title: Film Dynamics and Deposit Formation in Evaporating Multi-Component Liquids
Creators: name: Bender, Achim
Publisher: name: UNSPECIFIED
Year: 2020

description: Deposit formation from evaporating liquid films and drops is an important phenomenon in many industrial applications. In internal combustion engines, deposits form from fuel films on ports, cylinder walls, and pistons. In the exhaust gas treatment, deposits form from the urea-water solution, which is injected into the exhaust pipe to reduce nitrogen oxide emissions. In both cases, the deposit formation has a negative influence on the process efficiency and increases the emission of pollutants. The influence of physical parameters on the evaporation and deposition process is not understood. While some individual aspects of the evaporation and deposit formation process have been addressed numerically in the literature, the influence of some key phenomena remain unknown. Furthermore, a single multiscale model, taking all relevant physical processes and their interactions into account, is not available. Such a model would be important in order to gain a basic understanding of the process and to deduce strategies to avoid deposit formation in the future. This thesis is a first step in the development of such a model. Based on the analysis of previous investigations, it is concluded that the process can be separated into two stages. In the first stage, a thin liquid film, which is influenced by evaporation, turbulent shear stress, and chemical reactions, is present on a structured wall. This liquid film ruptures at some time during the process and then continues to evaporate in the second stage in which the deposits form. Long-wave theory is used to derive various models to investigate the evolution and stability of thin liquid films. These models consider a film on a heated or cooled structured wall evaporating into a pure vapor atmosphere or into an ambient gas, a liquid film with a time-dependent chemical reaction subject to a laminar shear flow, and a liquid film sheared by a turbulent shear stress from the gas flow. The resulting evolution equations are solved with a finite difference solver developed in this work. The linear stability of the solution is addressed and parametric studies are conducted. It is shown that the investigated physical phenomena have a big influence on the film stability and evolution and that there are strong interactions between the individual phenomena. This makes a full numerical simulation of the film development necessary. Evaporation and deposit formation from sessile binary drops are investigated with an arbitrary Lagrangian-Eulerian method. The mesh is deformed to follow the shape of the evaporating drop and the deposit shape. The developed model is validated against a correlation and experimental data. The results show that ring shaped deposits occur in the vicinity of the three-phase contact line in the absence of Marangoni flow. A parametric study for urea-water drops is conducted. The temperature of the wall, initial composition of the drop, and drop size influence the evaporation of the drop, the time of first deposit formation, and the deposit growth rate, as well as the resulting deposit shape. The deposit shape changes from a ring-shaped to a cap-shaped pattern with increasing importance of thermocapillarity.