In order to understand the shift in reactivity between the two
fuels at stoichiometric and lean conditions, OH sensitivity analyses
(not shown here) were performed for stoichiometric and lean
mixtures at 1300 K and 3 atm. The sensitivity analysis for 3-buten-2-one
reveals that two second order reactions involving oxygen
affect the OH concentration close to ignition. These two reactions
include the dominant chain branching reaction H + O2 ) OH + O
and C2H3 + O2 ) CH2CHO + O. For 2-butanone, only the chain
branching reaction (H + O2) affects the OH concentration with minimal
effect of the C2H3 + O2 reaction. When the equivalence ratio
decreases to 0.5, which basically doubles the O2 concentration since
the fuel is kept constant at 0.5%, two oxygen-containing reactions
speed up the ignition of 3-buten-2-one compared to only one such
reaction for 2-butanone. The ignition delay times of both ketones
decreases when U decreases to 0.5 as a result of the faster rate of
the chain branching reaction (H + O2) caused by the increased O2
concentration, however, the decrease in ignition delay times of
3-buten-2-one is greater because the additional oxygen reaction
(C2H3 + O2 ) CH2CHO + O) promotes reactivity. The importance of
C2H3 + O2 ) CH2CHO + O channel in 3-buten-2-one is justified because
C2H3 is a direct product of 3-buten-2-one decomposition
while relatively small amount of C2H3 is formed in 2-butanone system
through two subsequent H-abstraction reactions of C2H5.