An Experimental Study On The Characteristics Of Forest Fire In Canyon Terrain

On average,more than 220,000 forest fires occur in the world each year,burning about2.3%of the world’s forest area.It is important to study the spread pattern of forest fires,and forest fires in canyon terrain are an even more dangerous situation.In this paper,we use poplar wood shavings as fuel for simulation experiments in a canyon model built in the laboratory.The canyon model is able to adjust the slopeαand the angleδof the canyon（the angle between the fuel bed on one side of the canyon and the horizontal plane）.A series of experiments were conducted using a point fire source with varying slope and angle,observing the flame spread pattern by video data and temperature,calculating the fire spread rate by thermocouple data,and analyzing the flame change pattern by obtaining wind speed data in the axial direction of the canyon through a pitot tube.The fuel bed was adjusted at 0°,10°,20°,30°,40°,and 60°with zero canyon pinch angle,and uphill fire spread experiments were conducted at six slopes.The shape of the fire line was observed to change from a circle to a sharper triangle,and it was observed that the fire spread rate did not increase linearly with the slope,but changed exponentially,and the fitting equation for the slope and fire spread rate was obtained by nonlinear fitting:R=0.198+0.247e^0.054α.Three flame attachment states were observed in the experiment:at 0°and 10°,the flame almost did not When the slope increased to 20°and 30°,the flame tilted toward the fuel bed,and the fire spread rate increased slowly with the slope;when the slope was 40°and 60°,the flame was almost attached to the fuel bed,and the fire spread rate increased significantly and continued to increase with time.The wind speed was observed to increase with the slope in the direction of the fuel bed axis,and the airflow in front of the flame towards the unburned area was observed when the slope was greater than 30°.There was no clear pattern between the change in fuel width and fire spread rate at 0°slope,and when the slope was 20°the fuel bed width increased within a certain range and the fire spread rate also increased.In the case of varying only the canyon angle at a slope of 0°,it is observed that the fire spread rate increases with increasing canyon angle,but not linearly,but exponentially,and the fitted curve equation is:R=0.028+0.0023e^0.0112δ.When the angle reaches a critical value,the fire spread rate increases dramatically,which indicates that even without slope,the fire can break out when the canyon angle is large enough.This indicates that even without slope,an outbreak of fire can occur when the canyon angle is large enough.A strong axial wind blowing from the unburned area to the burned area was observed in the canyon with an increasing angle of inclination of 0°.The maximum slope line of the canyon exists when both slope and canyon angle exist（the line where the maximum slope angle of the canyon on the fuel bed is located）,the maximum slope line is parallel to the axis when only slope exists,and the maximum slope line of the canyon is perpendicular to the axis when only the angle of entrenchment exists.The shape of the fireline changes considerably with the canyon slope and the angle of entrenchment,and is equivalent to a conventional uphill fire when the angle of entrenchment is 0°or the slope is 0°.When the slope is greater than 0°but less than 30°and there is a pinch angle,the flame will deviate from the axis of the canyon and turn toward the direction of the maximum slope angle.However,when the slope is above 30°,the flame will deviate from the direction of the maximum slope line to the direction of the canyon axis.The rate of fire spread in the axial direction of the canyon increases with increasing canyon angle when the slope is non-zero,and the wind speed is observed to increase with increasing angle in the axial direction of the canyon.Slopes that would not otherwise experience fire outbreaks can also experience fire outbreaks at a given canyon angle resulting in a dramatic increase in fire spread rate.