Abstract:
An experimental and theoretical study of erosion in centrifugal compressor impellers is
presented. An experimental rig using laser anemometry techniques was employed to create
a database of particle restitution ratios for a range of materials. This data was unique in
that the particle rebound was measured in a quiescent condition where the aerodynamic
effects had been minimised, and also parametric factors not previously available were
included. These values were incorporated into the existing Particle Trajectory Code
developed by Cranfield University and Rolls Royce PLC. The code is used to calculate
the trajectories of discrete particles in three dimensional gas turbine geometries, and the
ensuing erosion. It was modified to include the effects of the periodic boundary
conditions, particle fragmentation, splitter blades, and variations in inlet dust concentration
profile. Flowfield calculations were performed on a Rolls Royce GEM-2 and splittered
GEM-60 impeller, which both represent the high pressure stage of the axial + centrifugal
compression system of GEM engines. A procedure developed by Tourlidakis, for the
analysis of steady viscous flow in high speed centrifugal compressors with tip leakage,
was used to generate the flowfields. The GEM-2 impeller flowfield was analysed at 1009c
speed, and validated with calculations and measurements which had been taken for
previous projects. Simulated erosion data under the same conditions was checked using
practical results obtained in a Rolls Royce PLC Helicopter Engine Environmental
Protection Programme, and good agreement was achieved. In order to provide a qualitative,
experimental assessment of erosion, a GEM-60 impeller was coated with four layers of
paint of different colours. Two sizes of quartz particle, each at three different vane
heights, were then seeded into the impeller while it was run cold at (the maximum) 70%
speed. The erosion patterns generated compared well with the results generated by the
Particle Trajectory Code.