Characterizing the Flaming Ignition of Cellulose Fuel Beds by Hot Steel Spheres

In this work, the flaming ignition of powdered cellulose fuel beds by hot steel spheres of various diameters and initial temperatures is studied. The spheres were heated in a tube furnace and then dropped onto a fuel bed, and the occurrence of flaming ignition or lack thereof was visually observed and recorded. A multivariable logistic regression method was developed and used to obtain an approximate ignition probability distribution and flaming ignition limits for the parameter space investigated. Each test was also recorded using high speed Schlieren videography, which gave qualitative insight into the ignition process. Ignition of larger spheres (>8.5 mm) appears to depend more strongly on sphere temperature than diameter. Alternatively, ignition by small spheres (<3.5 mm) appears less sensitive to temperature and more sensitive to sphere size. It was also confirmed that ignition cannot be predicted by bulk sphere energy alone. Flaming ignition occurs in the gas phase, in some cases away from the ball surface. Based on this and other evidence, it is proposed that ignition occurs when heat generation in the surrounding mixture of gasified fuel and air over-comes diffusive losses long enough for thermal runaway to occur. It is also noted that bulk energy may be important for predicting ignition by small spheres because of the increasing importance of the energy required to pyrolyze and produce an ignitable mixture.