Laminar burning velocities of ultra-lean iso-octane flames at atmospheric pressure: A comparative study with n-heptane flames

Abstract

In this study, the laminar burning velocity, $S_L$, of iso-octane, a key gasoline surrogate component, was measured at 298 K and 1 atm using the heat flux method, and comparative analyses were also conducted with the results of n-heptane flames from previous investigation. The experimental conditions covered ultra-lean conditions with an equivalence ratio as low as 0.5 that have never been reported before for the iso-octane flames, which was achieved through H₂ and O₂ enrichment. Similar to other fuels tested in earlier studies, a perfect linear relationship was found between $\ln \left(S_L\right)$ and $1/X_u$ ($X_u$ being the sum of mole fraction of fuel+oxygen in the unburnt mixture). This relationship was used to extrapolate or interpolate data under conditions that were unmeasurable due to the pulsating instabilities of the ultra-lean iso-octane flames, similar to the ultra-lean n-heptane flames, with carefully evaluated uncertainty. These results, together with a separate set of present measured iso-octane+air flame data with various equivalence ratios at 1 atm and 298 K, were compared with all available literature datasets, among which several more reliable sets were found more consistent and reliable for mechanism validation. Simulations were carried out using three literature models and an updated model from the authors, named Han. A $\sigma$ function was employed to quantitatively assess the standard deviation of the simulation results from experimental data using each model. The conditions of ultra-lean iso-octane, ultra-lean n-heptane, as well as iso-octane+air and n-heptane+air with large equivalence ratio spans were found with similar orders of the $\sigma$ function values from the four models. These findings, together with the similar instability performances of the ultra-lean n-heptane and iso-octane flames, were analyzed in terms of global flame characteristics and reaction sensitivities. It was interesting to find that, regardless of whether the conditions were ultra-lean and dominated by H₂/C1 reactions or fuel-rich where >C1 reactions became more important, the sensitivity values of laminar burning velocities for each reaction in iso-octane and n-heptane flames were nearly identical. Besides this understanding of the intrinsic physio-chemical nature of the flames, the updated Han model is found with the lowest $\sigma$ function values for all the conditions discussed in the present study, though with a compact size of 106 species and 508 reaction (74 species and 314 reactions if removing the nitrogen chemistry), which could help the design and optimization of relevant applications.