Experimental Study On The Characteristics Of Ridge-Canyon Fire Spread Under The Condition Of Slope

Surface fire spread is the most important form of forest fire.As the initial stage of fire development and the intermediate stage of transforming into more intense fires,it is particularly important to focus on surface fire spread.Topographic slope is an important factor affecting the spread of mountain fire.Most previous studies focus on upslope fire spread in a single terrain,but still few work is carried out on the spread of surface fire in complex terrain,especially,the terrain with several canyons and ridges.Therefore,in current study,the dead pine massoniana needles are used as the tested fuel,and the ridge and ridge-canyon fire spreads were experimentally studied with the various central slope angles(the angle between the ridge line or the canyon line and the horizontal ground.)and lateral slope angles(The angle between the bottom edge of the lateral fuel bed and the horizontal ground.).The purpose aims at the variation laws of the rate of fire spread,the fire line contour,the fire line angle,the flame tilt angle,the flame length,and the preheating mechanism with the change of the central and lateral slope angles.The difference between the spreads of ridge fire and canyon fire are also analyzed.The tests of ridge fire spread are performed by changing the central and lateral slope angles of the fuel bed.The results indicate that,after the ridge line is ignited,a drop-shaped fire line can be observed from the ignition point to the surroundings in a short time.When both the central and lateral slope angles are less than 10°,the upslope fire lines are parabolic.With the angles increase,the upslope fire line are converted from parabolic to V-shape.The fire line angle firstly increases and then keeps constant,and decreases with the increase of the central and lateral slope angle.When the central slope angle is small,the flame is short and almost vertical.As it increases,the flame tilts to the fuel bed,which promotes radiant heat transfer,increases the flame length and the combustion intensity,and causes the rate of ridge fire spread to increase.When the lateral slope angle increases or is even greater than the central slope angle,the component of buoyancy in the direction of the upslope along the direction of lateral slope of the fuel bed increases.This causes the flame to tilt inward the V-shaped fire line and be even attached to the fuel bed,and the rate of fire spread towards the end of the ridge line decreases.However,the flame length and the radiant heat transfer increases slightly.The tests of ridge-canyon fire spread are carried out by changing the central and lateral slope angles of the fuel bed.The results show that both the central slope angle and lateral slope angle have important effects on the characteristics of canyon fire spread.When the fire spreads from the lateral slope of ridge to the canyon line,two flame fronts can be observed.One spreads rapidly along the canyon line to the end,and the other develops rapidly along the maximum slope line of the canyon slope.The rate of canyon fire spread is large,resulting in incomplete flame combustion,and a large area of flame surface can be found behind the flame front,so the fire line contour is relatively fragmentary.With the central slope angle increases,the angle between the maximum slope line and the canyon line decreases,and the action trajectory of the flame front on the canyon slope is shifted to the canyon line.The fire line contour on the side slope changes from V-shape to hook shape,and the angle between the fire line and the canyon line decreases rapidly,while the flame length and combustion intensity increase significantly.When the lateral slope angle is small(less than 10°),the ridge fire spreads to the canyon line,and the fire line is almost parallel to the canyon line.Thus,the angle between fire line and the canyon line is very small.With the lateral slope angle increases,the fire line angle increases gradually.With the central slope angle is greater than 20°,the rate of canyon fire spread increases suddenly,and the instability of the fire spread also increased rapidly.The rate at the central slope angle α=40° is five times than the one at 20°.However,it decreases with the increasing lateral slope angle.The rate of fire spread on the lateral slope of canyon increases linearly with the central and lateral slope angle.During the preheating phase of the canyon fire,the effects of radiation and convection rapidly increases with the increase of the central and lateral slope angles.When the central slope angle is greater than 30°,the thermal environment of the flame front changes from radiation dominance to the mixing of radiation and convection,which is responsible for the sudden increase of the rate of canyon fire spread and much higher than the rate of ridge fire spread.