The mechanism of fuel surface cooling with water were studied experimentally by visualising a water droplet impacting on surfaces of burning wood. This droplet impact condition was investigated experimentally for the first time. The main focus of this current investigation is to provide a behaviour description of a water droplet impacting a burning solid surface, with the intent to clarify the differences between impingement on cold surfaces and burning real application materials such as wood. Wood was used in the tests to simulate the temperature and surface characteristics of burning specimens. Sizes of water drops were in a range between 0.95 mm and 2.1 mm, resulting in a Weber number from 52 to 264. Visualisation of impact proved to be a challenge because of the optical characteristics of wood undergoing pyrolysis, the required high shutter speed and the very small droplet size. Images were improved significantly and used for quantitative measurements through careful post-processing and the application of a sequence of enhancement techniques. A new mechanism of fire extinguishment is proposed based on the sealant effect of the water liquid layer. The results showed a clear divergence in the behaviour of water splash on burning wood compared to cold surfaces. Prompt splash, receding breakup, and deposition were observed to occur at the initial stage of the droplet impact. The critical Weber number at which the impact drop breaks up and splashes with sub-droplets was shown to be lower for the tests on burning surfaces. Different types of wood also showed the dependence of droplet dynamics. Furthermore, the increase in droplet impact velocity revealed that most of the wetting was achieved during the first few milliseconds after impact and resulted in faster evaporation. Finally, a conceptual model is presented to summarise the behaviour of a water droplet impact on wood surfaces.

通过观察水滴在燃烧的木材表面上的撞击，实验研究了水对燃料表面的冷却机理。首次对这种液滴的撞击条件进行了实验研究。本研究的主要重点是提供水滴撞击燃烧的固体表面的行为描述，以阐明撞击在冷表面上和燃烧真实的应用材料（例如木材）之间的差异。测试中使用木材来模拟燃烧样品的温度和表面特性。水滴的大小在0.95毫米至2.1毫米之间，韦伯数在52到264之间。可视化的影响被证明是一个挑战，因为木材经历了热解，所需的高快门速度和非常小的墨滴尺寸。通过仔细的后处理和一系列增强技术的应用，图像得到了显着改善，并用于定量测量。基于水液层的密封作用，提出了一种新的灭火机理。结果表明，与寒冷表面相比，燃烧木材上的水飞溅行为存在明显差异。在液滴撞击的初始阶段，观察到迅速的飞溅，后退的破裂和沉积。对于在燃烧表面上进行的测试，冲击滴破裂并飞溅到子滴的临界韦伯数表明较低。不同类型的木材也显示出液滴动力学的依赖性。此外，液滴冲击速度的增加表明，大多数润湿是在冲击后的最初几毫秒内实现的，并导致更快的蒸发。最后，提出了一个概念模型来总结水滴撞击木材表面的行为。