Air entrainment and oxygenation by overfalling nappes.

Abstract

This research analyzes air entrainment and flow conditions of overfalling nappes and the behavior of bubbles in the plunge pools leading to oxygenation of the water. A literature search indicated that these phenomena have never been analyzed as a whole. An experimental/analytical/numerical approach is used to estimate the rate of oxygenation by nappes over weirs. The experimental investigation was done at the Tennessee Valley Authority (TVA) Engineering Laboratory, Norris, Tennessee. An air-water concentration probe (developed by Lamb and Killen, 1950) was calibrated and used to determine contours of air concentration in the plunge pool. Representative bubble sizes were determined using high-speed photography. The air set in motion by the nappe was computed analytically by integrating the momentum and continuity equations over the air boundary layer. Comparison with the air in the plunge pool established the fraction of the boundary entering the plunge pool. A three-dimensional, fluid flow solver was used to compute a representative water flow field in the plunge pool. Using that water flow field, the trajectory and residence time of bubbles in the plunge pool were then computed by solving the two-dimensional Lagrangian equations of motion. Included in this calculation is the mass transfer of air from the bubble over its trajectory using an empirical mass transfer coefficient. From this calculation, the rate of oxygenation was determined for the overall plunge pool. The analytical results show good agreement with the experimental data collected by the author and with other findings documented in the literature.