Researches On The Flame And Flow Characteristics Of Fire Whirls

Frequently observed in forest fires, wildland fires and urban fires, fire whirl is a special fire phenomenon with intense rotation, which is induced by the complicated interaction between the combustion and the ambient rotational circulation. Compared to general pool fires, the burning rate, flame height, flame temperature and flame emissivity of fire whirls are highly increased, thus emitting much more radiative energy. Moreover, fire whirl is also a rotational vortex with complex structure. In the real fire scenario, the huge velocity field of fire whirls can lift up fuels around the flame and produce firebrands to create a large number of new fire ignitions, thus accelerating fire spread. Therefore, the research on combustion and flow characteristics of fire whirl can help us clearly understand this special natural phenomenon, and provide theoretical basis for prevention and suppression of fire whirls.The aims of this thesis are as follows:designing and constructing a new fire whirl facility in the open air by using air jet; studying the combustion characteristics of fire whirls such as burring rate, flame height and flame temperature, and finding the dependence of these dynamic parameters on flame size, heat release rate and ambient circulation; systematically investigating the radiative characteristics of fire whirl, including the flame emissivity, soot volume fraction and radiative heat flux, and developing a radiative model; examining the flow characteristics of fire whirl in detail, including axial, tangential and radial velocity distribution.The detailed work and results are summarized as follows.Firstly, a new air curtain fire whirl facility was constructed. The facilities previously used by other researchers to simulate fire whirl in laboratory can be divided into two types:mechanically driven facility and thermally driven facility. All the facilities are of the solid-wall enclosure type, for example the four-fixed wall structure. Due to the solid walls, it is difficult to examine the effect of fire whirl on exterior objects, especially the flame radiation. In this work, we used air curtains flowing upward, instead of the solid walls of four-fixed wall structure, to form a square enclosure and induce fire whirl. The characteristics of air curtains we used were analyzed in detail and the optimal tilt angle of the air curtain was determined to achieve the steady fire whirl phenomenon. According to the comparison of the mass loss rate data, the reproducibility of the experiments was verified.Secondly, the burning rate, the flame height and the flame temperature of fire whirls were systematically studied, and the correlations of these parameters with flame size, heat release rate and the ambient circulation were developed. The analysis of burning rate data suggested that the mass loss process of the fire whirls produced by the new facility can be divided into five stages including a long-term quasi-steady burning stage with a constant mass loss rate, and the burning rate data fit well with the model between the burning rate and the ambient circulation proposed by other researchers. According to dimensional analysis and data fitting, a correlation of flame height with the heat release rate and the ambient circulation is developed, whereby a normalized flame height can be defined. At the same time, by comparison between the data we obtained and the flame height model from other researchers, it was found that the flame height data can also fit well with the models in literature. The centerline temperature distribution showed that the temperature changes differently in continuous flame region, intermittent flame region and plume region. The radial temperature distribution is of the typical hump-type, which suggests the existence of the fuel rich region in the flame.Thirdly, the flame emissivity and soot volume fraction of fire whirls were analyzed in detail. Based on the grey body assumption and Kirchhoff s law, the flame emissivity was measured by using infrared method in the experiment, and a semi-empirical correlation of ε=1-e-3.68d with the effective extinction coefficient of3.68was developed to correlate the flame emissivity and the flame diameter. According to the radiation theory, the soot emissivity and gas emissivity were estimated, which suggested that the gas emissivity can be neglected. By the theoretical analysis on soot emission, the soot volume fraction was calculated. It was found that the soot volume fraction of fire whirl first increases slowly with normalized height and then has a tendency of decrease. Compared to general pool fires, the effective extinction coefficient of fire whirl is4-5times higher than that of pool fires; and the soot volume fraction is also larger than that of pool fires, which may be one of the reasons why the flame emissivity is higher than that of pool fires under the same flame diameter.Fourthly, a semi-empirical model was proposed to predict the radiant heat flux of fire whirl. In this model the flame is divided into multi-zones, and each zone is considered as grey body. The radiant heat flux from fire whirl can be achieved by the sum of the radiation from multi-zones. In the experiment the vertical and radial radiative heat flux were measured by flux sensors. The comparison of the measured radiation heat fluxes by sensors and predicted data by the model showed that the model can well predict the radiation from fire whirl. From the vertical and radial radiative heat flux distributions, the radiative fraction of fire whirl was calculated to be almost a constant of44%Finally, the flow characteristics of fire whirl was investigated in detail. The experiments were conducted by using a split cylinder. The velocity fields in the flame and that outside the flame were measured by using Particle Image Velocimetry (PIV) method. The flame wander behavior of fire whirl was also analyzed. The data analysis indicated that fire whirl is a steady vortex. In the vortex core, the fluid particles rotate as a solid body with a constant angular velocity, while the circulation in the core increases with2power of radial distance; Outside the vortex core, the fluid motion can be characterized as free vortex. In this region the tangential velocity is inversely proportional to radial distance, with a constant circulation. The vertical velocity profile fits well with Gaussian distribution, and at the centerline the vertical velocity reaches the maximum value, which means that the vertical velocity in fire whirl flame sharply decreases with radial distance and then it tend to be zero outside the flame. In the vicinity of the base table surface, the radial velocity first increases with radial distance and then decreases gradually. The effect of flame wander on radial velocity increased with height. The variations of the flame position in horizontal direction induced by flame wander were achieved and the probability density function was calculated, which indicated that with the radial distance the probability that the flame appears decreases and the probability density distribution fits well with Gaussian distribution.