Computational Assessment of Axial Compressor Flowfield Instability
At low flow rates, the flowfield of axial compressors becomes unstable. Two examples of this are rotating stall (a compression system level instability) and so-called “Rotating Instabilities” (a localized fluctuation in the tip flow region). This thesis presents full-annulus URANS computations demonstrating newly discovered properties of spike-type stall inception, Rotating Instabilities, and “multi-spikes” (a phenomenon related to Rotating Instabilities that has not been discussed in previous literature). It is shown, via the series of computations on a low speed rotor designed by Mitsubishi Heavy Industries (MHI) with nominal tip clearance (1.2%ctip), that spike-type stall inception is preceded by exponentially growing and circumferentially propagating long wavelength harmonic perturbations upstream of the rotor tip leading edge. These harmonic perturbations directly influence the circumferential variation of incidence and hence the blade which first exhibits leading edge separation that characterizes spike-type stall inception. The harmonic perturbations are supported by the rotor tip’s rolled over local instantaneous axisymmetric total-to-static pressure rise characteristic, which reaches peak pressure rise at a stable operating point prior to spike-type stall inception. The rolling over of the tip axisymmetric characteristic is caused by blockage due to increased tip leakage jet strength as the rotor mass flow rate is reduced, a phenomenon common to other rotors. These findings provide connections between spike-type and modal stall inception, in that both mechanisms involve pre-stall exponentially growing long wavelength perturbations and total-to-static pressure rise characteristics, at least over part of the span, rolling over prior to stall inception. In the series of computations of the MHI rotor with large tip clearance (4.5%ctip), Rotating Instabilities are captured with their characteristic frequency spectral “hump” (a property of Rotating Instabilities widely reported in the literature) reproduced. Rotating Instabilities are shown to begin to develop when the circumferentially averaged axial velocity above the blade tip becomes negative. The computations allow the structure of the Rotating Instabilities to be identified. During the development process, the tip leakage flow starts to oscillate, and rolls up into circumferentially propagating disturbances of a vortex tube nature with a circumferential spacing of between 1.16 to 1.65 blade pitches. This range in circumferential spacing of the Rotating Instabilities vortex tubes gives rise to the characteristic “hump” in the frequency spectra. These disturbances are confined within the top 15%span, exist downstream of the leading edge, and do not trigger rotating stall inception. Previously unreported “multi-spikes”, which are found to be adjacent spike-like disturbances measured in casing static pressure traces, occur when the mass flow rate is reduced from the Rotating Instabilities operating point. The computations show that “multi-spikes” consist of Rotating Instabilities disturbances (the vortex tubes) superposed with non-growing, circumferentially-propagating, long wavelength perturbations. The long wavelength perturbations create a pressure trough with a width of several blade passages (one-eighth of circumference in the case presented), which causes adjacent Rotating Instabilities disturbances to move upstream of the leading edge. Thus, the now-upstream vortex tubes are registered as adjacent spike-like disturbances by the upstream static pressure traces. The rest of the annulus, meanwhile, operates with Rotating Instabilities disturbances still contained within the blade passage downstream of the leading edge.