The flaming ignition behavior of powdered cellulose fuel beds by hot steel spheres of various diameters (2.38 to 15.9 mm) and initial temperatures (625°C to 1100°C) was studied using high speed schlieren videography. Understanding ignition in this scenario could offer insight into the mechanisms by which metal particles initiate wildland and Wildland Urban Interface (WUI) spotting fires. 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: sphere temperatures below 650°C did not ignite the cellulose and spheres with diameters below 3.18 mm and temperatures up to 1100°C did not ignite the cellulose either. The experiments suggest that the ignition time is considerably shorter (88 ms on average) in tests using spheres with diameters greater than 7 mm, while those with diameters less than 7 mm take considerably longer (262 ms on average) to ignite. Furthermore, qualitative analysis of high speed schlieren videos shows differences in pyrolysis and ignition behavior and suggests that, in these limiting regimes, there are different processes controlling ignition and that the particle requirements for ignition may be different.
展开▼
