Experimental Study On The Effects Of Fuel Load And Packing Ratio On Fire Spread Under Zero-Slope Condition

Surface fire is the most important form of the forest fire,for which fuel property is a key factor.The fuel load and packing ratio are two important fuel parameters.While the effect of packing ratio on fire spread has not been studied systematically.A method of controlling fuel bed packing ratio was presented in this paper.The wood excelsior was used as fuel.Fire spread experiments were performed under the zero-slope and no wind conditions,with different fuel loads(0.4,0.6,0.8,1.0,1.2 kg/m2)and fuel bed packing ratio(0.01,0.02,0.04,0.06,0.08,0.10,0.12).The flame length,flame tilt angle,rate of fire spread(ROS),residence time and mass loss rate were investigated under different fuel load and packing ratio conditions.The heat transfer mechanisms on the fuel bed surface and inside of the fuel bed were analyzed.The flame length,residence time,mass loss rate and ROS increase with the increase of fuel load.And the influence of fuel load on these parameters decreased gradually with the increase of packing ratio.The flame tilt angle decreased with the increase of fuel load when the packing ratio was less than 0.02.The flame tilt angle increased with the increasing fuel load when the packing ratio was 0.08.The diffrent intensity of buoyancy plume accompanying flame and pressure on both sides of the flame for different packing ratios lead to different flame tilt angles.The flame length and mass loss rate decreased with the increase of packing ratio.The packing ratio had little effect on flame length when the packing ratio was more than 0.1.The flame tilt angle increased with the increase of packing ratio when the packing ratio was more than 0.06.Especially,the flame tilt angle could reach 155° for a very large packing ratio,which differed greatly from the general zero-slope fire spread.The ROS decreased with the increase of packing ratio.This was because the increase of packing ratio led to the reduction of oxygen content in the fuel bed,which reduced the fuel combustion rate and thus reduced ROS.As the packing ratio increased,the residence time increased initially and then tended to be stable.The inflection point of residence time for different fuel loads located among 0.06-0.08 of packing ratio.Comprehensively considering the abrupt change of fuel consumption rate,it could be inferred that the fuel bed changed from thermal thin to thermal thick among 0.06-0.08.The total heat flux and radiant heat flux on the surface and inside of the fuel bed showed a decreasing trend with the increase of packing ratio.The fire spread process was dominated by the heat transfer inside the fuel bed when the packing ratio was between 0.06-0.08.The heat transfer effect of the flame on the fuel bed surface and inside the fuel bed are both important when the packing ratio was more than 0.06,which further confirmed the transition of the fuel bed from the thermal thin to the thermal thick.The heat convection on the fuel bead surface could not be neglected when the packing ratio was large.