An advanced axial-slot casing treatment on a tip-critical transonic compressor rotor Part 1: Unsteady hot wire and wall pressure measurements

Authors

C. Brandstetter, F. Wartzek, H.-P.Schiffer - TU Darmstadt
J.A. Streit. - TU München
F. Heinichen - Rolls-Royce Deutschland, Germany

Abstract

The application of casing treatments to compressor stages has proven suitable to achieve an extended operating range towards lower mass flow rates and to increase the stage pressure ratio. The responsible effect of the casing treatment on the blade passage flow is the reduction of blockage in the tip region mainly caused by the tip leakage vortex. In transonic compressors the tip leakage flow interacts with the shock induced by the subsequent blade. The position and strength of this shock effects the behavior of the vortex and can lead to higher blockage. At highly throttled operating points the shock detaches from the blade leading edge. Hence, the distance between tip leakage vortex development and the shock interaction is reduced. Aims of this investigation are to determine the effect of an advanced axial slot casing treatment on the shock detachment and to estimate the recirculation activity inside the slots. A casing treatment was applied on a modern one and a half stage high speed test rig with steady and unsteady instrumentation. The test rig allows the direct comparison with the conventional smooth wall configuration. Static wall pressure in the blade tip region was measured using unsteady pressure taps to resolve the shock structure close to the casing. Inside one casing treatment slot a hot wire probe was used to measure the recirculation velocity depending on the operating point. Additionally a five hole probe was used to measure the rotor outflow. The results show an increased operating range of the axial compressor towards lower mass flow rates along with a higher stage pressure ratio. A reduced detachment distance of the shock compared to the smooth wall configuration is observed at low mass flow rates. The dependency of the recirculation activity on the shock position is derived from the data. Over a wide range of mass flow rates the velocity inside the slot rises with the detachment of the shock and hence the pressure ratio. Only at highly throttled operating points the recirculation velocity decreases while the pressure ratio still rises. This is a possible indicator for repeated recirculation of flow through adjacent slots. The measurements are used as a validation for numerical simulations presented in Part 2 of this publication, showing a good agreement in the observed flow features including shock position. A comparison of numerical and experimental pressure distributions is given and the results are analyzed with similar methods to quantify the shock detachment and strength.