Integrated design of compressor transition ducts with swirling flow and aerodynamic lifting struts

To meet future environmental and economic targets aero gas turbines will require the development of new core engine technologies, higher overall pressure ratios and improved efficiencies. 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. However, this swirl must subsequently be removed within the s-shaped transition duct situated between the intermediate and high-pressure compressor spools, to present the downstream compressor with suitable inlet conditions. This is challenging due to the complex nature of the flow field which develops under the combined influence of pressure gradients and streamline curvature effects. Radial struts are commonly included in transition ducts to transfer loads and pass other secondary services. However, they traditionally serve no aerodynamic purpose and introduced parasitic losses and erode the stall margin. The work presented herein develops and validates a numerical methodology for the use of lifting struts to remove tangential momentum from the flow within an S-shaped compressor transition duct. Proof of concept, for an aggressive duct, was demonstrated using a fully annular test facility incorporating a 1½ stage axial compressor. The validated design methodology was then further developed and applied to a transition duct representative of a future very high-pressure ratio, high bypass ratio turbofan. Successful experimental assessment showed no evidence of flow separation although, overall, the introduction of struts increases loss. The design was shown to be relatively robust with only small changes observed at off-design conditions and the duct exit flow field remaining broadly similar. However, the presence of the struts had a notable upstream effect on the turbomachinery suggesting that a more integrated design approach to the strut and outlet guide vane (OGV) should be adopted. Consequently, the design concept was further developed to merge the struts into the upstream OGV row and employ a cyclic stagger to mask the strut pressure field from the upstream rotor. This also allowed the integrated strut to be move closer to the rotor giving a potential reduction in system length. A fully integrated design was developed for the high bypass ratio turbofan configuration and again experimentally validated. Results showed that large perturbations in the upstream pressure field were no longer observed at rotor exit. Additionally, the overall system loss was comparable to the non-integrated design.