Tip Running Clearances Effects on Tip Vortices Induced Axial Compressor Rotor Flutter

Abstract

The present study aimed at investigating numerically the effects of large blade tip running clearances on flutter stability of axial core multi-stage compressor rotor. During this study, the influences of aerodynamic boundary conditions, variable stator vane incidence and tip running clearances of upstream and downstream rotors on aerodynamic compressor flow and rotor flutter stability are thoroughly investigated. The simulations were carried out using an in-house 3-D aeroelasticity code. The steady-state-solution computations are performed on single-blade-passage-one-bladerow, stage-blocks and whole compressor models. These analyses included rotor blade models with nominal tip running clearances and artificially large tip clearances. Moreover, the effects of the variable stator vane incidences are assessed by performing steady-state-solution computations for nominal vane schedules and extreme vane malschedule. The first four flap and torsion vibration modes from finite element analyses are included in the unsteady flow computations and assessed for flutter stability. The results from the numerical investigations showed that the compressors with large rotor tip running clearances are susceptible to rotor tip flow induced flutter instability. The aerodynamic losses on the rotor with large tip clearances increase with other rotors having also large tip gaps. For the aerodynamic boundary conditions considered here, the simulations predicted flutter instability for the first flap vibration mode. The flutter instability predicted on the rotors with large tip clearances is driven by oscillating tip vortices on blade suction surface close to the blade tip leading edge. The flow in the rotor tip gap is mostly stalled and tip vortices oscillations are close to blade tip leading edge. The strength of these oscillating vortices appears to increase with increase in variable stator vane malschedule or negative incidence. Small changes in aerodynamic conditions can offset these instabilities. These studies indicate that the main ingredients for the occurrence of these phenomena are likely to be excessively large rotor tip running clearances combined with significant changes in flow incidence.