Experimental And Theoretical Study On Spotting Ignition

Spot fire, defined as a foreign body of small size arriving at the recipient fuel outside the main fire spread area and initiating a new ignition, is a fire phenomenon in large scale forest fires or Wildland-Urban Interface (WUI) fires. The foreign body in spot fire, at high temperature or in a state of flaming or glowing combustion, can be the lofted firebrands, the pieces of burning wood, the embers from the arcing between the power lines and trees, or the hot metal particles from fireworks, welding, grinding or arcing processes. The recipient fuel can be wildland fuels, building exterior (roof or wall) or building insulation materials. Spotting ignition by a foreign body is a significant mechanism of fire spread, as observed in many large-scale fires. Spotting ignition of wildland or building fuels is an important potential way leading to large scale forest and WUI fires.Spotting ignition is fundamentally different from the traditional flame contact or radiation driven ignition studied in the literature, and still presents significant knowledge gaps. The aim of the present work is to study the ignition process of building insulation materials and forest fuels by a hot particle and to provide a basic understanding of the ignition mechanism. The work of this thesis is summarized as follows.Thermal decomposition behavior during the ignition and burning processes of the low density building insulation materials were investigated under air atmosphere by using the non-isothermal thermogravimetry (TG) and differential scanning calorimetry (DSC). The decomposition kinetics in air was proposed by using the model-free (isoconversional) and model-fitting methods. The heat of decomposition was calculated from DSC measurements. By using the kinetic parameters of the oxidation step of building insulation materials and the heat of decomposition, the classical hot spot ignition theory predicted the critical condition for a hot metal particle to ignite building insulation materials. This was helpful for the preliminary understanding of the ignition risk of building insulation materials by a hot particle.In order to investigate hot particle ignition behavior, we set up an experimental platform and studied the ignition process of a widely used insulation material, expandable polystyrene (EPS) foam, by a hot steel particle under different conditions. In the experiments, a small spherical particle (6-14 mm in diameter) was heated to a high temperature (900-1100 ℃), and then placed on a bench-scale low-density (18 or 27 kg/m3) foam sample. We observed that flaming ignition could only occur on the foam surface during its rolling process (rolling ignition) or before it was fully embedded (embedding ignition). The measurements suggested that larger particles held lower critical temperatures for ignition, which decreased from 1030 to 935℃ for diameters increasing from 6 to 14 mm. Compared to higher-density forest fuels in the literature, the critical particle temperature of EPS foam is much higher, with a narrower transition region for ignition probability of 5-95% and has a weaker dependence on the particle size. Results also show that the sample density and thickness have a negligible influence on the ignition probability and mass-loss ratio. Theoretical analysis suggested that the hot particle acts as both heating and pilot sources, and the ignition of EPS foam is controlled by the competition between the mixing time and the residence time.According to the experimental observations, a numerical model was proposed, taking into account the reactant consumption and volatiles convection of expandable polystyrene decomposition in air. Three regimes, no ignition, unstable ignition and stable ignition, were identified, and two critical particle temperatures for separating the three regimes were determined. Comparison with the experimental data shows that the model can predict the range of critical ignition temperatures reasonably well.Considering the discrepancy of the decomposition and combustion property between the building insulation materials and wildland fuels, laboratory experiments were performed to investigate the ignition of the moist pine needle bed by a hot steel particle under wind. A spherical particle (6-14 mm in diameter) was heated to high temperature (600～1100 ℃) and then released on a bench-scale pine needle bed with the moisture content ranging from 6% to 35% under different ambient wind speeds (0-4 m/s). Several ignition phenomena including direct flaming, smoldering and smoldering-to-flaming transition were observed and discussed. The critical particle temperature for sustained ignition was found to decrease with the particle size and increase with FMC as Tp,crt= 1800(1+4 FMC)/d+500 [℃], and the maximum heating efficiency of particle is found to be ηsp=10%. As the particle size increases, the influence of FMC becomes weaker. Two different flaming ignition delay times were measured, and both decreasing with the particle temperature and wind speed, while increasing with FMC. The proposed heat transfer analysis explains the ignition limit and delay time as well as the interaction between flaming and smoldering. The theoretical analysis also suggests that the hot particle acts as both heating and pilot sources like a small flame for the direct flaming ignition, but only acts as a heating source for smoldering and its later transition to flaming.