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Abstract
A study is presented on the effect of oxidizer flow characteristics on the piloted ignition and opposed flame spread of a slab of PMMA heated by an external radiant flux. The objective is to establish the basis for a potential new lest method that may be used to determine the flammability performance of solid materials in terms of their ignition delay, critical heat flux for ignition, and flame-spread rate, for varied oxidizer flow conditions. The proposed Forced-flow Ignition and flame-Spread Test (FIST) follows the concepts of the LIFT (ASTM E 1321-93), and thus consists of a combination of ignition delay and flame spread experiments as a function of an externally applied radiant flux, but incorporates controlled forced convection as the predominant mechanism for the gas-phase transport of heat and mass. PMMA slabs were tested to assess the applicability of the methodology, and results are presented for radiant fluxes ranging from 0 to 35 kW/m2, forced oxidizer flow velocities of 1.0, 1.75, and 2.5 m/s. and natural convection. Results of the variation of ignition delay as a function of free stream oxygen concentration (18 to 45%) are also presented. Following the LIFT methodology, the ignition delay and flame spread data are used to assemble flammability diagrams for PMMA at different oxidizer flow velocities. Additional natural convection tests were performed in the LIFT apparatus with the purpose of establishing a base line for comparison of results. It is shown that the resulting FIST and LIFT flammability diagrams are similar. However, the diagrams as well as the parameters extracted from them depend on the oxidizer flow velocity, resulting in families of flow-dependent flammability diagrams. As a result of the use of forced convection, it is considered that the FIST is suitable for characterizing solid material flammability in mixed regimes ranging from natural to predominantly forced convection, and at different oxygen concentrations. This could provide a means to more accurately rank the relative flammability of solid combustible materials that would be used in environments where the oxidizer flow differs from air in natural convection, such as areas with significant air currents, vitiated environments, or microgravity applications (space based facilities).
