Experimental and Numerical Flow Analysis of Low-Speed Fans at Highly Loaded Windmilling Conditions

Abstract

This paper aims for the analysis of experimental and numerical results of windmilling flow topologies far from freewheeling condition. Two major cooling fans were investigated: a baseline design and an innovative one meant to reach good performance in both compressor and turbine modes. Experiments are conducted with global and local characterizations to determine energy recovery potential and local loss mechanisms. Also, tests were performed on a turbofan engine to confirm some trends observed on the cooling fans. The numerical study is carried out with mixing plane steady simulations, the results of which are in fair agreement with experimental data. The difference of local topology between freewheeling and highly loaded windmill demonstrates that classical deviation rules such as Carter's are not well-suited to highly loaded windmilling flows. Finally, under certain conditions, the minor influence of the stator on the rotor topology indicates that nonrotating elements can be considered as loss generators.

Generic Properties of Flows in Low-Speed Axial Fans Operating at Load-Controlled Windmill

“…This strengthens the necessity of developing deviation rules specific to loadcontrolled windmill to well predict the performance, as already stated in Ref. [15]. These corrections were established in an empirical way, and cannot be generalized without further analysis upon a large number of different machines.…”

“…A detailed description of the instrumentation regarding the planes 1, 2, 3, and 5 is available in Refs. [13] and [15]. In the present paper, a third-directional five-hole probe is used (Plane 4) around 27.5 mm downstream the trailing edge of the stator (Keller, -150/þ100 mbars, basic accuracy 0.047% FS, Rosemount -50/50 mbars, basic accuracy 0.15% FS).…”

“…From this point, a discrepancy arises and increases with the flow coefficient. Since the actual outlet flow angle is characterized by a highly radially distorted distribution [15], the prediction of the deviation is no longer accurate.…”

Assessment of Steady and Unsteady Full Annulus Simulations Predictivity for a Low-Speed Axial Fan at Load-Controlled Windmill

“…The overall performance is therefore not well predicted. At loaded windmill on small axial fans, numerical radial distributions at rotor outlet are no longer in accordance with the experimental ones, which worsen the discrepancy regarding the global performance (steady mixing plane simulation [8]). This comes from the fact that, in loaded cases, the flow on the rotor is more separated than at freewindmill.…”

Rotational Speed of a Damaged Fan Operating at Windmill

“…Despite the expectation of highly separated flow, full unsteady calculation was discarded due to high computational requirements. Dufour et al (2014) and Ortolan et al (2016) find steady RANS provides a reasonable approximation across the rotor, which remains the focus of this study in predicting rotational speed trends with damage. Mixing planes are applied at the interfaces between the rotor and stator domains.…”