Experimental and numerical study of soot formation in laminar n-butylcyclohexane and n-butylbenzene diffusion flames at elevated pressures

The soot formation characteristics of laminar nitrogen-diluted n-butylcyclohexane and n-butylbenzene diffusion flames were experimentally and numerically investigated at pressures from 2 to 7 bar. In the experiment, laser-induced incandescence (LII), time-resolved LII, and color-ratio pyrometry were used to measure soot volume fraction, soot particle diameter, and flame temperature. The results show that n-butylbenzene has a significantly higher soot propensity than n-butylcyclohexane. The soot growth and oxidation in both flames are enhanced with increasing pressure. The difference is that the promotion effect of pressure on the soot formation in the n-butylcyclohexane flame continues to weaken as the pressure increases, while this phenomenon does not occur in n-butylbenzene flames. Within the studied pressure range, the mean particle sizes (Dp_mean) in n-butylcyclohexane and n-butylbenzene flames show a good linear relationship with pressure. The pressure dependence of Dp_mean in n-butylbenzene flames is stronger than that of n-butylcyclohexane flames at pressures between 2 and 6 bar. The experiment and simulation results indicate that the enhancement of the promotion effect of pressure on the soot formation in the n-butylbenzene flame may be due to the combined effect of an increase in the soot surface reactivity and an increase in the number density of soot particles. The reaction pathway analysis suggests that the stepwise dehydrogenation reactions of cyclohexene are the main source of benzene formation in n-butylcyclohexane flames and pyrene is mainly formed via the reaction between indenyl and benzyl radicals in n-butylbenzene flames.