Laminar flame characteristics and kinetic modeling study of methanol-isooctane blends at elevated temperatures

Laminar flame speeds of methanol-isooctane blends were experimentally determined using the spherically propagating flame in a constant volume chamber at two initial temperatures (363 and 393 K), different blending ratios of methanol in liquid volume (0%, 20%, 40%, 80%, 100%), and over equivalence ratios of 0.7-1.6. Nonlinear methodology was employed to remove the stretch effect in the data processing. Results indicate that laminar flame speeds of methanol flame reach the peak at equivalence ratio around 1.2 and that of isooctane at equivalence ratio around 1.1. For the mixtures with less than 40% methanol, laminar flame speeds show moderate increase at all equivalence ratios. However, further increasing methanol addition will greatly accelerate laminar flame speeds at rich mixture sides but give slight change at lean mixture sides. Markstein length shows an increase tendency with the methanol addition at the equivalence ratios larger than a critical value while Markstein length gives a decrease tendency at the equivalence ratios smaller than the critical value. The critical equivalence ratio is between 1.2 and 1.3. Among the thermal effect, diffusive effect and kinetic effect, the kinetic effect was found to be the major factor bringing the variation of laminar flame speed with the variation of blending ratio. A kinetic model (IM model) was developed on the basis of the isooctane model of Chaos et al. (2007). The IM model shows good prediction on measured laminar flame speeds under all conditions. Reaction pathway reveals that the HCO and H productions are promoted while the productions of stable species are inhibited in the case of methanol addition into the isooctane at rich mixture sides, resulting in the laminar flame speed enhancement. These behaviors are verified from the sensitivity analysis and the concentrations of the reactive radicals.