Convection Ignition of Live Forest Fuels

Abstract

Wildland fires are an extremely costly and deadly problem. Crown fires, where live foliage ignites and burns, are particularly unpredictable -in part because live fuel ignition and combustion is poorly understood. Many wildland fire models assume radiation is the controlling heat transfer mechanism. However, there is a growing indication that radiation is insufficient to ignite the small, thin fuel particles that carry a wildland fire and that convective heating and flame bathing is a critical component. Unfortunately, ignition by convection heating of any fuel is poorly understood. Ignition of live forest fuels by any means is also completely unknown due to complicated moisture content and fuel chemistry considerations. To gain some insight into the wildland fire problem, an apparatus was built using two 6.5 kW electrical heaters to heat gas (air, nitrogen, etc.) over a range of temperatures from ambient up to 1200°C. The flow rate of these "airtorches" is adjustable. This apparatus was used to convectively ignite a range of both live and dead forest fuels. Fuels from all over the United States where used including Southern California, Utah, Florida, and Montana. To examine ignition threshold conditions and to have distinguishable differences in ignition times, air temperatures of 500°C and 600°C were used. The airflow rate varied slightly from 1.3 m/s to 1.4 m/s due to the density difference. Because live forest fuels contain large amounts of water, the evolution of both water and carbon dioxide was measured with time using a differential gas analyzer. Flaming ignition was seen for all dead fuels at 500°C, but the live fuels mostly showed glowing ignition. At 600°C, all fuels showed flaming ignition within 1-26 sec. Interestingly, all live fuels were still actively releasing water at ignition, implying there are steep temperature gradients within these physically thin fuels (i.e. not thermally thin). Simple heat transfer analysis in conjunction with the water evolution information was used to help explain the differences in ignition times due to fuel geometry.

Flammability of Two Mediterranean Mixed Forests: Study of the Non-additive Effect of Fuel Mixtures in Laboratory

“…Moreover, the flattened arrangement of the Qi leaves in the sample holder of the MLC device might have altered the transmission of the radiant heat through the sample, increasing the time for ignition. Reduced ignitability of fine forest fuels when submitted to radiant heat flux was experimentally observed by McAllister and Finney (2014) . Discrepancy observed at the laboratory scale comparing results from devices using radiant heat (i.e., MLC) to those using convection were partially attributed by these authors to the separation by air space of the typical forest fine fuel and because some particles can block a fraction of radiation.…”

Phases and Time-Scales of Ignition and Burning of Live Fuels

“…In some experiments, eruptive jetting occurred during transition and/or droplet burning phases. Eruptive jetting refers to the rapid ejection/burning of the gases from a needle/leaf to the surroundings [35,36]. An example of this process is shown in Fig.…”

“…However, LFMC might not be the only parameter for predicting the occurrence of some fires [18,19,27]. Other parameters that affect the burning of live fuel, such as foliar (leaf) chemistry [29][30][31][32][33] or heat flux rates [34][35][36][37], should be investigated [10,28]. Of note to this work, two modes of heat transfer are dominant in the spread of wildfires.…”

Mathematical Simulation of Forest Fuel Pyrolysis in One-Dimensional Statement Taking into Account Soot Formation

“…Experimental studies devoted to the investigation of the biomass ignition and combustion processes are also of great importance. The obtained data make it possible to determine the typical temperatures and ignition delay times of the samples and to estimate the regularities of this process [23][24][25][26][27][28][29][30][31][32]. It was noted that the ignition of living and dead fuels occurs in different ways due to the moisture content in the samples.…”