15th European Conference on Turbomachinery Fluid dynamics & Thermodynamics

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Nicolas Binder  - ISAE-Supaero, France
Lucas Sanchez - ISAE-Supaero, France
Adrien Thacker - ISAE-Supaero, France
Ludovic Rojda - ISAE-Supaero, France
Yannick Bousquet - ISAE-Supaero, France


The windmilling operation of fan is generally analyzed in the context of particular situations, such as the engine flame-out or severe transients. However, the free-windmilling operating point, for which no net work exchange occurs between the machine and the flow, is an essential feature of the map of any turbomachine stage. It sets the boundaries between compressor and turbine operating modes and gives access to physically-based extrapolations for operating characteristics. It is thus interesting to identify it, especially for off-design predictions. The free-windmilling regime is characterized by strongly negative incidences, usually more severe on the diffuser than on the rotor. Also, the coexistence of a turbine-like mode and compressor-like mode along the span is observed for conventional-design fans. Despite this complexity, recent literature reports a linear behavior for the flow close to the free-windmilling operation as a function of the flow coefficient. Consequently, the free-windmilling regime is reached for the same characteristic value of the flow-coefficient called Φp, whichever the rotational speed. The flow arrangement also keeps the same span-wise distribution if adequately normalized. The prediction of the value of Φp from a basic geometrical description of the rotor blade is within reach of low-order methods. However, the literature studies have been conducted for Reynolds numbers high enough to promote an early transition of the boundary layer and to avoid massive separations on the pressure side of the rotor blades. The present paper describes an extensive experimental analysis of the free-windmilling regime on a low-dimensional fan designed for cooling applications into aircraft. The WilLow test facility gives access to this regime for different mass-flows, i.e., different Reynolds numbers. For the highest Reynolds number configuration, it is demonstrated that the flow structure follows the theoretical expectations and that the linear behavior is observed. On the contrary, the lowest Reynolds number configuration induces some non-linear behaviors caused by the massive separation on the rotor blades. This latter completely recasts the structure of the flow, and the span-wise work distribution is affected. The linearity of the loading-to-flow coefficient is lost, resulting in a different value of Φp, which is no longer predictable by the available low-order methods. Finally, the ability of more accurate numerical simulations to capture this delicate physical effect is verified.


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