Interaction between flaming and smouldering in hot-particle ignition of forest fuels and effects of moisture and wind
Supan Wang A B , Xinyan Huang C , Haixiang Chen A D and Naian Liu A
+ Author Affiliations
- Author Affiliations
A State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230027, China.
B Jiangsu Key Laboratory of Hazardous Chemicals Safety and Control, College of Safety Science and Engineering, Nanjing Tech University, Nanjing, Jiangsu 211800, China.
C Department of Mechanical Engineering, University of California, Berkeley, CA 94720, USA.
D Corresponding author. Email: hxchen@ustc.edu.cn
International Journal of Wildland Fire 26(1) 71-81 https://doi.org/10.1071/WF16096
Submitted: 24 May 2016 Accepted: 25 October 2016 Published: 19 December 2016
Abstract
Ignition of natural fuels by hot metal particles from powerlines, welding and mechanical processes may initiate wildfires. In this work, a hot steel spherical particle (6–14 mm and 600–1100°C) was dropped onto pine needles with a fuel moisture content (FMC) of 6–32% and wind speed of 0–4 m s–1. Several ignition phenomena including direct flaming, smouldering and smouldering-to-flaming transition were observed. The critical particle temperature for sustained ignition was found to decrease with the particle size (d) and increase with FMC as 
(°C), and the maximum heating efficiency of particle was found to be 
. As the particle size increases, the influence of FMC becomes weaker. The flaming ignition delay times for both direct flaming and smouldering-to-flaming transition were measured, and decreased with particle temperature and wind speed, but increased with FMC. The proposed heat-transfer analysis explains the ignition limit and delay time, and suggests that the hot particle acts as both heating and pilot sources like a small flame for direct flaming ignition, but only acts as a heating source for smouldering. This study deepens the fundamental understanding of hot-particle ignition, and may help provide a first step to understanding the mechanism behind firebrand ignition.
Additional keywords: ignition delay, ignition limit, pine needles, smouldering-to-flaming transition, spotting.
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