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Numerical and Experimental Modeling of the Thermal-Saline Jet Discharge from : Thermal-Saline Jet Water Desalination Plants and Providing a Practical Solution to Reduce its Environmental Impacts

Azadi, Amin | 2023

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 56387 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Firoozabadi, Bahar
  7. Abstract:
  8. More than 80% of desalination units in the Persian Gulf use multi-stage flash (MSF) technology. The discharge of effluent from these units sometimes causes severe impacts to the marine environment due to the increase in temperature and salinity around the discharge point. In the current thesis, the geometrical, mixing and turbulence characteristics of the discharge of thermal-saline inclined jets under the water surface, similar to the effluent of MSF units, are numerically investigated by developing the buoyantBoussinesqPimpleFoam solver in the OpenFoam open-source software, using the LES turbulence model and the dynamic Smagorinsky subgrid scale and the UNESCO equation of state. The equations are solved implicitly and second-order backward, least squares, Laplacian and limited linear methods are used to discretize the time, gradient, Laplacian and convection terms, respectively. For this purpose, two dimensionless effective parameters of density ratio, which represents the ratio of buoyancy flux caused by heat to the buoyancy flux caused by salinity, and salinity Froude number are considered. The results show that the jet flow pattern (ascending, descending and neutral) is only dependent on the density ratio parameter, and jets with a density ratio greater than 1.3 are similar to jets with positive buoyancy and jets with a density ratio smaller than 1.2 are similar to jets with negative buoyancy. In addition, the density ratio between 1.2 and 1.3 separates ascending and descending jets, and in this range, jets often behave similar to neutral jets. In addition, it is shown that the vorticity criteria can well detect the change of jet behavior in the regions with momentum dominance and buoyancy dominance. The results show that in the momentum dominant region, the vorticity field has a clear boundary between the vorticity with opposite directions in the upper and lower halves as well as the two transverse halves of the jet, but with the weakening of the initial momentum flux and initiating the buoyancy dominant region, the vorticity field collapses and takes a completely random shape. In addition, in the initial areas of the discharge jet, the initial momentum flux dominates the jet behavior and jet moves in the direction of the nozzle. After that, with the weakening of the local momentum flux, the balance of the buoyancy fluxes determines the behavior of the jet. In these regions, the ratio of the buoyancy flux caused by heat to the buoyancy flux caused by salinity is greater than one for ascending jets, smaller than one for descending jets, and close to one for quasi-neutral jets. In the present thesis, the impact of the jet on the free surface is also modeled. For this purpose, the VOF free surface model coupled with SAS turbulence model is used. The simulation results show that for inclined dense jets impacting the free surface, investigating the dilution only at the return point of the jet to the bed is not enough and due to the formation of transverse plumes on the water surface and the drop of the dense flow from the surface toward the bed before return of the jet to the bed, the amount of dilution on the return plane should be evaluated from an environmental point of view. Moreover, in the present thesis, an inclined dense jet in the presence of obstacle bed is investigated experimentally and numerically. The results show that the concentration of the jet decreases after discharge due to the entrainment of the ambient fluid until shortly before impacting the bed. But close to the bed, due to the accumulation of saline fluid, the concentration starts to increase. The presence of an obstacle in the jet impact point to the bed causes the maximum dilution of the jet to decrease linearly with respect to the obstacle height. Numerical results similar to the experimental results show that the geometrical and mixing characteristics of the inclined dense jet in the upstream regions of the jet maximum height are independent of the shape of the bed and its topology
  9. Keywords:
  10. Flux Balance Analysis Method ; Environmental Impact ; Free Surface ; Inclined Negatively Buoyant Jet (INBJ) ; Large Eddy Simulation (LES) ; Thermal-Saline Jet ; Water Desalination

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