Cavitation Inception and Performance of a Centrifugal Impeller During Startup

Rapid acceleration of rocket engine turbopumps during start-up imparts significant transient effects to the resulting flow field, causing pump performance to vary widely when compared to quasi-steady operation. To improve turbopump design in response to the transient effects of start-up this paper presents a method to simulate turbopump startup using CFD. Cavitating pump performance is initially evaluated using a simulation with a constant outlet pressure boundary condition. Based on the difference between simulation inlet pressure and target inlet pressure, the defined pressure on the outlet boundary condition is modified. This process is repeated until simulation inlet pressure is essentially constant during start-up. Using this simulation method, the performance of a centrifugal turbopump during start-up is simulated. Reasonable solution convergence is reached in one single phase and four cavitating simulation iterations. After these five simulation iterations, the average error between inlet pressure and inlet target pressure is 10%. Cavitating simulation iterations 3 and 4 agree within 11% on average for inlet total pressure during startup, 0:1% on average for head coefficient, 13% on average for cavitation volume, 20% on average for flow coefficient, and 2% on average for RMS force on the impeller. The agreement between simulation iterations 3 and 4 suggests that a reasonable solution has been reached

A Numerical Study of the Development of Inducer Backflow

Inducers are used as a first stage in pumps to hinder cavitation and promote stable flow. Inducers pressurize the working fluid sufficiently such that cavitation does not develop in the rest of the pump, which allows the pump to operate at lower inlet head conditions. Despite the distinct advantages of inducer use, an undesirable region of backflow and resulting cavitation can form near the tips of the inducer blades. This backflow is often attributed to tip leakage flow, or the flow induced by the pressure differential across an inducer blade at the tip. We examine backflow of a single inducer geometry at varying flow coefficients with a tip clearance of τ = 0.32%, and no tip clearance. Removing the tip clearance prevents tip leakage flow. At all flow coefficients below design, we observe backflow penetrating up to 14% further upstream in the inducer with no tip clearance. The backflow region in the inducer with no tip clearance experiences higher velocities and extends further into the core flow. However, the inducer with tip clearance develops a larger vortex at the leading edge of the blades. A comprehensive analysis of these simulations suggests that diffusion as the working fluid is loaded onto the blades, not tip leakage flow, is the driving force for the formation of backflow