Experimental And Numerical Study On The Ignition Of Pine Needles By Firebrands And Thermal Radiation

In the Wildland-Urban Interface areas,once large scale forest fires happen,flame radiation and firebrands generated from burning trees could ignite the combustible materials,causing structure fire and then resulting in huge economic losses.In previous researches,firebrand ignition and radiation ignition are often studied separately while the combined effect is rarely investigated.In this thesis,experiments and numerical simulation have been conducted to investigate the ignition process of pine needle bed by hot particle and thermal radiation.The work of this thesis is summarized as follows.Laboratory experiments were conducted to investigate the combined effect of hot steel particle(simulating firebrand,8-12 mm in diameter,700?800?)and thermal radiation(simulating flame radiation,0?30 kW/m2)on the ignition of pine needle fuel bed.Ignition limit and ignition time of smoldering-to-flaming transition under various particle temperatures,particle diameters and radiant heat fluxes were discussed.Experimental results suggested the critical radiant heat flux under the combined effect decreased with the increase of particle diameter and temperature,and a linear relationship between the critical radiant heat flux and d(Tp-Tsm)was found through theoretical analysis.In the scope of this study,when thermal radiation was lower than the critical value of independent ignition process,the ignition time of smoldering-to-flaming transition was reduced with the increase of particle diameter for higher-temperature particle.As particle temperature decreased,the ignition time was independent of particle diameter.The results indicate that in the process of hot particle initiating smoldering ignition,there exists a critical heating distance which is not sensitive to the particle size and temperature.Only when the effective heating distance of the hot particle is greater than the critical value can the fuel bed be ignited successfully.When the fuel bed is ignited by hot particle and thermal radiation simultaneously,thermal radiation lengthens the effective heating distance of hot particle,which enables the particle with lower temperature to induce ignition.Based on the observed experimental phenomena,a two-dimensional model of smoldering ignition including water evaporation,fuel pyrolysis,fuel oxidation and char oxidation was established.Comparison with the experimental data showed that the numerical model can predict the ignition limit reasonably well.