Rotating instability (RI) is an obvious unsteady flow phenomenon occurring in the tip region of compressors, which is potentially linked to tip clearance flow noise, blade vibration and rotating stall/surge. The existing investigations for RI indicate the origins of RI are closely related to unsteady flow behaviors in given blade passages in the rotating reference frame, which depend on the design specifics of axial machines. However, no efforts are made to set up a quantitative link between the time scale of unsteady behavior in a given passage and the characteristic parameters of RI, let alone to define the fluid dynamic processes/events which are causally linked with the RI inception. This is the motivation for the current investigations.
In Part I, the experimental and numerical investigations are carried out to investigate tip flow unsteadiness in a subsonic axial compressor rotor. The measurement results show RI appears at operating points near the stability limit of the test rotor. It becomes more pronounced with mass flow rate decreased. The corresponding computational experiments show that flow unsteadiness in given passages also appears close to the stability limit with its initial origination confined to the tip region. The appearance of tip flow unsteadiness is accompanied by a phase lag pattern in different passages across the circumference similar to the detection of stall flutter. The well-developed Fourier-decomposed method is thus used to evaluate the mode characteristics of circumferential traveling waves. It turns out the circumferential traveling wave rotating against the rotor rotation direction with the mode order of 4 is prominent in the flow field with its frequency in the absolute frame equivalent to the mean frequency value of RI detected in measurements. The further analyses of the simulated flow fields indicate that tip flow unsteadiness in a given passage attributes to the periodic oscillation of “secondary clearance flow”, which induces a blockage tranfer across the passage. The mode order and propagation speed of RI depend on the blockage transfer induced by the periodic oscillation of “secondary clearance flow” between two neighbouring passages along the whole circumference. The investigation results presented in the paper implies that one of early ideas to interpret the origin of RI might be altered to such an extent that it contains any unsteady behavior associated with tip leakage flow, rather than limited to “periodical oscillation of tip leakage vortex”.