Importance of surface curvature in modelling droplet impingement on fan blades

Abstract

The surface roughening of fan blade leading-edges on large civil aeroengines, due to Water Droplet Erosion (WDE), can lead to a decrease in engine performance and increase the risk of flutter. To model the impingement of a high-speed water droplet on a surface, researchers have frequently assumed the droplet is (a) perfectly spherical, (b) impinging perpendicularly, on a (c) smooth, flat, homogeneous surface. However, the situation presented by the leading-edge of a fan blade calls into question one of these previously used assumptions. It is reasonable to assume a surface is flat, if in reality it is curved, when the radius of curvature of the surface is significantly larger than the droplet radius. In other contexts where WDE occurs, the typical droplet size has either been sufficiently small (e.g. droplet diameter in steam turbines) or the radius of curvature of the surface sufficiently large (e.g. wind turbine blades) that it has been sensible to assume the surface is flat; this does not appear to be the case for the WDE of fan blades.

The equations describing the kinematics of an impinging water droplet on a flat surface were reformulated for a curved surface. These novel equations were used, along with established conditions for the onset of lateral outflow jetting, to model the impingement of a droplet on a curved surface. European Aviation Safety Agency publications provided the input values typical for a large, high-bypass, civil turbofan, operating at 100% rpm. The results suggest the relatively similar radius of curvature of the leading-edge of a fan blade and radius of the impinging water droplet will significantly affect the onset of lateral outflow jetting. Jetting is predicted to commence substantially sooner; with the initial high-pressure stage expected to last less than half the time predicted for a flat surface. This substantially shorter initial stage is likely to have significant implications for the WDE that occurs as a result of multiple droplet impingements. Over the years, various attempts have been made to extend the frequently-used set of assumptions, described at the start of this abstract. However, we can find no studies where the effect of the curvature of the surface has been explored. Thus, currently, there are no appropriate theoretical treatments describing the liquid kinematics of a typical water droplet impingement on the leading-edge of a fan blade; this research seeks to rectify this.

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Identifiers

ORCID for Charles Burson-Thomas: orcid.org/0000-0001-9308-4669

ORCID for Robert Wood: orcid.org/0000-0003-0681-9239

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