Erosion rate prediction and correlation technique for ceramic surfaces exposed to high speed flows of abrasive suspensions

Abstract

We describe a simple method to predict (based on available erosion yield data) erosion rates for cylindrical ceramic targets (e.g. circular tube coatings, leading edges of turbine blades, or the ceramic-lined target zone inside CFBC-cyclones) exposed to high-speed abrasive particle-laden streams. Use is made of a convenient parameterization/extrapolation of published laboratory results giving, in effect, average erosion yields per particle impact (ϵp) for particular planar ceramic target and projectile materials over a range of impact velocities, Vp, incidence angles, θj, and particle sizes, νp. For a given target/flow geometry we reduce the engineering problem of predicting absolute target erosion rates to that of multiplying a readily calculated characteristic erosion rate by the universal dimensionless erosion rate functions explicitly approximated here in the limit of impacts by particles large enough to be undeflected or slowed down by the local target gas flow. Our characteristic erosion rate is that which would be associated with the mainstream abrasive particle volume flux if all particles struck at normal incidence with the mainstream velocity, U. Dimensionless erosion rate results are cast in terms of the following four dimensionless parameters characterizing the erodent/ceramic target system of interest: sensitivity (exponent n) of erosion yield to projectile incident velocity; sensitivity (exponent m appearing in (costm(θi)) of erosion yield to angle of incidence θi: sensitivity (exponent l) of erosion yield to projectile particle size (volume, νp); and the reference erosion yield, ϵp,ref (here, ϵp is evaluated at Vp = 100 ms−1, θt = o, and νp corresponding to dp = 100 μm). Based on our preliminary survey of available erosion yield experimental data, we provide a table givin “best-fit” values of the four parameters: l, m, n, and ϵp,ref required to complete a prediction of local and spatially-averaged erosion rates according to our present formalism. For the latter, useful closed-form approximations are provided for convex or concave cylindrical target geometries in the high Stokes number limit. Moreover, convenient correction factors are developed to account for a (Rosin-Rammler) particle size distribution in the erodent mainstream, and mainstreams not perpendicular to the cylinder axis. The more general case of arbitrary (finite) Stokes numbers is outlined. Using two numerical examples (convex leading edge coating on a turbine stator blade, and concave sector target zone in a CFBC-cyclone), we demonstrate that casting required erosion yield data in this suggested format greatly facilitates erosion design calculations for ceramic targets (or coatings) exposed to high-speed abrasive particle suspensions. Organizing empirical data in this manner will also facilitate the longer range goal of correlating each of the above-mentioned parameters with independently measurable physical properties of the participating materials.