Inlet distortion can severely affect the performance and the stability of engine fan operating at off-design conditions. In the framework of the emerging future architectures proposed to reduce the environmental impact of civil aviation, this condition can become inherently present, due to the adoption of Boundary Layer Ingesting (BLI) or ducted engines, tightly integrated or even completely embedded into the aircraft structure.Depending on the pattern and the level of distortion, different mechanisms arise across the rotor blades facing a nonuniform inflow, causing a change of the operating line and a reduction of stall margin. Several studies in the literature have been devoted to the analysis of transonic fans working with inlet distortion, employing a variety of methods. In order to study the phenomena that characterise the distortion transfer and the non-uniform blade excitation&nbsp;unsteady CFD calculations must be performed. The cost of full-annulus multi-row unsteady Reynolds-Averaged Navier-Stokes simulations (uRANS) is very high, and though Large-Eddy Simulations (LES) might be necessary to study specific cases or phenomena, with a cost becoming prohibitive. Simplified methods are therefore needed when a rapid assessment is sought, or the design of distortion-tolerant blades is faced.This paper proposes a study of a transonic fan subject to inlet distortion by employing steady RANS simulations and Mean-Line (ML) calculations. One-dimensional models based on conservation of mass, momentum and energy across the cascade and relying on empirical correlations for losses, flow angles and secondary flows are a useful design tool, that can reach high accuracy if properly calibrated. Based on the superimposition of several independent 1D solutions, the characteristic maps of engines with nonuniform conditions can be obtained, in the parallel compressor or the discretised compressor approach. In particular, the latter can be regarded as a quasi-2D method.Steady RANS simulations can generally match experimental operative maps of clean inlet configurations with sufficient accuracy. Despite not being able to correctly measure the interaction of the distortion across the cascade passages and predict the unsteady machine operation towards the surge, they are used as a benchmark in the present analysis. A total pressure deficit is considered in this study. A continuous pressure drop is introduced by means of a parametric function, allowing to control the extent and the penetration of the bubble, meant to mimic a detached inlet operation, where a recirculation region is formed in the intake lower lip.The study is made for a high-bypass modern transonic fan, for which a wide experimental database was available. The steady CFD model is validated for clean conditions and adopted to simulate the distorted inflow along the operating line. The performance drop and the change of the stall onset are analysed. Finally, the CFD-based inlet velocity profiles are used as boundary conditions for the quasi-2D discretised compressor method, based on a ML solver. A comparison between the two models is then established, highlighting the correspondence and the limitations of the two approaches, that in future studies shall be compared to more accurate unsteady models.

Inlet Distortion Simulations of a Transonic Fan With Cfd and Low-Order Methods

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

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