火旋风的火焰与流动特性研究

发布时间：2018-08-21 11:10

【摘要】：火旋风是一种燃烧和周围旋转环量相互耦合作用而诱发的具有强烈旋转的特殊火现象。它常见于森林、野外、森林城镇交界域火灾中。与普通池火相比,火旋风的燃烧速率,火焰高度,火焰温度,火焰发射率和辐射热流显著增高。而且火旋风自身也是一种结构复杂的旋转涡旋。现实火灾中,火旋风强烈的旋转速度场可以抬升起周围的任何物体并诱发飞火现象产生新的点火源,进而加快火灾的蔓延。因此,研究火旋风的燃烧和流动特性规律,有利于进一步清楚地认识这种特殊的自然现象,从而为火旋风灾害的预防和扑救提供可靠的理论依据,具有重要的基础科学意义和应用价值。本文的研究目标是,利用空气射流设计并构建一个开放环境的火旋风实验平台；研究开放环境实验平台产生的火旋风的燃烧速率、火焰高度和火焰温度等燃烧动力学特性,建立这些动力学特性与火焰尺寸、热释放速率和外围环量等参数的依赖关系；系统考察火旋风火焰的辐射特性,包括火焰发射率、炭黑体积分数和辐射热流密度等,并且建立辐射预测模型；详细研究火旋风火焰的流场特性,包括轴向、切向和径向速度分布等。本文具体工作包括：首先,建立了一个新型空气幕式火旋风实验平台。前人用于实验室模拟火旋风的装置主要分为机械驱动式平台和热驱动式实验平台,而且他们所用的实验平台都存在外围固壁结构,如四面墙式平台的外围固壁,这不利于研究火旋风对外界物体的影响,尤其是不利于研究火焰辐射。本文利用空气射流产生的空气幕取代了原有四面墙式实验平台的固壁,搭建了一个空气幕式火旋风平台。详细分析了所用空气射流的特性,确立了空气幕的最佳偏转角,以产生稳定的火旋风。通过燃料质量损失速率数据的比较,验证了实验的重复性。其次,系统研究了空气幕式平台产生的火旋风的燃烧速率、火焰高度和火焰温度等燃烧特性,建立了各参数间的依赖关系。对燃烧速率数据的分析表明火旋风的燃烧过程包括五个阶段,其中包括维持时间很长的准稳态燃烧阶段,而且燃烧速率数据可以很好地符合前人提出的燃烧速率与环量的关系模型。通过量纲分析和数据拟合,建立了火焰高度对热释放速率和环量这两个物理参数的依赖关系,并定义了一个标准化高度,同时将数据与前人的火焰高度模型进行了比较,发现本文获得的火焰高度数据也可以很好地符合前人的模型。火旋风火焰轴向温度分布表明其在连续火焰区、间歇火焰区和羽流区随标准化高度的变化不同。径向温度分布表明火焰内部富燃区的存在。然后,详细分析了火旋风火焰的发射率和炭黑体积分数。根据灰体假设和基尔霍夫定律,实验中利用红外技术方法测量了火旋风火焰的发射率,并且得到了一个发射率随火焰直径变化的半经验模型s=1-e-3.68d。基于火焰辐射理论估算了炭黑和辐射气体的发射率,发现辐射气体的发射率很小,可以忽略。通过对炭黑发射的理论分析,计算了火焰的炭黑体积分数,发现其随着标准化高度先增大,而后出现减小的趋势。通过与普通池火数据相比发现,火旋风火焰的平均熄灭系数是普通池火的4-5倍；而且火旋风的炭黑体积分数比池火的大,这可能是引起在相同的火焰直径下火旋风发射率更大的原因。再次,建立了一个分层辐射预测模型。模型中将火焰分成多层,每层火焰被当做是灰体,即火焰的发射是均匀的。通过计算每一层火焰向外的辐射热流,然后将多层火焰的辐射加和求得火焰的热流密度。实验中测量了火焰垂直方向和径向方向的热流密度分布。通过实验数据和模型模拟结果的对比,发现模型可以很好地预测火焰辐射热流密度的分布。根据辐射热流密度在垂直方向和径向方向的分布,得到了火旋风火焰的辐射分数约为44%。最后,借助圆柱壳体实验装置研究了火旋风火焰内外的流场特性。利用粒子图像测速方法(PIV)测量了火旋风火焰内部和外部的流场,并分析了火旋风火焰的漂移特性。结果表明：火旋风是一种稳定的涡旋,其涡核内部的流体做刚性旋转,具有恒定的角速度,其切向速度沿径向距离线性增大,环量与径向距离成二次方指数关系；涡核外部的流动是一个自由涡旋,其切向速度与径向距离成反比,在此区域火焰的环量是一个恒定值。火旋风轴向速度在径向方向的变化符合高斯分布,其沿径向距离逐渐减小,在中心轴线处达到轴向速度最大值。在紧贴壁面的边界层,径向速度随径向距离先增大,然后逐渐减小。随着高度的增加,径向速度分布受火焰漂移的影响增大,产生了很大的误差。获得了火焰漂移引起的火焰位置在y轴的投影变化的概率密度分布,发现沿着径向距离,火焰出现的概率逐渐减小,其概率密度分布呈现高斯分布特性。
[Abstract]:Fire whirlwind is a special fire phenomenon with strong rotation induced by the coupling of combustion and circumferential rotational circulation. It is common in forest, field, forest and town boundary fires. In real fires, the strong rotational velocity field of a fire whirlwind can lift up any object around it and induce a new ignition source, thus accelerating the spread of a fire. Special natural phenomena provide reliable theoretical basis for fire cyclone disaster prevention and extinguishing, and have important basic scientific significance and application value.
The aim of this paper is to design and construct an open environment fire whirlwind experimental platform by using air jet; to study the combustion kinetic characteristics of the fire whirlwind produced by the open environment experimental platform, such as combustion rate, flame height and flame temperature, and to establish these dynamic characteristics and flame size, heat release rate and peripheral circulation parameters. The radiation characteristics of fire whirlwind flame, including flame emissivity, carbon black volume fraction and radiation heat flux, are investigated systematically, and the radiation prediction model is established. The flow field characteristics of fire whirlwind flame, including axial, tangential and radial velocity distributions, are studied in detail.
The specific work of this paper includes:
Firstly, a new type of fire whirlwind experimental platform with air curtain is established. The former devices used to simulate fire whirlwind in laboratory are mainly divided into mechanical-driven platform and thermal-driven experimental platform. Moreover, the experimental platform they used has external wall-fixing structure, such as the external wall of four-wall platform, which is not conducive to the study of fire whirlwind. In this paper, the air curtain generated by air jet is used to replace the fixed wall of the original four-wall experimental platform, and an air curtain fire whirlwind platform is constructed. By comparing the data of fuel mass loss rate, the repeatability of the experiment is verified.
