INVESTIGATION ON MODE CHARACTERISTICS OF ROTATING INSTABILITY AND ROTATING STALL IN AN AXIAL COMPRESSOR

Rotating instability (RI) and rotating stall (RS) are two types of aerodynamic instability in axial compressors. The former features the side-by-side peaks below the blade passing frequency (BPF) in frequency spectra, and the latter represents one or more stall cells rotating in the compressor. This paper presents an experimental on the nearfield pressure and farfield acoustic characteristics of RI phenomenon in a low-speed axial compressor rotor, which endures both RI and RS at several working conditions. In order to obtain the high-order modes of RI and other aerodynamic instability, a total of 9 or 20 Kulites are circumferentially mounted on the casing wall to measure the nearfield pressure fluctuation using a mode order calibration method. Meantime in the farfield 16 microphones are planted to measure the acoustic mode order using the compressive sensing method. Through calibration the experiments acquire the mode orders generated by RI and the interaction between RI and BPF, which is higher than the number of transducers. As for RS, the mode decomposition shows a mode order of 1, indicating one single stall cell rotating in the compressor. This experiment also shows that amplitude of RI modes is decreased when RS occurs, but RS modes and RI modes will both be enhanced if the flow rate is further reduced. This experiment reveals that RI experiences three stages of “strengthen-weaken-strengthen”, and hence RI may not be regarded only as “prestall” disturbance.

Experimental Investigation of Rotating Instability in an Axial Compressor with a Steady Swirl Distortion Inlet

In this paper, an experimental study was carried out on the rotating instability in an axial compressor subjected to inlet steady paired swirl distortion. In order to deepen the understanding of the rotating stall mechanism under inlet steady paired swirl distortion, the dynamic-wall static pressure near the rotor tip was monitored to characterize the flow in the rotor tip region at different circumferential stations. In the experiment, the dynamic characteristics of the rotor tip flow field at a stable operating point and during the process from the stable point to complete stall were measured. The results indicated that for the compressor with a 2 mm rotor tip clearance, the inlet paired swirl distortion induced rotating instability (RI) near the stall point, causing the compressor to enter stall in advance. Compared with the RI intensity of the clean inlet, the distortion with a swirling blade stagger angle (αst) of ±20° increased the RI intensity up to 69.8%, while for αst equal to ±40°, the RI intensity increased at most by 135.8%. As the rotor tip clearance increased to 3 mm, the co-rotating swirl in the paired swirl distortion inhibited the appearance of RI, while the counter-rotating part aggravated the development of RI. At the beginning, the process of the compressor rotating stall involved the alternation of short-scale disturbance and long-scale disturbance. The co-rotating swirl weakened the perturbation propagated from the counter-rotating swirl sector. Once the inhibition was no longer present, the short-scale disturbance rapidly developed into a long-scale disturbance and then entered the rotating stall.

Investigation of tip leakage flow unsteadiness and rotating instability in a centrifugal compressor impeller

A phenomenon called rotating instability was rarely reported in a centrifugal compressor, although some associated discussions can be found in axial compressors. This paper presents a numerical investigation on the unsteadiness of the tip leakage flow and rotating instability in an isolated centrifugal compressor impeller. A three-dimensional, unsteady full annuls simulation is performed under three different operating conditions. The tip leakage flow unsteadiness at the impeller inlet and inside the impeller passage is detected by the pressure fluctuation. During the throttling process, the temporal and spatial propagation characteristic of the tip leakage flow unsteadiness under the near stall operating condition is revealed based on the wavelet analysis, Fourier transform, and dynamic mode decomposition. Furthermore, the relationship between the tip leakage flow unsteadiness and the rotating instability is also discussed. The results show that the pressure fluctuation mainly concentrates in the tip region at the near stall operating condition, which indicates the occurrence and circumferential propagation of tip leakage flow unsteadiness. The circumferential propagation of the tip leakage flow unsteadiness induces a significant circumferential rotating pressure wave with a wave number of 9 and approximately 48.2% of the rotational speed of the impeller. The characteristic frequency of the tip leakage flow unsteadiness agrees well with the dominant mode frequency of rotating instability. Therefore, the tip leakage flow unsteadiness is assumed to be responsible for the rotating instability. During the throttling process, the formation and development of the rotating instability at near stall operating condition can be divided into three stages based on signal analysis: prior to rotating instability, generation and development, and rotating instability.

Investigation on Mode Characteristics of Rotating Instability and Rotating Stall in an Axial Compressor

Rotating instability (RI) and rotating stall (RS) are two types of aerodynamic instability in axial compressors. The former features the side-by-side peaks below the blade passing frequency (BPF) in frequency spectra, and the latter represents one or more stall cells rotating in the compressor. This paper presents an experimental on the nearfield pressure and farfield acoustic characteristics of RI phenomenon in a low-speed axial compressor rotor, which endures both RI and RS at several working conditions. In order to obtain the high-order modes of RI and other aerodynamic instability, a total of 9 or 20 Kulites are circumferentially mounted on the casing wall to measure the nearfield pressure fluctuation using a mode order calibration method. Meantime in the farfield 16 microphones are planted to measure the acoustic mode order using the compressive sensing method. Through calibration the experiments acquire the mode orders generated by RI and the interaction between RI and BPF, which is higher than the number of transducers. As for RS, the mode decomposition shows a mode order of 1, indicating one single stall cell rotating in the compressor. This experiment also shows that amplitude of RI modes is decreased when RS occurs, but RS modes and RI modes will both be enhanced if the flow rate is further reduced. This experiment reveals that RI experiences three stages of “strengthen-weaken-strengthen”, and hence RI may not be regarded only as “prestall” disturbance.

