This study investigates the development of a scale-down approach for anaerobic digesters. Geometric similarity, flow regime, dimensionless cavern size and power consumption per unit volume (P/V) were ...identified as scaling criteria to design the pilot digester. At this pilot scale, a Computational Fluid Dynamics (CFD) model was validated using Particle Image Velocimetry (PIV) measurements and extended to the industrial anaerobic digester to describe its hydrodynamics and evaluate the robustness of the scaling criteria. The pilot scale CFD simulations were able to capture the main features of the flow, and model P/V and cavern diameter (Dc) with an average error of 4% and 3%, respectively. The comparison between the pilot and the industrial scale showed a deviation of 9% for P/V and about 5% for Dc and similar distributions of the dimensionless velocities inside the cavern, validating the scaling approach.
•A scaled down approach to viscoplastic flows in anaerobic digesters was developed.•The numerical model was validated at pilot scale by Particle Image Velocimetry data.•The model was successfully applied to industrial scale to describe its hydrodynamics.•Constant Re, P/V and dimensionless cavern diameter were found to be robust criteria.
This study investigates the rheological properties of rye and maize silage digestate from an industrial digester. In addition to the usual flow mode, oscillatory measurements were performed to ...highlight the visco-elastic characteristic of the digestate. Regardless of total solid (TS) content and solids size, results showed a shear-thinning behavior with a yield stress of the digestate, which was modelled by a Herschel-Bulkley equation. Furthermore, reducing the solids size by roughly a factor of 10 led to a 35-fold decrease in the viscosity at 20 s−1 for digestate with 8% of TS, which could improve the efficiency of mixing in digesters. On the other hand, the apparent viscosity of the digestate increases with increasing TS, which corresponded to a rise in cohesive energy. Equations linking rheological parameters and TS were provided. These models can be use in Computational Fluid Dynamics studies for a better modeling of the hydrodynamics within digesters.
Dimensionless flow curves of digestate at different solids concentration and size of RD: Raw Digestate, CMD: Coarsely Milled Digestate and FMD: Finely Milled Digestate. The dotted line represents the established model. Display omitted
•Both flow and oscillatory rheological measurements were applied to a silage digestate.•Establishment of a model describing agricultural digestate rheology.•Yield stress τ0 is related to total solid content TSC by an exponential model.•Reducing particle size by a factor 10 decreased τ0 by a factor 20 at a TSC of 8%.•Macroscopic flow behavior of digestate was related to the internal cohesive energy.
We numerically simulate a single-mode Rayleigh–Taylor instability between compressible miscible fluids with a highly accurate self-adaptive pseudospectral Chebyshev multidomain method in two ...two-dimensional boxes at small aspect ratios. The simulations are started from rest and pursued until the return toward mechanical equilibrium of the mixing. Four regimes—linear and weakly nonlinear, nonlinear steady bubble rise, return toward equilibrium, and finally a system of acoustic waves—can be identified. We show that this one-dimensional system of stationary acoustic waves is damped by the physical viscosity. This provides a reference solution.
