固体材料火焰传播过程中, 燃烧速率和火焰碳烟特性严重依赖于环境流动, 目前缺乏对实际环境中火焰传播过程中的燃烧速率和发烟特性的相关研究。该文利用地面常重力环境和落塔微重力设施, 开展聚甲基丙烯酸甲酯(PMMA)在不同流动环境中的燃烧特性实验。结果表明, 常重力环境中, 固体试样的质量燃烧速率与火焰化学计量轮廓面积具有良好的线性关系, 火焰中的碳烟浓度随环境流速的增大而减小; 微重力环境中, 火焰面积随气流速度非单调变化, 这与常重力环境中存在差异, 此外, 固体试样具有与常重力不同的烟点, 增大环境流速后, 碳烟浓度有增大的趋势。该研究可为常重力与微重力环境中的固体材料可燃性评价提供参考。

Objective: The flame spread rate, burning rate, and heat release rate are the key aspects of flammability, which determines the fire development process and the intensity of the heat release. The burning characteristics of a solid fuel strongly depend on the environmental conditions, such as the oxygen concentration, flow rate, and ambient pressure. Most studies have focused on the flame spread rate, and only a few have focused on the burning rate, heat release rate, and soot generation characteristics. When the burning rate of solid materials exceeds the smoke point, the distribution of soot within the flame and the volume fraction of soot undergo a large transformation, thus affecting the heat release rate and changing the flame propagation process. In addition, the generation and transport of soot are crucial for fire safety. An urgent need exists to understand the combustion and soot behavior during flame propagation in real fire scenarios. Methods: In this study, flame spread phenomena over a cylindrical polymethylmethacrylate (PMMA) at different airflow velocities have been experimentally studied under microgravity and normal gravity conditions. Microgravity experiments were performed in a drop tower. In microgravity experiments, flame spread in purely opposed flow was observed, and in normal gravity experiments, downward flame spread behaviors in the mixed flow with buoyancy-induced and forced flows were investigated. The airflow velocities used in both experiments were 1-35 cm/s, and the diameter of the solid sample was 2—10 mm. In the normal gravity environment, the variation in the sample mass during the flame spread process was recorded using an electronic balance, and the soot volume fraction inside the flame was tested using the light extinction method. In both sets of experiments, the luminescent flame and the stoichiometric flame contour photographed with the CH filter were recorded. Results: The flame area, which is estimated from the stoichiometric contour of the CH radicals of the flame, shows a good linear correlation with the measured mass burning rate. Meanwhile, the flame area decreases with increasing flow rate in a normal gravity environment, while in a microgravity environment, the flame area increases to a maximum value and then decreases with increasing opposed flow velocity, indicating a nonmonotonic variation trend. The soot formation of PMMA specimens depends on the diameter of the specimen and the flow conditions, and the experiments in normal gravity show that larger specimen diameters and lower flow rates favor soot formation. However, the flow velocities corresponding to the smoke points of PMMA specimens in different gravity environments are quite different. The flow velocities corresponding to the smoke points of specimens in microgravity environments are even lower. In normal gravity, the soot concentration in the flame decreases with increasing flow velocity. In contrast, in microgravity, solid materials have different smoke points, and the soot concentration increases with the convection velocity. Conclusions: The fuel burning rate and soot formation depend on the airflow velocity. The relationship between the flame area and the burning rate is independent of the fuel smoke point. Because of the variation in the flow condition, the resident time and oxidization time become different, resulting in variation in the soot formation characteristics.