Experimental And Theoretical Study On Transportation And Ignition Of Firebrands

The spotting process denotes that when a high-intensity forest fire occurs,flaming or glowing embers(e.g.,bark and branch),commonly called firebrands,are lofted by a fire plume and transported downwind to induce new fires ahead of the main fire.It is one of the primary ways leading to forest fires and Wildland-Urban Interface(WUI)fires.In addition,hot metal particles generated by the arcing between the power lines and trees,welding/grinding/arcing processes,and fireworks can also easily flight to initiate a new ignition,which is also a fire phenomenon called spot fire.Understanding firebrand generation,firebrand transportation,and firebrand ignition of fuel at the landing position is essential to identify the phenomena of spot fires.The flight distance of firebrands is an essential indicator that measures the risk of spot fires,and the ability of firebrands ignition at landing is the key to the formation of spot fire.During the transportation and ignition processes,the motion and combustion behavior of the firebrands affect their flight distance and ignition capacity.Therefore,the purposes of the present work include:1)revealing the mechanisms of transportation and combustion of hot metal particles and embers,and analyzing the effect of the main factors on firebrand flight process;2)studying flaming and smoldering spot ignition of natural fuels by a firebrand,and discussing the effect of fuel moisture content on ignition of forest fuels by a firebrand.The work of this thesis is summarized as follows:(1)Based on the heat transfer theory,a temperature degression model of a hot metal particle(no chemical reaction)during its flight process is built to discuss the temperature variation pattern.Then,taking aluminum particles commonly found in fireworks as an example,this study establishes a firebrand movement model,combing the temperature degression model to predict the safety distance to prevent hot aluminum particles from igniting building exterior insulation materials based on critical conditions for particle ignition and reveal the effect of firebrand diameter,firebrand initial temperature,and firebrand initial velocity on the safety distance.The coupling of movement model and temperature degression model preliminarily predicts the maximum flight distance(i.e.,safety distance to prevent the occurrence of WUI fires)of hot mental particles with different diameters igniting foam on different initial conditions.(2)The trajectory model of spherical metal particles was extended to cylindrical-disc wood particles.The main difference between these two kinds of firebrands is that wood particles burn during flight,whereas hot metal particles do not.A firebrand combustion model is constructed that contains an endothermic global pyrolysis reaction and an exothermic one-step char oxidation reaction.Numerical simulation is used to analyze the effect of ambient wind speed,initial particle diameter,and density on combustion.Moreover,the particle dynamics model is validated by the wind tunnel experiments.Modeling results agreed well with experimental results.Furthermore,this study simulates the process of firebrands generated by a wildland fire attacking an isolated building by using Fire Dynamic Simulator.The results show that the propagation distance and combustion behavior mainly depend on the ambient wind speed.(3)To further investigate the effect of ambient wind speed on the transportation and combustion processes of firebrands,this work experimentally studies the distributions of transport distance and residue mass of three disc glowing embers(about 1 g)with different sizes under two wind speeds of 5 m/s and 7 m/s.A bimodal(burning and extinction modes)distribution is found for tiny firebrands(12 mm diameter and 5 mm thickness)under a certain wind speed of 7 m/s.Especially,the transport distance of the extinction mode is 130 cm,lower than 200 cm of the burning mode.The proposed heat transfer analysis shows a critical wind speed dividing unimodal and bimodal distributions,above which the firebrands tend to be quenched by the wind cooling,showing bimodal distribution.The predicted critical wind speed of tiny firebrands(12 mm diameter and 5 mm thickness)is U*=5.9 m/s,which agrees with experimental measurements.The theoretical model can be used to predict the critical wind speeds of firebrands of different sizes,thus providing guidance for reducing the ignition risk of spot fires in practice.(4)In the stage of firebrands ignition at landing,this study experimentally investigates the ignition process of grass litter by an aluminum particle(1.6 mm～8 mm diameter)and a steel particle(1.6 mm～8 mm diameter)with the temperature of 500℃～1100℃.The results show that the ignition boundary for both flaming and smoldering follows a hyperbolic relationship between particle size and temperature,i.e.,ignition temperature decreases when particle diameter increases,and the temperature of smoldering ignition is lower than that of the flaming ignition.Furthermore,the aluminum particle melts during the heating process,increasing the energy of the aluminum particle.Thus,the ignition temperature of an aluminum particle is lower than that of an unmelted steel particle.Based on previous research,this study builds a theoretical model of flaming ignition by a hot metal particle,considering the melting of aluminum particle.The predicted results agree with the experimental results.In addition,based on a three steps reaction model of pyrolysis and combustion of forest fuel,this study establishes a theoretical model of smoldering ignition by a hot metal particle.The simulations agree well with the experimental results.(5)As the fuel moisture content(FMC)is essential in determining whether a firebrand will ignite the fuel,this study investigates the effect of FMC on ignition by a burning ember.By modifying the experimental setup of hot metal particle ignition,the effect of FMC(0.1%～55%)on the smoldering ignition of pine sawdust by glowing firebrands(1.59 mm～15.88 mm diameter)is studied experimentally.Under all experimental conditions,a firebrand can ignite a fuel bed with a maximum FMC of 40%.A larger firebrand is capable of igniting sawdust with a higher FMC,while a firebrand smaller than 3.17 mm in diameter is unable to initiate a smolder in a dry sawdust bed.Using an energy transfer model,an ignition boundary function,related to particle diameter and FMC,is determined,and a curve with 50%ignition probability is plotted.Moreover,the theoretical results validate and explain the experimental results of ignition limits of firebrands with different sizes on fuel beds with different FMC.