Air-Water Interface Area in Supercritical Flows Down Small-Slope Chutes

In storm waterways, at dam outlets and in water treatment plants, high-velocity supercritical flows are characterised by some air entrainment. The entrainment of fine air bubbles and the strong turbulent mixing contribute both to the air-water transfer of volatile gases (e.g. oxygen, nitrogen, VOC). As a result the downstream water quality is substantially affected. The report presents new experimental data obtained in a 25-m long channel with a 4-degree slope. The same channel was previously used by CHANSON (1995b). The present investigation is focused on the air-water flow properties. The air-water flow comprises a region of low-air-content (i.e. C < 0.3 to 0.4) with a bubbly mixture structure, and a high-air-content region characterised by air-water projections and foam structures. The analysis of the new data provides information on the air concentration distributions, the distributions of mean air-water velocities, the bubble chord length distributions and the air-water interface area. The results indicate a broad spectrum of bubble sizes extending over several orders of magnitude. The cumulative distributions of chord lengths follow approximately a Log-Normal distribution, Although the amount of entrained air is relatively small (i.e. typically Cmean < 0.12), the specific air-water interface area can reach over 100 m2 per unit volume of air and water. The study confirms the significant contribution of air entrainment to air-water gas transfer in supercritical chute flows