Ignition of fires

This paper discusses the mechanisms that lead to ignition of fires and the reasons behind the experimental correlations available in the literature. T he objective is to understand and quantify the physics of heat and mass tra nsfer and the chemistry of solid-phase decomposition and gas-phase runaway reactions that result in the appearance of a sustained gas-phase diffusion flame-a phenomenon identified as ignition. Both spontaneous (auto) and pilo ted (forced) ignition phenomena are discussed. Two types of materials commo nly found in building fires are considered-thermoplastics that melt and vap orize upon heating, and cellulosic materials that decompose and produce cha r. A general theoretical model is derived and specific numerical and analyt ical solutions are discussed in the light of experimental evidence and data . It is concluded that within the approximation of constant surface tempera ture at ignition, the ignition delay data may be correlated by a simple the rmal model based on inert heating of the solid. However, a significantly mo re complicated description which includes gas and solid-phase chemistry is required if the surface temperature at ignition is not constant.