Ignition and Spread of a smouldering Front in Porous Media Application to Zombie Fires Issue

**Ignition and spread of a "smouldering" front in porous media — Application to zombie fires issue —**: - Réf **ABG-129314** - Sujet de Thèse - 10/03/2025 - Autre financement public - Université de Poitiers - Institut PPRIME - Lieu de travail- Poitiers - Nouvelle Aquitaine - France - Intitulé du sujet- Ignition and spread of a "smouldering" front in porous media — Application to zombie fires issue — - Champs scientifiques- Sciences de l’ingénieur - Numérique - Physique - Mots clés- Fires, porous media, pyrolysis, numerical simulation, OpenFOAM **Description du sujet**: **Ignition and spread of a "smouldering" front in porous media** **— Application to zombie fires issue —** **1. Background** The risk of wildfires is particularly high in the Nouvelle-Aquitaine region of France. In recent years, several large-scale fires, such as the Landiras fires of 2022, have underscored the severity of this issue. Due to the geological specifics of certain areas in the Nouvelle-Aquitaine region, characterized by the presence of peat and lignite, a new type of fire propagation has been observed **:"the zombie fires"**. These underground fires pose a major challenge due to their difficulty in detection and control. They can reignite surface fires initially considered extinguished, as was the case in Landiras in 2022, where a resurgence led to the destruction of an additional 7,400 hectares. To address these challenges, the Nouvelle-Aquitaine region funds the PSGAR (Large - Scale Scientific Project) GRIFON project (Management of Multiple Forest Risks in Nouvelle - Aquitaine), which includes this work on zombie fires. **2. Research focus and methodology** The research project focuses on the experimental and numerical study of "underground" fires, also known as "zombie" fires. This type of fire typically develops in peat-rich soils with combustible organic matter, such as lignite. The underground combustion is governed by several coupled phenomena: - **Heat and mass transport** in porous media - **Pyrolysis reactions** of organic matter - **Oxidation reactions (smouldering)** involving oxygen diffusion within the porous medium (soil) This type of reaction, termed "smouldering" is characterized by slow propagation kinetics, facilitated by the highly exothermic nature of the process. These fires can persist for extended periods, even under unfavorable conditions (moisture, absence of external heat flux) and, in certain circumstances, reignite surface fires. The adopted methodology is based on a detailed study of three key processes: 1. **Ignition** of the smouldering process 2. **Propagation**of the reaction front (smouldering + pyrolysis) 3. **Surface ignitio**n and transition to a crown fire These three aspects will be studied using a combined approach: - An **experimental study** at the laboratory scale, aimed at reproducing and analyzing the development conditions of zombie fires in a controlled environment - A **numerical study** aimed at modeling underground fire propagation using advanced simulation tools **2.1 Experimental study** The study involves the design and instrumentation of an experi - mental setup ("zombie bench") to reproduce conditions conducive to the smouldering process in peat soils. The setup will include instrumentation combining several measurement methods: - **High-speed and thermal (IR) cameras**: to analyze the spatiotemporal dynamics of ignition and propagation of the smouldering front - **Radiative and convective flux meters**: to characterize the thermal balance at the surface of the porous medium - **Thermocouples**: to measure the thermal gradient within the sample and identify critical ignition temperatures - **Gas analyzers (Fourier Transform Infrared - FTIR)**: to monitor pyrolysis and oxidation reactions in real-time The experiments will aim to: 1. Study the influence of environmental conditions (temperature, humidity, heat flux) on the ignition of the smouldering front 2. Characterize propagation mechanisms based on the properties of the porous medium (porosity, composition,...) 3. Analyze conditions favoring the ignition of surface vegetation cover **2.2 Numerical study** The numerical approach involves the development and validation of an underground fire propagation model. This model will be implemented within the PATO (Porous Analysis Toolbox based on OpenFOAM) simulation code, originally developed by NASA for thermal shield calculations of spacecraft. Specific models have been implemented in the PATO code to address biomass combustion and fire propagation issues. The modeling will follow a multi-scale approach to account for the various physicochemical phenomena involved: —** Microscopic scale** ( 1 mm) : characterization of reaction kinetics by thermogravimetric analysis (TGA) - **Mesoscopic scale** ( 10 cm) : characterization of heat and mass transfers using a cone calorimeter. - **Macroscopic scale** ( 1 m) : complete modeling of the propagation process, considering couplin