CFD Analysis of Mechanical Mixing in Anaerobic Digesters

Abstract

A general computational fluid dynamics (CFD) model that characterizes mechanical mixing in anaerobic digesters was developed. In this model, liquid manure was assumed to be Newtonian (water) and non-Newtonian fluids dependent on total solids (TS) concentration. Commercial CFD software (Fluent 6.3) was used to simulate flow fields in a full-scale anaerobic digester with four mixing designs: (1) mechanical pumping by two 10 hp propellers in two external draft tubes, (2) mechanical stirring by two 10 hp side-entry propellers, (3) mechanical pumping by one 20 hp propeller in an internal draft tube, and (4) mechanical stirring by one 20 hp top-entry propeller. Power number and flow number of each propeller used in water mixing were validated against lab tests, and polynomial regression equations for predicting power inputs in non-Newtonian fluid mixing were derived. Through qualitative and quantitative comparisons of the flow fields, mechanical stirring by one 20 hp top-entry propeller was proposed to be the optimum mixing design at low TS levels, while mechanical stirring by multiple propellers was recommended at high TS levels.