Effects of Moisture on Combustion Characteristics of Live California Chaparral and Utah Foliage

• Main Author: Smith, Steven G.
• Format: Others
• Published: BYU ScholarsArchive 2005
• Subjects: forest fire; moisture content; ignition temperature; ignition time; wildfire; Chemical Engineering
• Online Access: https://scholarsarchive.byu.edu/etd/318; https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=1317&context=etd

Current fire-spread models are based largely on empirical correlations based on fires burning through dead pine needles. There is a need to increase the accuracy of modeling wildfires in live vegetation. This project investigates the quantitative and qualitative ignition characteristics of eight live fuels, four from southern California (manzanita, scrub oak, ceanothus, and chamise) and four from Utah (canyon maple, gambel oak, big sagebrush, and Utah juniper). Individual leaves were observed as they were exposed to hot gases from a flat flame burner. The broadleaf species from both California and Utah had noticeable surface changes during the ignition process. All fresh samples showed a color change on the leaf surface from a light dusty color to a dark wet color. This is likely due to the melting of the waxy protective layer. Samples of scrub oak, manzanita, ceanothus, canyon maple, and gambel oak at moderate moisture contents (50 to 75%) exhibited bubbling under the leaf surface. Liquid droplets were observed on the surface of Manzanita samples at moisture contents near 75%, while bursting was observed on the surface at moisture contents near 100%. This bursting is due to evaporation of the moisture inside the leaf causing internal pressures to exceed the surface strength of the leaf. Ignition was defined as the time when the first visible gaseous flame was observed near the leaf surface. Measurements of the time to ignition and the temperature at ignition were performed for all broadleaf species. A large degree of scatter was observed in the quantitative ignition data, due largely to variations in leaf thickness and moisture content. Time to ignition was found to correlate with sample thickness and the mass of moisture in the sample. Ignition temperature was constant for varying moisture mass but appeared to increase with thickness. The burning time, defined as the duration of a visible flame near the leaf, was found to correlate roughly with leaf mass. Several types of correlations were made to describe ignition temperature and ignition time as a function of leaf thickness and mass of moisture.