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

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.