Computational Fluid Dynamics

Flocculation Tank Simulation

Overview

Goals

The goals of the CFD work are fairly straightforward in scope. To properly form flocs, there needs to be an appropriate level of turbulent energy dissipation to enhance turbulent mixing. The levels throughout the flow need to be at a certain value to ensure that flocs are large enough that they will settle out in the sedimentation tank, while preventing any pre-sedimentation settling at the end of the flocculator or in the channels between the two sections. The rates also need to be such that turbulent eddies are strong enough to pull settled flocs off the bottom of the flocculator if they do end up settling. In order to ensure all these requirements, a full description of the flow through the length of the flocculator would be ideal, but the simulation time and processing power puts a limit on this ability. Thus, we look to model a certain number of baffles that will provide a reasonable description of the flow through the entire system. Work in previous years has been limited to simulations of five or less baffle turns meaning that analysis of the flow after significant development has not been examined. This was a primary goal for the fall of 2009. It was originally thought that a high level of turbulent energy dissipation at the beginning of the flocculator would lead to initial floc growth near the entrance with a tapering of levels to a much lower value near the end to prevent floc breakup. However, it was speculated that this particular distribution was leading to early settling of flocs before they ever entered the sedimentation tank. A more even dissipation of energy would ideally help break up flocs that become too large, but not be so high as to break up all reasonably sized particles. This scenario motivates the goal of modeling more of the flocculator and leads to two possible directions. One method involved increasing the number of baffles to greater than five and observe the outcome to see if after a certain number, the flow variables become steady between each turn. The other involved using a periodic condition where the fully developed case far from the entrance would be modeled using only one turn. The advantage of this method is less time to obtain a solution because the mesh has far fewer cells than a substantial number of baffles.

FLUENT Updates

Recently ANSYS acquired FLUENT and integrated it with the more user-friendly Workbench software. While this makes new projects much easier to set up when compared to the previous GAMBIT interface, it required a good deal of work to adapt previous work to the new system. Workbench is not a new tool itself, and ANSYS has used it for a number of years for its structural finite element solver. It allows the user to step through the process of geometry creation and meshing, setting up the physics, solving the problem, and postprocessing the results. Work began immediately on reproducing results from past years with the goal of ensuring the new system did not fundamentally change any of the results that had previously been obtained. It was clear early on that, while better than building geometry and meshes in GAMBIT, the new system left something to be desired. Certain common sense tasks that someone would likely want to perform in any CAD style program required nonintuitive methods. For example, when creating a baffle turn in the flocculator, our 2D model was best served by having an infinitely thin wall where the fluid would flow in opposite directions on different sides. The software help documentation was useless for providing an answer, and only after consulting technical support did we find that lines had to be drawn, created as separate bodies, and then projected onto the surface through which fluid would flow. Another such issue occurred while creating the near-wall mesh. In order to properly resolve the viscous sublayer (i.e. y+ values less than 5), cell size growth had to occur from an initial cell size (0.0003m) to the size used for meshing the flow's core that is "far" from the wall. This is implemented using a bias factor that governs the change in size of each cell when moving away from the wall. Unfortunately, there is no simple user input where first cell size, number of divisions, and total length are specified and the bias factor is simply computed from this information (GAMBIT actually had this ability which indicates ANSYS took this a step backwards in my opinion). Also the documentation doesn't provide this algorithm leaving it to the user to either figure it out independently or simply guess until the correct size is obtained.