UC Merced

Wildfires continue to pose increasing risks to ecological systems, human safety, and infrastructure, particularly in fire prone regions such as California. A substantial fraction of wildland and wildland-urban interface (WUI) fires are directly linked to human activities, including construction operations like welding and grinding, which generate hot metal particles capable of igniting dry vegetation. This dissertation systematically investigates the ignition behavior of grassy wildland fuels subjected to heated metal particles under varying fuel structures, particle properties, and wind conditions, including both steady and gusty flows. Through a combination of controlled bench-scale experiments, thermogravimetric analysis (TGA), and parametric studies, the research presents a unified understanding of the factors governing ignition, smoldering flaming (StF) transitions, ignition behavior, and fire spread dynamics across different ignition regimes.Initial experiments focused on the fundamental ignition behavior of wildland grasses such as Bromus, Avena wildoats, and Tall Wheatgrass-when exposed to stainless steel particles of varying size, carbon content, and quantity. Key findings indicate that increasing the number and diameter of particles significantly reduces the ignition temperature required for both flaming and smoldering ignitions. Furthermore, the carbon content of the particles was found to inversely correlate with ignition sensitivity: lower carbon stainless steel (e.g., 304C) induced ignition at lower temperatures due to superior thermal conductivity and faster heat transfer to the fuels. Among the fuels tested, Bromus exhibited the lowest ignition threshold (~320°C), supporting primarily flaming ignition, while Avena and Wheatgrass supported mostly smoldering ignition and required higher temperatures (380–400°C). The lignin content emerged as a key determinant: higher lignin in wheatgrass (20–22%) increased ignition temperature, reduced spread efficiency, and favored smoldering. This relationship between chemical composition specifically lignin and protein content and ignition behavior and the fuel characteristics were further supported by TGA data analysis.Building on the earlier findings, a follow-up study investigated the impact of fuel structure natural, shredded, and powdered form, on ignition behavior using the same species. Experimental results demonstrated that powdered fuels ignited at the lowest temperatures and exhibited the shortest burnout times due to enhanced surface area and heat transfer efficiency. Conversely, natural fuels showed greater variability in ignition behavior, supporting all ignition types including sustained smoldering and StF transitions.This work showed that fuel structure modulates combustion intensity and ignition pathway. Powdered fuels, while igniting easily, did not support prolonged smoldering due to compactness and limited oxygen penetration. Shredded fuels presented intermediate behavior. TGA further validated these differences, revealing distinct degradation profiles, with bromus undergoing mass loss at lower temperatures (~180–200°C), while wheatgrass decomposed above 270°C with higher DTG peaks, indicating greater thermal stability.Recognizing that real-world ignitions rarely occur in still air, a third set of experiments introduced controlled wind speeds (0.5–2.0 m/s) to assess their effect on ignition dynamics. As wind speed increased, time required for ignition, transition and burnout times decreased across all fuels, while peak ignition temperatures and heat release rates rose sharply. For example, in excelsior fuel, flaming ignition temperature increased from ~330°C to ~500°C between 0.5 and 2.0 m/s, while the HRR surged from ~3000 J/s to ~5200 J/s. The wind's dual role was evident: it enhanced oxygen availability and convective heat transfer but also caused rapid burnout by accelerating combustion kinetics. In fuels like avena and wheatgrass, higher wind speeds facilitated earlier StF transitions by raising pyrolysis rates and supporting faster oxidation. Rate of spread (ROS) was significantly influenced, with values reaching up to 1.2 cm/s in excelsior at 2.0 m/s wind.To simulate realistic, transient wildfire scenarios, the study introduced wind gusts (frequencies of 0.03–0.07 Hz) over constant wind speeds. Results showed that higher gust frequencies led to faster ignition, shorter transition times, and higher HRR. For instance, in wheatgrass, StF transition times dropped from 135 to 70 seconds and HRR rose from ~2000 J/s to ~2600 J/s as gust frequency increased from 0.03 to 0.07 Hz. Gusts enhanced combustion intensity by disturbing boundary layers, improving airflow penetration, and re-igniting partially extinguished smoldering fuels. The combined effect was a substantial reduction in ignition time and burnout variability. These results stress the importance of accounting for wind transience in fire prediction models, especially in WUI zones where fuel continuity and gusty conditions amplify ignition potential.This thesis presents a multi-dimensional view of ignition behavior in wildland fuels, highlighting how ignition is governed by an interplay between fuel chemistry, morphology, heat source characteristics, and environmental conditions. The combined results lead to several critical insights:Hot Particle Properties: Larger, low-carbon particles promote more efficient ignition via greater thermal mass and conductivity. Fuel Structure: Powdered fuels ignite at lower temperatures but offer limited combustion duration; natural fuels provide a fuller range of ignition types. Fuel Chemistry: Fuels with higher lignin and protein contents (e.g., wheatgrass) are more resistant to ignition due to higher activation energy requirements and reduced char reactivity.Wind and Gusts: Both steady wind and gusts accelerate ignition and transition dynamics, alter fire spread rates, and significantly influence HRR and combustion stability.This comprehensive body of work enhances predictive understanding of fire initiation from anthropogenic sources like construction sparks and provides valuable input for fire mitigation policies. It supports the development of safer operational guidelines for equipment usage near wildlands, improved fire spread models, and early-warning systems in WUI regions.