Direct Numerical Simulation of Nucleate Boiling in Microgravity

Prediction of heat transfer in nucleate boiling remains an open problem. CFD allows to perform simulations at industrial scales, but requires to model the wall heat transfer. The most advanced models are based on the heat flux partitioning between the latent heat due to the bubble vaporization and the sensible heat directly transferred to the liquid. They require a good prediction of the bubble growth rate, detachment diameter, which can be obtained with DNS. This thesis focus on such simulations. The solver DIVA has been developed at the IMFT and provides accurate simulations of bubble growth in contact line and micro-layer regimes. However, the contact line, which is the bubble interface in contact with the wall, requires a specific attention. This region is subject of significant heat flux and large variation of the contact angle. Several models have been developed to account for it. Yet to our knowledge, no clear coupling methodology between DNS and micro-region model has been proposed in the past. In this work, a coupling between these different scales is proposed. An implicit coupling has been developed between the micro-region model and the DNS, and required a deep computational work. This coupling is presented in a generic manner and can be done regardless of the chosen micro-region model. The convergence of the results is demonstrated in comparison with the RUBI experiment, developed for the study of boiling on an isolated site in microgravity, on board of the International Space Station. The configuration setup allows the measurement of wall temperatures and local heat fluxes through infrared thermography. These measurements are synchronized with bubble growth visualizations by high-speed camera. The coupling, along with the appropriate micro-region model, has significantly improved bubble growth predictions. Afterwards, a study of heat transfer between a superheated wall and a fluid in microgravity has been conducted. Without buoyancy, the bubble stays attached to the wall and thus the investigation is easier. Therefore, the study enables a better understanding of the parameters involved in the micro-region model. The prediction of bubble growth rates has been improved, and the physics at stake at the contact line is better apprehended.