Year of Award

2025

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Forest and Conservation Science

Department or School/College

W.A. Franke College of Forestry and Conservation

Committee Chair

Lloyd Queen

Commitee Members

Elizabeth Dodson, Kelsey Jensco, Anna Klene, Bret Butler

Keywords

Fire Behavior, fire environment, radiant heat transfer, convective heat transfer, wildland fuels, prescribed fire

Abstract

Extended fire seasons and increased fire severity in and around protected ecosystems and the wildland urban interface (WUI) will increasingly challenge fire suppression, impact communities and threaten property and resources. Addressing this growing threat through proactive management solutions that protect values at risk requires improved fire behavior prediction capabilities.

Here I explore the radiant and convective energy transfer mechanisms of fire ignition or extinction associated with fuel discontinuities. Through a series of wind tunnel experiments I identified energy and ignition thresholds unique to changing wind speed, fuel moistures and fire line characteristics, and developed a multi-variate linear regression model to aid fire management design and develop fuel treatments based on these results. Although radiation has historically been considered the driving force behind fire ignitions, I discovered that convective flame impingement frequency is critical for fire ignitions across fuel discontinuities. Additionally, I determined that the radiant and convective energy at the leading edge of a fuel gap are relatively equal but significantly differ at the leading edge of the receiving bed in favor of radiation by a ratio of 9:1. Even though the convective energy is approximately 10%, it still represents a significant difference in the Cross vs No Cross behavior. I determined that heat flux ≤35kW/m2 never results in a receiving bed ignition, peak heat flux > 80 kW/m2 always results in a receiving bed ignition and heat flux in between seemed to have a probability aspect of whether the fire would cross or not.

The patch fuel bed experiments were designed to simulate fuel mosaics and represent fuel discontinuities on the landscape. The results show a diminished energy threshold effect associated with fuel bed length and width. I concluded that the combined full bed and patch fuel bed experiments are statistically significant for predicting fuel gap thresholds based on wind speed, fuel bed width and fuel moisture. Furthermore, I learned that consecutive fuel breaks reduce the total energy at the second gap due to limited thermal energy, with radiation being reduced by half and a 7-fold reduction in convection. Finally, I tested the results developed from the laboratory experiments in the field at the tallgrass prairie to determine scalability. The peak flux measured from the field ranged from 60-140 kW/m2 as compared to peak heat flux for the wind tunnel experiments of 10-100 kW/m2. Using the trailing edge 35kW/m2 No Cross limit, I concluded from my model a maximum fuel break threshold to range from 3-6 meters.

The findings from this effort have identified key physical processes driving ignition as a function of fuel spatial distribution under a range of environmental conditions. Knowledge of the physical processes controlling gap bridging will substantially enhance our ability to predict fire ignition and the rate of spread in discontinuous fuels under a wide range of fuel and environmental conditions. New knowledge regarding the physical processes controlling ignition will facilitate development of operational models that can more realistically predict fire behavior and rate of spread in discontinuous fuel.

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© Copyright 2025 Daniel Michael Jimenez