ANALYSIS OF SOOT SURFACE GROWTH PATHWAYS USING PUBLISHED PLUG-FLOW REACTOR DATA WITH NEW PARTICLE SIZE DISTRIBUTION MEASUREMENTS AND PUBLISHED PREMIXED FLAME DATA

摘要

Soot particle size distributions were measured using thermophoretic sampling, followed by electron microscopy, at different residence times during soot formation in a plug-flow reactor (PFR) under two sets of premixed C_2H_4/air combustion conditions (1 atm, equivalence ratio 2.2, and 1520 K and 1620 K) for which published concentration profiles of gas species and soot mass are available. The data were used to calculate the Harris global rate coefficient, κ_(Harris), for C_2H_2 addition to soot. The C_2H_2-soot reactivity as indicated by κ_(Harris) found to oscillate significantly and to show a net increase with residence time in the PFR, neither of which behaviors would be expected if C_2H_2 were actually the growth reactant. In contrast, use of the data to compute the global collision efficiency for dichloromethane (DCM) soluble polycyclic aromatic hydrocarbon (PAH) addition to soot, γ_(PAH-soot) gives a soot reactivity with no net increase with residence time, and smaller oscillations explainable by the observation that higher molecular weight PAH fluctuate more than the total DCM soluble fraction. Interpreting the Harris C_2H_2 addition mechanism as a globalization of the PAH addition mechanism with constant soot reactivity indicates that the oscillations and net increase of the Harris C_2H_2-soot reactivity are consistent with the variations in the concentration of the PAH reactants and particle size. Also consistent with these results are previous observations that the Harris C_2H_2-soot reactivity in premixed flames decreases sharply with residence time. Analysis here of published data from premixed one-dimensional C_2H_2/O_2 and C_6H_6/O_2/Ar flames shows that the declining C_2H_2-soot reactivity agrees with the decline in the concentration of PAH reactants and increase in soot particle size, assuming a constant PAH-soot reactivity.