349613 Seasonal Effects on the Ignition Characteristics of Live Fuels in Wildfires

Monday, November 4, 2013
Grand Ballroom B (Hilton)
Victoria Lansinger, Jonathan Gallacher and Thomas Fletcher, Chemical Engineering, Brigham Young University, Provo, UT

Wildfires are incredibly complex and costly to fight. Models are used to predict fire spread and plan control strategies, but are often inaccurate and computationally expensive. Additionally, much of the research that contributes to fire spread models focuses on dead and dried fuels, while fewer studies have been conducted on the combustion behavior of live, water-rich plants, which make up a significant portion of wildfire fuels. In order to improve fire spread models and understand the physical phenomena governing live fuel combustion, we are studying the effects of seasonal changes in moisture content and different heating modes (convective and radiative heat transfer) on the ignition, burning, and devolatilization characteristics of live samples of several species common to fire-prone areas of the United States. Experiments are conducted in a glass cage over a flat flame burner (FFB) fueled by a stoichiometric mixture of methane and hydrogen gas with a surface temperature of approximately 1000 °C; the apparatus also includes a radiant panel (25-32 W/in2) adjacent to the cage that provides purely radiative heating to the sample. Over one year, a set of small samples (2-4 cm) of each participating species was burned each month, and raw data were collected via a video camera, IR camera, mass balance, and thermocouple to determine flame characteristics, surface temperatures, mass loss profiles, and gas temperatures, respectively. Data were collected and analyzed to determine the relationships between variables such as season, fuel moisture content, physical attributes, ignition time, ignition surface temperature, maximum flame height, flame duration, and mass loss rate.

The first year of experiments, which comprised three species native to the western United States, showed that ignition and burning characteristics depended on season, moisture content, and heating mode, but that each species responded differently. Moisture content varied predictably with season, reaching a peak during the spring and decreasing through the summer and fall. Ignition time and surface temperature behaved similarly, but with more variance, suggesting that moisture content is not the only contributing variable. Mass loss profiles also varied significantly with season and moisture content. Averaged over the year, the data indicated that heating mode (convective heating only with the FFB versus convective heating combine with radiative heating from the radiant panel) had a significant effect for some of the variables. For instance, the addition of radiation clearly decreased ignition time and increased ignition surface temperatures. However, the effect on variables such as maximum flame height and mass loss rate was unclear. The second year of data collection, which comprises seven species native to the western and southern United States, is ongoing. Thus far, the data display the same trends in the effects of season and heating mode, but also reveal regional patterns. Species collected from the same geographical area show similar burning behaviors, even between different types of species. In addition, the second year data includes fuel density measurements that do not correlate well with other physical attributes, but also display regional patterns. The results thus far demonstrate that while both seasonal changes in moisture content and differences in heating mode affect burning behavior, neither factor accounts for all of the discrepancies in the data. Additionally, neither factor appears to represent major changes in the physical phenomena governing the burning characteristics of live fuels.

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