Flow Physics During Surge and Recovery of a Multi-Stage High-Speed Compressor

Surge in a high-speed compressor can lead to violent disruption of flow, damages to the blade structures and eventually engine shutdown. The flow disruption caused by rotating stall and surge could potentially couple with blade vibration modes which can often not be identified during the design phase. As turbomachinery blade aerofoils are thinned to improve performance and reduce weight, aeroelastic challenges become more crucial which highlights the importance of accurate prediction of transient flow behaviour in such conditions. A significant amount of experimental research has contributed to the investigation and understanding of surge events. However, experimental investigations of deep surge are time consuming and expensive, especially in the event of blade damages. Numerical modelling such as unsteady CFD simulations can provide more informative understanding of the transient flow field during surge, however very limited publications can be found regarding deep surge and recovery of high-speed compressors.

This paper presents an investigation into the surge and recovery of an 8-stage high-speed axial compressor rig based on unsteady CFD computations. The CFD solver is based on the one equation Spalart-Allmaras turbulence model and the unsteady flow field is computed using URANS. Time history of the flow field during the surge cycles are obtained from numerical probes and are compared with experimental measurements where available. Surge is provoked by a gradual closure of exit throttle (at fixed speed) which led to the stall of mid-blocks. Immediately after surge inception, a compression wave travels from the mid-block to the front of compressor, which is shortly followed by a complete flow reversal. It will be shown that entropy perturbations generated due to flow reversal are convected to the front of compressor and reflected at the intake. The reflection is a compression shock wave which travels from the front to the back of compressor, bringing the reversed flow to a stop. For the case shown here, the flow recovery period of a surge cycle is accompanied by the formation of 3D vortex structures and large length-scale tip rotating stall, which persists and gradually disappears until the compressor recovers to its maximum flow capacity. The observations in the surge and recovery modelling are in good agreement with experimental measurements. Following the validation of the numerical model, additional work is performed to investigate the behaviour of compressor in successive surge cycles. The location for surge inception, strength of the surge shock wave and post-stall behaviour of the compressor in successive surge cycles were examined.