Flaming Ignition Behavior of Hot Steel and Aluminum Spheres Landing in Cellulose Fuel Beds

The ignition of combustible material by hot metal particles is an important fire ignition pathway that remains relatively unstudied. In this work, the flaming ignition behavior of powdered cellulose fuel beds by hot steel and aluminum spheres of various diameters and initial temperatures was studied. Understanding ignition in this scenario could offer insight into the mechanisms by which metal particles initiate wildland fires and fires in industrial settings. Earlier work on this topic has shown that ignition propensity has a relationship with the temperature and diameter of the sphere. However, little is known about the physical processes governing this relationship. This work provides further information regarding the conditions required for ignition, and useful observations for the development of a theoretical framework for predicting ignition propensity of combustible fuel beds. For the conditions tested, powdered cellulose ignition appears to exhibit limiting behavior in two regimes: for larger spheres, temperatures below 600 C did not ignite the cellulose and spheres with diameters below 2.38 mm for steel or 2.03 mm for aluminum and temperatures up to 1100 C did not ignite the cellulose either. We also observed that in the range of sphere diameters from 4-8 mm, aluminum spheres of a given diameter are more likely to cause ignition than their steel counterparts. This seems to be due to the fact that the aluminum spheres are molten at temperatures greater than 657.2 C; melting contributes to a spheres bulk energy through the latent heat of melting and allows for sphere deformation and splatter during impact. Furthermore, qualitative analysis of high speed schlieren videos shows differences in pyrolysis and ignition behavior and suggests that, different controlling processes may be at work for spheres of different sizes and for molten versus solid spheres.