Improving the Predictive Accuracy for Ketene in Diacetyl Laminar Premixed Flames: Experiment and Model Analysis.

Ketene is an important species in core mechanisms for the combustion of hydrocarbon and oxygenated fuels, but direct experiments with ketene are challenging to conduct due to its high reactivity. Diacetyl can be used as a precursor of ketene, and abundant ketene is present in premixed flames of diacetyl. However, predictions of ketene in diacetyl flames with previous models have significant uncertainties. The study of Sun et al. [Sun, W.; Wang, J.; Huang, C.; Hansen, N.; Yang, B. Combust. Flame, 2019, 205, 11-21, DOI: 10.1016/j.combustflame.2019.03.037] shows that the flame structure measurements should be performed under certain conditions to improve the predictive accuracy of ketene in diacetyl flames. In this work, the structures of three laminar premixed flames of diacetyl under atmospheric pressure in a range of equivalence ratios are examined with flame-sampling molecular-beam mass spectrometry (MBMS). With the new experimental data and the data available in literature, Bayesian analysis is performed to optimize the kinetic model. The obtained optimized model is compared with the original one, and the results show that the optimized model agrees better with the experimental data than the original one. The uncertainties of the rate coefficients of some key reactions are constrained with these experimental data, which eventually leads to smaller modeling uncertainties for ketene concentrations under studied conditions.

Propylene Oxide Addition Effect on the Chemical Speciation of a Fuel-Rich Premixed n-Heptane/Toluene Flame.

1,2-Propylene oxide (PO, C3H6O) is considered as a promising agent for improving fuel. In this work, the effect of PO additives on the species pool in a premixed burner-stabilized fuel-rich (ϕ = 1.6) flame fueled by n-heptane/toluene mixture (7/3 by volume of liquids) at atmospheric pressure is studied by the flame-sampling molecular beam mass spectrometry and numerical modeling in order to get insight into the chemical aspects of the influence of oxygenates with an epoxy group on the formation of abundant intermediates (including PAH precursors) during combustion of fossil fuels. The flames with various loadings of PO in the fuel blend (from 0 to 16.3% in mole basis) are examined, and detailed kinetic mechanisms available in the literature are validated against the measurements of mole fraction profiles of reactants, major products, and many intermediate species. A higher reactivity of the fresh mixture and a reduction in the peak mole fractions of intermediates playing an important role in PAH formation (benzene, styrene, ethylbenzene, phenol, acetylene, diacetylene, etc.) are observed when PO is added. This was found to be due to simultaneously two factors: the partial replacement of "sooting" fuel (toluene, which is the main precursor of these species) with oxygenated additive, and the changes in the flame radical pool caused by PO addition. Propylene oxide additive was found to change the ratio between H, OH, O, and CH3 toward an increase in the proportion of O and CH3. The detailed kinetic mechanisms considered in the work are found to overpredict the peak mole fraction of acetylene, a key species playing a crucial role in PAH growth. Its chemistry is revisited in order to provide a better prediction of C2H2 and, as a result, PAHs.