Studies of Wildland Fires: Inclined Flame Instabilities and Fire Exposure to Structures

Abstract

The present work investigates two aspects of the wildland fire problem: instabilities occurring in inclined laminar flames and the coupling between fire exposure and structure loss. The first study analyzes the structure and stability of a laminar diffusion flame that forms either on the topside or the underside of a semi-infinite inclined fuel surface. Experiments have found substantial structural differences between flames developing on the upper and lower sides of an incline. The effect of instabilities on flame morphology is investigated as they influence the development of structures, such as peaks and troughs in the flame, observed downstream. These structures influence heat transfer processes that govern pre-heating of downstream fuels and, as a result, drive flame spread. The solution of the stability eigenvalue problem determines the downstream location at which the flow becomes unstable. The analytical solution finds that instabilities emerge farther downstream in the flame for underside flames than for topside flames, in agreement with existing experimental observations.

The latter study applies reconstruction modeling to reproduce exposure conditions to structures from a wildland fire using the 2017 Northern California Tubbs Fire as a case study. The reconstruction simulates the distribution of embers, expanding the ability of fire reconstruction to represent conditions during the fire event which are not represented by the flaming fire front. Results from the Tubbs Fire simulation are used to provide exposure conditions to investigate the relation between exposure conditions, structure characteristics, and the damage sustained by a structure in the fire event. A methodology using fragility curves to estimate the probability of destruction, used for risk analysis in other disaster fields, is modified and developed here for application to wildland-urban interface fires. Results of the fragility analysis find that increased flame and ember exposure increase the likelihood of damage or destruction; however, there is a stronger relationship between ember exposure and destruction than between flame length and destruction. Relatively low levels of ember exposure still result in relatively high likelihoods of destruction, highlighting the importance of ember spread. Current models cannot model structure-to-structure fire spread; additional limitations are highlighted for future work.