Effect of Blade Damage on Low Pressure Ratio Fan Windmill Aerodynamics

Abstract

Fan windmill occurs when power to a turbofan is cut during flight and the airflow through the engine causes the fan to freewheel, often following fan damage. The fan rotational speed and drag during windmill determine the loads transmitted to the airframe, and these depend on the fan damage sustained. Idealized patterns of axisymmetric and non-axisymmetric damage have been studied using high-resolution measurements in a flow-field representative of a low pressure ratio fan in combination with steady Reynolds-averaged Navier–Stokes (RANS) simulations to understand the impact on the windmill flow field. Axisymmetric tip damage decreases the windmill rotational speed by 61% when 25% of the blade span is removed due to a shift in the zero-work radius. The reduced blade span creates an intense tip vortex and redistributes the flow, increasing the axial velocity above the damaged tip. For non-axisymmetric damage, there is still work output at the blade tip, such that removing 25% of half the blades with varying damage pitch to chord reduces the fan rotational speed by 9–13% compared to undamaged windmill. Phase-averaged hot-wire measurements show that this additional rotor work is due to radial flow redistribution combined with turning of the flow in the passages above the damaged blades.