Vortex breakdown in swirling pipe flow of fluids with shear-dependent viscosity

Laminar pipe flow with a controllable wall swirl has been studied both numerically and experimentally to explore the behavior of inelastic shear-dependent fluids. The pipe consists of two smoothly joined sections that can be rotated independently about the same axis. The circumstance of flow entering a stationary pipe from a rotating pipe (decaying swirl) has been investigated. Numerical parametric studies using both a power-law model and a simplified Carreau model are conducted to investigate the effect of shear-thinning and shear-thickening on the flow structure and the critical swirl ratio required to induce the breakdown at a range of Reynolds numbers. A new method of scaling (i.e., a new Reynolds number) is presented that accounts for the shear-dependent viscosity. Using this Reynolds number, the data for all fluids (shear-thickening, Newtonian, and shear-thinning) approximately collapses. Experimental visualisations using an aqueous solution of xantham gum confirm the conclusions drawn from the numerical results.