The Inertial Deposition of Fog Droplets on Steam Turbine Blades

Abstract

A theoretical approach for calculating the rate of deposition of fog droplets on steam turbine blades by inertial impaction is described. Deposition rates are computed by tracking a number of droplet path lines through a specified blade-to-blade vapor flowfield and identifying the limiting trajectories that just intersect the blade surface. A new technique for performing the calculations efficiently has been developed whereby the mathematical stiffness of the governing equations is removed, thus allowing the numerical integration to proceed stably with comparatively large time increments. For high accuracy, the vapor flowfield is specified by a quasi-three-dimensional flow calculation involving both meridional and blade-to-blade plane calculations. Results are presented for two representative “test cases,” namely the final stage blading of the low-pressure cylinder of a 500 MW turbine and a typical stage in a high-pressure wet steam turbine. The effect on the deposition rate of fog droplet size and blade profile geometry is investigated for both on- and off-design flowfields. Comparisons are made with the predictions of a simplified theory for inertial deposition and the effect of blade rotation in flows with high pitch angles is discussed.