Double Diffusive Effect on Desalination Discharges

In this paper, we report the behavior of a double-diffusive jet with a stability ratio $R=1.0\text{,}$ i.e., the jet is neutrally buoyant. Two cases are examined through laboratory experiments: (1) a rectangular surface jet and (2) a submerged round jet. The surface jet experiments had a discharge Reynolds number of approximately 3,500 and a range of temperature rise of 3–14°C with the corresponding salinity for density compensation. The results show that salt fingering occurred shortly after the channel exit and led to the development of positive (upward) buoyancy near the surface but negative buoyancy in the jet core. Eventually, the jet halted its surface propagation and plunged downward towards the bottom at a distance of approximately $x/D_0=6\text{.}$ The plunge was more pronounced with higher $\Delta T\text{.}$ At the plunge point, the surface density deficiency due to the excess temperature was as high as 0.15%. For the submerged round jet experiments, three Reynolds numbers were chosen at approximately 400, 2,000, and 8,000. At high discharge velocity, the jet behaved close to a normal round jet near the exit, but started to bend downward near the end of the monitoring range around 100 diam. With lower initial velocities, however, the effect of salt-fingering was obvious, and the jet curved downward significantly with distance. Analysis is performed to show that the resulting trajectory is consistent with the buildup of negative buoyancy flux with distance due to salt-fingering.