Experimental and Numerical Investigation of the CO2 Dilution Effect on Laminar Burning Velocities and Burned Gas Markstein Lengths of High/Low RON Gasolines and Isooctane Flames at Elevated Temperatures

The exhaust gas recirculation effect on the laminar burning velocity (SL) and flame stability of CO₂-diluted isooctane/air and high/low research octane number (RON) gasoline/air mixtures was investigated by studying spherically expanding flames at a constant pressure of 1 bar and 373–473 K. Measurements were carried out in a constant volume combustion vessel. The experimental setup and methodology were validated by comparing the isooctane results with the existing literature. The results of measurements showed that flame speeds of commercial gasolines do not vary significantly with RON whereas SL values of isooctane flames are consistently slower than those of gasoline. A temperature increase of 100 K (373–473 K) yielded 51–56, 50–54, and 48–51% increases in laminar burning velocities for the high RON gasoline, low RON gasoline, and isooctane, respectively. Investigation of burned gas Markstein lengths (Lb) revealed that the flame stability of the three fuels deteriorates considerably with increasing equivalence ratio; however, the burned gas Markstein lengths were not significantly affected by the 100 K increase in initial temperature. Similar to the laminar burning velocity, the burned gas Markstein lengths of the high and low RON gasoline flames were very close, but the isooctane/air flames had slightly higher burned gas Markstein lengths. The CO₂ addition at 473 K yielded 41–46, 42–44, and 46–49% decreases in laminar burning velocities for the high RON gasoline, low RON gasoline, and isooctane, respectively, due to a combination of dilution, thermal-diffusion, and chemical effects. The chemical effect had a minor contribution of 23–25% on the decrease in the laminar burning velocity at lean and stoichiometric conditions, and its impact dropped to only 15% for rich mixtures. The addition of CO₂ also postponed the initiation of cellular formation by increasing the Lb values of the three fuels. Numerical analysis was performed with the CHEMKIN software [Kee et al. PREMIX: A Fortran Program for Modeling Steady Laminar One-Dimensional Premixed Flames; Reaction Design: 1985] using the chemical mechanisms of Chaos et al. [Int. J. Chem. Kinet.2007, 39, 399], Kelley et al. [Proc. Combust. Inst.2011, 33, 501], and Jerzembeck et al. [Combust. Flame2009, 156, 292] to assess the accuracies of the mechanisms for isooctane flames at the experimentally investigated conditions. The current experimental data was most consistent with the mechanism of Chaos et al. with slightly slower SL values at ϕ > 1.2.