The formation of water droplets within condensing steam turbines is a complex process that occurs at supersaturated, non-equilibrium conditions and is influenced by, inter alia, the presence of foreign bodies (e.g., dust particles) and other steam impurities, and the unsteady segmentation of blade wakes by successive blade rows.  
The latter is often referred to as \`wake chopping', and its effect on the condensation process is the subject of this paper.  
On-site optical measurements conducted in the late 1980s on operational turbines (e.g. Young et al. (1988)) revealed droplet size distributions that are much broader and with larger average diameters than those observed in (steady flow) condensing nozzle experiments.  
A number of authors  (Bakhtar and Heaton (2005), Petr and Kolovratnik (2000), Guha and Young (1994)) have suggested that wake-induced unsteadiness may be a contributing factor towards this discrepancy.  
The practical significance is that thermodynamic \`wetness losses' (which constitute a major fraction of the overall loss) are strongly affected by droplet size.  
Likewise, droplet deposition and the various ensuing two-phase phenomena (such as film migration and coarse-water formation) also depend on the spectrum of droplet sizes in the primary fog.   

The theory behind wake-chopping models is that the high levels of dissipation within the segmented wakes lead to temperature fluctuations that suppress and postpone the condensation process.  
This results in nucleation occuring in regions with a range of expansion rates which in turn leads to a broader spectrum than would otherwise occur within a steady flow.  
The majority of wake-chopping models presented in the literature (e.g., Petr and Kolovratnik (2000), Guha and Young (1994)) adopt a stochastic approach, whereby large numbers of fluid particles are tracked through (some representation of) the turbine flowfield, assigning a random number at each successive blade row to represent the particle's pitchwise location, and hence its level of dissipation.  
The work described here follows a similar approach but includes  
(a) a comprehensive study of the sensivity to key model parameters (e.g., blade wake shape and wake decay rate);  
(b) an assessment of the impact of circumferential pressure variations;  
(c) a study of the implications for wetness losses and  
(d) a study of the implications for deposition rates.


The Influence of Wake Chopping on Wet-Steam Turbine Modelling

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

Turbo Expo - Virtual Conference

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.