Interfacial instabilities in two immiscible flows in an annular duct: Shear-thinning fluids surrounded with Newtonian fluids

Abstract

In this paper, the stability of two co-axial immiscible fluids flowing in an annular duct is investigated. The inner layer consists of a shear-thinning fluid, which is surrounded by a Newtonian liquid annulus in the outer layer. A constant pressure gradient is applied to drive the flow in the annular channel. Linear stability analysis is employed to investigate the shear-thinning effect on the Rayleigh-Plateau instability and the interface wave instability. Results show that the Rayleigh-Plateau mode can be enhanced and the topological structures of the marginal stability curve of the Rayleigh-Plateau mode can be significantly changed by the shear-thinning effect. When the shear-thinning effect is strong, a case study shows that the Rayleigh-Plateau instability can be slightly suppressed by the viscosity stratification in the inner layer. The shear-thinning effect has a dual influence on the interface wave instability. It can either enhance or suppress the interface wave instability, depending on the thickness ratio and viscosity ratio between the outer layer and the inner layer.