Seminar: Faxén Seminars

Free and load-controlled windmilling operation of compressors

Speaker: Guillaume Dufour
Organization: ISAE-SUPAERO, Toulouse, France
Time: 2017-12-12 13:00
Place: Teknikringen 8


The windmilling regime of a turbofan corresponds to a freewheeling mode of the fan rotor, driven by the ram pressure at the inlet. This operating mode is encountered in the context of in-flight shut-off of an engine, and raises several issues, such as the determination of the resulting rotational speed and drag. However, the windmilling operation of fans is not limited to turbo-engines: with the development of more-electrical aircraft, some cooling fans, which are required for ground operation but normally not used during flight, are now considered as potential energy generation devices. The fan has thus to operate in a turbine-like mode, with a significant resistive load on the shaft, which can be considered as a load-controlled windmilling regime. More generally speaking, energy recovery during the descent of distributed/electric propulsion is a much related issue. The understanding, prediction and modeling of such free- and load-controlled windmilling operation of compressors will be the focus of the presentation. To this end, two experimental facilities of the ISAE-Supaero Turbomachinery and Propulsion Group will be used to support the discussion: a cooling fan and a full-engine test bed (the DGEN-380). As discussed in the literature, windmilling operation is associated to largely negative incidence angles on both the rotor and the stator of the fan stage, with possible massive flow separations and unsteady effects. Moreover, the fan operates in a mixed fashion: the inboard sections operate in compressor mode, while the outboard sections operate in compressor mode. First, the analysis of experimental and numerical results will aim to deepen our understanding of such a regime, and the predictive capability of steady and unsteady CFD methods will be assessed. The results show that a fairly accurate prediction of the mean flow can be obtained with a standard RANS approach, though complex unsteady flow may develop. Then, an original concept of a dual fan, designed to operate efficiently both as a compressor and as a turbine depending on the conditions, will be presented. Experimental results show that eliminating the mixed-operation across the span at the free-windmilling operating point is a design target that ensures good efficiency of the fan in load-controlled windmilling (turbine) mode. Finally, the focus will be on integrated predictions, for which a specific modeling approach is retained: the body force source-term modeling approach. The basic principle of the BFM approach is to replace turbomachinery rows by source terms in the problem equations, thus alleviating the need to actually mesh blades and thereby reducing the computational cost. Validation of the BFM results for the free-windmilling operation of the fan stage of the DGEN engine will be presented. The results show that the model can reproduce the fairly complex operation of a free-windmilling fan with sufficient accuracy. Predictions of the rotational speed of the low-pressure shaft of the engine will then be proposed and validated against engine experimental data. Assuming zero-work exchange across the rotor, a transient equation for the rotational speed is derived and included in the time-marching process to the steady state, so that the rotational speed is an output of the BFM simulation. To conclude the presentation, perspectives towards full-engine modeling will be discussed, with an overview of ongoing studies to account for distortion effects.