Unsteady Wetness Effects in LP Steam Turbines

Abstract

One of the unresolved issues for condensing flow in large steam turbines is correct prediction of the droplet size distribution. Optical measurements taken in the later stages of LP steam turbines have shown that the time-averaged droplet size spectra are much broader with larger average diameters than predicted by most theoretical and computational methods. Previous work has suggested that the broad distributions might stem from unsteadiness created by the interaction between successive blade rows — the so-called ‘wake-chopping’ effect. The current paper presents preliminary results of multi-stage CFD calculations aimed at investigating the impact of such unsteadiness on the condensation process. A method for calculating unsteady, viscous, condensing flows in multi-stage steam turbines is first outlined. This is based on an established single-phase flow solver with nucleation and droplet growth incorporated via moment evolution equations for the polydispersed liquid phase. The method can be used to compute two- and three-dimensional steady and unsteady flows, time accuracy being preserved for unsteady calculations by means of the dual time-stepping technique. Comparison between computed results and experimental data is presented for nozzle and cascade flows for the purpose of validation. Finally, results are presented for a two-dimensional unsteady multi-stage calculation, highlighting the impact of wake-chopping and related unsteady phenomena on the droplet spectra.