Understanding flame structure in wildfires using Large Eddy Simulations

Abstract

The structure of wildfire flames in the presence of crossflow was analyzed by utilizing suitable non-reacting numerical experiments with low speed flow over a hot isothermal horizontal semi-infinite surface. FireFOAM, a Large Eddy Simulation (LES) based solver developed by FM Global for fire protection engineering applications, was employed for all the calculations. Early-time dynamics of Rayleigh-Taylor Instability (RTI) was first simulated using Direct Numerical Simulations (DNS) so that the solver could be verified against Linear Stability Theory (LST). Then attention was given to late-time dynamics in order understand the different stages (e.g., appearance of secondary instability, generation of larger scales due to interaction between structures) involved in the development of the instability. The onset of thermal vortex instability, in a configuration with low speed flow over a hot isothermal semi-infinite horizontal plate, predicted using DNS was compared with the literature. Spatial evolution of various terms in the streamwise vorticity equation was used to identify the dominant mechanisms responsible for the generation/evolution of vorticity. Streamwise evolution of the instabilities was studied and the effects of the changes in temperature and orientation of the plate on the thermal instabilities were also investigated. Finally, a configuration with low speed flow over a hot isothermal semi-infinite horizontal strip was used to understand the effects of upstream Boundary Layer (BL) height and the length of the strip on both the thin horizontal and larger structures (analogous to Flame Towers (FT) observed in real wildfires and laboratory experiments).