Simulations of heat transfer with liquid-vapor phase change and heater wetting properties

Whoever has already prepared tea has observed that when a solid body (the sauce pan), in contact with a liquid (water), is heated at a high enough temperature, the liquid eventually boils. This well-known and so common phenomenon is actually a very complex mechanism that controls the overall heat transfer. Boiling uses the energy from the hot solid body to transform water into vapour, and the vapour bubbles subsequently detach from the horizontal plate thanks to buoyancy, therefore making room for cool water. But in some particular cases, a vapour layer is formed on the whole plate instead of bubbles. The heat transfer is therefore jammed as the temperature of the solid body is no longer buffered by the cool water layer. This so-called “boiling crisis” can lead to the overheating or even the melting of the solid body, with evident harmful consequences, particularly in nuclear power plants or industrial heat exchangers.

Despite its relevance for applications and its interest from a fundamental perspective, this problem is still only partially understood, due to the complex couplings existing between phase change, bubble dynamics and contact line dynamics (contacts between solid, liquid and gas). In particular, it has been recently pointed out that the evaporation greatly affects the local dynamics of bubbles by changing the forces that anchor a vapour bubble at the solid surface (i.e. wetting properties).

The objectives of the present projects are twofold:

• the development of a numerical tool for the simulation of the boiling of a liquid on a heated surface, build on an existing high performance multiphase fluid simulation codes able to take into account the fully coupled dynamics ;
• the understanding of the dynamics of the nucleated vapour bubbles on a surface with controlled and possibly heterogeneous wetting properties.

The strategy is to develop modules specifically designed for phase change and contact line movement. The code, YALES2, which is largely used in both academic and industrial projects, offers a solid ground for the proposed implementation, also maximising the possibilities of exploitation of the new modules for applications.

Once the phase change and contact line tools are developed, a fundamental study on the dynamics of vapour bubbles will be carried out within the proposed project, aiming at linking this microscopic dynamics to the heat transfer at larger scales.