Determination of smoldering time and thermal characteristics of firebrands under laboratory conditions

Abstract

The laboratory experiment was conducted to simulate the transfer of smouldering particles produced in forest wildfires by a heated gas flow. The pine bark pieces with the linear dimensions L=(15; 20; 30) mm and a thickness of h=(4−5) mm were selected as model particles. The rate and temperature of the incident flow varied in the range of 1–3 m/s and 80–85 °C, respectively. The temperature of the samples was recorded using a thermal imager. To determine the minimum smouldering temperature of pine bark, the thermal analysis was conducted. The minimum smouldering temperature of pine bark was found to be 190 °C. This temperature will cause thermal decomposition of bark only at the first stage (oxidation of resinous components). In the study the smouldering time, the temperature and the weight of samples were obtained and analyzed under various experimental conditions. The data analysis shows that the increase in the particle size leads to the decrease in their mass loss, and the rate change of the incident flow does not practically influence the mass change. For particles with the linear dimensions of 10 mm and 20 mm, the mass varies from 6% to 25%. The maximum mass loss is observed for the flows with a rate of 1 and 2 m/s. The results have shown that the increase in the particle size leads to the increase in the smouldering time. The position of the particle plays an important role, the effect of which increases with increasing the particle size. The calculations showed that the smouldering time of bark samples is long enough for the particles to serve as new sources of spot fires. The particles were found to be transported to a distance of 218 m from the fire line which can certainly influence the propagation of the fire front.

Introduction

During wildfires a huge amount of smouldering particles is produced and transported by the convection column and the wind [1], [2], [3] which leads to the formation of new spot fires and the change in the fire behavior. According to [4], spotting can be classified into three categories, depending on the distance and the distribution density: short distance spot fires (up to 750 m) initiated by smouldering firebrands produced by the fire front, average distance spot fires (1000–1500 m) and large distance spot fires (>5000 m) initiated by smouldering firebrands involved in the convection column. Earlier, physical models of the fire front propagation only considered convection and radiation. In the last decade new models looked at the medium and large distance spotting [5], [6]. However, physical models that would consider the effect of smouldering particles on short distance spotting are still lacking. The latest results of field experiments have shown that this mechanism is an integral part of wildfire propagation [7], [8], [9]. Therefore, in order to refine mathematical models, it is reasonable to study in detail the process of smouldering and combustion of these particles under the laboratory conditions and evaluate their energy characteristics.

The review of experimental studies has shown that many are devoted to studying the aerodynamic characteristics of particles, with only a few devoted to the specifics of smouldering in the air flow [10], [11], [12], [13]. These studies are focused on the determination of flight paths and lifetimes of burning particles. There is no assessment of temperature characteristics for the particles, and smouldering time is determined visually. In order to address these issues, a laboratory experiment was conducted to simulate the transfer of particles by a gas flow where infrared diagnostic methods and thermal analysis were used for the first time to determine the particles temperature, time and characteristics of smouldering in the air flow.