Dynamics of Multiphase Flows : Liquid - Particle Suspensions and Droplet Spreading

Raiskinmäki, Pasi Dynamics of multiphase flows: liquid-particle suspensions and droplet spreading Jyväskylä: University of Jyväskylä, 2004, 152 p. (Research report/Department of Physics, University of Jyväskylä, ISSN 0075-465X; 7/2004) ISBN 951-39-1967-6 diss. Most fluids that appear in nature as well in numerous applications in industry are multiphase fluids such as liquid-particle suspensions. This thesis considers two typical multiphase systems, a liquid-particle suspension in a (two-dimensional) shear flow and a liquid droplet on a porous surface. These systems are analysed by ab initio lattice-Boltzmann simulations. We analysed the dependence of the viscosity of non-Brownian liquid-particle suspensions on the concentration and shape of the suspended particles, and on the shear rate. We found the viscosity to increase with increasing shear rate. This phenomenon was related to enhanced solid-phase momentum transfer across the system. At least two mechanisms were shown to contribute to the solid-phase momentum transfer: clustering of particles and inertial effects. In shear flow suspended particles form dynamic linear clusters, whose size correlates with the viscosity of the suspension for Reynolds numbers Reγ . 1. For high Reynolds numbers Reγ > 1, when the cluster size remains almost constant, viscosity increases due to inertial effects. In the second system we were interested in the dynamic behaviour of liquid droplet impact on a porous substrate. We could explain, at least qualitatively, how the evolution of the wetted area is controlled by impact velocity, contact angle and surface roughness. We also used in this work a tomographic image of a sample of paper board as the substrate. We determined the volume penetrated by the droplet as a function of time, and compared it with the corresponding result of an analytical capillary model. Based on this comparison, it is evident that our simulations capture relevant mechanisms of the penetration process, and that this process can be understood by a properly formulated capillary theory.