Critical Ignition Conditions of Wood by Cylindrical Firebrands

Hamed Salehizadeh,Raquel S P Hakes,Michael J Gollner

doi:10.3389/fmech.2021.630324

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.

Highlights

Over the past few decades, losses from fires at the wildland-urban interface (WUI) have increased dramatically (Caton et al, 2016)
A comparison between heat fluxes measured by the 1.27 cm water-cooled heat flux gauge (WC-HFG) and an array of 16 thin-skin calorimeters (TSCs) found that heat fluxes measured by the WC-HFG were slightly lower than those measured by the TSCs (Hakes, 2017). It is unclear whether these results indicate cooling from the 1.27 cm WC-HFG or a heating time-lag associated with the TSCs
Heat fluxes and temperatures from the inert tests are compared with time to smoldering or flaming ignition in the ignition tests to shed light on the phenomena leading to ignition of different materials

Summary

Introduction

Over the past few decades, losses from fires at the wildland-urban interface (WUI) have increased dramatically (Caton et al, 2016). While early works by Cohen (2008) and numerous investigations by Maranghides and Mell (2011), Maranghides et al (2013), Maranghides et al (2015), and Maranghides and McNamara (2016) have shown that firebrands are a major source of structural losses, our understanding of the physical mechanisms by which firebrands generate, loft, and eventually ignite structures is still in its early stages (Caton et al, 2016; Manzello et al, 2020). To better understand potential metrics to quantify critical ignition conditions for structural fuels exposed to firebrand piles, this study investigates the heating from and ignition by firebrand piles under wind on a small-scale, building on initial work by Hakes et al (2019). The goal of this study is to quantify heat flux conditions at ignition of dense fuels representative of WUI fuels

Objectives

Methods

Results

Conclusion