Study On The Dynamic Process Of Single Drop Impact On Charring Flammable Solid Surface

As the global environmental problem such as climatic anomaly, global warming, haze, etc become more and more series for the air pollution, while the traditional Halon fire extinguishing agents become the assistant to break the ozone. Water mist fire expression technology is regarded as the main alternative technology of halon extinguishing agent for many merit such as environmental friendly, high suppression effectiveness, high speed fire suppression and reasonable price, so it has been widely pay attention to.Many fire research workers focused on improving the performance of water mist fire suppression and the mechanism of water mist fire suppression for it is very important in social security. While focus on the research of drop impinges mechanism upon the heated solid surface can make the recognition of the microscopic of fire extinguish by using water mist more clear.Firstly, a small scale experiment of pure water and water with4%AFFF drop impacting upon normal and heated charring solid surfaces such as wood and paper surfaces was considered. A high-speed photography equipment was used during the experiment. The results show that heated charring phenomena on paper surfaces has a big influence on the droplet dynamic process. Some vapor bubbles occurred on the heated charring archival paper surface. The critical Weber nμmber of drop splashing is also affected by the heated charring characteristics.Compare with the dynamic progress of pure water and water with4%AFFF drop impacting on the normal and heated charring Betula costata surface. The result shows that as the pure water drop impact on heated wood surface, it would evaporate soon and splash to a lot of second shot droplets which rebound to a big liquid column and produce a few waves. And a few milliseconds later, it stopped on the wood surface as a small liquid column for the effect of thermal decomposition of wood. This phenomenon is similar to Leiden frost phenomenon. Otherwise, the maximum spread factor of pure water impacting on charring wood surface is bigger than the normal one. While the dynamic process of water with4%AFFF drops impact on the hot wood surface was wet well with wood surface. This result explained that after the water mist added with4%AFFF additive, it can more efficiently wetted with solid surface, and the more rapid heat transfer with heated solid surface, cool down heated flammable solid surface from the microscopic mechanism.In order to study the dynamic process of drop impact on different temperature flammable solid surface and the solid surface characteristics effect on the whole process. Three kinds of wood such as Lemon-scented gμm, cedar wood and pine tree were considered for they are commonly used in buildings. All the wood surfaces were heated from26℃to240℃.The experiment result shows that the wood surface roughness increased with the surface temperature. The dynamic progress of water impinges on different temperature wood surfaces shows that:When the water drop impact on26℃-120℃Lemon-scented gum surface, no bubbles and drop splash occurred. While at the surface temperature of173℃, the drop splashed to second multi-satellite droplets at2ms. At35ms, crystal type small bubbles formed and the bubble increased as the temperature increased. When the Lemon-scented gum surface temperature increased to220℃, a lot of boil off gas formed around the surface. While on the biggest surface roughness pine tree surface, the drop splashed at75℃and also formed liquid column which firstly increased and then decreased as the surface temperature increased. However, on the smallest surface roughness cedar wood surface, neither did drop splash occur nor bubbles formed upon all of the different temperatures (26℃-240℃) cedar wood surface.