A General Study of Counterflow Diffusion Flames for Supercritical CO₂ Combustion

A counterflow diffusion flame for supercritical CO₂ combustion is investigated at various CO₂ dilution levels and pressures by accounting for real gas effects into both thermal and transport properties. The UCF 1.1 24-species mechanism is used to account the chemistry. The nature of important nonpremixed combustion characteristics such as Prandtl number, thermal diffusivity, Lewis number, stoichiometric scalar dissipation rate, flame thickness, and Damköhler number are investigated with respect to CO₂ dilution and pressure. The results show that the aforementioned parameters are influenced by both dilution and pressure; the dilution effect is more dominant. Further, the result shows that Prandtl number increases with CO₂ dilution and at 90% CO₂ dilution, the difference between the Prandtl number of the inlet jets and the flame is minimal. Also, the common assumption of unity Lewis number in the theory and modeling of nonpremixed combustion does not hold reasonable for sCO₂ applications due to large difference of Lewis number across the flame and the Lewis number on the flame drop significantly with an increase in the CO₂ dilution. An interesting relation between Lewis number and CO₂ dilution is observed. The Lewis number of species drops by 15% when increasing the CO₂ dilution by 30%. Increasing the CO₂ dilution increases both the flow and chemical timescales; however, chemical timescale increases faster than the flow timescales. The magnitudes of the Damköhler number signify the need to consider finite rate chemistry for sCO₂ applications. Further, the Damköhler numbers at 90% sCO₂ dilution are very small; hence, laminar flamelet assumptions in turbulent combustion simulations are not physically correct for this application. Also, it is observed that the Damköhler number drops nonlinearly with increasing CO₂ dilution in the oxidizer stream. This is a very important observation for the operation of sCO₂ combustors. Further, the flame thickness is found to increase with CO₂ dilution and reduce with pressure.