Multiport Diffusers for Dense Discharges

Comprehensive laboratory experiments on multiport diffusers for dense effluents such as brine into stationary receiving waters are reported. Tracer concentration fields were mapped in three dimensions by laser-induced fluorescence (3DLIF). The effect of port spacing is described by $s/d\mathsf{F}$ where $s$ is the port spacing, $d$ the nozzle diameter, and $\mathsf{F}$ the jet densimetric Froude number. For $\mathrm{s}/d\mathsf{F}>\sim 2$, the jets did not merge, and the results followed expected asymptotic solutions for single jets. For $s/d\mathsf{F}<2$ the jets merged, but did not follow expected asymptotic line source solutions. As the port spacing was reduced, the rise height and other geometrical variables decreased. The dilutions also decreased, but much more rapidly than predicted. These were caused by Coanda effects and merging. The Coanda effect caused an under pressure on the interior jet surfaces which caused them to curve more sharply inwards. This shortened their trajectories, reducing the external surface area available for entrainment. Jet merging restricted entrainment of clear water to the inner surfaces and exacerbated the Coanda effect. The implications for numerical modeling are discussed. Integral entrainment models that assume unrestricted entrainment water, neglect reentrainment, and neglect dynamic interaction (Coanda effect) between jets may significantly overestimate dilution. Insight into port spacing effects can be obtained from studies on positively buoyant jets. To prevent reduction in dilution attributable to restricted entrainment, it is recommended to maintain adequate port spacing so that $\mathrm{s}/d\mathsf{F}>\sim 2$.