Experimental study of smouldering wildfire mitigation: spread, suppression and transition to flaming

Abstract

Smouldering and flaming combustion are both important in wildfires, and one can lead to the other. Although the characteristic temperature, spread rate and power of smouldering are low compared to flaming combustion, smouldering is far more persistent. Peatland wildfires are dominated by smouldering combustion and have been reported to be some of the largest wildfires in terms of fuel consumption and pollutant emissions. Unfortunately, current fundamental understanding of smouldering wildfire is limited, resulting in ineffective mitigation strategies. In this thesis, novel experiments were developed to study three phenomena: smouldering peat fire, its suppression, and the transition from smouldering to flaming combustion. To understand the behaviour of smouldering under real conditions, the largest to-date peat wildfire experiment was conducted in Sumatra, Indonesia, investigating ignition, spread, emission and suppression. The fire was observed continuously for the first time propagating for 10 days during day and night times, and despite major rainfalls. Lab-scale experiments were conducted to study the suppression under variable flow rate and wetting agents. The minimum suppression column height at the lab-scale agrees with the field-scale experiment. The required volume of suppressant per mass of peat was constant, therefore suggesting a fundamental property, at around 5.7 L/kg-peat despite changes in flow rate and wetting agent concentration. To further understand the transition from smouldering to flaming combustion, a novel experimental rig was designed to investigate the role of wind. Compared to continuous wind exposure, transition from smouldering to flaming occurred earlier under pulse wind and depended less on sample size. The results in this thesis can help to improve prevention and mitigation efforts.