Investigation of Unsteady Premixed Micro/Macro Counterflow Flames for Lean to Rich Methane/Air Mixture

In the current work, an unsteady analysis of methane/air premixed counterflow flame is carried out for different flame conditions and stability parameters considering different strain rate values. The results are presented at unsteady and final steady conditions, and the impact of time-dependent regimes and variations in equivalence ratio, from lean flame to rich one, are analyzed. The governing equations including continuity, momentum, energy, and species are numerically solved with a coupled simple and Piso algorithm. It is also found that when the strain rate value is 1000 s⁻¹, for flame stability, the hydraulic distance of the microchannel must be at least 0.05 mm. Increasing the strain rate results in decreasing the time of stabilizing temperature distribution with a faster quasi-steady equilibrium. The necessity of time-dependent analysis is to comprehend the variations in the main factors of flame structure before reaching the finalized steady-state condition. Therefore, by designing an intermittent automatic valve, if the flow stops in time period of 0.0025 s and starts again, the formation of NO₂ and CO₂ will be reduced about 50% and 9%, respectively, in a case with a = 100 s⁻¹.