Analyse Local Instationnaire des mécanismes de la Crise d'Ebullition – ALICE

The main scientific objective of this project is to determine a predictive local correlation for the critical heat flux as a function of relevant physical quantities averaged at the bubble scale. It will serve as 'sub-grid model' that, once integrated in a two-phase averaged model, will enable the prediction of the boiling crisis occurrence in any specific situation. This local correlation will be determined through a novel multi-scale approach of the boiling crisis based on a thorough analysis of the unsteady dynamics of bubbles at very high heat fluxes. This multi-scale approach will include the coupled analyses of the contact line dynamics (microscopic scale), of a single bubble spreading (mesoscopic scale) and the interaction of several and many spreading bubbles (macroscopic scale). Two successive up-scaling steps will be applied: the microscopic contact line model will be integrated into the mesoscopic bubble growth model and the latter will be used in the bubble interaction model used to elaborate the local correlation. This project is divided into five tasks whose objectives are detailed in the following. Task 1. The objective of this task is to validate quantitatively the vapor recoil mechanism as the triggering effect of the spreading of a bubble at high pressure, i.e. in the vicinity of the critical point. This validation will be based on the comparison of experimental results with numerical results. The experimental results will be obtained thank to two experiments, DECLIC and OLGA (ESEME), that will allow to measure precisely the shape of the bubbles and the dry spots in contact with the heated surface. A model for the triple line will be developed (ESEME and LMS) and implemented in the Trio_U code (CEA) that already allows to perform direct numerical simulations of two-phase flows with phase-change. The numerical simulations will allow to account for the unsteady effects necessary to validate the model. The quantitative comparison of the experimental results and numerical results will be based on the time evolution of the shape of the bubble and of the dry spot. Task 2. The objective of this task is to validate the vapor recoil mechanism as the triggering effect of the spreading of a bubble at moderate pressure. This validation will be based on the comparison of experimental results and numerical results. The experimental apparatus BOUM (IMFT) will be used to measure the time evolution of the shape of the bubble and of the dry spot at low pressure (different pressures will be studied). Numerical simulations (CEA) will allow to compare the model and the experimental data. The numerical simulations will also allow to measure the forces applied to the bubble and thus to develop a force balance model (IMFT). Task 3. The objective of this task is to study the influence of the interaction of several bubbles on their spreading. Only low pressure studies will be conducted. The BOUM apparatus will allow to study experimentally the interaction of two neighboring bubbles. In particular, we will study if the presence of a second bubble rather tends to delay or not the appearance of a vapor film. On the QLOVICE facility (CEA), we will study the interaction of many bubbles. The distribution of the size of the hot spots and of the acoustic emission will be analyzed statistically in order to detect a possible power law characteristic of self-organized complex systems. Numerical simulations (CEA) will allow to study the model developed on configurations with a few interaction bubbles and then on configurations where the number and arrangement of the nucleation sites will be more and more complex. These results aim at developing a simplified model of interacting spreading bubbles necessary to task 4 (LMS). Finally, the effect of the surface roughness will be studied experimentally (IMFT and CEA) and a contact line model accounting for the surface roughness will be developed and implemented (ESEME). Task 4. The objective of this task is to use the simplified model of interaction spreading bubbles to construct a cellular automaton model and to determine a predictive model of the boiling crisis. We will work on the development of the first analytical models describing scale free fluctuations in the vicinity of the boiling crisis. The idea is, for the first time, to make a link between boiling crisis and self-organized criticality observed in many other complex systems. This innovative theoretical approach can be viewed as an up-scaling approach that allows one to determine large scale characteristics in the form of a power law starting from basic interactions at the scale of bubbles.