Aerodynamic Influence of a Bleed on the Last Stage of a Low-Pressure Compressor and S-Duct

This paper uses computational fluid dynamics to investigate the effect of an engine handling bleed situated on the outer casing downstream of the last rotor stage of a low-pressure compressor and upstream of the outlet guide vane and S-shaped duct. The model, validated against existing experimental data, utilized an unsteady Reynolds-averaging Navier-Stokes (RANS) solver incorporating a Reynolds stress closure to examine the unsteady component interactions. The results showed that at bleed rates less than 25% of the mainstream flow, the bleed effects were negligible. However, at higher bleed rates, performance was significantly degraded. A uniform flow extraction hypothesis was employed to separate the positional bias effects of the bleed from the bulk flow diffusion. This revealed that the bleed-induced radial flow distortion can significantly affect the outlet guide vane (OGV) loading distribution, which thereby dictates the position and type of stall within the OGV passage. Extraction of the rotor tip leakage via the shroud bleed, combined with the radial flow distortion, contributed to a 28% reduction in duct loss at 10% bleed and up to 50% reduced loss at 25% bleed. The actual amount of flow required to be extracted for an OGV stall to develop was 30%. That was independent of the bleed location and the type of stall. For bleeds up to 20%, the S-duct displayed a remarkable resilience and consistency of flow variables at duct exit. However, a stalled OGV deteriorated the radial flow uniformity that was presented to the high-pressure compressor.