A numerical investigation of steady and unsteady methane-air partially-premixed flames.

摘要

Partially-premixed flames are established when less than stoichiometric quantity of oxidizer is molecularly mixed with the fuel stream before entering the reaction zone where additional oxidizer is available for complete combustion. Partially-premixed flames are widely used in practical combustion systems for their stable combustion and low pollutant formation, such as cooking stoves, Bunsen burners, diesel engines and other liquid fuel combustors.; In this research, a numerical investigation of steady and unsteady methane-air partially-premixed flames established on a Wolfhard-Parker slot burner under normal and zero-gravity conditions is presented. Simulations are used to (1) examine the structure of 1- and 0-g partially premixed flames, (2) characterize the effect of gravity on the flame structure and interactions between two reaction zones, (3) analyze the effects of C₂ chemistry on the partially premixed flame structure, and (4) investigate the effects of coflow on the structure and stability of 1- and 0-g flames.; The results indicate that combustion occurs in two reaction zones, namely an inner rich premixed zone and an outer nonpremixed zone. Both CH₄ and O₂ are mostly consumed in the premixed reaction zone, while CO and H₂ are produced in this zone and then transported to and consumed in the nonpremixed reaction zone.; There are important quantitative differences in the partially premixed flame structure obtained by using the C₁- and C₂-mechanisms. The overall reaction rate using C₂-chemistry is faster at high level of partial premixing and slower at low level of partial premixing compared with that using C₁-chemistry.; The presence of gravity reduces the spatial separation and, thereby, enhances the interactions between the inner and outer reaction zones. In general, for low coflow velocities, there are remarkable structural differences between the 0- and 1-g flames. For high coflow velocity, however, the difference between the 0- and 1-g flames is mostly negligible.; A more prominent effect of buoyant acceleration is to induce self-excited, periodic oscillation of the 1-g flame. The transient phenomena and the relative amplitude of oscillations involved with flame flicker for the normal gravity case indicate that toroidal vortices roll up periodically due to the Kelvin-Helmholtz instability associated with the convective flow. The flames still contain two distinct reaction regions.