Body Force Modeling of the Fan Stage of a Windmilling Turbofan

Abstract

The determination of the rotational speed and massflow of a windmilling fan is critical in the design of the engine-supporting structure and the sizing of the rudder. Given the very high bypass ratio obtained at windmill (typically around 50), the flow in the fan stage and bypass duct is of prime interest. Classical CFD simulations have been shown to predict such flows accurately, but extensive parametric studies can be needed, stressing the need for reduced-cost modeling of the flow in the engine. A Body Force Modeling (BFM) approach to windmilling simulations is examined in the present contribution. The main objective is to assess the capability of the BFM approach to reproduce the aerodynamics of the flow in the fan stage of a turbofan at windmill, and to propose a method to predict the massflow and the rotational speed of the fan. The available global and local experimental data of a high bypass ratio geared turbofan (the DGEN 380) are used to validate the model. Furthermore, classical RANS simulations are also provided as reference simulations to assess the accuracy of the BFM results. It is found that the overall performance of the fan stage is well predicted by the BFM simulations, in particular at the lowest rotational speed. In terms of local validation, radial profiles are also found to be in good agreement, except close to the shroud due to massive flow separations in both the rotor and stator. A BFM simulation is about 10 times faster than the baseline CFD computation, making this approach very efficient in terms of accuracy-to-cost ratio. Finally, a zero work exchange model is embedded in the BFM computations, such that both the massflow and the rotational speed of the fan become outputs of the simulations. The predictions obtained by the present approach show good agreement (maximal discrepancy of 6.5 %) with engine experimental data.