flow coefficients due to the action of the backflow that displaces the tip vortex away from the adjacent blade. Tani et al [11] suggests that the tip vortex interaction is not the primary cause for the onset or RC. This conclusion was based on unsteady Reynolds Averaged Navier Stokes (RANS) calculations of a three bladed inducer.…”

“…RC occurs only at high flow rates, and when the flow coefficient is reduced the instability is either absent or changes entirely in nature. Tani et al [11] clarified the reasons for this behavior, suggesting that RC is caused by the negative flow divergence caused by the blade cavity collapse that leads to reduced incidence for the adjacent blade. When the cavity fills about 65% of the blade pitch, the turning of the flow at the back of the cavity might interact with the adjacent blade, as shown schematically in Fig.…”