Abstract

Compressor aerodynamic instability, such as surge, has been attracting a broad interest in research because of its severe damage to engine performance and structural integrity. Therefore, the knowledge concerning surge flow mechanism and characteristics plays an essential role in taking effective measures of surge recovery. This paper aims to investigate the surge characteristics and distinct instability evolution process of an axial-centrifugal compressor using URANS simulation. Firstly, the CFD model of a combined compressor consisting of one axial and one centrifugal stage was established and validated by experimental data, then the transient characteristics of the surge were obtained. It was found that when the surge occurs, the reverse flow firstly occurs at the diffuser, and an obvious surge cycle was observed at the outlet of the centrifugal stage. In contrast, the pressure at the outlet of axial stage varied linearly with the mass flow without hysteresis phenomenon. When the reverse flow fully developed, the area with reverse flow showed an S-shaped meridional distribution, and higher reverse flow velocity existed at the regions near rotor hub, stator tip, and impeller tip. As a result, these regions were at risk to the detrimental effects of reverse flow and thus, suffer unsteady aerodynamic forces. Furthermore, a full-annulus model was established to study the development of the flow field during the surge inception process. The results showed that rotating stall was first observed near the axial stator hub, while the flow inside the centrifugal stage, worked normally. It indicated that the strong pressurization effect of the downstream centrifugal stage stabilized the operation of the combined compressor even though the upstream axial stage operated at rotating stall.

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