The rebound of non-spherical particles in gas-turbine components

Authors

Jack Gaskell - Department of Engineering ScienceUniversity of Oxford
Matthew McGilvray - Department of Engineering ScienceUniversity of Oxford
David Gillespie - Department of Engineering ScienceUniversity of Oxford

Abstract

Particulate deposition is known to cause significant damage to gas turbine components. Modelling the location and quantity of deposition is desirable to optimise component geometry and quantify cost of ownership effects that arise from the ingestion of atmospheric contaminants. Previous work has shown that the particle trajectory and angular velocity post-rebound – following collisions with a wall - can have a profound effect on the likelihood of subsequent downstream deposition. Therefore, accurately predicting the rebound of particles from a wall is crucial to model particle trajectories, erosion and deposition. Current bounce-stick modelling generally assumes the presence of spherical particles that are not representative of real atmospheric contaminants at the point of impact with the substrate. This work uses FEA simulations to compare the rebound of spheroids of different aspect ratios to that of spheres, and hence assess the applicability, and errors, associated with the use of spheres in models of atmospheric particulate collisions.   Simulations were performed of 3D prolate spheroids of aspect ratios 1.9 and 4.6 colliding with a half-space at different orientation angles relative to the surface. The spheroid geometries are typical of some observed atmospheric particulates. In all spheroid simulations the major axis was taken to lie in the axial flow – wall normal plane, consistent with simple fluid drag considerations, and previous observations. Oblique and normal collisions were modelled in ANSYS Mechanical and validation was obtained by comparison of spherical predictions to experimental data for equivalent diameter spheres and similar material pairs. The coefficients of restitution and angular velocities of the spheroids differ from those of spheres and vary significantly with particle orientation. In many instances, during the first collision the particle centroid does not obtain a normal velocity away from the surface, leading to multiple bounce events. Sensibly the changed aspect ratio causes the magnitude of angular velocities to increase significantly after impact, and on occasion to change its sign. This will alter the lift induced on the particle in the fluid boundary layer during rebound. Previous work has shown this to significantly change the downstream particle trajectory and subsequent deposition. Normal coefficients of restitution (CoR) were generally lower than those of equivalent spheres while tangential CoRs were higher, leading to shallower trajectories post-rebound.  The physical mechanisms for this behaviour, and how they can be captured in a low order model of rebound behaviour are discussed.