Direct numerical investigations of non-Newtonian drop oscillations and jet breakup

Abstract

Creating particles with well defined properties is important to many applications in modern life. A state of the art method to produce particles is spray drying. This involves solutions with non-Newtonian properties. Spray drying is defined by the droplets from the spray, therefore, it is important to understand the breakup of non-Newtonian jets. This thesis focuses on the numerical investigation of non-Newtonian behaviour of liquid jets and single drops. Simulations are used for the investigations of the underlying mechanisms of breakup. Direct Numerical Simulation (DNS) provides the necessary accuracy to capture the complex and highly transitional multiphase flows. The Volume of Fluid (VOF) approach is used to capture the two phases and the shear thinning viscosity is calculated with the Carreau-Yasuda model. Non-Newtonian drop oscillations are investigated. The mechanics of oscillating, shear thinning drops are explained. These drop oscillations are compared to Newtonian drops from theoretical fluids and the analytical solution from the linear theory for drop oscillations and the frequency and amplitude of the oscillations are analysed. Drops initiated with the shapes of higher order oscillation modes are investigated. The primary jet breakup of non-Newtonian liquids is investigated. The mechanisms of breakup of shear thinning jets are analysed for different breakup regimes. The influence of different parameters on the stability of the jet is investigated with a special focus on different shear thinning behaviours. The results are compared to jets from theoretical Newtonian fluids. A set of characterisations of the liquid surface are used for comparisons. For jets in the higher breakup regimes, the jet angle and the resulting drop sizes are investigated. The investigation in this thesis provides a better understanding of the fundamental processes involved in non-Newtonian jet breakup and a basis for technical and efficiency improvements to spraying applications.