Investigation on laminar burning velocities of benzene, toluene and ethylbenzene up to 20 atm

The laminar flame propagation of benzene, toluene and ethylbenzene was investigated at Tu = 423 K, Pu = 1-20 atm and various equivalence ratios in a high-pressure constant-volume cylindrical combustion vessel. The laminar burning velocities (LBVs) were observed to increase in the order of toluene, ethylbenzene and benzene at both atmospheric and high pressures, and the Markstein lengths were evaluated to be similar among the three aromatic fuels. Three recently developed kinetic models of aromatic fuels were used to simulate the experimental data. Sensitivity analysis was performed to investigate the kinetic effects on LBVs of the three aromatic fuels under various conditions. Different from the cases of alkane LBVs, the reactions of fuel-specific species are important for the LBVs of the three aromatic fuels at 1-20 atm, especially for the LBVs under rich conditions. In particular, the pyrolysis reactions in the sub-mechanism of aromatic fuels play a dominant role in negatively sensitive reactions under rich conditions, indicating prominent inhibition effects of aromatic flame chemistry on the LBVs. The sensitivity variation analysis and normalized sensitivity variation analysis indicate that the importance of most of the small species and fuel-specific species reactions are enhanced with increasing pressure, especially under rich conditions. The pressure effects of the flame chemistry on the aromatic LBVs strongly depend on the reaction circumstance. A series of chain termination reactions and chain propagation reactions producing less active radicals have large negative sensitivity coefficients and are generally more susceptible to the pressure variation than the positively sensitive reactions. The modeling analysis indicates that aromatic flame chemistry inevitably changes to resist the enhanced combustion intensity with increasing pressure.