Surge poses a significant challenge to the safe operation of compressors. Identifying surge precursors is a valuable approach for surge warning and has been extensively investigated, though it remains unresolved. This paper experimentally examines the flow unsteadiness in centrifugal and axial subsonic compressor setups. To quantitatively assess flow unsteadiness and identify surge precursors, approximate entropy (ApEn) is applied to pressure and sound signals. The signals are collected under two compressor operating conditions: (1) stable operating points, where the valve at the compressor outlet remains fixed, and (2) the transient process during which the valve opening is gradually reduced until surge occurs. In the stable operating region, the lowest ApEn values are observed at medium flow rates, indicating high signal regularity and low flow unsteadiness. This behavior is attributed to the optimal incidence angle at the inducer inlet, corresponding to highly stable internal compressor flow. As the compressor operating point transitions from a stable region to surge onset, ApEn initially decreases due to the weakening of high-frequency unsteady flow, then increases just before surge occurs. The ApEn assessments reveal a similar trend in both centrifugal and axial compressors. These findings suggest that the variation in flow unsteadiness can make ApEn a promising method for quantifying the flow unsteadiness and identifying the surge precursor.

The interactions of tip leakage flow with mainstream and shock wave result in larger aerodynamic losses and blockage in high loading compressors and tend to be one of the triggers for flow instability. In order to attenuate the influence of leakage flow and improve the stall margin of highly loaded compressor, the new rotors surrounded by tip winglet are investigated by a numerical method. The tip winglet is designed by extending the flat blade tip section in outer 1.5% span of rotor blade. As the angle between the leakage flow and main flow decreases due to winglet, the losses and flow blockage have been weakened near stall condition, and the stall margin of new rotor with pressure-side winglet has an increase of over 10%. The migration and accumulation of low-energy fluids near the corner of casing endwall are affected significantly by tip winglet. As a result, the pressure-side winglet causes an increase of static pressure near the casing corner of pressure surface. Although the driving pressure difference near the leading edge of blade has increased slightly in the tip region, the strength and massflow rate of leakage flow appear to be decreased. As the leakage flow weakens in the new rotor with pressure-side winglet, its interaction with mainstream and shock has been suppressed obviously, and the delay of rotating stall occurs as well. Moreover, the flowfields of the new rotor with pressure-side winglet have been simulated at 40%, 60%, and 80% design speed. It is shown that the flow blockage and losses in the tip region have also reduced near stall point, and an improvement of overall performance is present in the new rotor with pressure-side winglet. All the changes of tip flow structure caused by winglet benefit to an increase of aerodynamic performance of new rotor at full rotational speed range.

Surge waves are commonly modeled as one-dimensional plane waves in the understanding of compressor surge. However, this assumption does not adequately represent the physical phenomena observed under specific conditions. This study performed surge experiments on a centrifugal compressor, measuring pressure changes around surge onset with fast dynamic pressure transducers arranged at the same impeller inlet section but at different circumferential positions. The pressure signals are analyzed by the Hilbert–Huang transform, the ensemble empirical mode decomposition method, and the phase locking value concept. With them, instantaneous phase information was extracted, surge signal components were successfully isolated, and non-zero phase differences were identified. Those results demonstrate that surge waves exhibit significant circumferential propagation characteristics, challenging the conventional one-dimensional plane wave assumption. The proposed analytical approach was further validated with experimental data from two pressure-field microphones in an axial compressor experimental setup, thus confirming its validity in analyzing surge waves and the correctness of conclusions drawn from the centrifugal compressor. In conclusion, the novelty of this study is to offer experimental evidence and theoretical insights into the non-axially symmetric propagation of surge waves and an understanding of the fluid physics underlying compressor surge.