Effects of Non-Axisymmetric Volute on Rotating Stall in the Vaneless Diffuser of a Centrifugal Compressor

Abstract

Rotating stall is an important unstable flow phenomenon that leads to performance degradation and limits the stability boundary in centrifugal compressors. The volute is one of the sources to induce the non-axisymmetric flow in a centrifugal compressor, which has an important effect on the performance of compressors. However, the influence of volute on rotating stall is not clear. Therefore, the effects of volute on rotating stall by experimental and numerical simulation have been explored in this paper. It’s shown that one rotating stall cell generates in a specific location and disappears in another specific location of the vaneless diffuser as a result of the distorted flow field caused by the volute. Also, the cells cannot stably rotate in a whole circle. The frequency related to rotating stall captured in the experiment is 43.9% of the impeller passing frequency (IPF), while it is 44.7% of IPF captured by three-dimensional unsteady numerical simulation, which proves the accuracy of the numerical method in this study. The numerical simulation further reveals that the stall cell initialized in a specific location can be split into several cells during the evolution process. The reason for this is that the blockage in the vaneless diffuser induced by rotating stall is weakened by the mainstream from the impeller exit to make one initialized cell disperse into several ones. The volute has an important influence on the generation and evolution process of the rotating stall cells of compressors. By optimizing volute geometry to reduce the distortion of the flow field, it is expected that rotating stall can be weakened or suppressed, which is helpful to widen the operating range of centrifugal compressors.

Evolution of unsteady vortex structures and rotating stall cells in a centrifugal compressor with vaneless diffuser

“…Comparing with the experiment, the most obvious difference lies in the medium-frequency range, where the energy distributes in a broad range between IPF and BPF. This difference can also be reflected by Figure 8 in Niu et al (2020). At monitor III, the peak of fs = 5.7 IPF, which is also found at monitor II, replaces fr and becomes dominant.…”

“…The current case is based on the experiment of an in-house designed centrifugal compressor, consisting of an impeller, a vaneless diffuser and a volute as described in detail by Niu et al (2020). The bolt and nut at the inlet hub are also considered as illustrated in Figure 1(a).…”

“…The intense secondary flow in the impeller also affects the rotating stall in the diffusor by causing circumferential distortion of its inlet flow, resulting in the hub-corner separation on diffusor vanes (Marsan et al, 2015). The circumferential non-uniformity for most centrifugal compressors is caused by volute governs the evolution of the rotating stall cells (Niu et al, 2020;Yang et al, 2019). The generation and disappearance process of rotating stall cells are observed due to the pressure distortion along the circumferential direction in a vaneless diffuser.…”