Experimental And Modelling Study On Downslope Forest Fire Spread

The terrain slope is a key factor affecting the forest surface fire behavior.Previous study on the slope effect mainly focused on the upslope fire,while the downslope fire spread has attracted less attention even though it represents a large percentage of wildfire spread way.The heat transfer mechanism of this process is poorly understood and very few fire spread models have paid attention to the downslope fire.In this thesis,the effect of slope on the downslope fire spread behavior with line ignition was studied.The experiments were conducted with four slopes(0°,10°,20°,30°)and two fuel loads(0.4,0.8 kg/m2)by using the pine needles(Pinu.s massoniana)as fuel.The rate of spread,flame geometry parameters(such as flame length and flame angle),mass loss of pine needles,the flow field around the flame front,the temperature distribution on the surface and inside of the fuel bed and the radiation heat flux ahead of flame during downslope fire spread were analyzed.The experimental results show that the mass loss of fuel in the flame front increases linearly with time,which means the fire spread rate can reach a quasi-steady state in this size of fuel bed.With the increase of the slope,the rate of fire spread firstly decreases and then increases,which is consistent with the result described in the previous literature.The flame angle increases linearly with slope and the flame length is less affected by the slope.According to the measured flow velocity data,there exists a reverse flow in front of the flame and the velocity increases with slope.This suggests that the reverse flow of cold air will play a role of convection cooling on the unburned fuel.The combustion interface shape was reconstructed from the temperature curve of thermocouples placed at the top and bottom of fuel bed.The result shows that the combustion interface is perpendicular to the fuel bed,so that the fuel bed can be seen as a thermally thin type in this situation.The radiative heat flux data measured by the meter placed at the end of the fuel bed shows that the radiative heat flux under 300°slope is larger than that for 0° slope for two fuel loads.However,the view factor between the flame and the radiative heat flux meter surface decreases with the slope.By analyzing Stephen-Boltzmann law,it can be deduced that this opposite trend of heat flux and view factor with slope may be caused by the flame emissivity.The reduction of view factor on the amount of flame radiation can be supplemented by the increase of flame emissivity,which causes the measured flame radiation heat flux is larger for high slopes than zero slope and affect the rate of downslope fire spread.Based on the above analysis of heat transfer process of downslope fire spread,the energy balance equation of unburned fuel element ahead of the flame front was established to predict the rate of downslope fire spread.The flame radiation,combustion zone radiation,natural convection cooling and radiation heat loss of fuel elements were considered in the model.The model calculation results show that the flame radiation plays a dominant role in the downslope fire spread process,while the combustion zone radiation could not be neglected in the near flame region.