Investigations on double-state behavior of the counterflow premixed flame system

摘要

The counterflow flame system established between lean-methane-air and lean-hydrogen-air streams is investigated experimentally and numerically. A two-dimensional model known as UNICORN was used for the simulation. Detailed measurements for temperature and species concentrations were obtained along the centerline using Raman spectroscopy. A double-state behavior for this flame system was identified in the numerical simulations, which was later confirmed by the experiments. For the given flow conditions, the flame system can have either a single-flame or a double-flame structure depending on the way those conditions were achieved. Detailed comparisons were made between measurements and calculations for the two flame structures. Calculations for various lean methane-air mixtures and stretch rates were performed to understand the double-state behavior of counterflow premixed flames. It was found that the flame system exhibits double-state behavior only for leaner (φ_(CH_4) < 0.74) methane-air mixtures. Aerodynamic and chemical structures of the flames in different stretch-rate regimes were analyzed. When stretch rate on the flame system is increased, the flame transitions from a double-flame to a single-flame structure due to aerodynamic-cooling process. When stretch rate is decreased, the flame does not transition back to the double-flame structure due to stretch effects on molecular diffusion. However, for (φ_(CH_4) > 0.81), decrease in stretch rate increases flame temperature due to lack of stretch-induced cooling and returns the flame structure to a double-flame one. For a narrow range of equivalence ratios (0.74-0.81) counterflow premixed flames exhibit a hysteresis property.