Mixing of viscous non‐Newtonian fluids plays an important role in many industrial processes (wastewater treatment, methanization, etc.). In some cases, mixing by gas injection can be more interesting than mechanical mixing. The present study focuses on the gas injection in yield stress fluids. The influence of the air flow rate, fluid rheological properties, and geometrical configuration on an air jet impinging the bottom wall of a tank containing a yield stress fluid has been considered. Focus has been placed on the air cavity present at the injection point. The trends of two key parameters of the cavity have been characterized: its maximum diameter and frequency detachment. Correlations based on the characteristic dimensionless numbers governing the flow have been derived. These correlations show that the apparent viscosity has an effect on the cavity's frequency but a low influence on its diameter which is mainly governed by the air flow inertia.
This study is in line with two previous studies by the same authors on gas injection in yield stress fluids. Gas is injected toward the bottom wall of a prismatic tank containing a yield stress fluid. When rising toward the free surface, trains of bubbles generate fluid recirculation in the tank. Two experimental colorimetric methods are introduced and validated in order to quantify the recirculation liquid flow rate as well as the time evolution of the extent and shape of the mixed volume. The influences of the injection flow rate, fluid rheology, and reactor size have been quantified. Correlations based on the characteristic nondimensional numbers of the flow have been developed to predict the downward liquid flow rate as well as the mixed volume. A model for estimating the mixing time is also developed and compared to experimental results.
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