(355f) Investigation of the Influence of Different Mesh Types on Accuracy and Calculation Time in Stirred Slim Multi-Stage Impeller Vessels

Slim reactors are widely used in the chemical process industry, e.g. for polymerization reactions. The worldwide growing demand of polymers leads to higher production rates which could be achieved by increasing the reactor size. Due to space limitations and equipment transport issues the vessel diameter is limited but the reactor height can be increased leading to slim reactors up to a height-to-tankdiameter ratio of H/T = 5.

For the design, scale-up and optimization of such reactors, Computational Fluid Dynamics (CFD) becomes more and more important because it is an inherent safe and inexpensive method which gives a lot of detailed information about otherwise invisible phenomenon. It has  already successfully used for this purpose (e.g. S. Maaß et al., 2011; A. Newale et al., 2014).

Nevertheless there are still challenges to master. In the work by Nunhez et al. which was presented at the AIChE annual meeting 2012 several aspect were discussed, amongst others the right choice of turbulence model and the influence of meshes made of hexahedrons or tetrahedrons for a single-stage impeller reactor with H/T=1.

In this contribution we investigate aerated slim multi-stage impeller reactors with H/T>1 and compare the numerical with experimental results in terms of power input, mixing time, velocity distribution and gas holdup. The gas and the water phase are modeled with the Eulerian Multi-Phase model (EMP) while for the stirrer rotation the Multiple Reference Frame (MRF) as well as the Rigid Body Motion (RBM, sometimes also referenced as sliding mesh) approach is used. All calculations are done with STAR-CCM+ by CD-adapco. 

A comparison of accuracy and overall calculation time is done for meshes predominantly made of tetrahedrons, hexahedrons and a third cell type, polyhedrons, which are known to combine the advantage of an automated mesh generation with an reduced overall calculation time. While the accuracy in most cases is quite acceptable the calculation time differs significantly between the different mesh types.                 

_{Maaß, S., Eppinger, T., Altwasser, S., Rehm, T. and Kraume, M. (2011), Flow Field Analysis of Stirred Liquid-Liquid Systems in Slim Reactors. Chem. Eng. Technol., 34: 1215–1227.}

_{Newale, A., Dewan, Y., Aglave, R. and Eppinger, T. (2014), Simulation Based Optimization of Stirred Reactors, AIChE Spring Meeting 2014, New Orleans}

_{Nunhez, J.R., Kresta, S., Spogis, N., Machado, M.B. and Gomez, J.L. (2012), Investigating On the Influence of Mesh Density, Turbulence Models and Numerical Aspects of CFD Modeling in PBT Impellers Using Non-Structured Meshes, AIChE Annual Meeting 2012, Pittsburgh}