USE OF MIXING TUBES ON MARINE OUTFALLS

Abstract

To optimise dispersion from marine outfalls multiport diffusers have been developed. The addition of mixing tubes to such outlets can create an ejector action and so cause pre-mixing before discharge to the receiving mass of water. The characteristics of such water jet-pumps in this submerged condition have been derived elsewhere, but are applied herein to the dilution of effluents. For a range of jet to mixing tube area ratios optimisation has been carried out on a computer, thus indicating the densimetric Froude numbers and depth ratios at which dilution exceeds that for the plain jet. Even for a stagnant ambient medium dilution in experiments exceeds that predicted, possibly because of macro-turbulence not accounted for in the theory available. Turbulons developed in a mixing tube are larger than any emerging from a plain smaller jet and may thus promote better mixing from its exit to the sea surface. Mixing tubes have obvious applications in shallow water and where an effluent is particularly obnoxious.