Ramifications of including non-equilibrium effects for HCO in flame chemistry

Abstract

The formation and destruction pathways of the formyl radical (HCO) occupy a pivotal role in the conversion of fuel molecules (and their intermediates) to eventual products CO and CO2, and therefore, HCO has been a prescient indicator for heat release in combustion. In this work, we have characterized the impact of including non-equilibrium effects for HCO, i.e. “prompt” dissociation of HCO to H+CO, in simulations of laminar flame speeds for archetypal hydrocarbon and oxygenated molecules relevant to combustion. Prompt dissociation probabilities for HCO were systematically applied to all elementary reactions that included this radical (as either a product or reactant) in literature combustion kinetics models. Simulations with the prompt HCO dissociation corrected models predicted a 7–13% increase in laminar flame speeds at 1atm for the fuels characterized here (CH4, n-C7H16, CH3OH, CH3OCH3) relative to the predictions using the original models. It is evident that simulations of other fuel-air flames at 1atm will be similarly impacted, suggesting the indispensability of incorporating these non-equilibrium effects for predictive flame modeling. Simulations of higher pressure (10atm) heptane-air flames predicted a more modest effect (<5%) of incorporating these non-equilibrium effects. Additionally, species profiles in low-pressure (0.03atm) flames of CH2O and auto-ignition delay simulations (1.4atm) for CH2OO2Ar mixtures were also impacted to a noticeable extent. Lastly, it is also worth noting that prompt dissociations are a ubiquitous feature of all weakly-bound radicals; the kinetics of many of which (C2H3, C2H5, CH3O, CH2OH, etc.) are central to our current understanding of combustion chemistry. Theory/modeling studies are in progress to address the relevance of prompt dissociations in these weakly-bound radicals to combustion modeling.