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Numerical Investigation of the Seabed Inclination Effects on Mixing Characteristics of the Brine Discharged Jet from Desalination Plants

Habibi, Shayan | 2021

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 54749 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Firoozabadi, Bahar
  7. Abstract:
  8. A large number of desalination plants have been established to overcome the drinkable water scarcity in the world. These plants produce saline water as effluent, which is often discharged into local sea water. One of the applicable methods to dispose these kinds of effluents is to install an inclined nozzle below the water’s surface. Hence, an inclined negatively buoyant jet (INBJ) forms which is often fully turbulent due to the high discharge velocity. Local environmental characteristics affect the dynamics and mixing behavior of INBJs. One of these characteristics is the sea bed slope, on which the present study has been focused. Large eddy simulations (LES) are performed in the present study to investigate the behavior of 30°, 45°, and 60° INBJs with the bed slopes of 0°, 5°, 10°, 15°, and 20°. The present results show that increasing the sea bed inclination is considerably favorable in terms of mixing. 20° increase in the bed slope results in 167.75%, 81.13%, and 107.43% increase in the maximum centerline dilution for 30°, 45°, and 60° INBJs, respectively. In addition, increase in the bed slope reduces geometrical characteristics of INBJs, but the trend is severely related to the nozzle inclination. It is observed that variations in the bed slope do not alter the geometrical characteristics of a 45° INBJ before the impact point. On the other hand, non-dimensioned length of the return point for 30° and 60° INBJs is reduced by 18% and 25%, respectively by 20° increase in the bed slope. Investigating the normalized dilution at the return point and the mean concentration distribution on the bed for all computational cases reveals that discharging dense effluents into the ambient water where the sea bed slope is higher is environmentally favorable. Also, similar to horizontal bed cases, 60° nozzle inclination is favored even for sloped bed cases.In the present study, for the first time, coherent structures of INBJs are classified and their evolution along the jet path is interpreted with the changes in the flow regime from momentum-driven to buoyancy-driven. The Q-criterion with the instantaneous and mean vorticity fields are used to identify four types of coherent structures, namely shear-driven vortices, counter-rotating vortex pair (CRVP), buoyancy-driven vortices, and density current vortices. Comparing the shear and buoyancy-driven vortices it is observed that buoyancy-driven vortices are bigger and weaker than shear-driven ones, due to smaller local Reynolds number. Increasing the bed slope increases the distance between vortex rings in the momentum-driven regime. Also, along the jet path, vorticity reduces more rapidly by increasing the bed slope and thus, the homogeneity and isotropy of the buoyancy-driven flow increases in downstream
  9. Keywords:
  10. Brine Discharge ; Large Eddy Simulation (LES) ; Turbulence ; Coherent Structures ; Sea Bed Slope ; Inclined Negatively Buoyant Jet (INBJ) ; OpenFOAM Software

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