Measuring air-water interface area in supercritical open channel flow

In storm waterways and at dam outlets, high-velocity supercritical flows are characterized by substantial air bubble 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). The paper describes new experimental data obtained in a 25 m long channel with a 4° slope. The analysis of the data provides new information on the air-water flow properties and on the distributions air-water interface area. Although the amount of entrained air is 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 results are compared with an earlier prediction (Chanson, 1994) and confirm the significant contribution of air entrainment to air-water gas transfer in supercritical chute flows.