Chemically-Passive Suppression of Laminar Premixed Hydrogen Flames in Microgravity

Effects of chemically-passive fire suppressants on laminar premixed hydrogen flames were investigated by combined use of microgravity experiments and computations. The experiments used a short-drop free-fall laboratory facility that provides at least 450 ms of 2 10 g. Near -limit laminar burning velocities were measured for outwardly propagating spherical stoichiometric hydrogen-air flames with varying concentrations of He, Ar, N2 and CO2 as suppressants. Burning velocities were also computed using the steady, onedimensional laminar premixed flame code PREMIX. Both measured and computed results showed the suppressants to increase in effectiveness in the order He, Ar, N2 to CO2. The differences in effectiveness are shown to result from increased quenching of reactions by the increased specific heat due to the suppressant and from changes in the transport rates near the flame. The addition of suppressant generally decreased Markstein numbers, which made the flames more susceptible to preferential -diffusion instability. This effect increases flame speeds and tends to counteract the effect of suppressant to reduce laminar burning velocities. The concentration needed for each suppressant to prevent flame propagation was also determined. Far from this flammability limit, agreement between measured and computed laminar burning velocities was good, but for near -limit flames the computed velocities were significantly lower than measured values. These near-limit differences may be due to thirdbody recombination rates for H+O2+M=HO2+M reactions, and in particular to the thirdbody chaperon efficacy of various species M.