Evaluation of Free-Windmilling Speed based on Flow Self-Similarity of Wide Chord Fan Blade in High-Bypass Turbofan

This paper explores the aerodynamic differences of the wide-chord fan rotor under design and windmilling operation and reveals the windmilling characteristics and the changes in the overall rotor performance under different windmilling speeds. The torque characteristic curves and the operation boundary of windmilling fan are obtained through three-dimensional computational fluid dynamics (CFD) calculations. It is found that the radial profiles of aerodynamic parameters such as blade flow angle, load coefficient, total pressure loss coefficient, axial velocity, and entropy loss coefficient at rotor exit collapse nearly to the same curve respectively for different windmilling conditions, which indicates that the windmilling flow has a strong self-similarity. Meanwhile, there exist three operating modes along the blade span. The blade operation moves gradually from compressor-like regions near the hub to turbine-like regions near the shroud, in which the stirrer mode is the transitional stage between the two. In addition, the formation mechanism and evolution process of the tip leakage flow at windmill are emphatically analyzed. The results show that due to the reversal of pressure difference, two kinds of tip leakage flow in opposite directions will be generated, among which the intensity of the main leakage flow near the leading edge (LE) at pressure surface (PS) is much higher than that of the secondary leakage flow generated near trailing edge (TE) at the suction surface (SS). With the increase of windmilling speed, the intensity and influence range of both leakage flows are significantly expanded. Finally, this study proposes a mathematical model for predicting the windmilling speed of a high-bypass ratio turbofan based on the self-similarity property of windmilling flow and one-dimensional semi-empirical windmilling mass flow function. The model does not need to manually specify the mass flow and experimental data. It can be conveniently applied to the early stage of the high-bypass ratio turbofan development and the evaluation of the safety characteristics during the engine windmilling operation.