The effects of naphthalene-addition to alkylbenzenes on soot formation

Naphthalene and alkylbenzenes are present in practical transportation fuels. This study investigates the impact of naphthalene addition to alkylbenzenes on soot formation. Naphthalene was added to two kinds of alkylbenzenes, namely, 1,2,4-trimethylbenzene and n-propylbenzene. Because they are isomers, the effect of molecular structure is isolated. The sooting characteristics of naphthalene-added alkylbenzenes are compared to pure alkylbenzenes in laminar coflow flames. The fuel and carbon mass flow rates were kept constant for all cases. The soot volume fraction measurements show that n-propylbenzene is sensitive to naphthalene addition. In contrast, no significant changes in soot volume fraction were observed for the 1,2,4-trimethylbenzene flames. A slight increase in primary particle diameter was observed for both naphthalene-added n-propylbenzene and 1,2,4-trimethylbenzene, suggesting that naphthalene promotes soot surface growth. The calculated number densities show that naphthalene addition promotes soot nucleation for n-propylbenzene but not for 1,2,4-trimethylbenzene. The flames were simulated with the CoFlame code with the CRECK mechanism. The model partially agrees with the experimental results, as the model agrees with the case of 1,2,4-trimethylbenzene but underestimates the effect of naphthalene addition to n-propylbenzene. More understanding of the PAH formation beyond naphthalene is required. In conclusion, the study suggests that the effect of naphthalene addition on soot formation is fuel-type dependent.