Subcooled boiling flow in liquid-cooled internal combustion engines

Sammanfattning: Road transport sector contributes significantly to emission of carbon dioxide and other greenhouse gases, which negatively impact the global climate. Efficient management of energy, irrespective of the type of propulsion, has the potential to minimize fuel consumption and to reduce emission of greenhouse gases. This makes thermal (energy) management an indispensable part of automotive propulsion research and development. Cooling plays an important role in protecting the components from failure due to extreme thermal loads. An efficient cooling strategy, such as precision cooling, removes the excess heat precisely from the parts experiencing critical temperatures, without over cooling the component. The thesis focuses primarily on numerical methodologies to explore the potential of local nucleate boiling for efficient cooling of internal combustion engines. Nucleate boiling is a heat transfer phenomenon involving a phase change process, where the liquid coolant vaporizes in the form of bubbles close to the heated surface. Occurrence of nucleate boiling, locally in the vicinity of hot spots, offers a significant potential for efficient precision cooling, but at the risk of encountering film boiling. Film boiling is encountered as a consequence of excessive boiling which leads to coalescence and agglomeration of vapor bubbles, resulting in formation of a thin vapor film, next to the heated surface. On account of the low thermal conductivity of the vapor, film boiling prevents cooling and could potentially lead to material failure. Therefore, tapping the potential of controlled local nucleate boiling is a preferable approach. In the current work, a new semi-mechanistic wall boiling model is proposed that not only estimates the occurrence of boiling, but also the boiling heat flux and the extent of boiling. It is vital to know the extent of boiling in order to avoid, with sufficient margin, the risk of encountering film boiling. The proposed model is validated with results from channel flow experiments available in the literature. Further, the model is implemented in real engine simulations in both single phase and multiphase Computational Fluid Dynamics (CFD) frameworks. The model performance is evaluated by comparing the results of the simulations with relevant measurements. The model estimates the wall boiling heat flux with reasonably good accuracy and indicates the occurrence of excessive boiling with sufficient margin for industrial applications.