Characterisation of rain erosion at ex-service turbofan blade leading edges

Abstract

Rain erosion of turbofan blades creates problems in the aeronautics community, since the changes of profiles on leading edges affect aerodynamic performance that subsequently leads to significant efficiency drop for the aircraft engine. Water-hammer pressure induced by droplet impingements is primarily responsible for the damaged induced during the initial compression stage. As the surface roughened, the damage mechanism is then governed by lateral flow jetting assisted with hydraulic penetration. Studies on WDE are restricted to laboratory testing with simplified conditions and samples. Hence, this will be the first study to demonstrate real-life WDE damage on real turbofan blades during their ‘complicated’ in-service conditions. A set of Ti-6Al-4V ex-service turbofan blades are sectioned to examine their in-service degradation. Different parts of the blade, with impact velocities ranging from 144 m s⁻¹ to 396 m s⁻¹, were selected to study the effect of impact velocity on rain erosion damage. Erosion morphology and the damage mechanisms are characterised with Alicona profilometer and SEM. The results reveal that rain erosion occurs exclusively at the leading edges. Increase of rain erosion severity with increasing impact velocity is identified. The normalised volume loss is proportional to Vⁿ, where V is the impact velocity, ranging from 286 m s⁻¹ to 348 m s⁻¹, and the exponent (n) is estimated to be around 8 for Ti-6Al-4V ex-service turbofan blade, which agrees with the literature from laboratory tests. The damage features during early incubation period cannot be detected due to the roughening of the leading edges. However, intergranular fracture is detected at the tip of the leading edge with less severe damage. In the steady state, material removal appears to be due to coalescence of microcracks to form erosion craters. The cracks continue to propagate from the side wall and bottom of the erosion craters, attributable to the joint effect of lateral outflow jetting and hydraulic penetration induced by repetitive droplet impingements. Thus, the present research is an invaluable opportunity to validate the results obtained from laboratory tests by comparing with the real-life WDE, which then helps to further elucidate the damage mechanisms behind WDE.

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