Radiant smoldering ignition of plywood

by Gratkowski, Mark T.

This paper investigates the thermal conditions at the surface and at depth of 1.8 cm (3/4-inch) maple plywood exposed to heat fluxes between 6 and 15 kW/m2 in the cone calorimeter for up to 8 hours. The minimum heat flux for unpiloted smoldering ignition was 7.5 kW/m2 and compared favorably to classical self-heating theory. The role of selfheating was explored via temperature measurements distributed within the specimens. Elevated subsurface temperature profiles indicated self-heating was an important ignition factor resulting in ignition at depth with smolder propagation to the surface and into the material. The ignition depth was shown to be a function of the heat flux with the depth moving towards the surface as the heat flux increased. Supporting work included sensor calibration testing, mass loss rate analysis, char depth testing and computer modeling. The calibration testing showed optical pyrometer temperature measurements compare favorably to those of surface mounted thermocouples. Mass loss rate analysis was found to be a lagging indicator of smoldering ignition. The char depth tests showed that the rate of change of the temperatures recorded at depth increased around the time the derived char front passed. Computer modeling (HEATING) of a heat flux applied to the plywood for conditions similar to the performed ignition tests compared favorably to experimental data for sub-critical incident heat flux temperature profiles, excepting surface temperatures. For heat fluxes near critical, the model correctly predicted thermal runaway below the sample surface. At higher heat fluxes simulation results indicated surface ignition at times significantly earlier than experimental results. i