Oscillating grid turbulence in shear-thinning polymer solutions

Abstract

Oscillating grid apparatuses are well known and convenient tools for the fundamental study of turbulence and its interaction with other phenomena since they allow to generate turbulence supposedly homogeneous, isotropic, and free of mean shear. They could, in particular, be used to study turbulence and mass transfer near the interface between non-Newtonian liquids and a gas, as already done in air-water situations. Although frequently used in water and Newtonian fluids, oscillating grid turbulence (OGT) generation has yet been rarely applied and never characterized in non-Newtonian media. The present work consists of a first experimental characterization of the flow properties of shear-thinning polymer (Xanthan Gum, XG) solutions stirred by an oscillating grid. Various polymer concentrations are tested for a single grid stirring condition. The dilute and semidilute entanglement concentration regimes are considered. Liquid phase velocities are measured by Particle Image Velocimetry. The existing mean flow established in the tank is described and characterized, as well as turbulence properties (intensity, decay rate, length scales, isotropy, etc.). OGT in dilute polymer solutions induces an enhanced mean flow compared to water, a similar decay behavior with yet different decay rates, and enhanced turbulence large scales and anisotropy. In the semidilute regime of XG, turbulence and mean flows are essentially damped by viscosity. The evolution of mean flow and turbulence indicators leads to the definition of several polymer concentration subregimes, within the dilute one. Critical concentrations around 20 ppm and 50 ppm are found, comparable to drag reduction characteristic concentrations.