For many years, live fuels were treated as moist dead ones when modelling their burning behavior and their role in wildfires has been controversial in the past decades. More recently, researchers have reported discrepancies in the burning of this foliage compared to the dead, for instance, live fuels can sustain and spread the flame at much higher moisture contents, and they burn with popping and snapping sounds. Statistical research of fire spread and correlation of different parameters to the live and dead fuels’ moisture contents was done in the past, however, there is a lack of fundamental understanding of the live fuels ignition and spread. At IR4TD, we have utilized hybrid infrared-schlieren visualization and scaling analysis technique  in live fuel fire research to explain this fundamental difference between the fire-spread behavior of live and dead fuels and clarify previously unexplained phenomena associated with their burning. We discovered using the visualization and theoretical scaling that live fuels exhibit micro-explosive behaviors which can heat the adjacent fuels by intermittent flame impingements; therefore, can spread fire in a discrete mode. This latter conclusion is crucial to fire spread modeling and can explain why live fuels can sustain flame at higher moisture contents and even higher gust conditions when compared to dead fuels. This will also affect what can be used as a live fuel surrogate when the intent is to examine how real fires spread.
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1. Darwish Ahmad, A., et al., Ignition and burning mechanisms of live spruce needles. Fuel, 2021. 304: p. 121371.