Experimental and numerical studies of two-stage methanol flames

Abstract

A laminar counterflow configuration is investigated in which a fuel-rich methanol spray is transported by air against an apposing air stream. The fuel-stream equivalence ratio ranges from 1.6 to 3.0, and the fuel-side strain rate from 50 s−1 to 100 s−1. Under these conditions, there is a vaporization plane in the fuel stream at which the spray disappears, a pale green fuel-rich premixed flame in the fuel stream, and a brighter blue diffusion flame in the vicinity of the stagnation plane. Temperature profiles are measured by thermocouples, and concentration profiles of stable species are measured by gas chromatography of samples withdrawn by a fine probe. Computational methods are employed to calculate the flame structure, with detailed chemistry and transport included. Chemical-kinetic descriptions available in the literature predict the premixed flame to be more than 0.5 mm closer to the diffusion flame than observed experimentally and give nearly twice the measured peak CH4 concentration and more than twice the measured concentration of C2H2 in the premixed flame. Modification of the rate data by introducing a temperature-dependent branching ratio to the isomers CH3O and CH2OH, in the H attack on CH3OH, patterned after the known variation in the OH attack and by including the step CH3O+M→CH2OH+M, produces good agreement between all experimental and computational results. A reaction path for methanol in these flames is suggested, including routes to CH, important for prompt NO formation.