POD modal analysis based on LES of a baffled stirred tank

Stirred tank reactors are widely used for mixing operations in chemical, biochemical and process engineering. The tank configuration, working fluid (more often than not non-Newtonian) and flow regime are frequently conditioned to the desired products. A typical configuration consists of a fixed cylinder with a lid coupled with four-equally-spaced baffles and a Rushton-type impeller. In turbulent regime, the flow field can be decomposed into mean flow, organized motion (induced by the impeller blades) and turbulence. Reduced order modelling approaches such as proper orthogonal decomposition (POD) are practical tools for separating the different type of motions and to extract information from the velocity fields by ranking modes according to their contribution to the total kinetic energy. Only until recently, different research groups have started to apply POD to three-dimensional (3D) flow fields in stirred tanks. Data of large eddy simulations (LES) corresponding to a laboratory-scale, baffled, stirred tank with a Rushton-type impeller is considered to perform a POD modal analysis. In the simulations, the tank operates either at 600 or 800 rpm and with water and 0.2 wt% carboxymethyl cellulose (CMC) solution as working fluid. The study focuses on the most energetic modes and their corresponding 3D reconstructed velocity fields. To the authors' knowledge, the present investigation is the first attempt to apply POD to LES of a baffled stirred tank operating in turbulent flow conditions (at least in the region near the impeller) and not only for a working fluid having Newtonian properties but also for a fluid exhibiting shear-thinning behaviour (CMC).