Sensitivity of Turbine Efficiency and Flow Structures to Varying Purge Flow

Abstract

In many turbines, a small amount of air is ejected at the hub rim seal to prevent ingestion of hot gases into the cavity between the stator and the disk. This paper presents an experimental and computational investigation on the sensitivity of a 1.5-stage high-work axial turbine to varying purge flow rates equipped with nonaxisymmetric end walls. The paper gives a correlation of the total-to-total efficiency for different rates of purge flow injection while providing a physical explanation of the responsible mechanisms. The experimental data revealed a transitional behavior of the hub passage vortex when the purge flow injection rate reached 0.9% of the main mass flow. The increase of purge flow caused a lifting off of the rotor passage vortex. The result of this event was an unstable passage vortex resulting in a lower streamwise vorticity and higher unsteadiness. Additionally, a number of subharmonic frequencies appeared at an injection rate of 0.9% that were absent in the other injection rate cases. From the computations it could be seen that the injection created additional normal vorticity at the rotor leading edge. Through turning around the rotor leading edge, a streamwise vorticity component was introduced. The sensitivity to injection in terms of created vorticity was much larger below an injection rate of 0.9% than above.