The role of particles in the inhibition of premixed flames by iron pentacarbonyl

Laser light scattering has been used to investigate particle formation in F e(CO)(5)-inhibited premixed flames in order to understand the influence of metal and metal oxide condensation on flame inhibition. In premixed CH4-air flames, particles form early in the flame zone, nucleate and grow to a pea k scattering cross section, then disappear as the temperature increases to its peak value. Downstream in the postcombustion gases, the peak scattering signal is several orders of magnitude larger than the peak value near the main reaction zone of the flame. Thermophoretic particle sampling and numer ical estimates indicate nanoparticles with diameters between 10 and 30 nm. As the mole fraction of iron pentacarbonyl in the flame is increased, a con centration is reached at which both the burning velocity becomes constant a nd particle nucleation begins. A model of an ideal heterogeneous inhibitor shows that radical recombination on particle surfaces alone cannot account for the magnitude of the observed inhibition. Measurements in three CO-H-2 flames with similar adiabatic flame temperatures but different burning velo cities demonstrate the importance of residence time for particle formation in premixed flames. (C) 2000 by The Combustion Institute.