Droplet Characterization in the Wake of Steam Turbine Cascades

In low-pressure steam turbines, water droplet formation on the surfaces of stationary stator blades can lead to erosion on downstream turbine blades and other equipment. One property that affects the size of the droplets that are formed is the adhesive forces between the water and the surface of the stator blade. The adhesive forces hold the droplets to the surface where they may combine, forming increasingly larger droplets. Eventually, the aerodynamic forces will tear the droplets off the surface, carrying them downstream.

To study the effect of stator surface properties on the droplet size distribution, four cascades of stator blades were tested with a low-speed, cold-flow steam turbine simulator. A non-intrusive optical system was used to detect and measure the droplets.

Of the four cascades tested, the baseline cascade that showed obvious surface roughness had the largest average droplet size. The cascade that had been sandblasted smooth formed smaller average droplets than the baseline cascade but larger average droplets than the cascade that was coated with a proprietary, glass-like coating. This coating was designed to minimize the surface tension between the surface and the water droplet. The fourth cascade was coated in a superhydrophobic granular coating. This coating appeared to work so well to reduce the droplet size that no droplets could be detected by the imaging system. This untested cascade could not be considered in the quantitative analysis.