The effect of moisture content and thermal behaviour on the ignition of

Abstract

Fuel moisture content is one of the key parameters controlling the flaming ignition of wildland fuel. However, the role of fuel moisture content in assessing the flammability of different fuel curing (dead and live fuel) is still not well understood. This paper presents the results of ignition tests of fuel beds consisting of dead and live Eucalyptus saligna leaves under a wide range of moisture contents. External heat flux and fuel moisture content are shown to significantly influence time to ignition and mass loss rate at the ignition of Eucalyptus saligna leaves, thus illustrating distinctive heating processes in the fuel bed. The thermal behaviour of the leaf bed before ignition is analysed using the analytical solution to the heat conduction equation, as the classical ignition correlations yield inconclusive results. This approach allows identification of thermally thick and thin behaviours for distinct ranges of heating, with the transition (thermally intermediate) region observed at higher external heat fluxes for higher moisture content. Additionally, a flammability assessment based on time to ignition confirms the inadequacy of the common assumption that live fuel can be considered as moist dead fuel.

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

“…As expected, increasing the moisture content and thickness of foliage increases the ignition time of live fuels [17][18][19][20][21]. Moisture evaporating from fuels can impact burning by diluting flammable pyrolyzates (i.e., gas around the live fuel), absorbing thermal energy, possibly altering combustion reactions, and ultimately delaying the time to ignition in dead and live fuels [11,[17][18][19][20][21][22]. In some instances, live fuels have displayed blistering on the surface of the fuel, droplet ejection, or bursting of the structure while burning [19][20][21][22].…”

“…As a result, many studies have investigated the influence of LFMC on the flammability of fuels. As expected, increasing the moisture content and thickness of foliage increases the ignition time of live fuels [17][18][19][20][21]. Moisture evaporating from fuels can impact burning by diluting flammable pyrolyzates (i.e., gas around the live fuel), absorbing thermal energy, possibly altering combustion reactions, and ultimately delaying the time to ignition in dead and live fuels [11,[17][18][19][20][21][22].…”

“…Live fuel moisture content (LFMC) plays an influential role in the ignition and burning behavior of fuels [11][12][13][14][15][16][17][18][19][20][21][22][23]. As a result, many studies have investigated the influence of LFMC on the flammability of fuels.…”

“…Moisture evaporating from fuels can impact burning by diluting flammable pyrolyzates (i.e., gas around the live fuel), absorbing thermal energy, possibly altering combustion reactions, and ultimately delaying the time to ignition in dead and live fuels [11,[17][18][19][20][21][22]. In some instances, live fuels have displayed blistering on the surface of the fuel, droplet ejection, or bursting of the structure while burning [19][20][21][22]. Note, however, that changes in ignition phenomena caused by differences in species composition, moisture content, or environmental conditions have not been established.…”

A Review of Leaf-Level Flammability Traits in Eucalypt Trees

“…The relationship between FMC and ignitability seems to be linear, with higher FMC leading to longer time to ignition (i.e., reduced flammability) [40]. Researchers [51] found that higher FMC in fresh leaves and dead leaves (i.e., litter) led to reduced ignitability regardless of the intensity of the heat flux. Specifically, it was found that fresh eucalypt leaves required significantly higher temperatures of ignition (472 • C vs. 377 • C) and heat fluxes (24.5 kWm −2 vs. 15.5 kWm −2 ) when compared with dry leaves.…”