A study on the flow physics of a supersonic partial admission impulse turbine for different tip clearances

Compared to traditional turbines, supersonic partial admission impulse (SPAI) turbines have significantly different flow physics owing to partial admission configuration and high supersonic flow at the rotor entrance. The objective of the present study is to reveal the intrinsic flow physics of an SPAI turbine for different rotor tip clearance dimensions. Numerical simulations were carried out based on the unsteady Reynolds-averaged Navier-Stokes (URANS) method in an SPAI turbine whose time-averaged relative Mach number at the rotor inlet was up to 2.1. The partial admission configuration led to a severer blockage in the hub region of the rotor entrance than that in the mid-span and tip regions. As a consequence, corner separation occurred near the leading edge of the rotor hub. The tip leakage flow was divided into two vortices. Due to the strong shock/tip leakage vortex interaction, the first vortex broke down and disappeared immediately. The second vortex originated from the leading edge shock foot, migrated downstream toward the mid-span (near the rotor trailing edge). It subsequently interacted with the trailing edge shock and exhibited an expansion process without experiencing a breakdown. Under the unsteady conditions, slight variations were observed in the shock train at the nozzle as the rotor passed. In contrast, due to the partial admission, the blade loading and the rotor leading edge shock experienced significant periodic variations. The rotor blade exhibited the maximum loading when the bulk flow originated from the nozzle flowed toward the rotor blade, whereas the minimum loading was achieved when the nozzle induced wake streamed toward the rotor blade. The leading edge shock/tip leakage vortex interaction occurred periodically, resulting in a periodic vortex breakdown. For the lowest efficiency condition, the highest total to static pressure ratio was achieved and the maximum intensity was exhibited for the leading edge shock of the rotor blade. This generated a significant loss due to the shock and the shock-induced flow separation.