Flow And Mixing Characteristic Of Non-Newtonian Fluid In A Stirred Tank

As a fundamental unit operation, stirring is widely used in industry production, most of which involves Non-Newtonian fluids. However, the complex rheological characteristic of Non-Newtonian fluids makes it difficult to implement experimental investigation, most of the researches focus on flow characteristics in stirred tank are computational simulation rather than experimental investigation. To build up a better acknowledgement of Non-Newtonian flow characteristics in stirred tank, this article measured the flow characteristics of shear-thinning liquids in a stirred tank.In this paper, particle image velocity (PIV) techniques is introduced to investigate the flow characteristics of Sodium Polyacrylate solution in a stirred tank with Rushton impeller. The main issue of this paper is to what extent Reynolds number and flow index affect Non-Newtonian flow characteristics, and by means of changing impeller rotation rate and polymer concentration, the Reynolds number and flow index is changed. The profiles of mean velocity, rms fluctuating velocity and the turbulent kinetic energy (TKE) are showed to measure the influence.It is revealed in this study that when Reynolds number varying from 900 to 5500, the flow of Sodium Polyacrylate solution is transitional near the impeller, especially when Re<1500, the flow is near laminar, when Re>5000, the flow is near turbulent. At the level of impeller disc center-line, the radial component of dimensionless mean velocity correlate with Reynolds number, it increase with the increase of Reynolds number. Otherwise, the radial component of dimensionless mean velocity correlate with the flow index, and the flow index has a stronger influence on the mean velocity than Reynolds number. The dimensionless rms fluctuate velocity almost proportional to the Reynolds number, the Reynolds number effect is stronger than on the mean velocity. The distribution of turbulent kinetic energy (TKE) is slight asymmetric at the level of impeller disc center-line, and by observation of phase-resolved distribution, this asymmetry comes to maximum at phase angle 9=50°. Unlike in Newtonian flow, the highly turbulent zone in Non-Newtonian fluids generate after the dissipation of trailing vortex, while in Newtonian flow the highly turbulent zone generate with the generation of trailing vortex. In the blade region, the shear strain rate of pseudo-plastic liquids increases with the increase of Reynolds number. When near laminar, the shear strain rate distribution is more uniform, and when near turbulent, there exists two high shear strain rate region on both sides of the disc center-line, however the shear strain rate is fairly low at the level of impeller disc center-line. In the range of this experiment, the distribution of dimensionless racial velocity and turbulent kinetic energy of Non-Newtonian liquid is similar to Newtonian liquid which Reynolds number equal to its 1/10.