Experimental Investigation of the Effect of Bleed on the Aerodynamics of a Low-Pressure Compressor Stage in a Turbofan Engine

Abstract

Abstract The compression system in modern turbofan engines is split into several stages linked by s-shaped transition ducts. Downstream of the low-pressure system, a handling bleed is often required for off-design performance and/or to extract ice/water and foreign debris prior to the air entering the high-pressure compression stages. The inclusion of this bleed and various structural vanes can introduce unwanted component interactions and compromise the aerodynamic performance of the upstream low-pressure compressor stage and downstream transition duct. This paper presents an experimental investigation of the aerodynamic performance of a compressor transition duct and bleed for a very high bypass ratio turbofan. A fully annular, low-speed test facility incorporating a 1½ stage axial compressor was used to examine the mean and unsteady 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 the King strut, caused a similar flow distortion and redistribution in the OGV like the Regular struts, and had otherwise imposed a negligible effect on overall performance over a range of rotor flow coefficients. The addition of bleed had a more notable effect, generating an increasing outboard bias in the rotor efflux, as the flow migrated towards the offtake. At the design flow operating point, the OGV were relatively insensitive to this until the highest bleed rate (18%) where evidence of stall was observed. At a lower operating point, the change of rotor swirl and additional OGV incidence caused earlier onset of stall and a full OGV stall was observed above 10% bleed. Increasing bleed was observed to cause a gradual increase in duct loss up to the point of OGV stall when losses increased more rapidly.