Heat and Mass Transfer During Piloted Ignition of Cellulosic Solids

This study presents the results of an experimental and theoretical investigation into the heat and mass transfer processes that occur during piloted ignition of cellulosic materials. The experiments were conducted on both horizontal and vertical samples of thick pieces of wood initially at ambient temperature. The time required to ignite the sample in the presence of a pilot flame and the surface temperature-time history of the sample were recorded. The data so obtained were used to identify the critical surface temperature at ignition. These results compare well with those previously reported in the literature. Based on the experimental observations an approximate analytical model for the piloted ignition process was developed. The model predictions are in good agreement with the experimental measurements. This model is based on the following observations: (i) Critical conditions at ignition are achieved solely by external radiation and surface radiant emission plays a dominant role in determining the surface temperature at ignition, and (ii) although the heat lost by the flame to the solid at the instant of ignition is significant and may cause thermal quenching its contribution to the enthalpy rise of the solid is negligible. Also, to simplify the description of the gas-phase combustion process the concepts of nearly constant limit diffusion flame temperature at extinction and nearly constant heat of combustion of oxygen for most hydrocarbons are profitably employed. The approximate model presented here provides simple formulas that show good agreement with the experiments. It also provides some justification for the use of critical surface temperature and critical fuel mass flux as ignition criteria.