Asymptotic Structure of Methanol-Air Diffusion Flames

摘要

The asymptotic structure of counterflow nonpremixed methanol flames is analyzedusing a reduced four-step mechanism, deduced from a starting mechanism containing twenty-seven elementary reactions. The four overall steps represent (1) fuel-consumption reaction, where the fuel reacts with radicals to form H2 and CO, (2) water-gas shift reaction where CO reacts with radicals to form CO2, (3) radical-recombination reaction, and (4) chain-branching reaction. The outer structure of the flame is the classic Burke-Schumann structure governed by the overall one-step reaction CH3OH + 1.5O2 yields CO2 + 2H2O. The inner structure is presumed to consist of two layers, an inner layer of thickness of order d, and an oxidation layer of thickness of order e. The asymptotic analysis is performed for e between d and 1. The structure of the inner layer is presumed to be influenced primarily by the fuel-consumption reaction and the radical-recombination reaction, and the equations governing the structure of the inner layer are similar to those for premixed, methanol flames analyzed previously using the same reduced chemical-kinetic mechanism. Therefore, results of this previous analysis modified for diffusion flames are used here. In the oxidation layer all radicals are presumed to be in steady-state. In the analysis of the structure of the oxidation layer, a parameter D appears, which is roughly proportional to the square of the ratio of the thickness of the region where the global recombination reaction occurs, to the thickness of the region where the water-gas shift reaction is not in equilibrium. The structure of the oxidation layer is resolved by treating the quantity D to be of order unity, and in the limit for large values of D.