As expansion proceeds in a steam turbine, pressure and temperature fall; beyond a certain point the steam crosses the saturation line and begins to condense. The last stages therefore operate in two phases.
Two distinct losses
Thermodynamic loss. Condensation is not in equilibrium; the steam first subcools and then condenses through rapid nucleation. That irreversible process generates entropy and costs efficiency.
Mechanical loss. The droplets that form lag the steam; they exchange momentum as they strike the blades and produce an additional loss.
Erosion
The more physical problem is this: droplets collect on the stationary blades and form a film, the film breaks up at the edge into coarse droplets, and those droplets strike the rotating blades at high relative velocity. The result is progressive material loss at the blade leading edge.
Countermeasures — moisture removal slots, stator heating, resistant coatings, inter-stage spacing — are all geometric decisions that can be assessed by analysis.
Modelling
Non-equilibrium condensation. A model assuming instantaneous phase change at the saturation line cannot see the subcooling region or the nucleation; the efficiency prediction comes out optimistic.
Two size populations. The small nucleated droplets and the coarse droplets shed from the stator are of different orders and behave differently. Working with a single mean diameter makes the erosion prediction meaningless. See the multiphase note.
Droplet tracking. The trajectories of the coarse droplets are tracked as a discrete phase; the drag coefficient must be computed as a function of Reynolds number [1].
Transient interaction. Impact on the blades is part of the stator-rotor interaction, and a steady average erases it. See the rotating machinery note.
Erosion predictions are weak in absolute terms; comparative use and calibration against measured operating data are needed.
References
- Morsi, S. A. & Alexander, A. J. (1972). An investigation of particle trajectories in two-phase flow systems. Journal of Fluid Mechanics, 55(2), 193–208. DOI: 10.1017/S0022112072001806
- International Towing Tank Conference. ITTC Recommended Procedures and Guidelines 7.5-03-02-03: Practical Guidelines for Ship CFD Applications. ITTC.