Molecular Structure Effects On Laminar Burning Velocities At Elevated Temperature And Pressure

The laminar burning velocities of 45 hydrocarbons have been investigated in a constant volume combustion vessel at elevated temperature and pressure. The mixtures are ignited in the center of a spherical vessel at an initial temperature of 450 K and pressure of 304 kPa. Data have been acquired over the stoichiometry range of 0.55 ≤ ϕ ≤ 1.4. The burning velocity is determined from a thermodynamic analysis of the pressure vs. time data. The results for alkanes and alkenes are consistent with trends previously identified in the literature, i.e., alkenes are faster than the corresponding alkane with the same carbon connectivity. For both alkanes and alkenes, branching lowers the burning velocity. In addition, terminal alkenes and alkynes are found to be slightly faster than internal alkenes and alkynes. The present study includes broader coverage of aromatics than previous literature reports. The burning velocities for aromatics show a strong dependence on the type and site of alkyl substitution; methyl substitution lowers the burning velocity more than substitution with larger alkyl groups. For multiple methyl group substitution, meta substitution lowers the burning velocity more than ortho/para. The physical and chemical kinetic bases for the variation of burning velocity with molecular structure are discussed with the aid of elemental flux analyses of simulations using detailed chemical kinetic mechanisms. A consistent trend is identified in which “fast” burning fuels have a higher flux into decomposition pathways that yield H atoms and C₂ fragments, while “slow” fuels have a higher flux into pathways that form CH₃ radicals. The data and analysis presented in this paper provide a comprehensive, fundamental basis for relating fuel structure effects to combustion efficiency and emissions.