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The influence of fan root flow on the aerodynamics of a low-pressure compressor transition duct

conference contribution
posted on 05.04.2019 by Duncan Walker, I. Mariah, D. Tsakmakidou, H. Vadhvana, C. Hal
To reduce fuel-burn and CO2 emissions from aero gas turbines there is a drive towards very-high bypass ratio and smaller ultra-high-pressure ratio core engine technologies. However, this makes the design of the ducts connecting various compressor spools more challenging as the higher required radius change increases their aerodynamic loading. This is exacerbated for the duct which feeds the engine core as it must accept the relatively low-quality flow produced by the fan root. This is characterised by a hub-low pressure profile and large secondary flow structures which will inevitably increase loss and the likelihood of flow separation. Additionally, the desire for shorter, lighter nacelles means that the engine intake may be unable provide a uniform inlet flow to the fan when the aircraft is at an angle of attack or subject to cross winds. Any inlet distortion this generates can also further degrade the quality of the flow entering the core of the engine. This paper uses a combination of experiments and CFD to examine the effects of the inlet flow on the aerodynamics of an engine section splitter and transition duct designed to feed the low-pressure spool of a high bypass ratio turbofan. A fully annular test facility incorporating a 1½ stage axial compressor was used to compare the system performance of a rotor that produced a nominally flat profile with one that had a notably hub deficient flow. A RANS CFD model, employing a mixing plane between the rotor and Engine Section Stator (ESS) and a Reynolds Stress turbulence model, was then validated and used to further investigate the effects of increased inlet boundary layer thickness and bulk swirl distortion at rotor inlet. Overall, changes to the inlet condition were seen to have a surprisingly small effect on the flow at duct exit – i.e. the flow presented to the downstream compressor. Changes to the inlet did, however, generate increased secondary flows and degrade the performance of the ESS. This resulted in notably increased total pressure loss; in excess of 12% for the hub-low inlet and in excess of 30% at high inlet swirl where the flow in the ESS separated. However, the increased ESS wake structures, and the enhanced mixing, delayed separation in the duct suggesting that, overall the design was reasonably robust, albeit with a significant penalty in system loss.


It was funded by the Aerospace Technology Institute as part of the iCORE (Integrated Core Technologies) program.



  • Aeronautical, Automotive, Chemical and Materials Engineering


  • Aeronautical and Automotive Engineering

Published in

Proceedings of ASME Turbo Expo 2019: Turbomachinery Technical Conference and Exposition (GT2019)


2B: Turbomachinery


WALKER, A.D. ... et al., 2019. The influence of fan root flow on the aerodynamics of a low-pressure compressor transition duct. Presented at the ASME International Gas Turbine and Aeroengine Congress and Exposition, Phoenix, AR, USA, June 17 – 21.




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© Rolls Royce

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This paper was published in the Proceedings of ASME Turbo Expo 2019: Turbomachinery Technical Conference and Exposition (GT2019) and is available at

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Book series

ASME paper;GT2019-92048

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Paper No: GT2019-92048, V02BT42A008




Pheonix, Arizona, USA


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