Impact of Tip Clearance Size and Rotation Speed on the Surge Onset in a High Pressure Centrifugal Compressor

Abstract

The results presented in the paper aim at investigating the impact of tip clearance size and rotation speed on the surge onset in a transonic single-stage centrifugal compressor composed of a backswept splittered unshrouded impeller and a vaned diffuser. For that purpose, various slow throttle ramps into surge were conducted from 100% to 60% design speed of the compressor and two different tip clearance heights were investigated. The 1MW LMFA-ECL test rig was used to carry out the tests in the compressor stage. Unsteady pressure measurements up to 150 KHz were carried out in the inducer (i.e. the entry zone of the impeller between the main blade leading edge and the splitter blade leading edge) and in the diffuser thanks to nine and fifteen static pressure sensors respectively. At cruise rotation speed (92.7% of the nominal rotation speed), the surge is triggered by a boundary layer separation on the diffuser vane suction side whatever the tip clearance height may be. No precursor of surge or pre-surge activity has been recorded in the diffuser or in the impeller. The surge reveals a spike-type inception and the tip clearance increase does not change the path into instability. At lower rotation speeds high frequency disturbances (nearly half the BPF) have been recorded in the inducer before surge. These disturbances can be understood as “tip clearance rotating disturbances” because they are generated at the leading edge of the main blades and move along the tip clearance trajectory. These disturbances reveal a very unstable behavior while the compressor runs into a stable operating point even if the flow at the tip of impeller is dramatically affected by these disturbances. But these disturbances do not trigger the surge which always originates in the diffuser.