On the equivalent effect of initial temperature and pressure coupling on the flame speed of methane premixed combustion under dilution

This study was conducted with methane as fuel and CO₂ as diluent gas. The equivalent effect of initial temperatures (323–423 K) and pressures (0.1–0.3 MPa) coupling on methane-air premixed combustion flame propagation speed (i.e. increasing or decreasing initial temperature and pressure simultaneously to have same flame propagation speeds when equivalence ratio and fraction of CO₂ are unchanged) was investigated under a series of fractions of CO₂ (0%–16%) and equivalence ratios (0.9, 1.0, and 1.1). Laminar burning velocities of different test conditions with similar flame propagation speeds were investigated and flame instability was also analyzed. The results show that similar flame speeds can be obtained when elevating initial temperature and pressure synchronously under constant equivalence ratio and the fraction of CO₂. Similar flame propagation speeds can lead to close laminar burning velocities and similar flame structures. Flame speed under higher initial temperature and pressure is more sensitive to temperature and pressure, but dilution can suppress this sensitivity. Flame instability is stronger under higher initial temperature and pressure. Hydrodynamic instability dominates and it is mainly influenced by flame thickness. Differences are also shown in pressure and temperature during non-laminar stage. Dilution can enhance the difference in combustion duration.