Mechanisms of operational parameters influencing aeration performance in inverted umbrella aerators: an experimental study supported by CFD analysis

ABSTRACT The graphical abstract presents a comprehensive overview of the inverted umbrella aerator study. The top left illustrates a schematic of the aerator in a 5m multiplication sign 4m multiplication sign 1.2m tank, with its variable immersion depth spanning -60 to +80mm. Adjacent to this are three color-coded turbulent kinetic energy (TKE) contours corresponding to immersion depths of +40, -20, and -60mm, visually emphasizing that both TKE intensity and fully developed vortex structures reach their maximum at the -20mm condition. Below these, two line graphs summarize the key performance trends: standard aeration efficiency (SAE) and oxygen mass transfer coefficient (KLa20) both peak within the -40 to -20mm range, whereas the power number (Np) increases monotonically with deeper immersion. An accompanying caption synthesizes the main finding: optimal aeration performance is achieved between -40 and -20mm, and immersion depth should be prioritized over power adjustment for energy-efficient operation. A 'depth-first, power-secondary' stepwise control strategy is recommended. To improve aeration energy efficiency and guide its operational optimization, this study examines the effects of motor power load and immersion depth on aeration performance. A combined experimental and numerical approach was employed to analyze a 4:1 scale model of an inverted umbrella aerator. Key indicators (Re, KLa20, SAE, Np) were systematically evaluated; the main findings are: (1) Re is primarily governed by immersion depth, with motor power playing a secondary role, particularly at low loads. (2) Immersion depth determines the optimal performance range, while power mainly modulates the magnitude within that range. The optimal performance occurs at immersion depths of h/D ≈ –0.05 to –0.025. Importantly, Np should not be used alone to evaluate aeration performance, as its minimum does not correspond to conditions of maximum KLa20 or SAE. (3) CFD simulation results indicate that the optimal immersion depth maximizes turbulent kinetic energy (TKE) and generates a concentrated dissipation zone at the free surface, promoting bubble breakup and interfacial renewal. In contrast, both positive and excessive negative immersion depths degrade performance, due to inadequate TKE and reduced effective interaction area, respectively. A ‘depth-first, power-secondary’ stepwise control strategy is recommended, involving first adjusting the immersion depth, then modifying motor power.