Internal Flow Structures in Tip Clearance Based on Time-Resolved Schlieren Visualization and Numerical Simulations

Abstract

Tip clearance flow is a critical feature of the blade tip region in turbomachinery considering the flow losses and instabilities. Simplified blade tip models were tested in a transonic wind tunnel with flow visualized by time-resolved schlieren and numerical simulations. The in-clearance flow structures and their evolution were revealed. The discussed Reynolds and Mach numbers at the clearance exit ranges were 1.4×105≤Re≤3.7×105 and 0.19≤Ma≤0.63. Focus was put on the movement of the pressure-side separation bubble and over-tip shock waves. The results show that a compression zone was gradually established near the suction-side edge as blade loading increased. Unstable shock waves emerged in this region and propagated upstream at the critical state. Because the upstream turbulence was suppressed by the flow acceleration near the separation bubble, the stable range of the separation bubble boundary gradually increased with the increase of flow velocity until highly unstable shock waves appeared in the clearance. Then the stable range was determined by the range of shock oscillation. Meanwhile, the size of the separation bubble decreased monotonically with the increase of blade loading and maintained a linear relationship with the compression coefficient. By additional numerical simulations, the size of the separation bubble was revealed to be insensitive to the change of temperature. This implies that, when discussing the location and size of the in-clearance separation bubble, measurement data from cool flow can be applied for hot flows. This study is aimed at revealing the flow structures in the clearance flow, specialized for potential applications in turbomachinery with blade tip clearance. Wind tunnel experiments and numerical simulations were adopted to achieve these goals. Based on the results, one can get ideas about what kind of flow structures may exist in the blade tip clearance, what their relationship is with blade loading, and how they change with respect to temperature.