Schulte, Kathrin (Dr.-Ing.)Potyka, Johanna2025-07-162025193093422Xhttp://nbn-resolving.de/urn:nbn:de:bsz:93-opus-ds-160900https://elib.uni-stuttgart.de/handle/11682/16090https://doi.org/10.18419/opus-16071The interaction of two immiscible liquids in a gaseous environment is relevant for different technical applications. An example is the water injection into engines which can help to reduce emissions and increase efficiency. Another application is the rapid production of equal-sized capsules for the precise dosage and for the protection of the active substance of medicine or of food additives. The outcome of two colliding immiscible liquids' droplets as an elementary process has to be predicted for the design of such applications. Binary collisions of droplets can either result in merging or separation into two or more daughter droplets. The consideration of different properties of the liquids for both droplets breaks the symmetry of the collision process. Merging, crossing, single reflex and reflexive separation are reported in the literature as the possible outcomes of head-on collisions of immiscible liquids' droplets. These differences of immiscible compared to identical liquids' droplet collisions are only scarcely investigated in the literature. Numerical simulations allow for detailed insights into the collision process, but methods suitable for the prediction of three-dimensional and immiscible liquids' droplet collisions combined with relevant topological changes during the droplet collision process were not yet described. Therefore, a simulation tool for fully three-dimensional direct numerical simulations (DNS) of immiscible liquids' droplet collisions has been developed within this thesis and successfully applied to predictions of the immiscible liquids' droplet collision process and outcome. Existing and new methods were enhanced, developed and combined to enable massively parallel and efficient three-dimensional DNS of the interaction of two immiscible liquids of arbitrary three-dimensional geometry in a gaseous continuous environment. A main challenge of modelling immiscible three-phase flow, as compared to two-phase flow, is the representation of the geometry of thin films and contact lines, especially in three-dimensional domains. Therefore, the Piecewise Linear Interface Calculation (PLIC) and the Volume of Fluid (VOF) advection were adapted for three-phase cells. Additionally, a modified Continuous Surface Stress (CSS) model was introduced to model the surface and interfacial forces at liquid-liquid-gas contact lines and thin films. This model allows for simulations of immiscible liquids' interactions with topological changes. The modified CSS model requires an adaptation of the density and viscosity computation throughout the entire momentum equation solver. This consistent use of properties has been key to enabling momentum and energy conserving simulations of immiscible three-phase flow. The methods for the simulation of immiscible liquids' interaction were implemented in the multiphase flow solver Free Surface 3D (FS3D). The simulation results of several validation test cases were compared to analytical solutions. First order convergence was achieved for the PLIC reconstruction and VOF advection as well as for the liquid lens test. Furthermore, experimentally investigated cases of droplet-droplet and droplet-jet interactions have been reproduced by DNS. The morphology of selected cases as well as full regime maps for merging and separation agree with experiments. This shows that the developed methods enable the prediction of a wide range of three-dimensional interactions of immiscible liquids. Additionally, the increased efficiency of the new methods has been a key aspect to enable parameter studies within feasible compute times and simulations of large systems like droplet-jet interactions. Based on the analysis of immiscible liquids' droplet collision DNS resulting in different liquids' distributions after separation, a scaling approach was developed. This approach employs two dimensionless numbers to classify the distribution after the separation from quantities known before the collision. Furthermore, a new distribution resulting after the separation of two droplets of partially wetting liquids was discovered through the DNS. The simulation results provide detailed information on the whole collision process. Simulation results of immiscible and identical liquids' droplet collisions have been compared. The evaluation of the energy budgets throughout representative droplet collisions underlined the increased complexity in the collision process of immiscible liquids compared to identical liquids. A larger number of influencing factors on the collision process and outcome originates from different viscosities, densities and the wetting behaviour of the two interacting liquids, which should be accounted for in future analytical models. Summarised, this thesis provides numerical methods for the predictive three-dimensional simulation and a basis for a universal description of three-dimensional liquids' interactions with or without topological changes in gaseous environments.eninfo:eu-repo/semantics/openAccess620doctoralThesis