Review of the Transition From Smouldering to Flaming Combustion in Wildfires

Abstract

Wildfires are uncontrolled combustion events occurring in the natural environment (forest, grassland, or peatland). The frequency and size of these fires are expected to increase globally due to changes in climate, land use, and population movements, posing a significant threat to people, property, resources, and the environment. Wildfires can be broadly divided into two types: smouldering (heterogeneous combustion) and flaming (homogeneous combustion). Both are important in wildfires, and despite being fundamentally different, one can lead to the other. The smouldering-to-flaming (StF) transition is a quick initiation of homogeneous gas-phase ignition preceded by smouldering combustion, and is considered a threat because the following sudden increase in spread rate, power, and hazard. StF transition needs sufficient oxygen supply, heat generation, and pyrolysis gases. The unpredictable nature of the StF transition, both temporally and spatially, poses a challenge in wildfire prevention and mitigation. For example, a flaming fire may rekindle through the StF transition of an undetected smouldering fire or glowing embers. The current understanding of the mechanisms leading to the transition is poor and mostly limited to experiments with samples smaller than 1.2 m. Broadly, the literature has identified the two variables that govern this transition, i.e., oxygen supply and heat flux. Wind has competing effects by increasing the oxygen supply, but simultaneously increasing cooling. The permeability of a fuel and its ability to remain consolidated during burning has also been found to influence the transition. Permeability controls oxygen penetration into the fuel, and consolidation allows the formation of internal pores where StF can take place. Considering the high complexity of the StF transition problem, more studies are needed on different types of fuel, especially on wildland fuels because most studied materials are synthetic polymers. This paper synthesises the research, presents the various StF transition characteristics already in the literature, and identifies specific topics in need of further research.

Ignitibility of structural wood products exposed to embers during wildland fires :

“…Santoso et al [14] define the transition from smoldering to flaming combustion as a quick initiation of homogeneous gas-phase ignition following smoldering combustion. During smoldering combustion, the fuel undergoes rapid oxidation followed by a sharp temperature rise.…”

“…Several studies have reported on effects of wind speed on smoldering ignition of substrate fuels including porous fuels [104,105], and solid wooden substrates [14,21,29,31,32,97,98]. It is generally concluded that airflow has two competing effects on the smoldering combustion of fuel substrate.…”

“…This kind of exposurewhere a combustible solid substrate is in direct contact with a smoldering ignition sourceis not included in the existing test methods (except for roofing and decking materials). This is notable because burned structures within the WUI communities have been lost long after they might be considered safe from direct flaming exposures [3,6,8,9,[14][15][16]. Accumulation of embers in a corner or crevice configurations is very likely, and the probability of ignitions in such configurations is very high [11,17].…”

Investigating Coupled Effect of Radiative Heat Flux and Firebrand Showers on Ignition of Fuel Beds

“…2). Figure 2 offers comparison of firebrands in actual WUI fires [30] and firebrands from the firebrand generator. The experimental facility was modified for this work with the new aspect being the addition of a radiant panel to provide radiant heat to the fuel beds.…”

“…Comparison of firebrands in WUI fires (left) and firebrands from a firebrand generator under a 4 m/s wind (right). Image in left is from[30] showing WUI fires in California, USA 2018.effect of radiative heat flux and firebrands on ignition, and therefore the wind speeds were carefully selected. Based on scoping experiments, firebrands are able to ignite fuel beds of wood pieces within 40 min under both wind speeds without radiative heat flux.…”