Secondly, the combustion rate, flame height and flame temperature of a fire whirlwind produced by an air-curtain platform are studied systematically, and the dependence of the parameters is established. The burning rate data can well fit the relationship model between burning rate and circulation proposed by predecessors. By dimensional analysis and data fitting, the dependence of flame height on the two physical parameters of heat release rate and circulation is established, and a standardized height is defined. The data is compared with the previous flame height model. It is found that the flame height data obtained in this paper are also in good agreement with previous models. The axial temperature distribution of fire whirlwind flame shows that it varies with the normalized height in the continuous flame region, intermittent flame region and plume region.
According to the grey body hypothesis and Kirchhoff's law, the emissivity of fire whirlwind flame was measured by infrared technique, and a semi-empirical model s=1-e-3.68d was obtained. The carbon was estimated based on the flame radiation theory. It is found that the emittance of black and radiated gases is very small and can be ignored. The volume fraction of carbon black in the flame is calculated by theoretical analysis of the emission of carbon black. It is found that the emittance increases at first and then decreases with the normalized height. It is 4-5 times that of ordinary pool fire, and the volume fraction of carbon black of fire whirlwind is larger than that of pool fire, which may be the reason why the fire whirlwind emittance is higher at the same flame diameter.
Thirdly, a layered radiation prediction model is established, in which the flame is divided into several layers and each layer is regarded as a grey body, i.e. the flame is uniform. The radiant heat flux of each layer is calculated, and then the radial and vertical directions of the flame are measured. By comparing the experimental data with the simulated results, it is found that the model can well predict the distribution of the radial and vertical radial heat flux, and the radial radial fraction of the flame is about 44%.
Finally, the flow field inside and outside the flame of a fire whirlwind is studied by means of a cylindrical shell. The flow field inside and outside the flame of a fire whirlwind is measured by particle image velocimetry (PIV) and the drift characteristics of the flame are analyzed. With constant angular velocity, the tangential velocity increases linearly along the radial distance, and the circulation and radial distance are exponentially quadratic; the flow outside the vortex core is a free vortex, and the tangential velocity is inversely proportional to the radial distance, and the circulation of the flame in this region is a constant value. In the boundary layer close to the wall, the radial velocity first increases with the radial distance, and then decreases gradually. With the increase of height, the radial velocity distribution is affected by the flame drift, resulting in a great error. The probability density distribution of the projection change of the flame position on the y-axis shows that the probability of the flame appearing decreases gradually along the radial distance, and the probability density distribution presents a Gaussian distribution.
【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】：2015
【分类号】：X932

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