Control of Rotor Tip Leakage Through Colling Injection from Casing in a High Work Turbine: Computational Investigation Using A Feature-Based Jet Model

Abstract

This paper presents a study of the impact of casing injection on the blade tip leakage. The investigation includes both the flow physics and the consequences on overall performance. With the use of three-dimensional CFD together with a feature-based injection model, the flow structure related to the injection is revealed inside the tip region. The effects on performance of different injection configurations are compared to a baseline configuration without injection. The predictions are validated with an extensive experimental database made available in a companion paper (GT2007-27269). A detailed investigation of flow physics changes in the tip region is presented for one injection configuration. It appears that the tip leakage vortex is shifted axially downward when injecting from the casing. Special attention is laid on the unsteady flow field that establishes due to the relative motion of the rotor and the stationary injection holes on the casing. Both time averaged data and instantaneous snapshots are used to reveal that the flow in the tip gap region, especially near the pressure side lip, appears to get a fluctuating character. It turns out that the tip gap mass flow is reduced by about 11% relatively to the base line when injecting from the casing. In addition, it fluctuates by as much as 1.4% over its mean value. As a corollary a fluctuation of the total injected fluid mass flow is found, of an order of 2.2%. A flow model synthesizing the observed phenomena is presented.