The Effect of Mean Air Concentration on the Aeration Efficiency in Skimming Flow

The flow over stepped chutes can be classified into skimming flow regime, transition flow regime, and nappe flow regime. The aeration efficiencies of the three flow regimes differ significantly due to the different air entrainment mechanisms in the nappe, transition, and skimming flow regimes. It is well known that aeration efficiency increases in the stepped chutes due to earlier self-aeration and slower flow velocities compared to the smooth chutes. This paper investigates the aeration efficiency of stepped chutes, particularly the effect of mean air concentration. The results showed that mean air concentration had a significant effect on aeration efficiency. They also showed that the aeration efficiency increased as the mean air concentration increased. A design equation was also developed to predict the aeration efficiency in the stepped chutes as a function of mean air concentration and roughness Froude number.