Oscillating grid generating turbulence near gas-liquid interfaces in shear-thinning dilute polymer solutions

Abstract

Understanding the behavior of liquid phase turbulence near gas-liquid interfaces is of great interest in many fundamental, environmental, or industrial applications. For example, near-surface liquid side turbulence is known to enhance the mass transfers between the two phases. Descriptions of this behavior for air-water systems exist in the literature, but the case of turbulence in a shear-thinning liquid phase below a flat gas-liquid interface has never been considered to the best of our knowledge. This paper consists in an experimental characterization of low Reynolds number, oscillating grid generated, nearsurface turbulence in shear-thinning dilute polymer solutions, in the surface-influenced and in the viscous sublayers. The energy transfer mechanism, known in the water case, is evidenced in dilute polymer solutions. A horizontal damping mechanism, similar to the one introduced by surfactants, is evidenced. The evolution of the viscous sublayer depth can be explained by both viscous and shear-thinning effects, and it appears that a critical polymer concentration may exist within the dilute regime.

Oscillating grid turbulence in shear-thinning polymer solutions

“…We notice that the change in trend trend happens at higher depths (smaller z) for the vertical scale L z zz than for the horizontal one and that this depths increases with polymer concentration. This is consistent with the fact that vertical velocity structures are kinematically constrained by the horizontal interface, and larger for large polymer concentrations (to be detailed in Lacassagne et al 79 ).…”

Turbulent mass transfer near gas-liquid interfaces in water and shear-thinning dilute polymer solution

“…Viscoelasticity is checked to be negligible at this concentration [ 35,25]). Moreover, in the context of the near surface turbulence results [19], this low concentration allows for comparable viscous sub-layer depth and turbulence properties within the viscous sub-layer between water and XG solution, and hence the mass transfer events are compared in equivalent hydrodynamic conditions. Here the viscous sub-layer depth is 5.10 mm for water and 5.64 mm for the DPS case (see table 1).…”

“…Turbulence is generated in the liquid phase by an oscillating grid device previously described and used in [25,37,19,38,39], and sketched in figure 2.…”

“…What is known of turbulence action on dissolved gas at the interface can thus be summarized as follows (see figure 1). A turbulent eddy coming from the bulk enters the surface influenced region and is deformed by surface action [4,5,19]. If its typical size is large compared to the apparent scalar sub-layer depth, it mainly contracts the scalar sub-layer thus sharpening the concentration gradient and increasing mass transfer.…”

“…In a recent work [19], the liquid phase velocity data of the present set of measurements presented has been used to extend this description to the case of shear-thinning dilute polymer solutions (DPS). A correlation for the evolution of the viscous sub-layer depth in the dilute regime of shear-thinning inelastic xanthan gum (XG) solutions has been derived.…”