Blade Lean and Shroud Leakage Flows in Low Aspect Ratio Turbines

Blade lean, i.e., nonradial blade stacking, has been intensively used over the past in the design process of low aspect ratio gas and steam turbines. Although its influence on turbine efficiency is not completely understood, it has been proved as an effective way of controlling blade loading and secondary flows on blade passage endwalls. Three-dimensional blade designs in modern industrial practice are usually carried out using clean endwalls. The influence of the leakage flows on three-dimensional blade design is traditionally neglected. This paper presents an experimental study where two different stator blades, with different levels of compound lean, were tested in a low speed three-stage model turbine with the shroud leakage flow geometry representative of industrial practice. The experimental measurements were compared with numerical tests, conducted on the same blade geometries. The influence of the compound lean on the stator flow field was analyzed in detail. In order to analyze the combined effects of both the stator hub and the rotor shroud leakage flow on the blade lean, in the second part of the paper a numerical study on a two stage turbine with both leakage flow paths representative of a real turbine was carried out. Performance of three different stator blade designs (two different levels of compound lean and a straight blade) was investigated. The aim of this study is to understand the mechanism and the consequence of the stator blade lean on stage performance in an environment with leakage flows and associated cavities.