Low-frequency fluctuation propagation of rotating stall in the centrifugal compressor and pipe system

The occurrence and development of centrifugal pump rotating stall is closely related to disturbance in the incoming flow. The seal clearance leakage is an inherent disturbance in the impeller incoming flow of centrifugal pumps. In order to investigate the seal clearance leakage influence on the rotating stall characteristics, the flow field numerical simulation of a centrifugal pump under rotating stall flow conditions with and without seal clearance are carried out. It is found that under the 0.4Q0 rotating stall condition with seal clearance leakage, the “λ” special shaped high-velocity region distribution along the blade leading edges are strengthened, which lead to larger scale of stall vortices. And the periodicity of the separation vortex evolution is more obvious under the condition with leakage. For a specific stall vortex development process, at the initial format stage, there is a notable disturbance characteristic of medium frequencies band that of 2–3 times the rotation frequency in the pressure fluctuation near the blade leading edges, which is significantly different from that under the condition without leakage. The vorticity analysis shows that this disturbance is closely related to the leakage induced vortices. Affected by the seal clearance leakage disturbance, the stall rotating propagation frequency is reduced from 0.143 times the impeller rotational frequency to 0.103 times. Meanwhile, the fluctuation intensity in the impeller channels is greatly enhanced. Therefore, by judging the special fluctuation frequencies, regularity, and intensity trend in the monitoring signal, it can not only be used to guide the safe operation of centrifugal pumps under off-design conditions but also serve as the basis for judging the wear of the seal, which has important practical significance.

Investigation on the influence of seal clearance leakage on the rotating stall characteristics for a centrifugal pump

Tang,

Wang,

Wang

et al. 2024

Spatiotemporal characteristics of rotating stall in a centrifugal compressor with a volute at low-flow rate conditions

Li,

Zhang,

Liu

et al. 2024

In this paper, the characteristic of the internal flow structure and the spatiotemporal evolution of the rotating stall cell for a centrifugal compressor are investigated through the computational fluid dynamics approach at low-flow rate conditions. The qualitative study like generation mechanisms of the instable internal flow, the movement trend of the stall, and the spatiotemporal characteristics of fluctuation in the impeller are mainly focused in the present work. The stall cells in the impeller move along the impeller rotating direction, and significantly influenced by the volute tongue. The traveling mechanism of rotating stall is closely related to the volute tongue, which easily deteriorates the obvious distortion of the flow structure. Furthermore, the transition law of the rotating stall is quantificationally described through the density and velocity fluctuations. The stall evolution on the circumferential monitoring points is demonstrated by the pressure fluctuation both time and frequency scale, which verifies the influence of the volute tongue on the fluid structure in different operating conditions. In addition, the damage of the volute tongue that performed on the impeller passage and blade is analyzed specifically. The mean value of the fluctuation in the passage and the load on the blade decreases about 2.5% and 3.3%, respectively, after the blade passes through the volute tongue. The spatiotemporal evolution provides a profound understanding of the flow characteristics of stall evolution within the centrifugal compressors.

Investigating three-dimensional vortex evolution in centrifugal pump under rotating stall conditions using tomographic particle image velocimetry

Tong,

Shang

2024

A rotating stall in centrifugal pumps commonly occurs under off-design operations, which is a detrimental phenomenon leading to flow instabilities, pressure fluctuations, and reduced performance. A time-resolved non-intrusive three-dimensional (3D) flow visualization method is developed for investigating complex vortex structures in centrifugal pumps based on Omega vortex identification and tomographic particle image velocimetry (tomo-PIV). A special-made centrifugal pump prototype was developed with acrylic glass allowing for optical access. This method enables both qualitative and quantitative analysis of high spatiotemporal resolution on flow behaviors and dynamics under various stall conditions. The ultra-high sampling frequency realized over 40 time-consecutive observations per revolution under 0.2 Qd, 0.4 Qd, 0.6 Qd, and 0.8 Qd. It captures the instantaneous evolution of vortex structures that undergoes a growth–breakup transition within 7–9 ms. The rotating stall mechanism is revealed experimentally from the evolution of the vortex structure. Our analysis shows the tomo-PIV's additional velocity component aids in understanding the 3D characteristics of the stall. A substantial region of reverse flow in the z-axis direction is observed under 0.2 Qd. Vortex structures are more prone to blockage at the impeller inlet, exacerbating the stall phenomenon. As the flow rate increases, the velocity distributions across different layers exhibit a laminar characteristic with a more uniform profile. The vortex structures extend radially and migrate toward the outlet. The evolutions of the stall vortex, wake vortex, and inlet vortex share the same dominant frequency components (4.75fn and 5.25fn), but the flow rate affects the proportion of different frequency components.