Experimental study of water droplet impact on burning wood surfaces

Abstract

The mechanism of fuel surface cooling with water was 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.