The Deposition of Fog Droplets on Steam Turbine Blades by Turbulent Diffusion

Abstract

A theoretical approach for calculating the rate of deposition of fog droplets on steam turbine blades by turbulent diffusion is described. The theory is similar to that which has proved successful for predicting deposition of small particles in pipe flow and includes a recent correlation for the inertia-moderated regime. A reliable estimate of the blade surface shear stress distribution is required and is obtained by a quasi-three-dimensional inviscid flow calculation to give the blade surface velocity distribution, followed by a two-dimensional boundary layer calculation. The theory has been applied to two representative case studies. The first involves deposition on the final stage blading of the low-pressure cylinder of an operating 500 MW turbine, and the second concerns deposition in a high-pressure, wet steam turbine. Results are presented showing the effect of fog droplet size, surface roughness, and other flow parameters on the deposition rate. A comparison is made between the rates of deposition by diffusional and purely inertial mechanisms. In low-pressure turbines these are of comparable magnitude, but in high-pressure machines diffusional deposition may dominate.