Numerical investigation of air injection in the endwall region of a highly loaded compressor tandem stator configuration

Flow separation in highly loaded axial compressors remains a major barrier to performance, motivating the search for active flow control strategies. This study investigates air injection to energise low-momentum endwall flow in a tandem stator configuration, representing the first investigation of its kind for tandem vanes. A numerical investigation was conducted, starting with a smooth-casing reference case and progressing to parametric studies of slot geometry (inclination α , jet angle β , radius of curvature Rc , circumferential width wc ), relative injection mass flow rate mᵢₙⱼ/ mₛₜₐₗₗ and axial location ζ . The results show how each parameter influences efficiency and pressure ratio, yielding design guidelines: shallow α , moderate β towards the separation zone, relatively large Rc and a balanced wc – mᵢₙⱼ/ mₛₜₐₗₗ combination, best captured through the momentum coefficient Cᵤ and velocity ratio uᵢₙⱼ/u_∞ . Injection near ζ ≈ 1.2 (just upstream of separation) proved most effective, and off-design simulations showed larger efficiency gains towards de-throttled conditions, although stall margin was unaffected. Robustness was confirmed through turbulence-model comparisons and injector turbulence variations, which consistently reproduced suppression of suction-side separation. An integrated analysis of aerodynamic losses further showed that injection strategies remain beneficial when loss penalties are considered. The study thus establishes transferable guidelines for injector design in tandem stators, providing a foundation for future optimisation and experimental validation.