Effect of tip clearance on cavitating flow of a hydraulic axial turbine applied in turbopump

Abstract

The requirements of Liquid Propellant Rocket Engine (LPRE) are high for thrust, specific impulse, and flow rate; thus, its components also have strict requirements. For the turbopumps (TPs), this means high flow rate, high rotational speed, and high pressure ratio, which makes their operations susceptible to the cavitation phenomenon, as observed in two previous works. In the first, cavitation regions were observed in the first stage of the Space Shuttle Main Engine (SSME) Liquid Oxygen (LOX) booster turbine, for 3.0, 5.5, and 8.0% tip clearances (relative to rotor blade height), using monophase flow (Lindquist Whitacker et al., 2017). In the second, the simulations were performed with multiphase flow, producing results more physically coherent for the 3.0% gap configuration (Whitacker et al., 2018). The characteristics of both types of simulations in space propulsion applications still require better understanding. Therefore, to compare monophase and multiphase results at various operating points and turbine configurations, steady-state turbulent 3-D Computational Fluid Dynamics (CFD) simulations were performed, based on Reynolds-Averaged Navier-Stokes (RANS) formulation. The same three tip configurations for the turbine first stage were simulated, and the calculations were validated using experimental results from the National Aeronautics and Space Administration (NASA) (Boynton and Rohlik, 1976). This made it possible to verify the effect of the tip clearance on the machine performance and internal flowfield. When the gap increased, the pressure loading decreased in a large region of the blade tip, the interaction was greater between the Tip Clearance Vortex (TCV) and a vortex generated around the shroud cavitation region (Cavitation Vortex - CV), and this interaction moved towards the middle of the blade-to-blade passage. Thus, the losses increased and the efficiency decreased. Various comparative aspects between the simulations using both mono and multiphase numerical schemes are also discussed.