Experimental investigation of the effects of soil and environmental conditions on smouldering wildfires

Abstract

Smouldering peat-wildfires are the largest fires on earth and are responsible for vast economic damage, negative health effects, and significant quantities of greenhouse gas emissions. Despite their importance, limited research has focused on understanding their dynamics. Here I systematically study the influence of the three most important soil conditions: moisture content, inorganic content, and density, as well as three prevalent natural environmental conditions: wind, slope and temperature. Two novel experimental rigs were developed to study these conditions: the shallow reactor, which facilitating the simultaneous measurement of both horizontal and in-depth spread, and the Experimental Low-temperature Smouldering Apparatus (ELSA) which enables, for the first time, the experimental study of arctic wildfires by studying smouldering in low temperature conditions. The data generated is such that I put forward a new unifying theory of the governing parameters of smouldering spread which explains the influence of all three major soil properties. Horizontal spread was found to be controlled by heat sink density (the energy required to heat the soil to burning temperatures), while in-depth spread was governed by the organic density. The study of wind and slope revealed that forward and uphill spread significantly influenced horizontal spread rates due to improved heat transfer and oxygen supply, while downhill slopes and wind opposite to the spread direction had no significant effect. Evidence was found to suggest that spread on a slope can be explained as a function of the angle of spread direction relative to a horizontal plane. For the first time, I revealed that decreased soil temperatures resulted in deeper depth of burning, and heat losses reduced the critical moisture content from 160% (with an insulated reactor base) to 120% (with a cold reactor base). Spread rate and peak temperature where negligibly affected by soil temperature, even sustaining in frozen soil conditions. This thesis provides a comprehensive study of factors influencing smouldering wildfires, providing insight and data which support a new theory of smouldering spread, improving our understanding of smouldering dynamics.