Numerical modeling of plunging jets of brine: Mass transport and implications for desalination plant outfalls

Abstract

Plunging jets are used in many environmental and industrial applications to enhance mixing and mass transfer rates. One of the current challenges in applying plunging jet reactors for brine dispersal from desalination plants is that the density of brine causes the jet to drop straight to the seafloor. This hypoxic fluid disperses slowly and elicits a toxic effect on the local marine ecosystem. To provide new insights and improvements, we have developed a numerical model that considers the co-transport of brine in a two-phase air–fluid system. In our model, Navier–Stokes describes the transport of fluid, and Nernst–Planck describes the transport of dissolved brine. One of the key observations we made is that brine convection is characterized by competition between the positive buoyancy of air–fluid mixtures and the negative buoyancy of brine–fluid mixtures. Depending on the jet flow rate, the brine would either (1) ascend radially from the plume or (2) drop straight downwards. In our experiments, we demonstrate the same behaviors. Ultimately, it may be possible to reduce the destructive effects of high-density brine impinging on the seafloor by optimizing the jet to promote air entrainment, thus maintaining the system in its radial mixing regime.