Buoyant-flow downward flame spread over carbon fiber reinforced plastic in variable oxygen atmospheres

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This study explored the downward flame spread over carbon fiber reinforced plastic (CFRP) sheets under buoyant flow in variable oxygen concentrations. Tested CFRP sheets were fabricated by laminating two unidirectional carbon fiber (CF) sheets impregnated with epoxy resins and curing them in a high-temperature furnace. The fabricated CFRP sheets were combusted in a glovebox which allowed oxygen concentration to vary. Epoxy resin sheets were also tested to investigate the effect of CFs. SEM images of burned and unburned CFRP sheets showed that the flame spread over the CFRP sheets was driven via the pyrolysis of the impregnated epoxy resins. The limiting oxygen concentration (LOC) of the CFRP sheets was 31%, while that of the epoxy resin sheets was 19%. The difference in flammability would be produced by the different thermal inertia. Contrary to the order of flammability, the flame spread rate of the CFRP sheets was higher than that of the epoxy resin sheets in more than 31% O2. In-plane temperature distributions visualized via an IR camera suggested that the CFs conductively transferred heat from flame forward, thereby accelerating the flame spread. In other words, the CFs acted as heat conductors, such as metal cores in electrical wire fires. To predict the flame spread behaviors of the CFRP sheets, this work developed a novel simplified flame spread model that involves solid-phase heat transfer. According to the model with a simulated surface temperature profile, the analytical solution of flame spread rate was derived by formulating energy balance in each zone. When the calculated flame spread rates were compared with the measured ones, a quantitative agreement was recognized. This model would be applicable to other thermally thin high-thermal-conductivity materials as well as the CFRP sheets and therefore contribute to the fire risk assessment of such materials.