Abstract
When a compressor is throttled to the near stall point, rotating instability (RI) is often observed as significant increases of amplitude within a narrow frequency band which can be regarded as a pre-stall disturbance. In the current study, a single compressor rotor row with varying blade tip clearance (1.3%, 2.6% and 4.3% chord length) was numerically simulated using the zonal large eddy simulation model. The mesh with six blade passages was selected to capture the proper dynamic feature after being validated in comparison to the measured data, and the dynamic mode decomposition (DMD) approach was applied to the numerical temporal snapshots. In the experimental results, RIs are detected in the configurations with middle and large tip gaps (2.6% and 4.3% chord length), and the corresponding characterized frequencies are about 1/2 and 1/3 of the blade passing frequency, respectively. Simulations provide remarkable performance in capturing the measured flow features, and the DMD modes corresponding to the featured RI frequencies are successfully extracted and then visualized. The analysis of DMD results indicates that RI is essentially a presentation of the pressure wave propagating over the blade tip region. The tip leakage vortex stretches to the front part of the adjacent blade and consequently triggers the flow perturbations (waves). The wave influences the pressure distribution, which, in turn, determines the tip leakage flow and finally forms a loop.
Highlights
Rotating instability (RI) is one of the typical dynamic properties in compressors
Based on the same test rig, Kameier and Neise [5] discovered that the pressure level increases within narrow frequency bands in a region below blade passing frequency (BPF) when the tip clearance is enlarged, and RI only occurs if a reversed flow condition exists in the tip gap
Rolfes et al [7] observed RI in a single stage build of the Dresden Low Speed Research Compressor (LSRC) when the rotor tip clearance was increased to 2.5% chord length, and the mean frequency of the hump decreased with the tip gap extended
Summary
Rotating instability (RI) is one of the typical dynamic properties in compressors. This phenomenon is observed near the stability limit of the compressor and can be regarded as a pre-stall disturbance [1]. Liu et al [4] performed measurements on a low-speed axial fan and found a narrow-band tip clearance noise component below BPF when the tip clearance was larger than or equal to 0.27% of the impeller diameter. They concluded that the mode orders of the disturbance and rotating stall are not harmonically related. Based on the same test rig, Kameier and Neise [5] discovered that the pressure level increases within narrow frequency bands in a region below BPF when the tip clearance is enlarged, and RI only occurs if a reversed flow condition exists in the tip gap. Apart from the occurrence in low-speed compressors, the RI phenomena were detected in high-speed axial compressors [9,10], transonic compressors [11,12], centrifugal environments [13,14], and steam turbines [15,16]
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