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. 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, and 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 (NLH) 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 interactions 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 CFD results for comparisons with experiments is discussed.

Experimental analysis of the unsteady, turbilent flow through the fan stage of a high-bypass turbofan in windmilling conditions

“…Profiles are non-dimensionalized with the engine inlet Mach number M 2A , measured at mid-span (h/h max = 0.5), in order to observe the inlet conditions dependency. Results are consistent with previous work (García Dufour et al, 2015). At engine inlet (Figure 3 (a)), the Mach number is uniform across the first 75% of the span, then decreases near the shroud.…”

“…Remarkably, a local minimum of incidence is observed around h/h max = 0.65, which matches the maximum of velocity deficit in the stator wake as described in Figure 3 (c). This explains the larger size of the stator separation around 60% span observed by Dufour et al (2015). Finally, similarly to the Mach number profiles, a fairly good self-similarity of incidence profiles is observed in the range of inlet flow parameters being tested.…”

“…This corroborates the existence of another flow separation on the stator blades. According to Dufour et al (2015), this flow separation has a more complex topology than on the rotor blade and takes the shape of a massive 3D separation. Indeed, the separation on the stator blade increases in size along the span and reaches a maximum at around 60% span, where the flow separates on the leading edge without reattachment.…”

“…The existence of flow separation on both the rotor and stator in windmilling conditions leads to complex unsteady interaction between the rotor and stator wakes. The recent numerical study of Dufour et al (2015) has shown that the rotor wake triggers a periodic movement of the stator flow separation leading to vortex shedding. In order to confirm experimentally this mechanism, the hot-wire signals have been post-processed using phase averaging method as suggested by Fernández Oro et al (2007).…”

“…These studies agree that the relative flow leaves the fan at nearly the metal angle, and that the bulk of the pressure loss occurs in the stator. A recent study (Dufour et al, 2015) has further shown that a steady numerical approach fails to reproduce some important features of the flow. The authors conducted unsteady simulations on a 2D section of the fan stage.…”