Session: 31-07 Compressor Off-Design Impacts & Stall Inception

Paper Number: 122107

122107 - Effect of Blade Damage on Low Pressure Ratio Fan Windmill Aerodynamics 

Fan windmill occurs when power to a turbofan is cut during flight and the airflow through the engine causes the fan to freewheel. This can arise following an in-flight shutdown after a bird strike event that causes significant fan damage. The fan rotational speed and the engine drag during windmill determine the loads transmitted to the airframe and these depend on the fan damage sustained.

This paper is the first detailed experimental and computational study of axisymmetric and non-axisymmetric blade damage effects on fan windmill aerodynamics. A low-speed rig has been developed that reproduces the windmill flow field of a representative low pressure ratio engine fan. This rig has been tested with several idealised patterns of axisymmetric and non-axisymmetric tip damage. High-resolution measurements have been combined with results from computational methods to understand how the damage configuration impacts the windmill flow field and operating condition.

Axisymmetric tip damage removes a portion of the fan blade that would normally generate work output in an undamaged windmilling fan. This forces the work distribution to move radially inwards causing a lower windmill rotational speed. The reduced blade span also creates an intense tip vortex and redistributes the flow, increasing the axial velocity above the damaged tip. For non-axisymmetric damage, the fan is still able to produce work at the blade tip. Phase averaged hot-wire measurements show that this additional rotor work is partly due to radial flow redistribution and partly due to turning of the flow which still occurs in the passages above the damaged blades. This leads to a radial work distribution and windmill rotational speed that is closer to an undamaged case than to an equivalent axisymmetric damage case.

For an axisymmetric damage case in which 25% of the blade span has been removed, the fan rotational speed was reduced by more than 55% from the undamaged case. For non-axisymmetric damage cases with the same 25% of blade span removed from half of the fan blades, the rotational speed was reduced by only 9-13% from the undamaged case.

Presenting Author: Sofia Medina Cassillas University of Cambridge

Presenting Author Biography: Sofia Medina Cassillas is conducting her doctoral research in the field of engine fan aerodynamics at the Whittle Laboratory in the University of Cambridge, as part of the Centre for Doctoral Training in Future Propulsion and Power. Using a combination of experimental measurements in an engine-representative rig and computational simulations she will investigate the aerodynamics of an engine fan in the windmill condition.
She completed her bachelor's and master's degree in Aerospace Engineering at the University of Bristol in 2019 and is an Amelia Earhart Fellow since 2021.

Authors:

Sofia Medina Cassillas University of Cambridge
Alejandro Castillo Pardo University of Cambridge
Cesare Hall University of Cambridge
Benjamin Mohankumar Rolls-Royce