Weakly Bound Free Radicals in Combustion: “Prompt”
Dissociation of Formyl Radicals and Its Effect on Laminar Flame Speeds
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Abstract
Weakly
bound free radicals have low-dissociation thresholds such
that at high temperatures, time scales for dissociation and collisional
relaxation become comparable, leading to significant dissociation
during the vibrational–rotational relaxation process. Here
we characterize this “prompt” dissociation of formyl
(HCO), an important combustion radical, using direct dynamics calculations
for OH + CH<sub>2</sub>O and H + CH<sub>2</sub>O (key HCO-forming
reactions). For all other HCO-forming reactions, presumption of a
thermal incipient HCO distribution was used to derive prompt dissociation
fractions. Inclusion of these theoretically derived HCO prompt dissociation
fractions into combustion kinetics models provides an additional source
for H-atoms that feeds chain-branching reactions. Simulations using
these updated combustion models are therefore shown to enhance flame
propagation in 1,3,5-trioxane and acetylene. The present results suggest
that HCO prompt dissociation should be included when simulating flames
of hydrocarbons and oxygenated molecules and that prompt dissociations
of other weakly bound radicals may also impact combustion simulations
