Interacting particles and droplets at low Reynolds numbers

Sammanfattning: The overall purpose with this work is to study the flow past particles and droplets using numerical simulations. Especially the interaction among particles and droplets have been a main objective as the results are to be used in order to improve already existing spray models. In order to represent the particulate and droplet phase, the Volume of Fluid (VOF) and the Volume of Solid (VOS) methods are used, respectively. The variations of the drag- and lift-coefficients of two fixed solid spherical particles placed at different positions relative each other is studied for particle Reynolds numbers of 50, 100, 200, 300 and 600. For Reynolds numbers of 100, 300 and 600, both a uniform and a pulsating inflow is applied to investigate the effects of flow field fluctuations. Independent of Reynolds number, particles in tandem formation are found to experience the largest effect in the drag, as compared to the single particle case. Depending on the flow situation in--between the particles for various particle arrangements, attraction and repulsion forces are detected. These forces may lead to unsteady dynamics of the particle arrangement. The simulations of the motion of particles near a solid wall show that single and dual particles settling under gravity is clearly affected by the wall. However, for side-by-side arrangements, the simulations show that the particle-particle interaction is stronger as compared to the wall effects. The simulations are carried out for Reynolds numbers in the range from 1 to 1000 and particle to fluid density ratios of 1.5 to 8. Both the Reynolds number and the density ratio are found to highly influence the falling motion. Simulations of single and dual droplets in a uniform flow for Reynolds numbers 100 and Weber numbers of 0.1 - 10 have been studied. For single droplets, Reynolds numbers of 50, 100 and 200 for the same Weber number range has also been investigated. The results are used in order to evaluate how droplet deformation can be corrected for in numerical models for droplets. A drag correction function is presented, showing similar trends when compared to other drag correlations found in literature. The results show the importance of accounting for the full interaction among droplets and particles. Such interaction has to be included even for rather dilute two-phase systems. The obtained data has already been used for correcting for the four-way interaction that is usually neglected in current models.