Investigation of the tip leakage flow at turbine rotor blades with squealer cavity

Understanding of the tip leakage flow (TLF) in turbine rotors is one key aspect in the design for improving the efficiency of turbines. This requires measurements and simulations of the TLF, especially when investigating new rotor blade designs with blade tip treatments. However, flow measurements in the tip gap of a rotating machine are highly challenging because of the small gap size of about 1 mm and the high unsteadiness of the flow requiring a high temporal resolution of about 10 μs. For this purpose, an optimized non-intrusive measurement concept based on frequency modulated Doppler global velocimetry is presented, which fulfills the requirements. Three component velocity fields of the TLF were obtained in a turbine test rig at a blade passing frequency of 930 Hz. The rotor blades were equipped with a squealer tip, and the TLF in the squealer cavity region was successfully measured. The measurement agrees well with calculated results showing gradients in the tip gap above the squealer cavity. Furthermore, the development of the tip clearance vortex was resolved at the suction side of the blades.

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