Experimental investigation of the effect of bleed on the aerodynamics of a low-pressure compressor stage in a turbofan engine

The compression system in modern turbofan engines is split into stages linked by transition ducts. Downstream of the low-pressure system, a handling bleed is often required and, in conjunction with structural vanes, can introduce component interactions which compromise aerodynamic performance. In this paper a fully annular, low-speed test facility incorporating a 1½ stage axial compressor is used to examine the flow in the last stage of a low-pressure compressor and the downstream transition duct. The transition duct incorporated load bearing struts, including a so-called king strut with twice the thickness of the regular struts. The bleed utilized a 360° annular slot located on the casing immediately downstream of the low-pressure rotor and upstream of the outlet guide vane. The results showed that both the regular and king strut caused a similar flow distortion in the vane row but overall imposed a negligible effect on overall performance. The addition of bleed had a larger effect, generating an increasing outboard bias at rotor exit as the bleed flow migrated towards the offtake. At the design operating point, the outlet guide vanes were relatively insensitive to this until the highest bleed rate (18%) where evidence of stall was observed. At a lower operating point, a modification to the rotor swirl caused additional incidence onto the vanes resulting in earlier onset of stall; a full stall was observed above 10% bleed. Increasing bleed caused a gradual increase in duct loss up to stall when losses increased rapidly.