Intermittent fireline behavior over porous vegetative media in different crossflow conditions

Abstract

Detailed physical understanding of fire spread is important in the face of the increasing frequency of wildland fires around the globe. Historically, fire spread across porous vegetative media has been considered a continuous phenomenon. Most studies neglect the influence of flame pulsations on the ignition of fuel particles ahead of the fire front, hence approximating the fire spread to a steady and continuous process. This research explores the dynamic nature of fire propagation by experimentally examining the instantaneous flame pulsations and their impact on the ignition of virgin fuel particles. Fire spread experiments were conducted over a longleaf pine needle (Pinus Palustris) testbed under varying crossflow conditions. In addition to introducing a flame tilt, the presence of crossflow strongly enhances the pulsating nature of a free-burning fire. The flame region of influence ahead of the fireline was augmented by the flame tilt and flame pulsations thereby leading to point ignitions at a distance. If sustained, these flash ignitions merge with the fireline, leading to flame spread in the form of leaps. Fire behavior was evaluated by conducting detailed image analysis of videos acquired by placing various cameras around the testbed. Additionally, local temperature and flow velocity were measured by placing a series of thermocouple trees and bi-directional probes within the fuel bed. A curved flame profile was observed under wind-aided conditions, and the curvature was seen to increase with the increasing velocity. Alternatively, a flat temperature profile was observed for no wind conditions. Under forced flow conditions, the bi-directional probes within the testbed measured the flow blocking effect (drag forces) and the presence of flame greatly enhanced the local flow velocity.