The most effective and economical way to dispose brine from large and medium size desalination plants is via submerged outfalls employing single port or multiport diffusers. The brine is denser than the sea water, so it tends to sink downwards driven by the negative buoyancy: consequently, after the initial turbulent dilution the brine reaches the sea bottom, where the possibility for additional mixing is low. This implies a local increase in salinity and potential environmental hazards to sensitive species. As a consequence, the outfall position and the diffuser parameters should be properly designed according to the largest dilution achievable before the brine sinks to the sea floor, combined with the lowest economical costs. As dilution is proportional to the distance covered before sinking to the floor, the diffusers are typically inclined upwards, with an angle to the horizontal, to increase this path. Under these conditions, these kinds of release are neither axisymmetric nor Gaussian and, consequently, the classical one-dimensional integral equations for simple jets and plumes cannot lead to precise results. Here we present the results from series of laboratory experiments of the release of brine from submerged outfalls, under the whole range of practical conditions (angle to the horizontal of the diffuser, density of the brine, etc.). The experiments were performed by means of LIF (Laser Induced Fluorescence) and RIV (Robust Image Velocimetry), two non-intrusive techniques that allow the measurement of concentrations and velocity fields. These results have been compared with previous available results and summarized in equations linking the diffuser parameters to the dimensions of the region interested by the release and to the dilution, useful to optimize these parameters under given environmental and economical constrains. The dilution shows a direct proportionality to the densimetric Froude number and a parabolic dependence on the angle of release to the horizontal.
On the optimization of the diffuser parameters for the release of brine from submerged outfalls
FERRARI, SIMONE;BADAS, MARIA GRAZIA;QUERZOLI, GIORGIO
2012-01-01
Abstract
The most effective and economical way to dispose brine from large and medium size desalination plants is via submerged outfalls employing single port or multiport diffusers. The brine is denser than the sea water, so it tends to sink downwards driven by the negative buoyancy: consequently, after the initial turbulent dilution the brine reaches the sea bottom, where the possibility for additional mixing is low. This implies a local increase in salinity and potential environmental hazards to sensitive species. As a consequence, the outfall position and the diffuser parameters should be properly designed according to the largest dilution achievable before the brine sinks to the sea floor, combined with the lowest economical costs. As dilution is proportional to the distance covered before sinking to the floor, the diffusers are typically inclined upwards, with an angle to the horizontal, to increase this path. Under these conditions, these kinds of release are neither axisymmetric nor Gaussian and, consequently, the classical one-dimensional integral equations for simple jets and plumes cannot lead to precise results. Here we present the results from series of laboratory experiments of the release of brine from submerged outfalls, under the whole range of practical conditions (angle to the horizontal of the diffuser, density of the brine, etc.). The experiments were performed by means of LIF (Laser Induced Fluorescence) and RIV (Robust Image Velocimetry), two non-intrusive techniques that allow the measurement of concentrations and velocity fields. These results have been compared with previous available results and summarized in equations linking the diffuser parameters to the dimensions of the region interested by the release and to the dilution, useful to optimize these parameters under given environmental and economical constrains. The dilution shows a direct proportionality to the densimetric Froude number and a parabolic dependence on the angle of release to the horizontal.
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