Experimental and Numerical Analysis of Unsteady Flow Structure in a Centrifugal Compressor With Variable Vaned Diffuser

Abstract

The unsteady flow at small flow rates is always the most important of typical unsteady phenomena in centrifugal compressors, since it is closely related to the operating safety and efficiency. To study the mechanism of stall and surge generation, an experimental research on an industrial centrifugal compressor with variable vaned diffuser is carried out to study the unsteady flow structure from design point to surge. A multi-phase dynamic pressure measurement is conducted, based on 23 dynamic pressure sensors mounted on the shroud side casing surface of the compressor. The sensors are circumferentially distributed in a non-uniform manner at seven different radial positions, including the impeller region, the vaneless region and the diffuser throat region. Real-time data is recorded during the whole valve-adjusting process. The characteristics of pressure fields at some specific operating conditions are focused on, especially the pre-stall, stall precursor, stall and surge conditions. According to the multiphase data association, the originating position of the stall precursor can be determined. The features of the unsteady flow structure are also obtained, such as the surge pattern and the propagation direction of stall cells. In addition, when the diffuser vane setting angle (OGV) is turned up, the core factors to trigger total instability will change. In order to visually show how the tip leakage and separation vortex in the impeller gradually affect the flow structure in the vaned diffuser region and even the whole machine, numerical simulation and dynamic mode decomposition (DMD) method are used to study the flow mechanisms. The numerical simulation result is well matched with the experimental result. With the help of the DMD method, a few low-frequency tip leakage vortex structures are extracted from the unsteady numerical result over a period of time, which correlate with the experimental result. Meanwhile, on this issue, the feasibility of dynamic experimental analysis combined with multi-channel numerical simulation analysis is verified and discussed. Through the two analytic methods, a detailed understanding of the unsteady flow structure in the centrifugal compressor with variable vaned diffuser is obtained.

Numerical Study of the Improvement in Stability and Performance by Use of a Partial Vaned Diffuser for a Centrifugal Compressor Stage

“…The flow decomposition technique is used to distinguish different flow patterns that occur within a compressor. POD and DMD methods are commonly applied to pressure fields to study the internal flow of fluid machinery in the literature [25][26][27][28][29]. The modal decomposition methods are suitable for the study of pressure loss and flow instability caused by stall cell and impeller-diffuser interaction near the shroud side of the diffuser.…”

“…The pressure field for DMD modes of these two frequencies at 0.95 span is shown in Figure 22. Similar to the POD method, the DMD method is time-dependent, which describes the evolution characteristics of the vortex structures at a specific frequency during a period [29]. It can be seen from Figure 22a that the pressure fluctuation caused by the stall cells mainly appears in the diffuser passages, especially in the TVD, where the largescale vortex structures block part of the diffuser passages.…”

Numerical Investigation of Transient Flow Characteristics in a Centrifugal Compressor Stage with Variable Inlet Guide Vanes at Low Mass Flow Rates

“…The results show that the compressed DMD approach can save 88.4% of the calculation time, and it is found that the origin of the diffuser stall lies at the hub surface with a fixed circumferential position near the diffuser outlet. To visually show the influence of impeller clearance vortex and separation vortex on the internal flow of vaned diffuser, the flow mechanism in the compressor was studied by the POD and DMD method [29]. Previous studies have successfully placed and characterized the coherent flow structures in the compressor stage under the stall condition by using the POD and DMD method [30].…”

Experimental and Computational Analysis of the Unstable Flow Structure in a Centrifugal Compressor with a Vaneless Diffuser

“…Unstable flow phenomena, like stall and surge, has always been a hot topic in the industry of compressors, but so far there are still many unresolved issues [1,2]. In the recent researches on centrifugal compressors, an insight into the internal unstable flow structure, especially at the tip clearance, played an important role in improving the stability of the overall machine [3][4][5][6].…”