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.

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Smoldering combustion of natural organic layers like peatlands leads to the largest fires on Earth. The multidimensional smoldering structure of peatlands is observed in the field, but the mechanism is poorly understood. In this work, a 2-D cylindrical axi-symmetric model that considers volume shrinkage is developed for the natural downward smoldering of peat with different inorganic contents. Numerical results show that the model captures the two-dimensional multisubfront structure of the peat smoldering process. For the peat with lower inorganic content, the predicted bidimensionality of wall-attachment is more obvious, the mass loss rate and the spread rate are higher, and the smoldering front is thinner. For the peat with higher inorganic content, smoldering is more sensitive to lateral heat loss. Both the inorganic content and the lateral heat loss have negative effects on the volume shrinkage, peak temperature, and in-depth spread rate. Additionally, two groups of experiments using peat samples with added inorganic contents of 0% and 66.7% were conducted in an uninsulated furnace. Comparison of the numerical and experimental data shows that the model predicts the smoldering characteristics very well at lower inorganic content and qualitatively well at higher inorganic content. The predicted effects of the inorganic content and lateral heat loss on the peat smoldering process provide a theoretical basis for fire-fighting measures of smoldering peatlands with different levels of inorganics.

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