Upslope fire spread over a pine needle fuel bed in a trench associated with eruptive fire

Abstract

Eruptive fire is a typical extreme fire behavior in wildland fuels, characterized by a sudden acceleration of fire spread over confined slopes in trench-like terrain (e.g. in a deep canyon). However, wildfire spread acceleration in trench configuration has received little study. This paper presents a systematic experimental study on the fire spread over a pine needle fuel bed in an inclined trench. The effects of aspect ratio (ratio of the trench side wall height to the trench width) and slope angle on the fire spread behaviors are investigated. It is indicated that flame attachment toward the fuel bed surface is induced by the combined effects of the slope and the air entrainment restriction due to the trench configuration. The flame attachment occurring at higher slopes with higher aspect ratios induces an acceleration of fire spread, causing the fire spread to undergo a rapid transition from a steady phase with lower rate of spread (ROS) to quick spread phase with much higher ROS. The fire spread acceleration induced by the flame attachment leads to remarkable enhancement of burning intensity, as verified by long flame depth and high mass loss rate during fire spread. The fuel consumption efficiency decreases almost linearly with increasing slope angle. Under higher slope angles (higher than 20○ in this work) the flame tilting induces hot gas flow ahead of the flame front, which enhances the convective heating. When flame attachment occurs, the spatial influence range of convective heating increases sharply. It is concluded that for eruptive fire, radiative heating and convective heating play a comparable role in fuel preheating. Experimental data suggest that when flame attachment occurs the convective heating is greatly enhanced and becomes an important mechanism of fuel preheating. This is inferred to be a major potential mechanism for eruptive fire in trench configuration.