Free-Windmilling Speed Prediction Based on Aerodynamic Similarity of an Axial Wide-Chord Fan

This paper explores the windmilling operational boundary and aerodynamic characteristics of an axial wide-chord fan rotor according to the zero-torque characteristic. The interesting results show the radial profiles of the aerodynamic parameters at the rotor exit nearly collapse to the same curve for different windmilling speeds, which indicate that windmilling flow has a strong similarity. Meanwhile, there are three different operating modes along the blade span. The formation mechanism and evolution process of the tip leakage flow at the windmill are also analyzed. The reversal of the pressure difference will generate two kinds of tip leakage flows in opposite directions. A clear blockage zone appears near the blade tip under the joint action of the two kinds of tip leakage flows. Finally, this study proposes a mathematical model to predict the windmilling speed of a high-bypass-ratio turbofan based on the aerodynamic similarity of windmilling flow and the one-dimensional semiempirical windmilling mass flow function. This can be applied to the early stages of high-bypass-ratio turbofan development and can evaluate safety characteristics during engine windmilling operations.