Abstract
The S-shaped inlet of the modern military aircraft propulsion system suffers from severe inlet distortion during flight maneuvering. The complex distorted flow field is generally decomposed into total pressure and swirl distortion, both of which could substantially degrade the performance and stability of an aero-engine. In the ground test of fans and low-pressure compressors, how to simulate the complex inlet distortion occurring in flight is non-trivial. In this paper, a subsonic S-shaped inlet test rig was constructed, and eight configurations of curved-edge plate-type distortion generators were installed in the intake channel to create a complex distorted flow field downstream. Results show that the distorted flow field is dependent on the circumferential position of the blocking plate due to the duct curvature. Prominent vortex structures caused by the pressure gradient and the cross secondary flow exist between the main flow and separated flow. Conventional distortion indices DC, CDI/RDI, and the SAE AIR1419/SAE AIR 5686 standard system are implemented to represent the total pressure distortion and swirl distortion and the effects of the size and location of the curved-edge plate on distortion intensity and distribution are investigated. Additionally, this paper also studied the spatial variation of the total pressure and swirl distortion between the duct circular exit section and the annular section, termed as spatial transition characteristics. This spatial transition process can be used in the future to specify a real inlet flow condition for stability assessment of a fan or low-pressure compressor. According to the experimental results, the circumferential total pressure distortion index is approximately unaltered along the contraction duct, while the radial index reduces by 62% on average. The spatial transition of the swirl distortion differs for bilaterally symmetrical and asymmetric swirling fields. The overall swirl intensity index decays by 37%∼70% only for bilaterally symmetrical modes, while being the same for bilaterally asymmetric approximately. At last, the generation and evolution mechanism of the above complex distorted flow field is analyzed theoretically.
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