Nucleation phenomena in a multi-stage low pressure steam turbine

Abstract

During the expansion of steam in the low pressure (LP) stages of steam turbines, the originating two-phase wet-steam mixture causes considerable thermodynamic losses as well as aerodynamic losses. The reduction of these loss mechanisms is the subject of research project at the Institute of Steam and Gas Turbines, Aachen University. A three-dimensional nucleating wet steam flow with homogeneous/heterogeneous nucleation in the three front stages of an industrial LP-steam turbine is investigated numerically. The steady, viscous, and compressible metastable steam flow calculations are performed with a Navier—Stokes flow solver incorporating the IAPWS-IF97 steam tables. A union numerical approach for both the homo- and heterogeneous nucleation occurring on soluble nuclei is employed to capture the effects related to the nucleation phenomenon. The model links the interfacial surface tension, the size of nuclei, the chemical characteristics of the substances forming the droplets, and the expansion rate with the nucleation rate. In order to take into account the additional viscous effects due to shrouded bladings, the open shroud cavities are modeled in detail. Droplet density spectra, radial droplet number, droplet diameter, and wetness fraction distributions at the exit of the third stage are calculated. It is shown that impurities can cause nucleation to appear at lower supersaturations with higher nucleation rates compared to homogeneous nucleation of pure steam. In this way, thermodynamic and kinematic relaxation losses are reduced. Owing to the dissipative viscous effects near the endwalls, the nucleation fronts exhibit convex shapes. They are locally bound within the region of high expansion rates in the second stage's nozzle guide vane. For both heterogeneous and homogenous nucleating flows the wetness is highly dispersed with narrow droplet density spectra behind the three front stages.