An experimental study on the process of mixing and dilution for the discharge of dense effluent

Introduction: In recent years, scarcity of freshwater resources and the increasing water demands due to population growth and industrialization, have turned the issue of supplying drinking water into a global subject. Therefore, exploiting unconventional water resources, such as saline and brackish water using emerging technologies for desalination, has emerged as a promising solution in coastal areas. Desalination through Reverse Osmosis (RO) technology besides freshwater produces brine effluent as a byproduct, which has more salinity and density than the feeding water. Improper disposal of this effluent into coastal bodies will have serious environmental impacts on the receiving environment and can severely affect the aquatic ecosystem. To prevent the negative impacts, the effluent is discharged through submerged nozzles diagonally, with a high initial velocity and momentum, at a distance far enough from the shore. Using this method, the outflow is mixed with the seawater due to disturbances and the concentration is reduced down to the tolerance of the marine environment. In this study, the results of an experimental study were reported in the stationary environment to investigated the time evaluation of the discharge, and the process of mixing and dispersion for 60o inclined dense discharge.

Methodology: The planar laser-induced fluorescence technique (PLIF) has been used to capture the flow central plane in this study. The system consisted of two swift scanning mirrors to provide a flat laser sheet across the centerline of the flow. The laser sheet was formed by the oscillation of a 100 milliwatts green Diode-pump solid-state laser (DPSSL) beam with 0.5mm width. With an infinitesimal quantity of a fluorescent dye (Rhodamine 6G), the discharged effluent would be fluoresced under the laser. The reflected light is captured by a CCD camera (Mars 640-300G 1/4"@4.8um) in the grayscale form at the rate of 100 frames per second. The procedures were controlled by a computer server equipped with an I/O board and controlling software and the images were continuously downloaded to the hard disk of the server for later processes. The captured images were then modified and calibrated for laser attenuation and sensor response for each pixel using clear and dyed water of known concentration.
Using this technique, the system can illuminate the instantaneous behavior of the flow and the production, development, and dissipation of turbulent eddies along the flow. By capturing the flow instance behavior, the formation of turbulent eddies and their impacts on turbulent diffusion and flow mixing and entrainment have been investigated. Also, concentration fluctuations at the centerline, the effects of Kelvin-Helmholtz instability, and shear entrainment on the flow mixing process were discussed.

Results and Discussion: By illuminating the flow behavior, the development of flow regime in jet and plume-like region and the formation of instabilities, and the dissipation of eddies were studied‎. For this purpose, the instantaneous images of the flow evolution for different times were extracted and depicted using a non-dimensional parameter developed specifically for this purpose. The different processes of flow mixing and dilution along the jet and plume ‎regions were analyzed by describing the physics of eddies formation and dissipation long the inertial subrange. The formation of flow packets out of the main path is affected by the intensity of ‎velocity fluctuations. The vortices take their energy from the averaged velocity and transfer it from the biggest formed scale i.e. integral scale to the smallest vortices i.e. Kolmogorov's viscous subrange in a cascade of energy. The Energy Cascade is basically an energy spectrum that characterizes the turbulent kinetic energy distribution as a function of length scale. The scales of turbulent structures are directly a function of velocity fluctuation in each region and direct the process of the entrainment that led to the increases of dilution from the nozzle tip to the seafloor. These days, numerical simulations are becoming a common way for modeling the brine discharge in the marine environment. It is time-consuming and needs high expertise. The models are usually unsteady and after full development, the flow time-averaged image of the last 45 to 60 seconds is used for identifying the flow geometrical and mixing behavior. Using these experiments, it observed that the non-dimensional time required to flow fully developed and reach the impact point is about T*=5.7. Knowing that will help engineers find the optimal duration of time needed for the simulation and modeling.

Conclusion: It is known through the basics of physics that the maximum range for projectile motion happens when it is launched at an angle of 45 degrees. However, the previous experiments exhibited that the maximum dilution at the impact point occurs at the nozzle with 60o inclinations where the flow path is the longest. It was carefully examined by explaining the complex action of turbulence on flow mixing and dilution. The formation of eddies initially begins due to Kelvin-Helmholtz instability and it leads to velocity fluctuations with different time and length scales in the body of flow. The result is concentration dispersion with different magnitude along the flow path in inclined dense discharges.