The function of stirring is to create uniform, homogeneous glass (homogenization in glass industry). Stirring equipment operates at high temperatures and under high mechanical stresses, so stirring devices have to be robust and often involve large amounts of platinum or platinum alloys. Although stirrers, stirrer bars, blenders, homogenizers, screw plungers and plunging stirrers currently used are generally effective in operation, reliable and with predictable lifetime, lower cost and improved operational efficiency are main targets for the future.
Figure 1: Path lines
NOGRID's stirrer application in the meshless CFD simulation software NOGRID points analyzes continuously, batches operating stirrer systems and finds the optimal stirrer design to reach the best product quality. We found out that the best measure of quality for a certain stirrer device is the deformation of a body, which flows through the stirrer system. We integrated the local deformation of a body over the complete path through the mixing cell. The amount of the deformation indicates how this body was stretched. A high deformation means that striae, which pass the stirrer, are stretched to a very long body and as a result disappear and don't influence the product quality anymore.
Figure 2: Path lines at the end of the analysis
In the glass industry homogenization systems are used to improve the product quality. In practice it can be seen that, depending on the forming process, a stirrer system can both generate and remove glass defects. Looking at physical modelling of stirrer systems it can easily be shown, how particle tracers follow the flow and what happens, if the tracers run through the vicinity of the stirrer blades. The starting position of the tracers may vary, but iso-kinetic conditions must be applied. Supposing that density and viscosity of the tracers correspond to the fluid properties, physical modelling of stirrer systems shows a lot of effects, which help to understand the flow in stirrer systems.
To obtain detailed information about the mixing quality of a certain stirrer system we use the mathematical simulation. Here we analyze particle tracers during their way through the mixing cell. Important results are the length of the path, the dwell time and deformation of a mathematical volume. Especially the information about the stretching of a certain volume along the particle path indicates the quality of the mixing process.
However, the material properties of real striae could differ from that of the glass melt. To model this effect we developed a tool, which allows the computation of a volume with its own density and viscosity. This volume doesn't follow the streamlines of the fluid any longer but finds its own way through the stirrer system, depending on the mechanical forces and on the wall distance.