Abstract
Three-dimensional aerators are often used in hydraulic structures to prevent cavitation damage via enhanced air entrainment. However, the mechanisms of aeration and bubble dispersion along the developing shear flow region on such aerators remain unclear. A double-tip conductivity probe is employed in present experimental study to investigate the air concentration, bubble count rate, and bubble size downstream of a three-dimensional aerator involving various approach-flow features and geometric parameters. The results show that the cross-sectional distribution of the air bubble frequency is in accordance with the Gaussian distribution, and the relationship between the air concentration and bubble frequency obeys a quasi-parabolic law. The air bubble frequency reaches an apex at an air concentration (C) of approximately 50% and decreases to zero as C = 0% and C = 100%. The relative location of the air-bubble frequency apex is 0.210, 0.326 and 0.283 times the thickness of the layers at the upper, lower and side nappes, respectively. The air bubble chord length decreases gradually from the air water interface to the core area. The air concentration increases exponentially with the bubble chord length. The air bubble frequency distributions can be fit well using a “modified” gamma distribution function.
Highlights
Cavitation erosion caused by high velocity flows is a common phenomenon in spillways or chutes with high-head dams
Full interfacial aeration is commonly observed at the free surface of the upper, lower and side nappe along the free jet (Figure 1)
The obtained air concentration distributions satisfy the analytical model of Chanson [12], Equation (4) for air bubble frequency distributions as observed in supercritical open channel flows and backward-facing step flows (Toombes and Chanson [19])
Summary
Cavitation erosion caused by high velocity flows is a common phenomenon in spillways or chutes with high-head dams. A commonly adopted counter-measure to reduce or prevent cavitation damage is aeration [1,2,3,4,5,6]. Three-dimensional aerators are used to enhance air entertainment into water and to prevent cavitation erosion on spillways. Full interfacial aeration is commonly observed at the free surface of the upper, lower and side nappe along the free jet (Figure 1). The effect of air entrainment is based on the jet disintegration process due to turbulence and secondary interactions with the surrounding atmosphere. Water 2018, 10, 1430; doi:10.3390/w10101430 www.mdpi.com/journal/water (a)
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