Eliminating air entrainment in complex siphon drainage systems: a physical modelling approach

The generation of persistent foam in the cooling water discharge of coastal power plants poses significant environmental and operational challenges. This phenomenon is fundamentally driven by a "hydro-chemical coupling" mechanism, where hydraulic air entrainment generates bubbles that are subsequently stabilized by surfactants in the seawater. While chemical defoaming is costly and potentially polluting, existing physical solutions often fail to address the complex, transient flow regimes found in tidal discharge systems. To address this, this study proposes an integrated "source control" strategy to eliminate hydraulic air entrainment. Using a comprehensive 1:20 scale physical model, we analyzed the air entrainment mechanisms in a complex discharge system comprising siphon wells, a long culvert, and a steep terminal drop. We designed and validated a combined abatement solution featuring a flow-limiting orifice plate in the siphon well and a novel three-stage de-aeration well at the outlet. Experimental results demonstrate that this configuration effectively eliminates air entrainment for over 95% of the operational period. Furthermore, the hydraulic impact is minimal, with the upstream backwater rise remaining below 0.5m even under extreme high-tide conditions. This study establishes a robust, environmentally friendly engineering methodology for managing foam in complex hydraulic systems dominated by large tidal variations.