Laminar Burning Velocity and Markstein Length of CH₄/CO₂/Air Premixed Flames at Various Equivalence Ratios and CO₂ Concentrations Under Elevated Pressure

Biogas is a renewable fuel predominantly composed of carbon dioxide (CO₂) and methane (CH₄) in varying proportions. The effects of the varying CO₂ proportion need to be clarified for the development of engines. The laminar burning velocity and the burned gas Markstein length of premixed CH₄/CO₂/Air were measured with CO₂ concentration ranging from 0.3 to 0.7 dilution ratios. The equivalence ratio was varied from 0.8 to 1.2, the initial pressure was set at 0.5 MPa, and the temperature was set to 298 K. The experiment was performed using a high-pressure constant volume combustion chamber. One-dimensional simulation of the flames was conducted using GRI-Mech 3.0. The results showed a reduction in the laminar burning velocity of CH₄/CO₂/Air mixtures with an increase in CO₂ dilution ratio. A non-monotonic relationship was discovered between measured Markstein length and CO₂ dilution ratio with different equivalence ratios. It was found that an increase in the CO₂ dilution increased the response of the flames to stretch. For the lean and stoichiometric flames, the Markstein length was nearly constant with CO₂ dilution of 0–0.5 and decreased with CO₂ dilution of 0.7, suggesting an increase in susceptibility of the flame to the intrinsic flame instability. This was found to be mainly due to an increase in the Zel’dovich number and a decrease in the effective Lewis number with CO₂ dilution. The Markstein length of the rich flame increased with CO₂ dilution as it was more sensitive to CO₂ dilution. Thermo-diffusive effects and pure stretch effects had similar influences on the burning velocity of the rich flames with an increase in stretch rate.