Effects of Moisture on Combustion of Live Wildland Forest Fuels Effects of Moisture on Combustion of Live Wildland Forest Fuels

by Pickett, Brent M

Current operational wildland fire models are based on numerous correlations from experiments performed on dry (dead) fuel beds. However, experience has shown distinct differences in burning behaviors between dry and moist (live) fuels. To better understand these fundamental differences, an experiment was designed to use a flat-flame burner to simulate a moving fire front which heated and ignited a stationary, individual fuel sample. Samples included various U.S. species from the California chaparral, the intermountain west, and the southeastern regions. Temperature, mass, and video images were recorded throughout each experimental run from which numerous data values were obtained such as time to ignition, ignition temperature, flame height, time of flame duration, and mass release rates.

Qualitative results showed various phenomena such as color change, bubbling, bursting, brand formation, and bending; these phenomena were species-dependent. Quantitative results showed differences in the ignition values (time, temperature, and mass) among species. It was observed that all moisture did not leave the interior of the sample at the time of ignition. Also, from the temperature history profiles, no plateau was observed at 100°C, but instead at 200-300°C. This indicates a need to treat evaporation differently than the classical combustion model. Samples were treated with solvents in attempt to extract the cuticle from the surface. These treated samples were compared to non-treated samples, though no significant combustion characteristics were observed. The time of color change for the treated samples varied significantly, indicating that the cuticle was indeed removed from the surface.

Two-leaf configurations were developed and compared to determine combustion interactions between leaves. A second leaf was placed directly above the original leaf. Results showed that the time of flame duration of the upper leaf was significantly affected by the presence of the lower leaf. Causes for the prolonged flame were found to be the consumption of O2 by the lower leaf and the obstruction provided by the lower leaf, creating a wake effect which displaced hot gases from the flat-flame burner as well as entrained surrounding room temperature gas.

A semi-physical model based on fluid dynamics and heat and mass transfer was developed that included the observed plateau at 200-300°C, rather than at 100°C; this was done for both the single- and two-leaf configurations. Another model using a statistical approach was produced which described the combustion of a bush that incorporated data obtained from the experimental results. Overall burning times and percentage of fuel consumption were obtained for various fuel loadings using this statistical model.