Theoretically predicting the flame-spread limit of carbon-fiber-reinforced plastic

This work studied the flame-spread limit of carbon-fiber-reinforced plastic (CFRP) sheets and attempted estimating it theoretically. First, to measure the limiting oxygen concentration (LOC), the CFRP sheets with different carbon fiber (CF) orientations were combusted under variable opposed-flow velocity (Vg) and oxygen concentration. The flammability decreased as the CF orientation angle (θ) was increased. Increasing θ facilitated conductive heat losses sideways through the CFs, resulting in the low flammability. The LOC decreased with increased Vg. A flame leaned toward the solid surface as Vg was increased, leading to the high heat transfer rate from the flame. The increased heat transfer rate accelerated the flame spread to decrease the LOC. The decrease in the LOC was similar to the trend in the radiative regime for polymers. Next, the LOC was estimated via an equation that describes the radiative extinction. The calculated flame-spread limit, however, did not quantitatively agree with the measured LOCs, because that equation was derived from the conventional flame spread model that does not consider the solid-phase heat transfer. Nevertheless, the solid-phase heat transfer was dominant in the flame spread over the CFRP sheets. Then, a novel equation on the radiative extinction was formulated according to the flame spread model of CFRP. Although the response of the flame spread rate and the LOC to Vg was well simulated, a good agreement between the computed and measured LOCs was still not obtained. This is probably because the observed extinction of the CFRP sheets would not be governed by the radiative extinction, though both the radiative and blow-off extinctions are dominant. Therefore, to obtain the correct flame-spread limit, it is necessary to develop and solve an equation that considers both extinctions with true properties of the CFRP sheets.