An experimental and analytical study of tip clearance effects in axial flow compressors

An analytical and experimental study of the performance changes and flow effects of rotor tip clearance variation in axial flow compressors is presented. The analyses demonstrate that previously unrecognized loss characteristics and flow behavior trends can be identified in a wide range of published tests of clearance effects. The performance and flow data are correlated at constant power conditions or nondimensionally in terms of the test compressor's geometry and wall flow conditions to observe the trends. Flow field changes caused by increasing the tip seal clearance of a low speed, large scale axial compressor having a symmetric velocity diagram were examined experimentally. The measurements were made at several clearances for one rotational speed. Synchronized blade-to-blade measurements of case wall static pressure and skin friction under the rotor tip of the second stage were collected. The detailed surveys showed the tip local blade-to-blade flow pattern to be dependent on the rotor blade's position relative to the upstream stator. Preliminary case wall skin friction traces showed a similar dependence and indicated a region of stagnant flow several blade thicknesses wide exists on the case wall beside the blade suction side. The stagnant region's position corresponds with a minimum wall static pressure trough standing off the blade suction surface. The blade loading at the tip was found to alter its form depending on the passage throughflow, stator relative position and tip clearance height Detailed measurements and correlative results are presented. The detailed measurements were evaluated in the context of the flow mechanisms causing the efficiency losses, the models typically used to quantify tip clearance effects, computations of the tip local flow field and the possibilities for improved tip local blade and case wall geometry design for axial flow compressors.