Study on the performance deterioration of mixed flow impeller due to change in tip clearance

Performance of mixed flow compressor with un-shrouded impeller strongly depends upon unsteady, asymmetrical flow fields in the axial directions. The flow through the mixed flow impeller is complex due to three-dimensional nature of geometry. In mixed flow impeller, there are clearances between the rotating impeller blades and the casing as the high pressure ratio compressors are usually open shrouded impellers. As a result, certain amount of reduction in the performance is unavoidable due to clearance flows. In the present investigations, numerical analysis is performed using a commercial code to investigate tip clearance effects on through flow. The performance of mixed flow impeller with four different clearances between impeller and stationary shroud are evaluated and compared with experimental results. The impeller performance map was obtained for different operating speeds and mass flow rates with different tip clearances. The result shows that the tip leakage flow strongly interacts with mainstream and contributes to total pressure loss and performance reduction. The pressure and performance decrement are approximately linearly proportional to the gap between impeller and stationary shroud. The analysis showed scope for improvement in design of the compressor for better performance in terms of efficiency and operating range.