【作者】 武金模；

【导师】 刘乃安；

【作者基本信息】 中国科学技术大学 ， 安全科学与工程， 2014， 博士

【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。更多还原

【Abstract】 In wildland fires, a lot of attention addresses surface fire spread, which is significantly influenced by ambient wind, moisture content of fuel and terrain or slope. In previous studies, most experiments were conducted under slope ranges lower than30°, and the effect of slope were considered very simply in many fire spread models. In this work, wind-driven (0-5.5m/s) fire spread experiments were conducted in combustion wind tunnel, using grasses (Hulunbeir Grassland) with various moisture content (4%-32%) as fuels, and a series of fire spread experiments with a linear flame front were performed on a6m long combustion platform with slopes from zero until32°, using pine needles of Pinus sylvestris (Jilin city, Jilin Province) as fuels. The temperature above the fuel bed was measured using K type thermocouples in wind-driven and upslope fires. Additionally, the fuel mass consumption in flaming fire spread, the velocities of the inflow around the flame, and the heat fluxes (total and radiant) near the end of the fuel bed were also measured in upslope fires.The rate of fire spread (ROS) is calculated using the characteristic ignition time of fuel extracted from temperature curve. The overall ROS calculated in global scale of fuel bed shows that the fire spread can be regarded as a steady process. However, the local ROS calculated in the region between adjacent thermocouples shows higher fluctuations as compared to the overall ROS. It is suggested that the fuel bed should be long enough to achieve a stable fire spread.In wind-driven fires, ROS is obviously enhanced by ambient wind, and increases linearly with windspeed under4.6m/s. ROS is also remarkably influenced by moisture content of fuels, but varies in a reverse way as wind. According to experimental data, a semi-empirical model including windspeed and moisture content is developed, which agrees well with the experimental results.By experimental observations and data analysis, three regions of slope are identified in upslope fires:a) low slope region of0°-20°with smaller ROS; b) moderate slope region of25°-29°with gradual increase of ROS; c) high slope region of30°-32°for rapid fire spread. It is revealed that the role of slope in ROS involves two types of mechanisms. First, the slope of fuel bed naturally reduces the angle between flame front and fuel bed. Especially this is the governing mechanism in lower slopes. Second, the slope induces upslope wind which is dominant in higher slopes.By using pitot tubes data, it is revealed that the upslope wind promotes the burning especially in high slopes. The velocity data shows that weak reverse inflow exists in front of the flame, and at the same time the upslope wind induced by the flame itself exists behind the flame front. The significant differences between the velocities of the reverse inflow and the upslope wind are suggested to be the major cause for the forward tilting of the flame front. The mass loss rate measured by load cells varies with slope in a similar way as ROS. However, the fuel consumption efficiency varies in a reverse way, from nearly unity at zero slope to0.1at slope of32°. This implies that the assumption of thermally thin fuel layer in modeling study is effective in low slope region, while it fails in higher slopes.The experimental data of heat flux meters and reverse inflow velocities by pitot tubes are used to further interpret the transfer mechanisms in fuel preheating of fire spread with a linear flame front. The results indicate that besides radiation heating, both natural convection cooling and flame-induced convection cooling exert impacts, but with different spatial influence ranges. Natural convection takes effect in a region from far field to near field (close to the flame), while flame-induced convection cooling (induced by reverse inflow) takes effect within the region close to the flame. With increasing slope angles, the convection cooling is dominated by natural convection under lower slope angles, and is jointly influenced by natural and flame-induced convections (mixed convection) under higher slope angles, in which flame-induced convection plays a dominant role.A theoretical fire spread model for a linear flame front based on energy conservation and heat transfer analysis is developed, in which radiation, convection and radiation loss are considered in details. A natural cooling convection and the fuel consumption efficiency are involved in the model. The new model is found to fit well with the experimental data of ROS, and its reliability especially for higher slopes is verified by comparisons with other models.更多还原