A CFD study on the change of scale of non-Newtonian stirred digesters at low Reynolds numbers

Biogas from anaerobic digestion of agricultural waste is proving to be a convincing way to reduce greenhouse gas emissions. To optimize the process energy efficiency, the CFD simulation of the laminar non-Newtonian fluid mixing in the digester would be an effective method, but the adoption of appropriate spatial discretization at the production scale is currently impossible. For this reason, the identification of change of scale rules for an effective design and for preliminary laboratory scale experimental investigations is still of paramount importance. This work is aimed at the identification of a methodology for the scale down of an industrial stirred anaerobic digester with a volume of 1500 m3, for which CFD simulations have an unacceptable computational cost. The investigation is based on the simulation of three different scale down geometries. The different blade rotational speeds were determined from four different change of scale approaches, which enforced constant blade tip speed, constant shear rate close to the blades, constant Reynolds number and constant power per unit volume, across the different digester sizes. The volume distributions of velocity magnitude, shear rate and shear stress can be exploited to assess the presence of dead zones or localized region where biogas production may be inhibited. The effect of the different change of scale rules on the local instantaneous fluid dynamics were quantified and discussed, finding that both the non-dimensional velocity and non-dimensional shear rate fields are constant across the different scales, when the Reynolds number, based on the Metzner and Otto concept, is constant.