Study on the mechanism of molten aluminum droplets igniting EPS foam

Abstract

Molten metal droplets with higher fire risk may cause large fires. The ignition by molten metal droplets is more complicated than by solid inert spots due to the latent heat release of solidification and fluid spread involved. There are few studies on the ignition by molten metal with liquid behavior at present. The mechanism of molten aluminum droplets igniting EPS foam was studied in this work. A small spherical aluminum particle (4-12 mm in diameter) was heated and melted to a pre-set temperature (740-1000 °C) and was dropped to a foam sample at a fixed height (20 cm). It was observed that the droplet became a splat after a collision due to the flow of liquid aluminum. The splat diameter was recorded to analyze heat transfer during the collision process. Results show that as the diameter increased from 4 to 12 mm, the critical temperature to ignite decreased from 975 to 760 °C. Compared with solid hot particles, the critical temperatures for ignition and fire spread by molten aluminum droplets were much lower. The internal energy of droplets was also investigated, and the energy per unit contact area was found to be a critical parameter (about 2.5 MJ/m2), which determines whether the ignition will occur. The diffusion of combustible gas was numerically studied and was found consistent with the trend of experimental results. Moreover, the effects of temperature and the initial size of droplets on gas mixing time were also theoretically analyzed. The competition between the gas mixing time and the splat residence time explained the variation of critical ignition temperature with droplet size.