Taylor-Couette flow of shear-thinning fluids : primary and higher-order instabilities*

Taylor-Couette flow is mathematically well-defined flow that occurs throughout industry and is often used as a mixer in chemical engineering. Many of these applications involve non-Newtonian fluids, which is predicted to have a strong effect on the primary and higher-order instabilities that are essential for effective mixing; however, there is a clear lack of experimental data on this subject. We examine the behaviour of Newtonian and non-Newtonian (shear-thinning) fluids in Taylor-Couette flow for a range of Reynolds number (up to 1000) using a Particle-Image Velocimetry and flow visualisation (for phenomena occurring at short and long time-scales, respectively). Using this combined approach, we identify the variations in flow topography, the critical Reynolds number, frequency and amplitude of various instabilities. We show that shear-thinning causes vorticity to become concentrated in narrow low-viscosity bands and increases the wavelength (vortex size) of the instability. For strongly shear-thinning fluids, the wavelength becomes dependent on Re, with the transitions occurring through the spontaneous merge or splitting of vortices. This process is strongly hysteretic and has not previously been observed in Taylor-Couette flow.