Flow physics during surge of an axial-centrifugal compressor

Abstract

Aerodynamic instability design is one of the crucial elements when designing a compressor. Unstable phenomenon such as surge harms the compressor in both performance and structure integrity. The complex unsteady flow process during instability strongly relates to the compressor design. This paper investigates the transient instability process using URANS on a three-stage-axial and one-stage-centrifugal combined compressor under its off-design condition. Results show that the compressor suffers two surge patterns. The mild surge happens first, with a higher pressure oscillation frequency than the traditional mild surge. Then, with the decrease of valve opening, the deep surge is initiated, and the axial stages work at the peak pressure ratio because the mass flow rate is limited by the choked radial diffuser. Moreover, the aerodynamic blade loads are analyzed and find that the IGV and first rotor have the largest unsteady force among all blade rows. According to the flow field, the root of the rotor and the tip of the stator suffer the most serious impulse caused by reversed flow during surge cycle.