Multistage interaction and transonic flow effects in a high-speed axial compressor

Experiments are performed in an advanced design 1&1/2 stage high-speed axial compressor to investigate multistage interaction and transonic flow effects at both design and part-speed compressor operating conditions. This investigation includes measurements of the rotor generated unsteady aerodynamic forcing functions to the upstream IGV and downstream stator, the resulting IGV and stator unsteady aerodynamic response, and Particle Image Velocimetry measurements of the time-variant IGV and stator vane-to-vane flow fields at several time instants over one interaction cycle. A highly unsteady flow field occurs at transonic operating conditions due to the reflection and diffraction of the rotor leading edge shocks by the upstream IGV row, with a time-dependent reflected wave pattern established in the upstream vane passages. The impact of the rotor shocks with the upstream IGV row generates very high levels of unsteady loading in the vane trailing edge region. Increasing the IGV-rotor axial spacing did not significantly reduce the unsteady aerodynamic loading caused by these interactions, with harmonics as high as 8x blade-pass frequency still present in the vane response spectrum. Transonic rotor operation also has a significant impact on the unsteady aerodynamic response of the downstream stator due to phenomena associated with the intra-stator transport of the chopped rotor wake segments. In the stator reference frame, the rotor wakes have a slip velocity relative to the mean flow that causes the low momentum wake fluid to migrate across the vane passage and accumulate on the stator pressure surface as the chopped wake segments are transported downstream. This results in the generation of counter-rotating vortices on each side of the chopped wake segment which convect downstream with the mean flow and act as an additional source of unsteadiness to the vane pressure surface. Rotor-IGV and rotor-stator interactions also generate static pressure fluctuations that act as an additional unsteady aerodynamic forcing function to the downstream stator. The spatial periodicity of these acoustic interactions is over the entire annulus due the unequal number of blades and vanes in the compressor, with the amplitude of the acoustic excitation to the downstream stator varying from vane-to-vane around the compressor annulus