CONTEXT

Hydrogen is a key asset for the energy transition, but it still poses major scientific and safety challenges. Colorless and odorless, hydrogen leaks easily, ignites at low concentrations and temperatures, and can lead to the propagation of rapid deflagrations as well as detonations, a dangerous type of supersonic combustion. Understanding the mechanisms involved in the transition from deflagration (slow flame) to detonation (supersonic flame accompanied by a shock wave) is therefore vital to the safety of hydrogen production facilities (electrolyzers) and the nuclear industry. In the accidental scenario of loss of cooling and core meltdown, oxidation of uranium rod cladding can lead to the release of hydrogen. It was the subsequent explosion that led to the loss of containment and release of radioactive material at Fukushima and Three Mile Island. Hydrogen risk management is therefore one of the major challenges for nuclear safety.

The main mechanism behind the deflagration -> detonation transition is the presence of obstacles along the flame path. These generate vorticity, which increases the surface area of the flame and accelerates the reactive wave. When obstacles are in sufficient number and proportion, a runaway effect and wave reflections can lead to a shock-chemical reaction coupling: detonation is born, propagating at several kilometers per second. Unfortunately, it's impossible to avoid the fact that industrial plants are cluttered with obstacles: pipes, buildings, machines, walkways, structures... and present this type of scenario.

Conversely, a very densely-packed, porous environment can, on the contrary, smother a too-rapid flame and allow the reverse transition from detonation to deflagration, which is less dangerous in nature. For example, detonation can be attenuated by passage through a porous matrix, or when a porous medium is placed along the walls during propagation in a tube. A crucial safety question then arises: under what circumstances does an obstacle accelerate or slow down a flame? Can porous media be designed to stop dangerous flames?


OBJECTIVES

The aim of this thesis is to approach this question from three angles:

1/ on the one hand, via the preparation and execution of experimental tests on the CEA Saclay hydrogen explosion test bench (SSEXHY). These include:
- exploring different geometries for porous media, based on parameterizable topologies. These porous matrices will then be 3D printed via metal additive manufacturing;
- prepare instrumentation for the SSEXHY explosion test bench, featuring a visualization section using a Schlieren technique coupled with an ultra-fast camera capable of several million images per second;
- post-process the results of the shock and pressure sensors and the OH*-filtered photomultipliers.

2/ secondly, via numerical simulations of the DNS or LES type on research calculation codes. For example, we might be interested in :
- the influence of porous obstacle geometry (shape, porosity, hydraulic diameter, etc.) on flame propagation speed and deflagration<->detonation transitions;
- the influence of the 2/3D character of porous materials;
- the proposal of new criteria for selecting mesh refinement levels to capture the phenomena of interest.

3/ Finally, theoretical modelling of the problem from the point of view of volume-averaged equations will be carried out, with the aim of developing simplified, predictive models of the behavior of porous flame arresters.