The need to reduce fuel-burn and CO2 emissions, is driving turbofan engines towards very high bypass-ratios with a geared architecture and smaller ultra-high-pressure ratio core engine. However, the increased radial offset between compressor spools leads to a more challenging design for the compressor transition ducts. To minimise weight these ducts must achieve the radial turning in a short a length as possible, but this leads to string curvature induced pressure gradients increasing the aerodynamic loading and likelihood of flow separation. For the duct connecting the low-pressure fan to the engine core this is exacerbated by the poor-quality flow generated at the fan hub which is characterised by low total pressure and large rotating secondary flow structures.A previous paper from the authors examined this numerically in order to design modifications for an existing low-speed fully annular test facility such that the rotor produced these large structures. The current paper presents an experimental evaluation of the new rotor design and examines the effect of the increased loss cores on the performance of a set of outlet guide vanes and an engine representative compressor transition duct. Aerodynamic data were collected using both miniature five-hole probes, for the time-averaged pressure and velocity field, and phase-locked hot-wire anemometry to capture the rotating secondary flows produced by the rotor. Analysis of the experimental data was supported by time-resolved uRANS CFD to aid in understanding the development of the secondary flow structures. Results showed that these structures promoted mixing through the outlet guide vanes increasing the momentum exchange between the core and boundary layer flows. The guide vane secondary flows were reduced in size but moved radially inboard reducing the hub-wall boundary layer thickness. Measurements within the duct showed a continued reduction in the hub-wall boundary layer suggesting that the duct was further from separation. Consequently, a 12.5% shorter duct was designed using CFD and experimentally evaluated. The experimental data showed this more aggressive duct was very close to separation in the critical second bend on the hub-wall. However, by duct exit the flow recovered to produce a more uniform flow. Overall the data suggest that the large loss cores generated at the fan root should enable a length saving in the duct of close to 10% which will have a significant cumulative effect in terms of reduce of fuel burn and CO2 over the operational life of an engine.

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

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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