The Influence of Fan Root Flow on the Aerodynamic of a Low-Pressure Compressor Transition Duct

Abstract

To reduce fuel-burn and emissions, there is a drive toward higher bypass ratio and smaller high-pressure ratio core engines. This makes the design of the ducts connecting compressor spools more challenging as the higher radius change increases aerodynamic loading. This is exacerbated at inlet to the engine core by fan root flow which is characterized by a hub-low-pressure profile and large secondary flow structures. Additionally, shorter, lighter nacelles mean that the intake may not provide a uniform inlet flow when the aircraft is at an angle of attack or subject to cross winds. Such inlet distortion can further degrade the flow entering the engine. A combination of experiments and computational fluid dynamics (CFD) has been used to examine the effects on the aerodynamics of an engine section splitter (ESS) and transition duct designed to feed the low-pressure spool of a high bypass ratio turbofan. A test facility incorporating a 1½ stage axial compressor was used to compare system performance for a flat rotor exit profile to one with a hub deficient flow. Validated Reynolds averaged Navier–Stokes (RANS) CFD was then used to further investigate the effects of increased inlet boundary layer thickness and bulk swirl distortion at rotor inlet. These changes were seen to have a surprisingly small effect on the flow at the duct exit. However, increased secondary flows were observed which degraded the performance of the ESS and significantly increased loss. Nevertheless, the enhanced mixing delayed separation in the duct suggesting that overall the design was reasonably robust albeit with increased system loss.

Experimental Investigation of Secondary Flows and Length Reduction for a Low-Pressure Compressor Transition Duct

“…The slower hub speed of large fan blades and secondary flows generated at the hub typically give rise to a hublow total pressure profile at rotor exit. Walker et al [1] investigated the effect of such a profile compared to a nominally flat profile and observed an increase in overall duct loss. However, they also observed modified secondary flows through the ESS which promoted momentum exchange between the core flow and the hub boundary layer.…”

“…Flow field data in the duct suggested this moved the duct further from separation and that potentially a shorter, more aggressive duct could be used. However, the experimental configuration used by Walker et al [1] did not produce representative rotor secondary flows and loss cores. Hence Tsakmakidou et al [12] built on the work by numerically examining the parameters which affect the generation of secondary flows through the rotor.…”

“…Hence Tsakmakidou et al [12] built on the work by numerically examining the parameters which affect the generation of secondary flows through the rotor. The main aim of Tsakmakidou et al [12] was to design suitable modifications to the test rig compressor used by Walker et al [1] such that the rotor secondary flows were larger and more representative. They found that this could be achieved by a combination of reduced solidity, increased diffusion factor (reduced blade count) and increased hub inlet boundary layer thickness.…”

“…All experimental data were obtained on the low-speed test facility (Fig. 6), housed at Loughborough University, and used previously by Walker et al [1]. For a more detailed description the reader is referred to [1].…”

Review on the aerodynamics of intermediate compressor duct

“…-Turning strut could not remove swirl completely, hence, some sort of secondary flows were found at duct outlet. Walker et al [98] Incorporated the engine suction splitter (EES) -ESS showed a very small effect on flow development and due to wakes produced by ESS, flow did not move towards the separation. -The rapid mixing of wakes enhanced the duct loss by 12%.…”