Suppression of Coarse Droplet Formation From Gas-Sheared Liquid Sheets by Controlling the Trailing Edge Shape and Wettability

Abstract To mitigate the erosion issue in steam turbines, it is crucial to suppress the formation of coarse droplets spreading from the trailing edge upstream as well as from the shroud wall. The droplet size will be highly dependent on the trailing edge shapes and wettability, while the quantitative measurement under well-controlled flow conditions is limited, and hence, the theoretical prediction model including the effect of controlled trailing edge has yet established. We have developed an experimental setup, in which the shearing gas is fully developed and the liquid sheet on a wall is in uniform thickness along the transversal direction, allowing for the quantitative and general discussion from the wavy liquid sheets to the fragmenting droplet in a consistent manner. Here, we perform the visualization experiment on an acrylic pipe flow at the trailing edge thickness of 0.5 mm, 1.0 mm, and 2.0 mm, and the tapered angle of 10 deg and 30 deg, and at the wettability of hydrophobic condition. We successfully quantify the extending ligament from the trailing edge and the following droplet statistics using a high-speed imaging technique. By implementing the hydrophobic tapered wall, we clearly find that the circumferential wavelength between two ligaments on the trailing edge becomes short by minimizing the accumulated liquid on the edge stimulated by the Rayleigh-Taylor instability, leading to the formation of small droplets and suppression of coarse droplets. We finally formulate a theoretical model to predict the mean/maximum droplet sizes and distributions with considering the shape of trailing edge and wettability, which is validated by the present experimental results.