The effects of dilution with nitrogen and steam on the laminar burning velocity of methanol at room and elevated temperatures
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Dilution of flames using recirculated exhaust gas or water vapor is an important NOx reduction technique in various combustion applications. Dilution, however, can also lead to combustion instabilities due to reduced burning velocities. For methanol/air flames this subject is largely unexplored. Therefore, the current work examines the effects of diluting methanol/air flames with nitrogen and water vapor both experimentally and by simulation. Using the heat flux method, the laminar burning velocity u(l) of methanol/air mixtures was measured at p = 1 bar, T-u = 298-358 K, phi = 0.7-1.5, molar water vapor contents up to 20% and molar excess nitrogen contents of almost 10%. Simulations were performed using the methanol oxidation mechanism of Li et al. (IJCK 39: 109 (2007)). Excellent agreement between experimental and computed results was found for lean mixtures. For rich mixtures the mechanism of Li et al. overestimated the laminar burning velocity up to 5%. The effect of dilution on u(l) was well predicted for both diluents. The relative impact of thermal and chemical effects of dilution was estimated computationally. For both N-2 and H2O the chemical effect was shown to be negligible for diluents ratios considered here (<20%). Based on the modeling results, an explicit correlation was proposed that describes the effect of dilution on u(l) in terms of diluent molar content, diluent specific molar heat capacity, equivalence ratio and unburned mixture temperature. Very good agreement was obtained between the correlation and the modeling data. The effects of unburned mixture temperature on the laminar burning velocity of methanol were analyzed using the correlation u(l) = u(l0) . (T-u/T-u0)(alpha). The current experimental results showed that the power exponent alpha reached a minimum for phi = 1.2, which was well reproduced by the modeling. The modeling results indicated that alpha increases as the mixture gets more diluted.