Dissertation Defense – Raquel Hakes

Title: Studies of Inclined Flame Instabilities and the Relationship Between Wildland Fire Exposure and Structure Destruction

Day/Time: Friday, July 23, 2021 at 12:00 pm EST

Committee members: 
Dr. Michael Gollner (Chair)
Dr. Anya Jones (Dean’s rep)
Dr. Ken Kiger
Dr. Johan Larsson
Dr. Elaine Oran, Arnaud Trouvé

Abstract: The present work investigates two aspects of the wildland fire problem: the structure and stability of inclined flames and the application of reconstruction fire modeling to a new methodology of structure level risk analysis in the wildland-urban interface. The work analyzes the structure and stability of a laminar diffusion flame that forms either on the top of or beneath a semi-infinite inclined fuel surface. Experiments have found substantial structural differences between flames developing on the upper and lower sides of an inclined fuel surface. These differences cannot be explained with current analytical models of steady semi-infinite flames, which provide identical solutions for both configurations. The role of instabilities is investigated as they influence the development of structures, such as peaks and troughs in the flame, observed downstream after the transition to turbulence. These structures influence heat transfer processes that govern pre-heating of downstream fuels and, as a result, drive flame spread. The formulation used to investigate instability formation utilizes the limit of infinitely fast reaction, taking into account the non-unity Lewis number of the fuel vapor. The solution of the stability eigenvalue problem determines the downstream location at which the flow becomes unstable, characterized by a critical Grashof number. The analytical solution finds that instabilities emerge downstream in the flame, developing farther downstream on the lower side of the incline as compared with the flames developing on the upper side of the inclined surface, in agreement with existing experimental observations.

The latter study investigates the use of 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, small pieces of burning material or “firebrands” lofted in the fire plume, which can ignite upon deposition. Including embers expands 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 in a subsequent study investigating 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 fire exposure (represented as flame length) 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 damage or destruction. It is also found that relatively low levels of ember exposure still result in relatively high likelihoods of destruction, highlighting the importance of ember spread. Limitations still exist, such as the inability to model structure-to-structure fire spread, but are highlighted as needed for future work.