CFD simulation of mixing in egg-shaped anaerobic digesters

• Published in: Water research (Oxford) 2010-03, Vol.44 (5), p.1507-1519
• Main Author: Wu, Binxin
• Format: Article
• Language: English
• Subjects: anaerobic digesters; Anaerobiosis; animal manure management; Applied sciences; CFD technique; Computational fluid dynamics; Computer Simulation; Digesters; Draft tubes; Egg-shaped digester; equations; equipment design; Exact sciences and technology; General treatment and storage processes; Liquids; Mathematical models; Mechanical draft tube mixer; Mixing energy level; Models, Chemical; Non-Newtonian fluid; optimization; Other industrial wastes. Sewage sludge; Pollution; Propellers; Reproducibility of Results; Rheology - instrumentation; Rotation; simulation models; Specifications; Thermodynamics; Wastes; Water treatment and pollution
• ISSN: 0043-1354; 1879-2448
• DOI: 10.1016/j.watres.2009.10.040

A computational fluid dynamics (CFD) model that characterizes mechanical draft tube mixing in egg-shaped anaerobic digesters was developed. Simulation of flow patterns were carried out with a propeller rotating from 400 to 750 rpm, assuming liquid manure to be Newtonian (water) and non-Newtonian fluids depending on the total solids (TS) concentration. Power number and flow number of the propeller in water mixing were validated against lab specifications and experimental data from a field test. The rotational direction and placement of the propeller were examined to identify the primary pumping mode and the optimum position of the propeller fixed inside the tube. Quantitative comparisons of two mixing methods and two digester shapes indicated that mechanical draft tube mixing is more efficient than external pumped recirculation, and that the egg shape provides for more efficient mixing than the cylindrical shape. Furthermore, scale-up rules for mixing in egg-shaped digesters were investigated.