Analysis of characteristics and mechanism of flow unsteadiness in a transonic compressor

This paper reveals the flow mechanism of unsteady flow oscillations at small mass flow rate conditions in a transonic compressor rotor. A series of unsteady RANS simulations were performed to achieve time-accurate and time-averaged flow fields. The predicted results were validated by experimental data. The present analysis indicates that the tip leakage vortex (TLV) breakdown occurs with the operating condition approaching the stability limit due to an interaction between the TLV and the shock wave. A bubble-type vortex breakdown is first observed. However, the TLV breakdown does not directly cause flow oscillations. Only if the breakdown region develops beyond a threshold volume value (9.15e−9 m3 in current case), it will influence the static pressure distribution at the pressure side of blade tip region. After that the flux of tip leakage flow, the strength of TLV, the shock/TLV interaction are varied, which in return changed the size of the breakdown region in the adjacent passage. In such a way, a self-sustained system which causes periodical flow oscillations is established. When the compressor is further throttled, the bubble- and spiral-type TLV breakdown occur alternatively, which induces a change in the frequency of the periodical flow oscillation. Although the characteristic frequency of flow oscillation changes with the transition of operating conditions, the trigger for flow unsteadiness is the same as that in the bubble-type breakdown case, namely the changes of static pressure distribution at the pressure side of blade tip. Moreover, during the transition from bubble- to spiral-type breakdown, the breakdown region significantly increases, resulting in a more severe pressure fluctuation in the flow field.