The opposite influences of flame suppressants on the ignition of combustible mixtures behind shock waves

The experiments were performed to investigate the influence of the flame suppressants CCl4, CF3H, and C2F4Br2 on the ignition of hydrogen-oxygen, acetylene-oxygen and methane-oxygen mixtures behind shock waves. The temperature dependencies of the ignition delay times and temperature profiles in the induction zone were measured. The experiments show that CCl4 and C2F4Br2 inhibit the ignition of hydrogen and acetylene and promote the ignition of methane. The influence of CF3H on the inhibition is much smaller. A qualitative kinetic analysis suggests that at low temperatures typical for hydrogen and acetylene ignition, the flame suppressants participate in only chain termination reactions inhibiting the ignition. For methane ignition at higher temperatures, the pyrolysis of admixture results in the formation of active radicals, which initiate chain reactions of combustion. The results prove that the chemical stability of the suppressants molecules is essential for successive chemical suppression of ignition under given conditions. The strong inhibiting effect of C2F4Br2 on ignition of H2/O2 mixtures behind shock waves is notable.