Non-equilibrium wet-steam calculations of unsteady low-pressure turbine flows

Abstract

Accurate prediction of the droplet size distribution in steam turbines is crucial for the correct analysis of wetness losses and other two-phase effects. Measurements taken in the later stages of low-pressure turbines have shown broader size distributions with larger average sizes than those predicted by numerical methods. One hypothesis is that the broad distributions stem from the unsteady interaction between blade rows. Wake segmentation in successive rows means that fluid particles passing through the machine have different dissipation histories which in turn cause a wider range of nucleation and droplet growth rates. In the present paper, a method for modelling 3D unsteady multistage condensing flows is described and applied to a five-stage model turbine. One-tenth of the annulus of the turbine is modelled, and an integer blade number ratio between rows is achieved by scaling the blade profiles, keeping the pitch-chord ratio and stagger angle constant. The results are compared with pressure measurements and droplet Sauter-mean radii obtained from experimental optical data. Generally, the level of agreement is reasonable, but with some discrepancies in the detailed streamwise and spanwise trends. Calculations of this type are at an early stage of development and improvements in mesh quality and boundary and interface treatments will be required before firm conclusions can be drawn.