Numerical study of non-Newtonian fluid droplet dynamics

Abstract

The liquid droplet dynamics are important in many applications such as inkjet printing, spray coating and spray cooling. There are two key phenomena involved in such applications: droplet formation from a nozzle and droplet impact on a solid surface. Droplet formation from a nozzle and droplet impact on a solid surface have been studied mainly for Newtonian fluids by many researchers. However, many fluids used in industrial applications are complex non-Newtonian fluids. Therefore, more research associated with non-Newtonian fluids is necessary. This dissertation focuses on the numerical investigation of the droplet dynamics of non-Newtonian fluids. A particular non-Newtonian fluid type known as a yield-stress fluid (or viscoplastic fluid) exhibiting shear-thinning behavior is investigated. Numerical simulation is performed using a volume-of-fluid model, and the presence of yield-stress and shear-rate dependent viscosity is modeled using the Herschel–Bulkley rheological model. This work is divided into two parts: droplet impact on a solid surface and droplet formation from a nozzle. Firstly, the impact dynamics of yield-stress fluid droplets on a solid surface are numerically investigated. The numerical results are found to be in qualitative agreement with experimental data in the literature. By performing extensive numerical simulations varying the impact velocity, rheological parameters, and surface tension, the influence of these parameters on the impact dynamics are evaluated, and the dominant parameters that govern the spreading and relaxation phases are determined. The results show that while the spreading behavior is determined by the power-law index n, the non-Newtonian Reynolds number Ren, and the Weber number We, the retraction behavior is determined by the non-Newtonian capillary Can and the Bingham-capillary number . In addition, the scaling law that predicts the maximum spreading diameter is proposed. Secondly, the drop-on-demand droplet formation dynamics of yield-stress fluids are numerically investigated. The influence of rheological parameters on the droplet formation dynamics of yield-stress fluids is investigated. In particular, the yield-stress t0, consistency factor K, and power-law index n are varied separately to determine their independent influences on droplet formation behavior. As a result, the qualitative effects of above parameters are found and the non-dimensional parameters that govern the droplet formation dynamics are determined.