Validation and flow structure analysis in a turbofan stage at windmill

Abstract

In the present study, the flow through the fan stage of a high bypass ratio turbofan at windmill is studied numerically. First, steady mixing plane simulations are validated against detailed experimental engine test-bed measurements, at several locations within the fan stage and close to the core/bypass flow splitter. Good agreement between the numerical and experimental results is obtained for the global operating point in the fan map. For the two windmilling points considered, it appears that as the flight Mach number is reduced, the windmilling operating point moves closer to the stability limit of the fan. For the radial profiles, good agreement is found downstream of the rotor. Downstream of the stator, it is shown that the steady approach fails to reproduce some important feature of the flow. A local flow analysis is proposed, evidencing several characteristics of the flow in windmilling: in the rotor, the size of the separation zone is found to increase from hub to tip, due to a solidity effect. In the stator, massive flow separation occurs at mid-span, which leads to the formation of two streamwise counter-rotating vortices. Then, the nonlinear harmonic method is applied to a section (at 70% of the relative span) of the fan stage. A modal analysis is performed, showing a specific behavior at windmill: the massively separated flows in the rotor and the stator entail strong rotor/stator interaction modes. Finally, the unsteady flow pattern is examined: the velocity defect of the rotor wake, which periodically increases the flow angle on the stator, is shown to trigger a periodic movement of the reattachment point at the trailing edge of the stator, associated with vortex shedding from the lower side of the vane. The implication of this qualitative flow behavior on the method to extract computational fluid dynamics results for comparisons with experiments is discussed.