外界风和坡度条件下地表火蔓延的实验和模型研究

发布时间：2018-04-14 04:26

本文选题：地表火蔓延 + 线形火前沿　；参考：《中国科学技术大学》2014年博士论文

【摘要】：地表火蔓延是野火最主要的火灾形式,而外界风速、燃料含水率和地形坡度是影响火蔓延的重要因素。前人工作中,对坡度作用的研究仍不够充分,对坡度影响火蔓延的物理机制的认识尚不深入。本文在燃烧风洞中使用草原植被燃料(呼伦贝尔草原)开展不同风速(0-5.5m/s)和含水率(4%-32%)条件下的火蔓延实验；基于坡度可调火蔓延燃烧平台,重点开展松针(樟子松Pinus sylvestris,吉林省吉林市)燃料床上线形上坡(0°-32°)火蔓延实验。两种实验中通过布设多根K型热电偶观测火蔓延速率(Rate of fire spread,简写为ROS))；另外在上坡火蔓延实验中,使用称重板、皮托管和热流计分别测量火蔓延过程中的燃烧特征、火焰周边流场结构以及火前传热特征。从热电偶测温曲线中提取燃料着火的特征时刻并利用线形拟合方法计算全局ROS(火前沿在蔓延稳定阶段或距离内的整体上表征速度)；在相邻热电偶间距内使用数学平均方法计算局部ROS(火前沿在蔓延过程中不同空间位置处的推进表征速度)。全局ROS在整体表现出稳定一致性,而局部ROS表现出围绕全局ROS上下波动的特征,并且波动的程度随风速和坡度的增加而增大。分析指出室内实验中燃料床的长度是制约开展稳定火蔓延实验的最主要因素。对于草原植被燃料火蔓延实验,风速极大促进ROS,在4.6m/s以下ROS随风速线形增加；而燃料含水率对于火蔓延产生显著抑制作用。根据实验结果,从能量分析出发建立了一个综合考虑风速和含水率影响的半经验半物理模型,模型的计算结果与实验测量值吻合。对于上坡火蔓延实验,ROS随坡度增加。实验坡度范围被划分为三个区间。低坡度区：0°-20°,ROS随坡度缓慢微弱增长；中坡度区：25°-29°,ROS随坡度明显增大；高坡度区：30°-32°,ROS急剧增加。坡度的作用被归纳于两个方面：坡度自身和上坡火诱导产生斜坡风。坡度自身造成火焰与燃料床的夹角减小增强了火焰的辐射传热能力,从而导致ROS增加。斜坡风造成火焰进一步向前倾斜并促进燃烧,造成类似外界风的效果。其中前者在低坡度区发挥主要作用,而后者随着坡度的增大逐渐占据主导地位。斜坡风由火焰前后温度场的不对称和空气卷吸能力的差异引起,皮托管测量结果验证火焰后方斜坡风的存在。上坡线形火蔓延实验中,与火蔓延同向的斜坡风不能穿透火焰面,火焰前方只存在微弱的逆向(与火蔓延方向相反)卷吸气流。随着坡度的增大,斜坡风增加更加迅速；解释了低坡度下火焰面竖直而更大坡度下火焰额外倾斜的现象。上坡火蔓延中不同位置的燃料失重速率存在散布性,与局部ROS的波动特征相对应。燃烧速率随坡度的增大而增加,与ROS具有类似的变化趋势。有效燃烧消耗率随坡度的增加而减小,分析认为在模型研究中低坡度下燃料床符合“热薄型"通用假定；而在更大坡度下燃料床表现出“热厚型”特征。辐射和总热两种热流计的测量表明,在线形火蔓延的预热阶段,辐射热占据主导地位,而对流冷却同样发挥了重要的作用,在量级上与辐射热损相当。在低坡度下对流表现为自然对流冷却的形式,它在火前的长距离范围内产生影响；而在高坡度下则表现为强迫对流和自然对流的混合对流冷却形式,其中强迫对流冷却由火前的逆向卷吸造成并且只在火前的短距离内发挥作用。基于能量平衡,建立了一个考虑辐射热、对流热和辐射热损的物理模型,模型中引入自然冷却和有效燃烧消耗率。模型预测结果与实验测量吻合良好,准确映了ROS随坡度急剧增加的特征,较好地解决了其他模型在较大坡度下ROS预测明显偏低的问题。
[Abstract]:Surface fire spread is the main fire form of wildfire , while the external wind speed , fuel moisture content and terrain slope are the important factors that influence the spread of fire . In the former work , the research on slope effect is not enough , and the understanding of the physical mechanism of slope influence fire spread is not deep . In this paper , the fire spread experiment under different wind speed ( 0 - 5.5m / s ) and water content ( 4 % -32 % ) is carried out in the combustion wind tunnel .
Based on the slope - adjustable fire - spread combustion platform , the fire spread experiment of the linear upslope ( 0 掳 - 32 掳 ) on the fuel bed of pine needle ( Pinus masstris , Jilin City , Jilin Province ) was mainly carried out .
In addition , in the fire spread experiment of the upslope , the combustion characteristics of the fire spread process , the structure of the flow field around the flame and the heat transfer characteristics of the fire are measured by using the weighing plate , the Pitot tube and the heat flow meter .

