Effect of CO2 and N2 dilution on laminar premixed MTHF/air flames: Experiments and kinetic studies

The effect of N2 and CO2 dilution on MTHF/air premixed flame propagation was investigated at initial temperature and pressure of 393 K and 0.1 MPa, respectively. The flame speed measurements were conducted on spherically expanding premixed flame in a constant volume vessel using high speed Schlieren technique. The level of dilution was varied from 5 to 15% at equivalence ratio (ϕ) ranging from 0.8 to 1.5. CO2 has a larger impact on flame speed reduction than N2, where the peak flame speed at ϕ = 1.1 dropped from 0.58 m/s to 0.44 m/s and then to 0.24 m/s as the CO2 dilution increased from 0% to 15%. Markstein length was found to increase with dilution in the premixed flames which could be attributed to the decrease in the probability of hydrodynamic instability owing to increasing flame thickness especially for rich conditions. Numerical simulations using two detailed kinetic mechanisms showed good agreement with the experimental measurements. Comparison between the kinetic effects and physical dilution effects of CO2 on flame speed showed that the latter dominated overall, with the relative contribution of kinetic effects being highest for Φ = 1.1. Reverse CO + OH ⇔ CO2 + H was identified as the key reaction directly affected by CO2 dilution leading to CO2 dissociation and H consumption. The competition for H radical with primary chain branching/propagating reactions hindered the MTHF/air combustion in CO2 diluted flames.