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

Abstract

Laminar flame speeds and Markstein lengths of H2/CO with CO2 dilution were measured at normal and elevated pressures (0.1MPa, 0.5MPa, 1.0MPa) and temperatures (298K, 375K, 450K) 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.