Under construction: Projected finish date: 5/18/2008 Sunday

CFD Simulation Scientific Paper (By: Jorge Rodriguez, Yong Sheng Khoo)

Title: Better Understanding of Flocculation Process through CFD Simulation

Abstract

Flocculation is one of important process used by Aguaclara to treat water. The process involved particles collide in the solution and come together to form floc. This paper aims to present a methodology of using CFD to better understand the phenomena that take place in the flocculation tank. Better understanding of the flocculation process will enable the team to better design the water treatment system.

Introduction

Flocculation is direct process of particle collision. Past research has shown that fluid shear plays important role in causing the collision. Hence using CFD, the process in the flocculation tank is simulated. The ultimate result of this research is to find the strain rate in the flow that causes the shearing process which influencing the particle collision. The CFD tools that were used in the modeling process were Gambit and FLUENT. Gambit was used to create the geometry of the flocculator, set up boundary conditions and creating mesh while FLUENT was used to plug in the data and initial conditions to obtain solution.

Modeling Approach

The real life flocculation tank designed by Aguaclara involved a 180 deg turn over few dozens baffles. For validating result and modeling purpose, a simple 180 deg turn over two baffles was modeled. The first step was to set up the geometry of the of the simple flocculator. For future comparison with the experimental data, the design parameters for the pilot plant test flocculator was modified and used.

Figure 1: Geometry of Flocculator

The design parameters used are:

Height: 1m
Clearance: 0.15m
Baffle width: 0.1m
Velocity inlet: 0.1m/s

With the geometry, the mesh for the model can then be set up.

Figure 2: Meshing Parameters (Click on the figure to see the original size)

Figure 2 shows the meshing parameters that were used. The boundary layers was first establish at all the wall surfaces. The boundary layer was set up such that the solution obtain in gambit would provide result of y+ of less than 5. After that, the mesh edges were set up such that they will provide higher mesh resolution near the turn and lower resolution far from the turn. With the initial meshing conditions set up, all the faces were then meshed.

Figure 3: Mesh of the Model (Click on figure for original size)

Figure 3 shows the overall mesh of the flocculator model. As can be seen, the mesh is fine near the turn and at the wall. The final step at this point was to set up the boundary conditions of the system.

Figure 4: Boundary Conditions

Figure 4 shows the boundary condition that was used for modeling. For a flocculator, there is an in flow and out flow of the fluids. Since inlet velocity inlet was known from the experimental data, the inlet was set to the Velocity Inlet type boundary condition. The outlet was set to Pressure Outlet boundary condition type, the atmospheric pressure.

Figure 1 shows the mesh that was generated in Gambit. Right hand side of the figure shows the mesh near the wall. There are more refine meshes near the wall in order to be able to solve the viscous sublayer. Also note that in general, there is no discontinuity or sudden jump in mesh size throughout the model to prevent error in simulation. The mesh is also more refine at the lower part of the flocculator. This is where the turning occurs. Since this will be the region where most fluiditic activity take place, a finer mesh is employed in this region.