Providing effective constraints for developing ketene combustion mechanisms: A detailed kinetic investigation of diacetyl flames

Ketene is an important combustion intermediate, but due to its high reactivity, it has been difficult to validate the corresponding kinetic sub-mechanisms with direct experimental measurements. It is shown here the possibility to refine the ketene combustion model through relevant experimental and modeling analyses of diacetyl [2,3-butadione, (CH3CO)2] combustion. The consumption of diacetyl leads to abundant ketene under high-temperature oxidation conditions. From the aspect of the hierarchical structure of a kinetic mechanism, the ketene sub-mechanism acts as a part of a secondary mechanism in the kinetic scheme exploring diacetyl combustion. Therefore, this strategy to understand the kinetics of ketene through studying the combustion chemistry of diacetyl should be based on a good understanding of the diacetyl combustion kinetics. To this end, we investigated the flame chemistry of diacetyl through experimental, modeling, and theoretical approaches. Flame-sampling molecular-beam mass spectrometry with single-photon ionization was employed to measure the detailed chemical structure of a premixed diacetyl flame stabilized at the pressure of 18.0 Torr and the equivalence ratio of 1.2. The quantitative speciation measurements were essential in identifying important reactions in the fuel sub-mechanism, and the corresponding rate coefficients were obtained through high-level theoretical calculations. A kinetic model was proposed, which showed satisfactory performances in predicting the measured flame structure. With the diacetyl flame chemistry better understood, global sensitivity analyses were carried out to suggest conditions for future experiments potentially facilitating the development of ketene sub-mechanisms.