Abstract
Air entrainment, or surface aeration, is common when the shear between a gas and liquid region is significant. Air entrainment is found in wind-driven flows at the gas-liquid interface of mixing systems and small and large-scale fermentation tanks. This gas-liquid mixing forms a multiphase flow, either promoting the separation of low-density components in a mixture or increasing oxidation, both of which could disrupt the yield of a fermentation process. This study utilizes a standard stirred tank reactor (STR) equipped with a Rushton-type disc turbine impeller and baffles to examine air entrainment at the gas-liquid interface. High-speed backlit imaging is employed to visualize and characterize the air entrainment. The impeller speed and distance between the impeller and the static liquid height significantly impact the amount of air drawn into the system. Substantial surface deformation is observed as a function of impeller speed and location, leading to different surface aeration mechanisms. An average imaging technique was used to characterize air entrained in the system. The results also show the fluid height and depth of entrained air penetration is affected by the impeller speed and location. Recommendations to quantify air entrainment magnitude due to surface aeration will be outlined and will be the focus of future work.
