Roles of CO₂ and H₂O in premixed turbulent oxy-fuel combustion

A series of Two-Dimensional Direct Numerical Simulation (2-D DNS) cases are performed to investigate the effects of CO₂ and H₂O on premixed turbulent oxy-fuel combustion (case oxy-H₂O, case oxy-CO₂) in a constant vessel. A DNS solver for low Mach reacting flow is developed based on open source code OpenFOAM. The chemical effects of CO₂ and H₂O are isolated by a pair of artificial species. 2-D DNS results shows the flame surface of oxy-CO₂ case trends to be wrinkled with small convex and concave structures due to a lower laminar flame speed and a smaller effective Lewis number. Moreover, the response of flame surface to the curvature is different with corresponding dilute agent. Owing to the high binary diffusion coefficient between H₂O and H, the negative curvature effect on fast diffusing species, e.g. H and H₂, are enhanced in oxy-H₂O, and results in a higher burning velocity. The effects of CO₂ on H transport are contrary to the effects of H₂O and the oxidation process is slowed due to the elementary reaction CO₂ + H ⇔ CO + OH, which result in a more uniform Heat Release Rate (HRR) distribution on flame surface. In addition, chemical reaction pathway analysis is performed to elucidate the chemical effects of CO₂ and H₂O. It is found that CO₂ and H₂O will cool down the temperature of burnt region compared with N₂. Finally, an alternative solution is raised to counteract the negative effect of CO₂ on flame speed, which should be useful in the development and design of oxy-fuel combustion.