The effect of hub leakage on the aerodynamic performance of high-pressure steam turbine stages

Abstract

In a steam turbine, leakage occurs at the gap between the rotor shafts and diaphragms and is injected into the main stream at the inlet of the rotor blade. Although this hub leakage flow is small, it affects the main flow, especially in the rotor passage. This study uses computational fluid dynamics to investigate the influence of hub leakage flow re-entry on the aerodynamic performance of turbine stages of a 10-stage high-pressure steam turbine operating at USC conditions. Each stage was individually investigated in regard to the influence of stage reaction on the efficiency drop. The flow field of the turbine without the leakage flow was analyzed first, and then the variations in flow phenomena and stage efficiency were investigated with increasing leakage flow ratio. Our analysis shows that the efficiency drop depends strongly on the stage reaction. The first stage designed with 21 % reaction showed an efficiency drop of approximately 1 % for 1 % increase in leakage flow, which is almost twice that of the last three stages with an average stage reaction of 48 %.