Numerical Simulation of Two-Phase Boiling Flows in Macro and Micro-scale Channels

Abstract

Two-phase boiling flows are used in a wide range of engineering applications. One of the most common applications is cooling systems. A refrigerant is injected into a closed-loop under saturation conditions extracting the unwanted heat from the corresponding system and boiling as a consequence. However, flow boiling is a complex process governed by several flow characteristics making it challenging to predict. It is strongly related to the turbulence and shear stress occurring between the two phases. Correlations are usually used to model this process. These correlations are based on global averaged parameters in a certain range of operating conditions, while boiling is also dependent on local effects. Thus, the implementation of a more sophisticated numerical approach is mandatory. Computational Fluid Dynamics (CFD) can provide the needed local flow characteristics to predict all aspects of boiling. In this study, a two-phase model implemented in an open-source CFD solver has been tested on two different fluids: Freon R12 and Carbon Dioxide CO2. R12 was chosen since it is one of the most common fluids employed in cooling technologies and its behaviour has been widely assessed experimentally. In addition, CO2 fluid has been selected as it is the new refrigerant with promising characteristics to replace CFCs such as R12. In particular, CERN/CMS is heading in that direction as they are updating their detector cooling system to CO2 evaporative cooling. The CFD model has been tested for the two fluids in different configurations and under various levels of simplification of the model. This work provides a detailed analysis of the key physical aspects dominating the boiling process such as turbulence and buoyancy effects. Finally, some recommendations on best practices in modelling boiling flows are provided.