Implementation of stability-enhancement with tip air injection in a multi-stage axial flow compressor

Abstract

Tip air injection for stall margin extension is experimentally investigated in a multi-stage axial flow compressor. By comparing the stall margin improvement (SMI) under tip air injection in different stages, injected air only upstream from the first stage can causes a 20% SMI. This is because stall inception appears in the first stage. From the trend curve of SMI versus injected momentum ratios applied in the first stage, two demarcation points are found that separate the curve into micro, macro, and overlarge injections. Micro injection with a small SMI only weakens the tip leakage flow. Macro and overlarge injections influence both the tip leakage flow and mainstream flow, which contribute to a large SMI. However, overlarge injection causes the second stage to stall first, and causes the SMI to remain constant with further increases in the injected momentum ratio. The outlet axial velocity and flow angle measured using a five-hole probe indicate that with the overlarge injection in the first stage, the separation of the rotor blade passage in the second stage is more adverse than those in the other stages. The adverse passage separation originating from the hub region results in the second stage losing its working capability and stalling first. Moreover, at this point, the blade tip injection in the second stage has no additional effect on the extension of the stall margin.