Investigation of Tip Clearance Flow Fields and Methods of Desensitizing Tip Clearance Effects in Turbines

摘要

An experimental and computational investigation was carried out during this reporting period to derive the flow field information at various axial and radial locations from the leading to the trailing edge in the tip clearance region of the rotor, and the casing static pressure was measured to derive the trajectory of the leakage vortex and the horseshoe vortex. The data provided here indicates that the tip leakage flow originates at approximately 50% of the chord and is confined to the tip clearance region due to the relative motion between the blade and the wall, as well as the secondary flow in the tip region. Both of these effects are responsible for keeping the tip vortex closer to the suction surface corner of the tip. The leakage flow losses are high, but are confined to an extremely small region near the tip of the suction surface as opposed to the secondary flow, which tends to increase the losses spread over a substantial portion of the passage. Therefore, the information provided here is substantially different from that derived on the basis of cascades testing. Numerical investigation of the flow field in the Penn State rotor based on the utilization of the Navier-Stokes solver has been carried out to gain a better understanding of the secondary and the tip leakage flow development. Analysis of the leakage flow development shows that the relative motion of the blade and the casing wall reduces the propagation of the leakage flow into the mainflow. The tip leakage vortex is confined to the suction surface corner of the casing. Most of the leakage losses are due to the mixing of the tip leakage vortex downstream of the trailing edge.