Modelling and Analysis of Turbofan Engines Under Windmilling Conditions

Engine windmilling and relight performance is a matter of safety in aviation. Two of the most important properties that engine designers must have a feeling for, even from the very early design stages, are the windmilling engine mass flow and internal drag. However, only empirical approaches are available so far to perform such studies. This work is focused on the development and validation of a first principles methodology for the prediction of the aforementioned properties, based on the simple frictional flow theory. Considerations about the modeling of the pressure loss factors are also made. Validation of the method is carried out by comparing the predicted windmilling drag and mass flow against test data of several high-bypass civil engines for a typical range of operating conditions. The results compare favorably with the test data, proving the robustness and reliability of this approach. Fully dimensionless parameters are defined to describe the windmilling performance, independent of the engine design and operating conditions. This research contributes to the preliminary engine development phases, because engine studies can be conducted using minimal geometrical information, allowing for a reliable prediction of the windmilling properties.