An aggressive S-shaped compressor transition duct with swirling flow and aerodynamic lifting struts

In a multistage intermediate pressure compressor an efficiency benefit may be gained by reducing reaction in the rear stages, and allowing swirl to persist at the exit. This swirl must now be removed within the transition duct that is situated between the intermediate and high pressure compressor spools, in order to present the downstream compressor with suitable inlet conditions. This paper presents the numerical design and experimental validation of an initial concept which uses a lifting strut to remove tangential momentum from the flow within an S-shaped compressor transition duct. The design methodology uses an existing strut profile with the camber line modified to remove a specified amount of the inlet tangential momentum. A linear strut loading was employed in the meridional direction with a nominally constant loading in the radial direction. This approach was applied to an existing aggressive S-duct configuration in which approximately 12.5° of swirl remains at OGV exit. 3D CFD predictions were used for preliminary assessment of duct loading and to determine how much swirl could be removed. Consequently, a fully annular test facility incorporating a 1 1/2 stage axial compressor was used to experimentally evaluate four configurations; an unstrutted duct, a non-lifting strut and lifting struts designed to remove 50% and 75% of the inlet tangential momentum. Despite the expected large increase in loss caused by the introduction of struts there was not a significant additional loss measured with the inclusion of turning. No evidence of flow separation was observed and the data suggested that it may be possible to remove more swirl than was attempted. Although the turning struts did not remove the entire targeted swirl due to viscous deviation the data still confirm the feasibility of using a lifting strut/duct concept which has the potential to off-load the rear stages of the upstream compressor.