Desensitization of over tip leakage in an axial turbine rotor by tip surface coolant injection.

摘要

Mechanical energy extraction in axial flow turbine rotors occurs through a change in angular momentum of the working fluid. The gap between the turbine rotor and the stationary casing is referred to as the tip gap. High pressure turbine blades are typically un-shrouded and pressure driven flow through the tip gap is termed as over tip leakage. Over tip leakage reduces efficiency of the turbine stage and also causes thermal distress to blade tip surfaces. The gap height typically increases over the operational life of a turbine, leading to increased efficiency drop. The thermal load on the tip surface also increases with increasing gap height and is exacerbated by the radial transport of high temperature fluid found in the core of the combustor exit flow. Thus over tip leakage not only decreases stage efficiency, but also constrains it by limiting the maximum cycle temperature.; Reducing the sensitivity of turbine performance to the effects of the tip gap is termed Tip Desensitization. An experimental investigation of tip desensitization through coolant injection from a tip surface trench was conducted in a large scale, low speed, rotating research turbine facility. Five out of twenty nine rotor blades, referred to as cooled blades, are provided with coolant injection at four locations, at 61%, 71%, 81%, and 91% blade tip axial chord length. At each of the first three locations the coolant jets are directed towards the blade pressure-side, while coolant is exhausted radially at the last location.; The sensitivity of total pressure defect, due to over tip leakage, to tip gap height is reduced by both coolant injection and roughening of the casing surface. The total pressure defect due to the large gap height of 1.40% blade height is reduced to levels comparable to the defect due to a gap height of 0.72% blade height. The strong total pressure gradient that characterizes the leakage vortex due to the gap height of 1.40% blade height is considerably diminished by both coolant injection and roughening of the casing surface. Coolant injection from 81% chord location is most effective in reducing both the total pressure defect and the total pressure gradient. Casing surface roughness significantly shifts the leakage vortex towards blade suction surface reducing its interaction with the upper passage vortex. The benefit of casing surface roughness is greater at larger gap heights. (Abstract shortened by UMI.)