Onset and Transition to Elastic Turbulence: Effects of Rheological Property Variations for Polyacrylamide-Water Solutions

摘要

Within the present investigation, the influences of rheological property characteristics on the onset and transition to elastic turbulence are considered. Properties under consideration include effective viscosity, relaxation time, general Weissenberg number, and a newly developed transition parameter Re_(ETC). Elastic turbulence phenomena occur when elastic polymer solutions within liquids are excited by a mechanical stress, and then exhibit highly non-linear and non-Newtonian behavior. The mechanical stresses act on the polymers within the flow stretching the polymers, causing a secondary flow. This secondary flow acts back on the polymers within the solution and stretches them further causing increased mixing and chaotic fluid motions. As a result, this flow transitions from laminar to turbulent flow through elasticity (rather than inertia) at very low Reynolds numbers. Different rheological property effects are induced using different concentrations of polyacrylamide in water, as different magnitudes of shear stress and strain rate are imposed upon the flow field. The result is changing polymer relaxation time, and alteration of the onset and extent of elastic turbulence within the flow field. This flow field is provided by a miniature viscous disk pump (VDP), which operates at different rotational speeds. The VDP consists of a disk that rotates above a C-shaped channel with inner and outer radii of 1.19 mm, and 2.38 mm, respectively. Fluid inlet and outlet ports are located at the ends of the C-shaped channel. Channel depths of 230 μm and 640 μm are employed to provide different ranges of shear stress distribution within the flow. As such, characterization and quantification of non-Newtonian fluid elastic turbulence effects are provided, relative to Newtonian flow behavior. Of particular interest are the rheological properties which are associated with the onset and transition of elastic turbulence, and the increased mixing, increased transport, increased diffusion levels, larger static pressure rise magnitudes, and increased magnitudes of effective viscosity, which occur subsequent to transition.