Multi-passage time-resolved CFD analysis of rotor tip stall inception and passive control in a highly-loaded axial compressor

Tip stall inception in a high speed jet axial rotor, if induced at a critical phase in a sortie, can compromise the stability of the engine and undermine the aircraft safety. Steady and time-resolved Computational Fluid Dynamics (CFD) is used to investigate the flow through a highly loaded axial rotor, at a stage pressure ratio representative of future highly manoeuvrable combat aircraft systems. The base flow from which tip stall onsets and the early stages of tip stall are modelled, reproducing the salient stages of the flow processes leading to an axial compressor rotating stall, by flow separation at the rotor tip. A casing treatment concept aimed at mitigating this stall type is explored by CFD. The treatment features a channel that provides passive flow control by flow recirculation. Some novel design aspects of the channel geometry are considered. A numerical investigation of the channel parameter space, by varying the axial length, achieved a 9.11% improvement in the compressor stall margin with respect to the baseline CFD prediction, without any adiabatic efficiency penalty. The time-dependent CFD simulations provided some useful insights on the stall inception process in the highly loaded axial compressor with the recirculation channel treatment.