Laboratory Study of Upward Infrared Radiation from Flaming and Smouldering Combustion of Natural Fuels

Most wildfires combine two types of combustion, flaming and smouldering, which are fundamentally different in chemical and physical terms, but can transition from one to the other. Flaming is characterised by high spread rate but short duration, whereas smouldering has a low spread rate but long duration. The distinction between them is therefore important for understanding wildfires, as well as for their detection and mitigation. Many current technologies for remote sensing of wildfire use infrared sensors but do not address the distinction between flaming and smouldering, and therefore false negatives in the detection of smouldering hotspots remain a technological challenge. In this work, we investigated the upward infrared radiation from smouldering peat, representing coarse fuels, and flaming pine needles, representing fine fuels, under laboratory-controlled experiments. We measured the radiation from a top view of the burning samples at eight different wavelengths commonly used in remote sensing of wildfires, spanning the short-wave (SWIR), mid-wave (MWIR), and long-wave (LWIR) infrared. We found that the spectral intensity of flaming is between 2 and 7 times higher than that of smouldering at all wavelengths. The highest flaming-to-smouldering intensity ratio was found in the mid-wave infrared, ranging from 4.4 to 6.5, making it the range most effective for distinguishing the combustion type. While both flaming and smouldering were detectable in the experiments, the low spectral intensity of smouldering, combined with the low spatial resolution of most remote sensing devices and the potential obscuration of the ground, might hinder detection in the field. Despite these limitations for field detection, our study provides new insights for the infrared characterisation of smouldering fires and for distinguishing them from flaming.