Investigation of the Thermal Processes of Cellulosic Fuels: Drying, Pyrolysis, Ignition, and Burning under Varied Physical Conditions

The flammability of small-scale live and dead fuels, especially at the leaf level, is a key factor influencing combustion dynamics in many scenarios, including wildfire behaviour. Given the importance of the flammability of fine fuels in these scenarios, this study investigates the flammability, desiccation, ignition, and thermal behaviour of moist fine fuels within a hot convective flow under varying spatial configurations. These configurations include different orientations of a single leaf-like element relative to the main convective flow and multiple leaf-like elements in different configurations. Convection is considered as it is the dominant heat transfer mechanism for porous fuels, such as are found in wildland environments. Computational Fluid Dynamics (CFD) simulations are employed to numerically examine the various cases. The simulations are carried out using FireFOAM, an open-source solver within the OpenFOAM framework, employing the Large Eddy Simulation (LES) turbulence modelling approach. The PIMPLE algorithm, combining the Pressure-Implicit with Splitting of Operators (PISO) and the Semi-Implicit Method for Pressure-Linked Equations (SIMPLE), is used for pressure–velocity coupling. The pyrolysis process is modelled as occurring perpendicular to the interface between the solid fuel surface and the surrounding hot gas region, to capture the thermal decomposition behaviour of the fuel. A single-step Arrhenius global reaction is used to simulate the chemical conversion of solid fuel into gaseous volatiles and residual char. The effect of different orientations of a leaf-like element relative to the convective flow was investigated over a range of different fuel moisture content values, and was found to have a significant effect on combustion behaviour by altering ignition timing and burning efficiency. Specifically, three different orientations of the leaf-like element were considered: horizontal, inclined (45°), and vertical, and the orientation was found to affect the drying process, pyrolysis rate, ignition, and combustion behaviour of the solid fuel. Five different fuel moisture content values, varying from 4% to 63%, were also considered in combination with the different orientations and were also found to influence the combustion behaviour. These simulations provided fundamental insights into the key physical mechanisms governing heat transfer and combustion at the leaf-scale, which could be extended to more complex fire propagation scenarios. Aerodynamic effects and the influence of turbulent eddies and wake interactions arising from the different spatial distances and proximities of fine fuel elements were also examined in detail from CFD simulations. The impact of these interactions on the thermal behaviour of the fuels is investigated by considering the effect of proximity of a second leaf-like element placed downstream from the first. This so-called Downstream Proximity Effect (DPE) was found to influence different thermal and combustion processes, such as desiccation, ignition, flame structure, temperature increment, volatile emission, char formation, and heat input rate. This study was then extended to investigate the influence of the lateral proximity of multiple leaf-like elements on boundary layer development, the modulation of convective heat transfer, and the drying, pyrolysis, and combustion processes. This so-called Lateral Proximity Effect (LPE) was also found to have a strong influence on the combustion processes such as char formation, volatile emission, and desiccation. Overall, this thesis provides a fundamental investigation into the thermal drivers and physical mechanisms that govern heat transfer and combustion behaviour using computational techniques to provide insights that can be extended to more intricate fire spread conditions.