Abstract
Negatively buoyant jets develop when fluids are released upwards into a lighter fluid or, vice versa, downwards into a heavier fluid. There are many engineering applications, such as the discharge, via submerged outfalls, of brine from desalination plants into the sea. Some concerns are raised about the potential negative environmental impacts of this discharge. The increase in salinity is the major cause for environmental impact, as it is very harmful to many marine species. The diffusers for brine discharge are typically inclined upwards, to increase the path before the brine reaches the sea bottom, as it tends to fall downwards driven by negative buoyancy. The negatively buoyant jet that develops conserves axisymmetry only when released vertically, so that it is not possible to use the well-known equations for axisymmetric jets. The main target of this paper is to investigate on a laboratory model the effects of different stratifications on the features of negatively buoyant jets. This has been done via a LIF (Light Induced Fluorescence) technique, testing various release angles on the horizontal and densimetric Froude numbers. Except for the initial stage, a different widening rate for the upper boundary and the lower boundary has been highlighted.
Highlights
Buoyant jets (NBJs) develop when fluids are released upwards into a lighter fluid or, vice versa, downwards into a heavier fluid.Negatively buoyant jets (NBJs) are implied in many fields, from the passive containment cooling system of nuclear power plants (Wang et al, 2017 [1]) to the volcanic eruption columns (Kaminski et al 2005 [2]), see Ferrari and Querzoli, 2010 [3], for a list
The discharge of brine from desalination plants is one of the most investigated, because of the concerns linked to the potential negative environmental impacts of the brine discharge (Campbell et al, 2005 [10]): this is mainly due to the increase in salinity which brine discharges impose on the receiving body, harmful to a lot of marine species, in particular to the Posidonia Oceanica
The jet axis was assumed to be coincident with the locus of the concentration maxima on jet crosssections. Detailed information on these parameters, as well as on the experimental set-up and data elaboration, can be found on Ferrari and Querzoli, 2010 [3] and on Ferrari et al, 2016 [19]. This paragraph is organised as follows: first of all, a discussion about the different stratifications, stable on the upper region and unstable in the lower region of the NBJ, will be given; the concentration profiles, orthogonal to the NBJ axis, at various distances from the outlet will be shown, in order to discuss their shape, with particular regard to the Gaussian shape hypothesis often employed in the study of these phenomena; lastly, the focus will be on the widening features of the NBJ upper and lower region
Summary
Buoyant jets (NBJs) develop when fluids are released upwards into a lighter fluid or, vice versa, downwards into a heavier fluid.NBJs are implied in many fields, from the passive containment cooling system of nuclear power plants (Wang et al, 2017 [1]) to the volcanic eruption columns (Kaminski et al 2005 [2]), see Ferrari and Querzoli, 2010 [3], for a list. As a matter of fact, NBJs conserve axisymmetry only as long as they are vertically released, so it is not possible to use the well-known equations for axisymmetric jets. As this lack in axisymmetry is mainly due to the different conditions of stratifications in the upper and lower boundaries of NBJs, the main target of this paper is to investigate the different effects of these stratifications on the features of NBJs
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