Unsteady aerodynamic interaction in regulated two-stage radial turbine at pulsating conditions

Regulated multi-stage turbocharging is an indispensable technology to achieve high boosting pressure and hence power recovery for High-Altitude Long-Endurance Unmanned Aerial Vehicle (HALE UAV). Inevitably, unsteady interstage coupling has become a key factor restricting the pursuit of higher aerodynamic performance in multi-stage turbochargers. This study is to investigate the unsteady gasdynamic behavior of two-stage radial turbines and aimed to obtained the knowledge of unsteady aerodynamic interaction at pulsating conditions. Results reveal an obvious difference in the performance discrepancy mechanisms of high-pressure turbine (HPT) and low-pressure turbine (LPT). HPT shows more unsteady than that of LPT according to the discussion on mass accumulation and local temporal gradient. Furthermore, this unsteadiness is drastically enhanced when the valve is open. For HPT with open valve, a sharp reduction of up to 10.7% in the cycle-averaged efficiency is observed. Both the volute and the rotor are the main components causing the deterioration of efficiency. Swirling flow precession leads to a dramatically loss generation in the volute. Leading-edge separation and tip leakage flow are the dominated factors in rotor. The former is primarily attributed to inlet swirling flow and the latter is predominantly attributed to the pulsating effect. For LPT, the performance is not sensitive to the valve state, with no more than 3% discrepancy on efficiency. At specific velocity ratio, the negative swirling flow tends to exert a positive effect to the rotor efficiency, whereas the positive swirling flow leads to a higher entropy generation in rotor passage.