The Tip Leakage Vortex Breakdown and its Possible Relationship With Rotating Instability in a Subsonic Compressor Rotor

The unsteady flow in the compressor at small mass flow rate has an important impact on the safety and efficiency of the compressor. Rotating instability was found in the experiment at near stall condition. Through URANS simulation, the origin of unsteady flow in an isolated subsonic rotor is studied. And the relationship between unsteadiness of tip leakage flow and rotating instability is revealed. With the deepening of the throttle, the flow field in the rotor changes from steady to unsteady. The intermittent spiral type breakdown of tip leakage vortex is considered to be the origin of the unsteady flow. Quantitative analysis of the tip leakage vortex shows the breakdown cycle caused by the interaction of the tip leaked vortex with the adjacent blade. When the tangential velocity and axial velocity of the leakage vortex reach a critical value, the tip leakage vortex will break. A radial vortex called back flow vortex will appear periodically after breakdown happens, which plays an important in rotating instability. The back flow vortex at upstream causes an overflow at adjacent blade leading edge, which results the next breakdown happens at downstream. Due to such feedback, the tip leakage vortex breakdown at two location alternately. A possible cause of RI was proposed: The spiral breakdown of the tip leakage vortex at different positions resulted in a cross-passage structure, which propagates into circumferential direction.

Investigation of rotating instability characteristics in an axial compressor with different tip clearances

It is believed that the rotating instability phenomenon originating in the compressor tip region is due to leakage flow, which is closely associated with the blade tip clearance. In this work, we have studied the correlation between the dynamic characteristics of blade tip flow and the size of tip clearance for a single-stage low-speed compressor rotor, so as to unveil the mechanism of rotating instability. The full-passage numerical simulations were carried out to obtain the variations in frequency, circumferential mode, and spatial flow field associated with rotating instability. The results of spatial mode decomposition with open clearance show the number of predominate instability modes identified are 25 and 30, respectively. By diminishing the blade tip clearance, all these unstable modes greatly diminished. The formation and propagation of the tip leakage vortex were described in detail to show the development of rotating instability. Two flow field reduced-order methods, proper orthogonal decomposition and dynamic mode decomposition, were used to analyze the flow field, energy proportion, and stability of related modes under different tip clearances. The results show that the first several modes with strong stability account for a large proportion of energy and make a major contribution to flow unsteadiness. The energy proportion and stability of rotating instability decrease as the tip clearance becomes smaller. The blade-passing frequency and its multiples emerge as the main components of the flow field.

Azimuthal Mode Characteristics of Rotating Instability in Axial Compressor Using Compressed Sensing Method

A signal reconstruction algorithm based on the compressed sensing (CS) theory with dual-uniform sampling point (DUSP) distribution is developed and applied to identify the circumferential mode of axial compressor. A regular failure signal pattern is found and the corresponding explanation is presented with validation. Circumferential mode analysis is applied to both numerical and experimental pressure fluctuation signals of rotating instability in the axial compressor tip region. For numerical calculations, the signal in the circumferential mode domain is reconstructed by the CS with random measurement points and DUSP respectively. The success rates and the reconstruction errors are discussed in details. It is shown that the circumferential mode reconstruction method based on CS combined with DUSP is capable to identify the complex flow modes in tip region of axial compressor. For the experimental results, high circumferential mode numbers are reconstructed based on dynamic pressure signals measured by DUSP. Circumferential mode analysis efficiency is thereby significantly improved. The time-resolved characteristics of the rotating instability (RI) is discussed. Moreover, a robustness analysis is conducted, demonstrating the ability of the CS-based method with DUSP to address fault sensor problems.

Experimental investigation of rotating instability in a contra-rotating axial flow compressor

The rotating instability in a contra-rotating axial flow compressor is investigated by experiments. Twenty-four pressure sensors were installed on the casing to capture the unsteady flow in the rotor tip region simultaneously. A double-phase-locking technique suitable for the contra-rotating compressor was proposed to characterise the static pressure contours of the rotor tip. The mean and root-mean-square pressure contours indicate that rotating instability occurs before the rotating stall happened, and the rotor tip clearance vortex is located upstream of the rear rotor leading edge plane before stall. Fourier spectrum shows that rotating instability and rotating stall both happened under the stall condition, and the frequency band of rotating instability does not change with the flow rate. In the front rotor, the frequency of rotating instability is half of the blade passing frequency. It is verified that the modal estimation method can be implemented by using the average azimuthal phase velocity, which significantly reduced the number of pressure sensors required. Modal estimation results show that each peak of the rotating instability frequency band corresponds to a unique dominant circumferential mode. By optimising average azimuthal phase velocity, an improved modal estimation method is obtained, which can further improve the reliability of the modal estimation results.