Numerical analysis of controlling cavitation instabilities in tandem cascades

Cavitation is an inevitable phenomenon that occurs when improvements such as performance enhancement and weight reduction are made to the turbopump in liquid-propellant rocket engines. Unsteady cavitation may cause oscillations (cavitation instabilities) in the turbopump. Accurate prediction and efficient suppression of cavitation instabilities are important for designing turbopumps. We performed a numerical simulation of the unsteady cavitation in tandem cascades and compared the results with those obtained for a single-stage cascade. The type of cavitation instability could be controlled by changing the front- and rear-blade chord lengths. When the clearance gap between the front and rear blades was located near the cascade throat entrance, rotating-stall conditions could be easily achieved, even at high flow rates. Cavitation surge and super-synchronous and sub-synchronous rotating cavitations were suppressed when the clearance gap was located at 40% of the total chord length. When the clearance gap was located inside the cascade throat, cavitation reached a steady state at the σ value where the cavity length equaled the front-blade length; then, cavitation instabilities and unsteady cavitation were suppressed in the low-σ region.When the clearance gap was located at 80%of the total chord length, cavitation surge was completely suppressed, although rotating cavitation occurred over a larger region.