林火蔓延过程中辐射换热和点燃特性分析

Analysis on Radiation Heat Transfer and the Associated Ignition during Forest Fire Spread

【目的】基于辐射换热原理和火行为模型,从理论角度研究林火蔓延过程中火源辐射换热规律以及辐射点燃的特点。【方法】林火蔓延过程中的燃烧区为辐射源,其向火蔓延前方的热辐射等效为矩形辐射面。矩形热辐射面的长度为火焰长度,宽度对应火线宽度;环境风速的变化导致热辐射面出现不同程度的倾斜。选取枯立木和枯倒木这2类林火环境中典型可燃物作为辐射接受体,并分别设定接受体微元距地面1.5 m高和贴于地面。将林火蔓延以及至林缘后熄灭过程中对目标物的辐射换热分2个阶段计算,由此推导出接受体的瞬态辐射换热通量和累积的辐射换热量的计算式,并相应构筑辐射点燃的判据。与此同时,结合以往对林火行为规律的认识建立起林火蔓延以及至林缘衰弱熄灭过程中火行为参数间的关系式以辅助模型计算。基于火灾安全计算的有效性原则,设定火灾场景为最糟糕情形,即火焰温度赋值为1 200 K、发射率为1.0,并且一般情形下热辐射在大气中透过率为1.0。计算过程中,取火焰火宽度50 m,火焰长度变化范围为5 m～40 m,而风速变化范围则为0 m·s^(-1)～10 m·s^(-1)。【结果】针对特定野外试验工况试算结果确认,辐射换热通量计算结果与野外测量数据高度一致,由此验证了辐射换热模型的可靠性。针对若干火灾场景开展系列计算表明,无论是接受到的辐射换热通量还是累积辐射换热量,枯立木相对于枯倒木都占有优势,但这种优势随环境风速增大有所削弱。火焰蔓延至林缘后对接受体的辐射换热为其能量积累做出主要贡献,是造成辐射点燃的主要能量来源。【结论】随着接受体与林缘间距离的增大,目标物的能量积累水平迅速降低;火焰长度和环境风速则通过改变热辐射辐射换热效率和持续时间以影响辐射点燃的条件。以林缘附近枯立木作为接受体开展热辐射点燃条件的计算证实,当树冠火焰长度增至40 m,30 m以上间隔都能有效避免辐射点燃现象。如果替换成生物防火林带,该间隔则可以大大缩短。本项工作凸显生物防火林带在阻隔林火中的重要性,同时为林火阻隔系统的设计以及林区设施的保护提供特定理论指导。

【Objective】 In the present work we studied the characteristics of the interception of the radiant heat flux by two typical wildland combustibles and the associated ignition phenomena during the spread of a wildland fire through theoretical calculation, based on the principle of radiant heat transfer and fire behavior model.【Method】 Thermal radiation was sourced from the burning zone in a forest fire, which was considered as an equivalent rectangular emissive surface. The length of the emissive surface corresponded to the flame length, whereas the width was determined by the fireline size;The rectangular emissive surface had a tilted angle varying with the environmental wind speed. Two typical combustibles around the edge of a forest, i.e. the dead standing timbers and the dead fallen timbers, were selected as the thermal attack targets with the receiver elements set at 1.5 m high above the ground and on the ground, respectively. By taking into account of the radiation heat exchange during the fire spread and the fire extinction process of the fire at the edge of a forest, a mathematical model was developed to quantify the transient rate of heat exchange between the flame zone and a target at the forest edge and then the accumulated amount of radiation heat intercepted by the target during a fire. Meanwhile, in combination with the existing understanding on forest fire behavior, the formulas were also derived to correlate the parameters of the fire behavior presented during fire spread and the fire extinction process, which enabled the proceeding of the thermal radiation transfer calculations and the evaluation of ignition conditions for various fire scenarios. In light of the principle of fire safety assessment with effectiveness, the worst fire scenario was considered during evaluation, where the flame had a temperature of 1 200 K and an emissivity of 1.0, and the atmospheric transmittance for thermal radiation was set at 1.0 as well. Furthermore, the flame width was evaluated at 50 m, and the flame length was ranged from 5 m to 40 m, whereas the wind speed varied from 0 m[DK]·s^-1 to 10 m[DK]·s^-1 to simulate drastic change of the environmental conditions.【Result】 The calculations for specific fire situations were performed and then compared with the available field test results. It was observed that the calculated radiant heat flux coincides with the field measurements, which confirmed the reliability of the model for determining the radiant heat transfer in forest fire. The examination of various practical fire scenarios indicated that a dead standing timber near the forest edge takes the advantage of both intercepting radiant heat flux and accumulation of radiation heat in comparison with a dead fallen timber as a rule, although this advantage is weaken with an increase in the environmental wind speed. The radiant heat flux received by individual targets reaches the maximum just after a fire source arrives at the forest edge, and the energy accumulation by a target at this stage plays a major role in the ignition of the target.【Conclusion】 With the increase in the distance between the set target and the forest edge, the levels of energy accumulation by the target decreased rapidly;meanwhile, the ignition of a target was altered by the flame length and the local wind speed owing to their roles in the change in the duration of thermal radiation and the radiant heat transfer efficiency between fire source and the target. A large amount of calculation results for the dead standing timber near the forest edge suggested that, a non-fuel interval with a width of above 30 m is sufficient to avoid the ignition triggered by thermal radiation from a forest fire with the flame length up to 40 m. However, in case of construction of a biological fire-prevention belt, the separation distance can be reduced to a large extent. This work provides a specific theoretical guideline for the design of forest fire barriers in forest areas as well as the protection of structures and facilities in wildland fire-prone areas.