Numerical Study of the Combustion Characteristics of Propane-Oxyfuel Flames with CO₂ Dilution

Oxyfuel combustion with carbon capture could be employed to reduce CO₂ emissions and eliminate thermal NO_x emissions from combustion systems. High temperatures associated with the use of pure oxygen as an oxidizer in combustion systems would require recycling CO₂ from flue gases to be used as a diluent, safeguarding the structural and material safety of the systems. The CO₂ in the O₂/CO₂ oxyfuel oxidizer mixture, being diluent, lowers the temperature as well as flame speed and, consequently, affects the combustion characteristics. In this study, we investigated, numerically, the effect of CO₂ dilution level on nonpremixed, swirl-stabilized, propane-oxyfuel flames in terms of the flame's macrostructure, temperature, and emissions. Results show that the flame transitions from a jet-like to a V-shaped flame consequent to fuel jet-vortex interaction and that this interaction can be employed in swirl-stabilized flame characterization. The jet-like flames obtained at low dilution levels were found to have the same nondimensional vortex strength of 0.2. Also, the flame transition coincides with a sudden increase in the vortex strength value from 0.2 to about 0.3 and continued to increase linearly with increase in CO₂ dilution level. The CO emissions increase with CO₂ dilution level due to the combined effect of cooling, low residence time, and CO₂ dissociation, up to the threshold of 50% CO₂ dilution level, beyond which it decreases due to drastic decrease in CO₂ dissociation that is attributed mainly to the cooling effect.