THE ASYMPTOTIC STRUCTURE OF PREMIXED METHANOL AIR FLAMES

The asymptotic structure of methanol-air flames is analyzed using a reduced four-step mechanism, for values of equivalence ratio psi from 0.52 to 1.0 and values of pressure p from 1 to 10 atm. The reduced mechanism was deduced from a starting mechanism, containing 22 elementary chemical reactions. In the analysis, the overall flame structure is subdivided into four zones-a preheat zone with thickness of order unity, an inner layer with thickness of order delta, an oxidation layer with thickness of order epsilon, and a postflame zone with thickness of order unity. The analysis is performed for delta much less than epsilon much less than 1.0. The inner layer is located between the preheat zone and the oxidation layer, and in this layer, finite-rate reactions related to the consumption of the fuel, to form primarily H-2 and CO and some H2O and CO2, are considered. In the oxidation layer finite-rate reactions related to the oxidation of H-2 and CO to H2O and CO2 are considered. Numerical integration of two coupled, nonlinear, second-order, ordinary differential equations is performed to resolve the structure of the oxidation layer. Analytical expressions are obtained for predicting the burning velocity, upsilon(u). At the stoichiometric conditions with p = 1 atm the model predicts values of upsilon(u) which are somewhat lower than those calculated from full numerical integration of the conservation equations. However, the model predicts the observed decrease in the value of upsilon(u) with decreasing values of psi and increasing values of p.