Flow patterns in agitated vessels

Abstract The flow patterns obtained when viscous fluids are agitated inside baffled cylindrical tanks have been studied in both Newtonian and non‐Newtonian systems. The experimental technique consisted of observing the motions of small tracer particles in highly illuminated, narrow beams of light. The results may be broken down into two major categories. The first was a qualitative comparison between the flow patterns obtained in non‐Newtonian and Newtonian fluids of the same general viscosity levels. This part of the study included observation of changes in the flow fields as one moves from laminar into turbulent conditions for both fluid systems. The second portion of the paper deals with quantitative determinations of local flow velocities, shear rates, and power‐dissipation rates in various parts of the vessel. The following conclusions may be drawn from these measurements. 1. Local fluid shear rates were found to be directly proportional to impeller speed, in both Newtonian and non‐Newtonian systems. As would be expected, the shear rates decreased more rapidly with increasing distances from the impeller in pseudoplastic non‐Newtonian fluids than in Newtonian systems. 2. The rates of local power dissipation decreased rapidly with distance from the impeller. 3. The fluid velocities in the horizontal plane of the impeller varied almost linearly with rotational speed in the Newtonian systems, in accordance with prior observations. On the other hand, movement in pseudoplastic systems increased exponentially with impeller speed. This effect, like the dampening of shear rates, is caused by the cumulative effects of local velocity and viscosity changes in these systems. (Increases in velocity decrease the fluid viscosity, which in turn causes further increases in velocity, etc.) This work serves to define pertinent problems which are of importance in determining mixing rates, on a microscopic scale. Efforts may now perhaps be turned more profitably than before toward macroscopic fluid‐mixing studies.