The dependence of the aerodynamic stability of fan blades with distorted inflow is studied as a function of the nodal diameter and the amplitude of the inlet perturbation. The analysis is conducted using a block-wise spatial Fourier decomposition of reduced-passages to reconstruct the full-annulus. The method represents very efficiently the unsteady flow generated by  inlet non-uniformities. The explicit spatial Fourier  approximation is exploited to characterize the relevance of each nodal diameter at the inlet perturbation in  the fan stall process and study the nonlinear stability in a harmonic basis. This approximation allows studying the contribution to stall of each individual circumferential mode separately.  The effect of the different nodal diameters of the inlet perturbation has been assessed for the NASA rotor 67. The maximum amplitude of total pressure distortion at which the compressor becomes unstable and triggers a stall process has been computed. It has been proven that despite the complexity of a screen-induced total pressure distortion the only relevant parameter of the problem is the nonlinear stability of the fan of the most unstable nodal diameter. The equivalence in terms of stability of a realistic inlet distortion screen and single harmonic distortion has been assessed running distortion screens of 60º and 120º.  It is concluded that performing a nonlinear simulation with a single harmonic perturbation is enough to assess fan stability. For NASA rotor 67, it is shown that the most unstable nodal diameter is the first. This study not only shows a reduction in computational time to assess the stability of a fan stage of  a factor of seven but also creates an efficient methodology for understanding  the nonlinear instability of fans due to inlet distortion profiles.


Nonlinear Stability Analysis of a Generic Fan With Distorted Inflow Using Passage-Spectral Method

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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