Desensitization of the Flowfield from Rotor Tip-Gap Height by Casing-Air Injection

Abstract

This paper presents an investigation that aims to desensitize the flowfield of an unshrouded high-pressure gas-turbine stage from the radial clearance between the rotor tip and the casing. A novel method of cooling-air injection from the rotor casing into the rotor tip region is applied. The injection opposes the tip-leakage flow and affects the development of the rotor secondary flow. In a previous investigation, an increase of stage efficiency of 0.55 percentage points was achieved with an injection of 0.7% of the core mass flow. The present study investigates the effect of the injection on the flowfield for two different rotor tip clearances. Measurements conducted with a two-sensor fast-response pressure probe provide data describing the time-resolved behavior of flow angles and pressures as well as turbulence intensity. The experimental investigation is done on the tip-gap heights of 0.65 and 1.00% span and injection mass flow rates representing 0, 0.7, and 1% of the turbine core mass flow. The results show that an increasing injection rate reduces the difference of rotor exit flow angle and flow unsteadiness between the two different tip-gap heights. The stage efficiency increases with injection for both tip-gap cases, whereas identical values are obtained at the injection rate of 1 % of the turbine mass flow.