Particle image velocimetry measurement of non-Newtonian fluid flow in stirred reactors

Abstract. Fluid in anaerobic digesters always exhibits non-Newtonian behaviors. Computational fluid dynamics investigation of fluid mixing in digesters requires particle image velocimetry (PIV) measurements for model validation. The PIV technique has aided to provide valuable information about the velocity field and turbulence quantities in regions of interest of the vessel, assisting in the understanding of the phenomena involved in the analysis of mechanical mixing for non-Newtonian fluids. This investigation takes into account the state of the art about the use of PIV applied to stirred vessels, and uses a typical baffled stirred vessel with a down-pumping pitched blade (45 inclined degrees) turbine to conduct a series of experiments. The assessment has been made with a solution of water mixed with three concentrations (0.2%, 0.3% and 0.4%, based on weight) of carboxy-methyl-cellulose (CMC) in order to deal with a shear thinning fluid. Data obtained from the experiments was served to know changes on the flow patterns caused by the CMC concentration; and was analysed by means of the velocity field and turbulent kinetic energy happening in the region of the turbine and its surroundings. Also, this study considers the effects on the flow patterns caused by a range of angular velocity (from 50 rpm to 300 rpm) of the turbine. Thus, having a thorough analysis on the effects of CMC concentration and angular velocity to better understand their impact on the flow pattern dynamics that could help to enhance evaluations of the performance on mechanical mixing.