CFD Simulation of Solid and Non-Newtonian Fluid Two-Phase Flow in Anaerobic Digesters

Abstract

A CFD model is used to study solid suspension in anaerobic digesters.Simulations of solid and non-Newtonian fluid two-phase flow are performed.Minimum impeller speed for avoiding solid accumulation in the digester is proposed.Major issues arising from the simulation results are discussed.Abstract. A principal goal of solid-liquid flow modelling is to predict the minimum power consumption for avoiding solid accumulation in stirred reactors. In this study, a computational fluid dynamics (CFD) technique was used to characterize solid suspension in anaerobic digesters with mechanical mixers where the liquid phase exhibited non-Newtonian fluid behavior. The commercial CFD code Fluent 16.0 was utilized to solve the multiphase flow. The impeller torque and cloud height as a function of impeller speed in a small tank were validated against the experimental data from the literature. The simulation results demonstrate that the impeller torques from CFD modeling match perfectly with those from measurements, and that the predicted cloud heights agree reasonably well with the measured ones. The solid-liquid flow in a large egg-shaped digester with a mechanical draft tube mixer was modeled where the impeller speed ranged from 400 to 750 rpm at 50 rpm increments. A combination of tangent line and uniformity index methods was used to predict the minimum impeller speed to maintain solid suspension in the digester. Moreover, the major issues arising from results were discussed. Keywords: Anaerobic digester, Computational fluid dynamics, Multiphase flow, Non-Newtonian fluid, Solid suspension.