Laminar flame speeds of H₂/CO with CO₂ dilution at normal and elevated pressures and temperatures

Laminar flame speeds and Markstein lengths of H₂/CO with CO₂ dilution were measured at normal and elevated pressures (0.1 MPa, 0.5 MPa, 1.0 MPa) and temperatures (298 K, 375 K, 450 K) using outwardly propagating spherical flame method. Simulation considering detailed chemistry and transport was conducted to examine the effects of radiation re-absorption of CO₂ dilution on flame propagation. The laminar flame speeds measured in experiments were compared with prediction by detailed chemistry and the performance of three different chemical mechanisms recently developed for syngas oxidation was discussed. It is found that the laminar flame speed decreases linearly with CO₂ dilution ratio. The Markstein length remains positive and decreases with pressure. At atmospheric pressure, increasing CO₂ dilution ratio and preheat temperature can reduce the Markstein length, and thereby promote the diffusive-thermal instability. The chemical effects of CO₂ dilution, including the direct reaction effect and the three-body effect, were quantitatively analyzed using the pseudo CO₂ strategy. The direct reaction effect of CO₂ dilution mainly reduces the CO oxidation rate through reaction CO + OH = CO₂ + H and competes H atoms with the main chain-branching reaction H + O₂ = O + OH, resulting in reduced laminar flame speed and flame temperature. And the three-body effect of CO₂ dilution prompts three-body reactions with negative influence on flame speeds. Numerical results demonstrate that both chemical effects on flame speeds are enhanced by elevating CO₂ dilution ratio and pressure but reduced by elevating temperature.