ID 167

Internal breakup of an inelastic and shear thinning non-Newtonian fluid by vortical flow of air

 

Abstract:

Breakup and atomization of jets of non-Newtonian fluids in general and gelled propellants in particular, is a challenging technological problem owing to the high viscosity and complex rheology. Viscous forces tend to stabilize the surface disturbances and prevent breakup. This stability of surface poses a major challenge to the atomization. The stability can however be broken if sustained interactions can be induced at the interface between a non-Newtonian fluid of high viscosity and vortical flow of air. We used three-dimensional coupled level set-volume of fluid (CLS-VOF) computations on a model inelastic and shear-thinning non-Newtonian fluid to analyze the instability of the viscous fluid-air interface. Airflow breaks into a system of many vortices and they continuously shear the surface of viscous fluid. This sustained shear triggers Kelvin-Helmholtz instability of the surface and causes undulations of the surface. Subsequent motion of the shear-thinned fluid in a narrow conical confinement could lead to internal breakup of the viscous fluid. Experiments using a two-fluid atomizer based on this method and a gelled propellant reveal that the internal breakup predicted in CLS-VOF computations is observed in practice. Complete breakup could be achieved downstream of the confinement such that the droplets produced in the spray had average diameter of 50 μm at low-to-moderate air-viscous fluid ratios by mass.