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Experimental study of the performance and stability of a low pressure axial compressor for a contra-rotating turbofan engine architecture

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VKI PHDT 2013-07, Nicolas Van de Wyer, Experimental study of the performance and stability of a low pressure axial compressor for a contra-rotating turbofan engine architecture, ISBN 978-2-87516-046-1, 207 pgs

Experimental study of the performance and stability of a low pressure axial compressor for a contra-rotating turbofan engine architecture

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Experimental study of the performance and stability of a low pressure axial compressor for a contra-rotating turbofan engine architecture
By Nicolas Van de Wyer

PhD Thesis from the von Karman Institute / Université Catholique de Louvain, December 2012, ISBN 978-2-87516-046-1, 207 pgs


Abstract

The new contra-rotating turbofan architecture leads to a substantial reduction of the pollutants and the noise emission of civil aircrafts. This architecture is based on two shorter fans, rotating in opposite direction at lower speed. Due to the mechanical link between the fan and the low-pressure compressor, its speed is also reduced. In order to keep the pressure ratio constant, without weight increase, the low-pressure compressor stages are designed with an unusually high loading coefficient and flow coefficient, which leads to a reduced stability margin.

The purpose of this thesis is to present a complete characterization of the first stage of the low-pressure compressor for the new contra-rotating turbofan architecture in terms of global performance, unsteady flow field and instability phenomena. This work will be the basis for future research in this field, considering the low number of publications on low-pressure compressors and the absence of references concerning the low-pressure compressor of contra-rotating turbofans. The characterization is performed on a half scale model in the VKI R4 test facility. This facility is a closed loop, high speed compressor rig, allowing the investigation of the influence of the Reynolds number on the performance. The effect of a circumferential groove casing treatment on the performance, the stability and the unsteady flow field is studied. The measurements are compared to numerical simulations from various CFD codes using various turbulence models.

The very small size of the blades imposes a high level of miniaturization of the instrumentation, particularly challenging in the case of the unsteady measurements. The global performance of the compressor is derived from the steady conditions upstream and downstream of the stage at four operating points, from choke to near the stability limit. The simultaneous acquisition of the casing wall static pressure and the total pressure downstream the rotor allows a visualization of the tip leakage vortex in the three dimensions. The data processing developed for this visualization includes the phase lock averaging and a specific interpolation method taking into account the various phenomena driving the flow in these regions.

The performance of the compressor shows an important reduction of the pres- sure ratio between design and near stall conditions, caused by a flow separation along the suction side at the hub of the blade, typical of hub loaded compressor characteristics. At near stall conditions, the pressure defect region due to the flow separation covers 35% of the rotor pitch. The investigation of the instability in the compressor shows a modal stall inception process confirmed by several elements. The unstable regime is a rotating stall caused by the large separation at the hub of the blades. The stall cells’ configuration in the compressor is completely unusual, with a single full-span cell splitting in multiple full-span cells, equally spaced and of equal size. In the single cell configuration, the stall cell covers 80% of the circumference. The non tip-critical aspect of the compressor makes the effects of the casing treatment on the mid-span performance and on the stall margin negligible. Nevertheless, the casing treatment induces a reduction of the blockage in the tip region when the origin of the tip leakage vortex is located upstream the casing treatment.

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