Critical Ignition Conditions of Wood by Cylindrical Firebrands

Abstract

This study investigated the thermal conditions preceding ignition of three dense woody fuels often found on structures by firebrands, a major cause of home ignition during wildland-urban interface (WUI) fires. Piles of smoldering cylindrical firebrands, fabricated from wooden dowels, were deposited either on a flat inert surface instrumented with temperature and heat flux sensors or on a target fuel (marine-grade plywood, oriented-strand board, or cedar shingles) to investigate critical conditions at ignition. The former provided thermal data to characterize the time before and at ignition, while the latter provided smoldering and flaming ignition times. Tests were conducted in a small-scale wind tunnel. Larger firebrand piles produced higher temperatures at the center of the pile, thought to be due to re-radiation within the pile. Ignition was found to be dependent on target fuel density; flaming ignition was additionally found to be dependent on wind speed. Higher wind speeds increased the rate of oxidation and led to higher temperatures and heat fluxes measured on the test surface. The heat flux at ignition was determined by combining results of inert and ignition tests, showing that ignition occurred while transient heating from the firebrand pile was increasing. Ultimately, critical ignition conditions from firebrand pile exposure are needed to design appropriate fire safety standards and WUI fire modeling.

Ignitibility of structural wood products exposed to embers during wildland fires :

“…Hakes et al [29] and Salehizadeh [31] measured temperatures in a pile of embers using type K thermocouples and thin skin calorimeters. They recorded higher temperatures with larger piles and at the center of the pile.…”

“…However, it is the heat transfer from the ember to the combustible substrate it lands on that determines the ignition propensity of the substrate. Following the pioneering work by Manzello et al [21] and researchers [29,31,32] at the University of Maryland, recent studies [34,87] have reported on high resolution heat flux measurements from smoldering embers.…”

“…However, most of these data are qualitative, and the studies largely report on ignition probabilities of the fuel studied. Very recently, the research groups at the University of Edinburgh [23], University of Maryland [29][30][31][32], University of California Berkeley [33], and others [34] performed advanced experiments to study thermal characteristics of embers (e.g., ember temperature and heat transfer as well as factors and conditions that affect each one) and identify conditions under which the wooden substrate would ignite. These experiments provide insights into measurement techniques and associated uncertainties.…”

“…As mentioned earlier, embers tend to accumulate on solid substrates, particularly into gaps or re-entrant corner spaces. Hakes et al [29] and Salehizadeh [31] reported a drastic increase in heating from a pile of embers as compared to heating from individual embers; the effect of individual ember diameter was reported to be negligible while studying thermal characteristics of the pile of embers [29,32].…”

“…The above studies indicate that although physical characteristics of individual embers (e.g., size, shape, and mass) influence ember transport and spotting fires in porous vegetative fuels (e.g., pine needle beds), individual embers do not typically ignite combustible solids and these physical properties are not the key characteristics of interest when studying ember piles. Thus, when assessing the ignition of structural components (e.g., solid wooden substrates), properties of ember piles such as total mass, bulk density, pile height, and geometry of the deposited pile -in addition to wind speed [23,29,31,32,34] -play more decisive roles in influencing the thermal behavior (i.e., temperature and heat transfer) of smoldering embers and thus the response of the combustible solids upon which they land.…”