Studies Of Unsteady Tip Flow And Rotating Instability In Axial Compressors

Aerodynamic instabilities in compressors is always a focus of attention in academic and industrial community,because they have important influences on compressors' performance,noise generation and safe running.The best known aerodynamic instability in compressors is rotating stall.In recent twenty years,a new phenomenon of aerodynamic instabilities gained more and more attentions of researchers—rotating instability,which can be seen as a fluctuating flow disturbance propagating circumferentially around the rotor.The rotating instability usually occurs in axial compressors with large tip clearances at highly loaded operating conditions and is responsible for the intensification of the tip clearance noise.At specific operating conditions rotating instability triggers blade vibrations.As the trend is progressing toward smaller engine cores and larger tip clearances continues,more attentions is required for the phenomenon of rotating instability.The present thesis takes a low-speed axial compressor as the research object.The rotating instability and unsteady flow in the rotor tip region are investigated systematically.With the use of dynamic pressure measurements,time resolved PIV technique and fullannulus unsteady numerical simulations,the studies are performed on the rotating instability's frequency characteristics,circumferential propagating features,modal events and the relation between the rotating instability and the complex tip vortical flow field.Based on the detailed analysis of the rotating instability character and unsteady features of tip flow field,the different fluctuating flow structures are differentiated by its frequency.A circumferential propagating model for the fluctuating tip leakage flow has been proposed,which can explain the generation mechanism of rotating instability.1.An axial compressor test rig has been set up which is suitable for the investigations of aerodynamic instabilities.The aerodynamic performance tests were carried out on the compressor rotor,and the pneumatic parameters downstream the rotor were measured by a five-hole probe.Dynamic pressure measurements and time resolved PIV tests were performed on the test rig.Full-annulus unsteady simulations were adopted to acquire the flow details in the rotor.Monitor points were imposed in the relative and absolute coordinate frames in different ways to capture the fluctuating pressure signals.2.Using the dynamic pressure transducers distributed circumferentially on the rotor casing,the unsteady pressure signals in the rotor tip region have been measured.Through the frequency and correlated analyses,the rotating instability was identified in a range of operating conditions and the frequency and circumferential propagating features were analyzed.On the basis of validations of the numerical results,the pressure signals was gathered in the unsteady simulations in the same way as experiments.A consistent phenomenon of rotating instability as that in the tests was captured in the simulations.A modal decomposition technique was adopted to acquire the different azimuthal modes which compose the rotating instability disturbance.3.The velocity field at different planes along chord direction were measured by the time resolved PIV at different flow rates.With the use of pressure transducers arranged along chord and phase-locked averaged method,the pressure and RMS pressure on the blade-to-blade plane were acquired.Both the results of velocity field and pressure field tests showed that the fluctuating level of the rotor tip flow field enhanced as the flow rate reduced,and the fluctuating features were closely related to the tip leakage flow.4.Through the detailed analyses of the unsteady features of tip vortical flow field with the numerical results,two kinds of flow instability mechanism were found.One is the tip leakage flow oscillation,the other is the circumferential movement of the leading edge induced vortex.The frequency of the tip leakage flow oscillation is coincide with the rotating instability frequency.The leading edge induced vortex appears near the rotor leading edge at the near-stall condition,and it is generated under the interactions of tip leakage flow,incoming flow and rotor leading edge.The circumferential traveling of the leading edge induced vortex results a flow disturbance with low mode orders.5.The tip leakage flow oscillation has phase lag in neighboring blade passages,which can be seen as a flow disturbance rotating around the rotor.Based on the carefully examine of the phase lag phenomenon,the circumferential propagating model of tip leakage flow oscillation is set up with theoretical method.It is found that the rotating flow disturbance caused by fluctuating tip leakage flow contains different modes,and the calculation method of modal parameters of each mode is acquired.Further,the rotating modes caused by fluctuating tip leakage flow coincide with the rotating instability modes,which indicates that the circumferential propagating of the tip leakage flow oscillation is the flow mechanism of rotating instability.