Oxygen Transfer in Bubbly Turbulent Shear Flow

The equation for oxygen transfer in a bubbly air‐water turbulent shear flow in a vertical pipe is derived. Dimensionless variables that characterize the transfer coefficient are identified. Laboratory experiments are conducted for a range of water‐flow velocities and air‐flow rates, with and without tangential swirl. The overall transfer coefficient $\left(K_{L}a\right)$ is estimated from the measured DO (dissolved oxygen) response curves. The transfer coefficient $\left(k_L\right)$ is computed using measured bubble‐size distributions. For a given air‐water discharge ratio $\left(\lambda \right)$, a higher flow velocity yields a larger $k_L$ due to higher turbulence and bubble‐surface renewal. For a given flow velocity, as $\lambda$ increases so does $K_{L}a$, primarily because the total air/water interfacial area increases. The rate of increase of $K_{L}a$, however, diminishes for larger $\lambda$, because $k_L$ decreases with $\lambda$. In this case the decrease in $k_L$ results from a reduction in the turbulence intensity per unit volume of air. The transfer coefficient normalized by the mean flow velocity varies inversely with $\lambda$ and directly with the Reynolds number. The effect of the swirl on oxygen transfer is found to be insignificant.