If solids are not sufficiently suspended in biogas fermenters, sedimentation layers can form which can highly impair the effectiveness of these fermenters. Mechanical agitation is usually used to avoid this problem; however, non-Newtonian flow properties of the substrates complicate both the use and design of these agitation systems, so that mixing parameters are often inefficient and inadequate.
The subproject "Suspension modeling" within the FNR project Sens-O-Mix therefore investigates the interplay between non-Newtonian rheology, stirring technology and suspension. The aim is to derive optimized stirring parameters as well as the development of CFD models, which are suitable for a sufficiently accurate representation of the suspension processes in a stirred tank with non-Newtonian fluids.
The model development is based on an experimental investigation of suspensions in stirred tanks using laser-optical methods. Transparent solids as well as non-Newtonian liquids, whose refractive index (RI) is adapted to that of the solids, are used for this purpose (see Figure 1).
With these systems, information about local solid contents and fluid dynamics in the entire stirred tank can be determined. For this purpose, a laser sheet is created which optically cuts a stirred tank (s. Figure 1, left). By adding very small, density-matched tracer particles, which follow the flow of the liquid, fluid velocity can be determined (see Figure 1, center). Simultaneous measurement of the solid phase is enabled by the addition of a fluorescent dye dissolved in the liquid. This colorant is excited by the laser light, which enables a selective recording of the resulting fluorescent light by the use of a long-pass filter. Since the solids do not fluoresce, shadows are created wherever solid particles are present (see Figure 2, right).
With these images, velocities of the liquid phase can be determined by particle image velocimetry (PIV). Using particle tracking velocimetry (PTV), this can also be done simultaneously for the solid phase. This allows, for example, the investigation of flow and suspension conditions in a stirred tank in dependence of the rotational frequency of a pitched-blade turbine (see Figure 3).
Based on these experimental investigations, multiphase simulations are conducted in OpenFOAM, which in particular consider non-Newtonian rheology. Here, different multiphase methods (Euler-Euler, Euler-Lagrange, DEM) including non-Newtonian rheology models are tested for their validity. Finally, using validated models, stirring parameters are varied to derive rules for an efficient stirring operation in biogas fermenters.