Simulating Fluid-structure Iinteraction for Mixing Device Design

The changes in chemical production processes, in particular the shift from chemical production to biotechnology, puts more emphasis on shear rate control of mixing devices. At the same time, mixing devices should be highly optimized with respect to power consumption, both for economical reasons and due to the prevention of additional heat-up of the mixing process. In addition, most biotechnological mixing processes include a two-phase fluid consisting of a liquid phase and a gas phase. Computational fluid dynamics (CFD), has been established as a methodology that helps design mixing devices in the past years. Under the above conditions, however, the fluid flow simulation may no longer be independent of the associated structural deformation problem: the agglomeration of the gas phase in the wake of the impeller blades can significantly change the dynamic structural load of the impeller, even leading to premature fatigue failure of the impeller; for critical impeller blade designs, a moderate torsion of the blade may already significantly change the shear rate exposition near the leading edge of the impeller, thus leading to a decrease in the production rate of the microorganisms. This paper will therefore explain the procedure to perform a coupled Fluid-Structure Interaction (FSI) simulation, consisting of a part including the fluid flow simulation with the CFD code FLUENT, and a structural analysis part based on Finite Element Analysis. Particular focus will be given to numerical aspects influencing the quality of the results, such as meshing, coupling methodology, physical models used etc. In addition, the computational effort required for this kind of simulation will be reported and compared to the potential gain of the coupled simulation, i.e. increased product quality or output and enhanced device reliability.