CFD investigation of turbulence models for mechanical agitation of non-Newtonian fluids in anaerobic digesters

This study evaluates six turbulence models for mechanical agitation of non-Newtonian fluids in a lab-scale anaerobic digestion tank with a pitched blade turbine (PBT) impeller. The models studied are: (1) the standard k–ɛ model, (2) the RNG k–ɛ model, (3) the realizable k–ɛ model, (4) the standard k–ω model, (5) the SST k–ω model, and (6) the Reynolds stress model. Through comparing power and flow numbers for the PBT impeller obtained from computational fluid dynamics (CFD) with those from the lab specifications, the realizable k–ɛ and the standard k–ω models are found to be more appropriate than the other turbulence models. An alternative method to calculate the Reynolds number for the moving zone that characterizes the impeller rotation is proposed to judge the flow regime. To check the effect of the model setup on the predictive accuracy, both discretization scheme and numerical approach are investigated. The model validation is conducted by comparing the simulated velocities with experimental data in a lab-scale digester from literature. Moreover, CFD simulation of mixing in a full-scale digester with two side-entry impellers is performed to optimize the installation.

Research highlights►Evaluate six turbulence models for mechanical agitation of non-Newtonian fluids in anaerobic digesters, and validate the simulated velocities against the experimental data in a lab-scale digester. ►Develop an alternative method that calculates Reynolds number for the moving zone to judge the flow regime in the whole tank, and check the effects of discretization scheme and numerical approach on the predictive accuracy. ►Perform CFD-based optimum installation of impellers for mixing in a full-scale digester with two side-entry impellers.