Weiwei Cui

Accurate measurement of compressor noise imposes specific requirements on number and positions of microphones. However, the effect of microphone number and placement within the pressure field—typically inside the compressor inlet duct—has received little attention. In this study, acoustic experiments were conducted on a 1.5-stage axial compressor—comprising inlet guide vanes (IGVs), a rotor, and a stator—using two pressure-field microphones and thirteen free-field microphones. The noise measurements revealed a circumferentially non-uniform distribution of sound pressure level (SPL) along the inlet duct wall, despite the presence of uniform inlet and outlet boundary conditions. To complement the experiments, unsteady Reynolds-averaged Navier–Stokes (URANS) simulations were performed to predict the unsteady flow field and analyze the superposition of multiple rotor–stator interactions. The results indicate that the coupling of the two rotor–stator interactions in localized regions enhances the local pressure fluctuation. This localized enhancement of pressure fluctuation is identified as the primary mechanism responsible for the circumferential SPL variations in the inlet duct of the IGV–rotor–stator configuration. This finding provides evidence that a single pressure-field microphone at a fixed circumferential position is insufficient for accurately capturing compressor inlet noise levels—even under distortion-free inlet conditions—particularly in multi-row or multi-stage axial compressors.

Compressor surge poses significant risks to aerodynamic performance and structural safety, motivating the development of accurate early warning techniques based on signal analysis. This study identifies a previously under-recognized ultra-low-frequency surge precursor in a 1.5-stage axial compressor. Its under-recognition in prior studies is attributed to its close spectral proximity to the dominant high-amplitude surge signal, whose strong energy masks this weak low-frequency signature in conventional time–frequency representations. To resolve this faint precursor, a high-resolution signal-processing framework is established by integrating the fractional adaptive superlet transform—which has rarely been applied in compressor aerodynamic instability research—with complete ensemble empirical mode decomposition with adaptive noise. This combined approach provides the enhanced low-frequency resolution and robust mode separation required to isolate the precursor from the adjacent surge component. Using this framework, the precursor is extracted and characterized. It exhibits a characteristic frequency lower than the surge frequency, non-axisymmetric behavior, and a spiral upstream-propagating pattern, which cannot be identified using traditional Fourier- or wavelet-based techniques. Further analysis locates this low-frequency component to the vicinity of the outlet guide vane exit, which appears 20–100 rotor revolutions prior to surge onset and is closely associated with surge initiation. The results demonstrate that the proposed signal-processing techniques can reveal hidden instability signatures that are difficult to detect with conventional methods, and offer new possibilities for early surge detection, as demonstrated by the identification of a low-frequency surge precursor in the tested 1.5-stage axial compressor.

The clearance flow assumes a crucial role in determining the performance and operational stability of aero-compressors, and a novel method for generating tip-induced shock waves using a circumferentially gradually expanding gap structure is put forward to suppress the leakage flow with different intensities in a transonic rotor under variable rotational speed conditions. The findings indicate that as the rotational speed diminishes, the chordwise extent of tip region where the induced shock emerges contracts significantly in the new rotors, decreasing from 40% of the tip chord at 100% design speed to 15% at 75% speed. This is accompanied by a slight reduction in the shock intensity and a circumferential displacement of the shock position towards the tip clearance. These phenomena jointly lead to a gradual weakening of the shock wave's inhibitory influence on the supersonic leakage flow. Consequently, at 100% speed, the maximum increment in the stall margin reaches 8.89%, whereas the relative gains in both the stall margin and the massflow rate variation range decline to approximately 2% at 75% design speed. Nevertheless, a moderate increase in the circumferential clearance expansion can alleviate the speed-induced deterioration in the stall margins of new rotors. Moreover, at high rotational speeds, the dominant mechanism for mitigating leakage flow is mainly governed by the induced shock wave, while both shock wave and diffuser effects contribute in a synergistic manner at lower speeds. However, the effectiveness of the shock wave-based mechanism in suppressing leakage flow remains substantially greater than that of the diffuser effect.

The variable geometry orifice (VGO) is an active flow control technology designed to improve the turbocharger compressor stability margin at the low-end operating region of a turbocharged engine. As far as the orifice design, previous studies reported optimized design parameters such as the insertion angle and radial height of the orifice, but the gap between the orifice and the impeller inlet remains insufficiently explored. This study experimentally and numerically investigates a centrifugal compressor equipped with an orifice positioned at varying axial locations in front of the impeller. Both experimental and numerical results consistently demonstrate that translating the orifice plate axially toward the impeller inlet significantly enhances the compressor’s stability margin. Specifically, at 60,000 rpm, the surge-boundary mass flow can be increased by up to 14.75%, corresponding to an approximately 2.8% improvement in isentropic efficiency. Furthermore, Dynamic Mode Decomposition (DMD) analysis indicates that when the orifice is positioned close to the impeller, it effectively impedes backflow in the inlet duct and suppresses low-frequency, high-energy unsteady modes in the vaneless diffuser, thereby enhancing flow stability. These findings suggest that during designing a turbocharger compressor with VGO configuration, the orifice plate should be designed close to the impeller inlet.

Fast pyrolysis, rather than linear heating, favored the production of xylene. An increase in both cellulose and lignin content in the simulated biomass improved the yield of monocycle aromatic hydrocarbons via various mechanisms.

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.

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.