On many occasions, fan or compressor stages have to face azimuthal flow distortion at inlet, which affects their performance and stability. These flow distortions can be caused by external events or by some particular geometrical features. The aim of this work is to propose a joined numerical and experimental analysis of the flow behavior in a single axial compressor stage under flow distortion. The distortions are generated by different grids that are placed upstream to the rotor. Experimentally, the flow analysis is based on the measurements obtained by a series of unsteady pressure sensors flush-mounted at the casing of the machine rotor. URANS computations are conducted using the elsA software. The flow distortion is simulated by a drop of stagnation pressure ratio at the inlet boundary condition. The study is focusing first on the ability of a pressure drop, imposed as an inlet boundary condition in CFD, to reproduce accurately the effect of a flow distortion. The analysis is conducted using singular value decomposition (SVD) and dynamic mode decomposition (DMD). A special attention is then paid, on the experimental level, to the arising of rotating stall, from the onset of the instability up to completely developed stall cells.
In aircraft engines, compressor stages can encounter situations in which the flow is distorted at rotor inlet, for example during particular flight maneuvers, or due to the shape of the inlet of the airframe. The main objective of this paper is to investigate experimentally the effect of an inlet flow distortion on the internal flow dynamic and its consequences on the performance and operability of these machines. The distortion was generated by a porous plate grid installed upstream of the compressor. Eight total pressure rakes placed downstream of the grid were used to evaluate the distortion. Unsteady pressure measurements were performed on the casing at different axial and azimuthal positions to investigate the dynamic flow behavior at nominal conditions, at an operating point close to stall and during the transition to stall. 2D-2C PIV maps, synchronized with the runner position, were obtained on rotor blade-to-blade planes at three different spanwise positions (79%, 51%, 18% of the blade span). Three relative angular positions of the grid to the laser sheet were investigated at two different flow rates, namely the nominal flow rate and a flow rate close to stall. These three positions are corresponding to the laser sheet cutting through i/ the center of the grid’s wake and ii/ the two edges of the grid. The impact of the distortion on the performance of the compressor is analyzed and compared to existing models. The impact of the flow dynamic will also be considered, especially in operations close to the stall limit.
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