extracting the characteristic moment of the fuel ignition from the thermocouple temperature measurement curve and calculating the global ROS by using the linear fitting method ( the integral upper characterization speed of the fire front edge in the propagation stability phase or distance ) ;
A mathematical mean method is used to calculate the local ROS ( propelling characterization velocity at different spatial locations in the spread process ) using mathematical averaging methods within the interval of adjacent thermocouple . Global ros exhibit stable consistency in the whole , while local ROS exhibit characteristics that fluctuate up and down around the global ROS , and the extent of the fluctuations increases with increasing wind speed and slope . The analysis indicates that the length of the fuel bed in the indoor experiment is the most important factor limiting the steady fire spread experiment .

For the experiment of grassland vegetation fuel fire spread , the wind speed greatly promoted ROS , and ROS increased linearly with wind speed at 4.6m / s .
According to the experimental results , a semi - empirical semi - physical model considering the influence of wind speed and moisture content is established from the energy analysis . The results of the model are in agreement with the experimental measurements .

For uphill fire spread experiments , ROS increased with slope . The range of experimental slope was divided into three sections . Low slope area : 0 掳 -20 掳 , ROS increased slowly with slope ;
Middle slope zone : 25 掳 -29 掳 , ROS gradually increases with slope ;
High slope zone : 30 掳 -32 掳 , ROS sharply increased . The effect of slope is summed up in two aspects : slope itself and slope wind induced slope wind . The slope itself causes the angle between the flame and the fuel bed to decrease the radiant heat transfer capacity of the flame , thus leading to the increase of ROS . The slope wind causes the flame to tilt further forward and promote combustion , resulting in similar external wind effect . The former plays a major role in the low slope area , and the latter gradually occupies the dominant position with the increase of slope .

The slope wind is caused by the asymmetry of the temperature field before and after the flame and the difference of the air entrainment ability .
This paper explains the phenomenon of flame extra inclination under the vertical and larger slope of the flame surface at low slope .

The combustion rate increases with the increase of slope . The effective combustion consumption rate decreases with the increase of slope . The effective combustion consumption rate decreases with the increase of slope , and the analysis considers that the fuel bed in the model study accords with the general assumption of " thermal thin type " , and the fuel bed shows the " hot - thin type " characteristic under the larger slope .

The measurement of both the radiation and the total heat indicates that the radiant heat takes a dominant role in the pre - heating stage of the linear fire spreading , while the convection cooling also plays an important role in the magnitude of the convection cooling .
whereas under high slope , a mixed convection cooling form of forced convection and natural convection is presented in which forced convection cooling is caused by reverse entrainment before the fire and works only within a short distance before the fire .

Based on the energy balance , a physical model considering radiant heat , convection heat and radiant heat loss is established , and natural cooling and effective combustion consumption rate are introduced in the model . The results of the model are in good agreement with the experimental measurement , and the characteristics of the ROS increasing rapidly with the slope are accurately reflected , and the problem that the ROS is obviously lower in the other models under the larger slope is solved .

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：X932

【参考文献】