Effects Of Fuel Bed Width On Upslope Fire Spread:Experimental And Theoretical Study

Surface fire spread is the most important type of wildland fires in which slope is commonly accepted to be the essential environmental factor affecting the fire spread behavior and it has been extensively investigated.For experimental studies,laboratory or field experiments were restricted mainly to fires within limited fuel bed size,while the influence of fuel bed width was often ignored.An infinitely wide fireline was generally assumed in theoretical studies,leading to that the effects of fireline width on model predictions had not been investigated.The literature review showed that fuel bed width has a significant effect on the rate of spread(ROS)and flame dynamics,but the joint effects of fuel bed width and slope have been subjected to limited study.Besides,due to the lack of effective measurements on experimental data,the analysis and discussion were mostly based on qualitative descriptions or reasonable assumptions.Therefore,more elaborate experiments need to be carried out to further study the fuel bed width effects on upslope fire spread,to improve our knowledge of the fire spread and provide more valuable data for model validation.In this work,a series of experiments were conducted to study the effects of slope(0-30°)and fuel bed width(0.5?2.0 m)on fire spread behaviors with and without sidewalls.For upslope fires without sidewalls,the fireline shape was significantly affected by the slope angle.For slopes lower than 10°,a parabolic fireline contour remained during steady fire spread stage.For slopes higher than 20°,the fireline changed to a more pointed "inversed V-shape".The fireline angle was found to decrease linearly with increasing slope,which was almost not affected the fuel bed width.Variance analysis showed that the influence of slope and its interaction with fuel bed width on the flame length and flame tilt angle was significant.The influence of fuel bed width on flame length was significant.ROS was extracted from the output of streamwise distributed thermocouples.It was shown that the global ROS underwent an acceleration stage before the quasi-steady stage,which had a longer acceleration length under higher slope and larger fuel bed width conditions.The local ROS and residence time had pronounced fluctuation characteristics,and the average values for steady state were found to increase with increasing slope.The fuel mass loss rate increased with increasing slope,and the resulting fire induced flow became more significant,which indicated that lateral air entrainment and interactions between flames played a key role in the acceleration of ROS.Flame radiation was recognized as the dominant heat transfer mechanism when the fuel bed width was small.When the fuel bed width was larger than 1.5 m,a remarkable mixed radiative-convective regime existed for slopes higher than 25°.In the study of fuel bed width effects on upslope line fires,two sidewalls with the same height of fuel bed were used to obtain a uniform straight fireline.ROS was found to increase with increasing slope and fuel bed width,and the presence of sidewalls caused a marked increase in ROS.The slope factor in fire spread model was modified by the dimensionless fuel bed width,and the model was found to match the experimental results well.Flame length increased with increasing slope angle or fuel bed width,while flame tilt angle decreased with increasing slope angle.The effective fuel consumption ratio decreased with ROS,which indicated lower fractions of effectively heated fuel mass.The correlation of flame emissivity and flame depth was obtained and the predicted distribution of flame radiation was in good agreement with the experimental results.For low slope angles(<20°),the reverse airflow ahead of the flame front denoted the natural convective cooling effect,thus flame radiation was the dominant preheating mechanism.For high slope angles(>25°)and large fuel bed widths(>1 m),the flow velocity and heat flux data both indicated that the fuel preheating was controlled by both flame radiation and convective heating,and the range of convective heating increased with slope and fuel bed width.Based on the linear upslope fire spread model,flame radiation,combustion zone radiation,re-radiation and natural convection were considered as the main heat transfer mechanisms.The model was found to predict the effects of slope and fuel bed width on ROS well.The predicted fuel preheating temperature showed that the effective heat transfer range increased with increasing slope and fuel bed width.Combustion zone radiation was the dominant heat transfer mechanism during horizontal fire spread,while flame radiation gradually played a significant role when the slope increased.Based on this model,the fuel bed width threshold for fire spread dominated by flame radiation was found.A new approximate expression for view factor integration was proposed,and it was found to match the results of the numerical solution better.An expression was derived for the fireline width threshold,which was found proportional to the flame length and flame tilt angle.