Piloted ignition times,critical heat fluxes and mass loss rates at reduced oxygen atmospheres

Ignition, pyrolysis and burning of materials in reduced oxygen atmospheres occur when recirculating combustion gases are mixed with the air flowing into an enclosure. Still the incoming air can be sufficient for the complete combustion of the pyrolysis gases. Thus, for the prediction of fires in enclosures it is essential to understand the ignition and burning of materials in a reduced oxygen atmosphere even when plenty of oxidizer is available for complete combustion. Previous work employing gaseous fuels has shown that under these conditions, but before extinction, burning of gaseous fuels issuing from a nozzle is complete but radiation from the flames decreases owing to the reduction of their temperature. Complementary to that work, piloted ignition of solids is investigated here at reduced oxygen concentrations by measuring the ignition times and mass loss rates of the solid at ignition.These results were obtained in a cone calorimeter modified to supply air at reduced oxygen concentrations. Two types of plywood, normal and fire retardant 4 mm thick were examined at three imposed heat fluxes 25, 35 and 50 kW/m² and at oxygen concentrations of 21%, 18% and 15% by volume. Because heating at these heat fluxes and material thickness corresponds to intermediate thermal conditions (i.e. neither thin nor thick), novel analytical solutions are developed to analyze the data and extract the thermal and ignition properties of the material. The same novel solutions can be applied to modeling concurrent or countercurrent flame spread. Moreover, a theory for piloted ignition explains why the ignition times and mass pyrolysis rates are weakly dependent on reduced oxygen concentrations.