Kinetic effects of n-propylbenzene on n-dodecane diffusion cool flame extinction

Experimental and numerical investigations on the kinetic influence of n-propylbenzene on the extinction limit of n-dodecane/oxygen diffusion cool flames in the counterflow configuration have been conducted. Diffusion cool flame extinction limits were measured using planar laser-induced fluorescence of formaldehyde in nitrogen-diluted flames. Numerical computations using a jet fuel surrogate kinetic model were performed to predict flame extinction and to examine the nature of the kinetic coupling between n-dodecane and n-propylbenzene for both high-temperature and low-temperature oxidation. It is found that at a fixed mass fraction of n-dodecane, replacing nitrogen with n-propylbenzene increases the overall reactivity of the hot flame but dramatically decreases that of the cool flame. In hot flames (above 1600 K), at the same n-dodecane fuel concentration, oxidation of n-propylbenzene and its fragments contributes to the increase of the net heat release and the net production of the hydroxyl and hydrogen radicals. However, in cool flames (below 750 K), n-propylbenzene and its fragments scavenge the hydroxyl and hydroperoxyl radicals to produce stable aromatic intermediates, thereby inhibiting major heat release and radical pool formation. Extinction sensitivity analyses revealed the importance of changes in the radical pool population on cool flame extinction, as was demonstrated by the sensitivity of flame extinction to the competition between the low-temperature chain propagation and chain-branching reactions at near-limit conditions.