Experimental investigation on the smoldering limit of scraps of paper initiated by a cylindrical rod heater

Abstract

The ignition temperature for smoldering in scraps of paper has been determined experimentally for several bulk densities under different external air flows. Beds of compressed scraps of paper were packed into a vertical combustion chamber to achieve different bulk densities and the temperature of an electrically set rod-heater was heated to temperatures between 350 and 450 °C. Once the rod-heater had reached the set temperature, the electric current supply was then turned off and the heater was embedded into the middle of the vertical combustion chamber filled with a packed bed of paper scraps. We obtained the ignition limit of the scraps of paper for different bulk densities under different external air flows by recording the temperature evolution at different positions in the combustion chamber. Results show that without external air flow, the ignition temperature first decreased with an increase in bulk density, then after a critical value, an increase in bulk density led to an increase in the ignition temperature. Furthermore, we found that even under a quiescent case (without external air flow), for the cases where ignition occurred, the smoldering front first reached the bottom side of the combustion chamber, then propagated to the upper side. As for cases with external air flows, again the propagations of the smoldering front towards the bottom side of the combustion chamber were observed in the cases where ignition happened, and external air flows played different roles in ignition limit of paper scraps bed with low bulk density, medium bulk density and high bulk density regime, respectively.