Catalytic Inhibition of Laminar Flames by Transition Metal Compounds.
Catalytic Inhibition of Laminar Flames by Transition
Linteris, G. T.; Rumminger, M. D.; Babushok, V. I.
Progress in Energy and Combustion Science, Vol. 34, No.
3, 288-329, June 2008.
Sponsor:Department of Defense, Washington, DC
laminar flames; metal compounds; flame extinguishment;
catalytic inhibition; halon alternatives; fire
suppression; nanoparticles; fuel additives; soot
formation; premixed flames; flame retardants; ignition;
nozzles; metals; vapor phases; iron; tin; manganese;
kinetics; premixed flames; counterflow flames; diffusion
flames; chemical inhibition; condensation
Some of the most effective flame inhibitors ever found
are metallic compounds. Their effectiveness, however,
drops off rapidly with an increase of agent
concentration, and varies widely with flame type. Iron
pentacarbonyl, for example, can be up to two orders of
magnitude more efficient than CF3Br for reducing the
burning velocity of premixed laminar flames when added
at low volume fraction; nevertheless, it is nearly
ineffective for extinction of co-flow diffusion flames.
This article outlines previous research into flame
inhibition by metal-containing compounds, and for more
recent work, focuses on experimental and modeling
studies of inhibited premixed, counterflow diffusion,
and co-flow diffusion flames by the present authors. The
strong flame inhibition by metal compounds when added at
low volume fraction is found to occur through the
gas-phase catalytic cycles leading to a highly effective
radical recombination in the reaction zone. While the
reactions of these cycles proceed in some cases at close
to collisional rates, the agent effectiveness requires
that the inhibiting species and the radicals in the
flame overlap, and this can sometimes be limited by
gas-phase transport rates. The metal species often lose
their effectiveness above a certain volume fraction due
to condensation processes. The influence of particle
formation on inhibitor effectiveness depends upon the
metal species concentration, particle size, residence
time for particle formation, local flame temperature,
and the drag and thermophoretic forces in the flame.