Control of rotor tip leakage through cooling injection from the casing in a high-work turbine: Experimental investigation

Abstract

This paper presents an experimental investigation of a novel approach for controlling the rotor tip secondary flow by injecting cooling air from the stationary casing onto the rotor tip. It contains a detailed analysis of the unsteady flow interaction between the injected air and the flow in the rotor tip region and its impact on the rotor secondary flow structures. The experimental investigation has been conducted on a one-and-1/2-stage, unshrouded turbine, which has been especially designed and built for the current investigation. The turbine test case models a highly-loaded, high-pressure gas turbine stage. Measurements conducted with a two-sensor fast response pressure probe (FRAP) have provided data describing the time-resolved behavior of flow angles and pressures, as well as turbulence intensity in the exit plane of the rotor. Cooling air has been injected in circumferential direction at a 30° angle from the casing tangent, opposing the rotor turning direction through a circumferential array of 10 equidistant holes per rotor pitch. Different cooling air injection configurations have been tested. Injection parameters such as massflow, axial position and size of the holes have been varied to see the effect on the rotor tip secondary flows. The results of the current investigation show that with the injection, the size and the turbulence intensity of the rotor tip leakage vortex and the rotor tip passage vortex reduce. Both vortices move towards the tip suction side corner of the rotor passage. With an appropriate combination of injection massflow rate and axial injection position the isentropic efficiency of the stage was improved by 0.55 percent points.