The paper investigates effects of operating conditions, tip clearance sizes and external unsteady excitations on the unsteady tip clearance flow in an isolated axial compressor rotor by unsteady 3D Navier-Stokes simulations. The results show that the unsteady tip clearance vortex takes a periodic flow behavior in the rotor tip region. With the decrease of the flow coefficient, the unsteady tip clearance vortex is enhanced and its frequency becomes lower. A larger tip clearance size can cause bigger unsteady fluctuation amplitude and a lower fluctuation frequency of the tip clearance vortex at the near stall operating condition. The unsteady excitation with the natural frequency of the tip clearance vortex can enhance the unsteadiness of the tip clearance vortex and improve the overall rotor performance. The frequency of the unsteady tip clearance vortex is independent of external unsteady excitations with different frequencies.

Steady discrete micro air injection at the tip region in front of the first compressor rotor has been proved to be an effective method to delay the inception of rotating stall in a low speed axial compressor. Considering the practical application a new type of micro injector was designed and described in this paper, which was imbedded in the casing and could be moved along the chord. In order to verify its feasibility to other cases, such as high subsonic axial compressor or centrifugal compressor, some other cases have been studied. Experimental results of the same low speed axial compressor showed that the new injector could possess many other advantages besides successfully stabilizing the compressor. Experiments performed on a high subsonic axial compressor confirmed the effectiveness of micro air injection when the relative velocity at the blade tip is high subsonic. Meanwhile in order to explore its feasibility in centrifugal compressor, a similar micro injector was designed and tested on a low speed centrifugal compressor with vaned diffuser. The injected mass flow was a bit larger than that used in axial compressors and the results showed micro injection could also delay the onset of rotating stall in the centrifugal compressor.

In the course of advancing the understanding of the unsteady flow nature of compressor tip clearance flows, the present paper investigates the unsteady tip clearance flow in the second rotor of a two-stage low-speed axial compressor and its interaction with upstream and downstream stators. Numerical methods were adopted in the present study and the research focused on clarifying the unsteadiness of tip clearance flow behavior and its link to the change of rotor performance, subjected to the variables of axial gap sizes between the rotor and upstream and downstream stators. The result shows how and why the tip leakage vortex trajectory changes its shape with the change of gap size, and its impact on the rotor pressure rise characteristic. Within all the computed operating range, the pressure rise increases monotonically with the decrease of upstream axial gaps, but no monotonic variation was observed with the change of downstream axial gaps. This trend of performance change could be explained by the unsteady effect of upstream stator wakes, and the overall result is that the rotor performance was found to be more influenced by the upstream interaction than the downstream interaction. The frequency characteristic of the tip clearance vortex, under the influence of gap size and compressor operating condition, was also analyzed to provide a quantified estimation of its periodic flow behavior and a comparison with the recent results of other researchers.