Computational Fluid Dynamics Investigation of Turbulence Models for Non-Newtonian Fluid Flow in Anaerobic Digesters

In this paper, 12 turbulence models for single-phase non-Newtonian fluid flow in a pipe Ere evaluated by comparing the frictional pressure drops obtained from computational fluid dynamics (CFD) with those from three friction factor correlations. The turbulence models studied are (1)three high-Reynolds-number k-epsilon models, (2) six low-Reynolds-number k-epsilon models, (3) two k-epsilon models, and (4)the Reynolds stress model. The simulation results indicate that the Chang-Hsieh-Chen version of the low-Reynolds-number k-epsilon model performs better than the other models in predicting the frictional pressure drops while the standard k-epsilon model has an acceptable accuracy and a low computing cost In the model applications, CFD simulation of mixing in a full-scale anaerobic digester with pumped circulation is performed to propose an improvement in the effective mixing standards recommended by the U.S. EPA based on the effect of rheology on the flow fields. Characterization of the velocity gradient is conducted to quantify the growth or breakage of an assumed floc size. Placement of two discharge nozzles in the digester is analyzed to show that spacing two nozzles 180 degrees apart with each one discharging at an angle of 45 degrees off the wall is the most efficient Moreover, the similarity rules of geometry and mixing energy are checked for scaling up the digester.