Study On The Development Of Crown Fire And The Characteristics Of Fire Behavior

A wide range of effects were caused by canopy fire.In the process of combustion,the fire head formed by fire front often included burning branches,broken wood and flying fire formed by sparks,even igniting the surrounding trees,forming high energy fire characteristics and special fire behaviors such as flame fusion,fire explosion and fire whirlwind and so on.In this paper,the common green tree species were used as the experimental object to carry out the single tree crown fire burning experiments under the conditions of different moisture content,different tree heights and different tree species with the windless environment.Based on the single tree crown fire combustion experimental platform,the fire development and fire behavior characteristics measurement experiments were carried out.In the experiment,the fire behavior parameters such as mass loss rate,flame surface temperature,flame height,radiation heat flow intensity and fire duration were recorded by setting up multiple K-type thermoelectric power,and the energy release rate,heat release rate were calculated.The characteristic parameters of fire spread,such as fire spreading rate and fire line strength,were discussed.The experiment was carried out under the condition of no wind and no slope.The development stage of single tree crown fire was divided into four stages: the initial stage of ignition,the accelerated stage of development,the attenuation stage and the end stage of slow combustion.The results were showed that:(1)The mass change was not affected by water content,canopy height and tree species and the trend of mass change was "waterfall".(2)Mass loss rate,heat release rate,temperature of each measuring point on the flame surface,radiant heat flux and fireline intensity all increased first and then decreased.The order of three measuring points on the vertical of radiant heat flux was as follows: measuring point at 1/2 tree height > measuring point at tree height > measuring point at 1.5 times tree height.(3)The calorific values of Melaleuca bracteata,Sabina Chinensis cv.Kaizuca and Sabina Chinensis cv.Pyramidails were 19.468MJ/kg,19.873MJ/kg and 19.160MJ/kg,respectively.(4)The maximum flame height is about 2 times the height of the tree crown.The results of single tree crown fire combustion experiments with different moisture content were showed that :(1)the total mass change was basically at the same level.(2)The maximum mass loss rate,maximum heat release rate,flame height,fire duration and maximum fireline intensity all increased first and then decreased with the increase of moisture content,and the maximum heat release rate was 642.05 k W.(3)The maximum temperature of the flame surface was above 600℃,and the locations of the flame surface was all at the measuring point at the 1/2 tree height.(4)The measured maximum thermal radiation and the fire spread rate decreased with the increase of water content,and the fire spread rate ranged from 1.94m/min to7.35m/min..According to the experimental data under different tree heights,the following results were obtained :(1)The total mass change,maximum mass loss rate,energy release,maximum heat release rate,maximum thermal radiation measurement value,fire front spread rate and maximum fire line intensity all increased with the increase of tree height;(2)The fire duration increased first and then decreased with the increase of tree height;(3)The maximum heat release rate was556.84 k W,and the fire spread rate was between 1.28m/min and 5.74m/min.The results were showed that:(1)the order of the total mass change and the maximum radiant heat flux at the base of the crown was as follows: Melaleuca bracteata >Sabina Chinensis cv.Kaizuca > Sabina Chinensis cv.Pyramidails;(2)The maximum mass loss rate,the maximum heat release rate,and the fire duration were sorted by: Sabina Chinensis cv.Kaizuca > Melaleuca bracteata > Sabina Chinensis cv.Pyramidails;(3)The order of energy release,fire spread rate and maximum fireline intensity were as follows: Sabina Chinensis cv.Kaizuca > Sabina Chinensis cv.Pyramidails > Melaleuca bracteata;(4)The overall fire risk of Cypress chinensis is higher than that of the other two species.