Numerical Modeling and Investigation of Boiling Phenomena

The subject of the present thesis is the numerical modeling and investigation of boiling phenomena. The heat transfer during boiling is highly efficient and therefore used for many applications in power generation, process engineering and cooling of high performance electronics. The precise knowledge of particular boiling processes, their relevant parameters and limitations is of utmost importance for an optimized application. Therefore, the fundamentals of boiling heat transfer have been intensively studied in the last decades and are still subject of many ongoing research activities all over the world. In spite of this effort, many aspects of boiling heat transfer are still not completely understood. The difficulty is mainly due to the small length and time scales. In addition to highly resolved experiments, the numerical modeling of boiling heat transfer has been established in fundamental research during the last years. However, most of the existing models and methods are limited with respect to their applicability. Thus, 3D simulations of boiling in complex geometries cannot be handled by the existing methods. However, the use of complex heater geometries is one of the possibilities to fulfill the demand for more efficient heat transfer units and its numerical investigation is therefore desirable. Within the framework of the present thesis, a numerical model was developed which enables the simulation of boiling processes in arbitrarily complex geometries at a high level of accuracy. The model is based on the Volume-of-Fluid solver of the Computational Fluid Dynamics software OpenFOAM and resolves all relevant length and time scales. The latter particularly applies for the microscopic, but highly relevant, region at the 3-phase contact line where the liquid-vapor interface meets the wall. The present thesis contains a detailed description of the model and comprehensive information about its validation. Furthermore, several simulations of different boiling phenomena are presented. The simulation results are discussed and compared to mostly experimental data available in literature. Simulations were accomplished for nucleate boiling of single bubbles and merging bubbles, flow boiling in a near-wall shear flow, boiling in a structured micro-channel and film boiling of droplets (Leidenfrost phenomenon). Good agreement to existing data is achieved. Further, the simulation results enable a detailed analysis and a more comprehensive understanding of the transfer mechanisms. Hereby, the knowledge gained during highly resolved experiments can be extended significantly. The formation of an enclosed droplet within a merged bubble which was observed but not understood experimentally is an excellent example. The detailed analysis of the simulation results enable a clarification of the causes for the formation of the droplet and lead to a gain in knowledge which would not have been possible in the experiment. In summary, the present thesis includes the development, implementation and validation of a boiling model as well as a wide range of simulations on different boiling phenomena. The latter clearly demonstrate the potential of the numerical investigation of boiling phenomena in fundamental research and in the design of small boiling devices.