Experimental Determination of the Flow Field in the Tip Region of a LP-Steam Turbine

Abstract

Pneumatic multihole probes were applied to determine the local flow field in the tip region of a LP–steam turbine. For three tip clearances measurements were carried out varying the operating point of the turbine from full to partial load each. The maxima of the tip clearance flow and the resulting jetflow in the adjacent axial–radial diffusor show a strong dependency on the operating point, the width of the tip gap, and the axial distance to the rotor. In contrast, the radial extension of the jetflow is primarily influenced by the operating point and limited to the upper 10% of the flow channel. The leakage flow rate and the interaction with the main flow are increasing when widening the tip clearance which leads to a significant global efficiency deficit. In order to prevent a smooth flow through the tip gap, the design is changed by casing treatment upstream of the running wheel yielding a positive overlap of the gap. Its effect on the flow is investigated and compared to the other configurations of the turbine.