Flow physics during durge 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 conditions. Results show that the compressor suffers two surge patterns. The mild surge happens first, with a higher 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, analysis of aerodynamic loads reveals that IGV and the first rotor have the largest unsteady force among all blade rows. According to the flow field, the rotor root and stator tip suffer the most serious impulse caused by reversed flow during surge conditions.