Laminar burning velocity of diacetyl + air flames. Further assessment of combustion chemistry of ketene
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Ketene is important intermediate in high-temperature chemistry of several oxygenates, such as acetone, acetic acid, and diacetyl. Ketene reactions appear in the sensitivity spectra of calculated burning velocities of the first two species. To provide independent experimental data for validation of the ketene sub-mechanism, the laminar burning velocities of diacetyl + air flames at 1 atm and initial gas temperatures of 298 K, 318 K, and 338 K were determined for the first time. Measurements were performed using the heat flux method in non-stretched flames, stabilised on a perforated plate burner at adiabatic conditions. Recently developed detailed kinetic mechanism for acetic acid flames with updated ketene sub-mechanism has been extended by reactions of diacetyl and includes revised rate constants for reactions of acetaldehyde and acetyl radical. The model was first compared with available experimental data on ketene pyrolysis and oxidation. Its performance in prediction of C2 species formation was improved by significant reduction of the previously estimated rate constants of ketene reactions with CH3 and CH2 radicals. The updated mechanism was then compared with the new measurements for diacetyl and earlier data for acetaldehyde, acetone and acetic acid flames. The model closely reproduces burning velocity of diacetyl + air in lean and rich mixtures while underpredicts in stoichiometric and slightly rich flames. Performance of the model for acetaldehyde + air flames was much improved as compared to the Konnov mechanism version 0.6. Good agreement of the modelling with experimental data for acetone + air flames was also demonstrated. The disparity between predicted burning velocities of acetic acid and recent measurements did not change. The model was further examined using sensitivity analysis for these flames to elucidate common reactions affecting its performance. It was concluded that the mechanism performance in prediction of the burning velocities of acetic acid flames could be improved by revision of reactions between CH2CO and OH radicals, while keeping its agreement with other flames studied. Remaining uncertainties in the ketene sub-mechanism are outlined.