The effects (chemical, thermal, transport, and radiative) of CO2 added to the fuel side and oxidizer side on the flame temperature and the position of the flame front in a one-dimensional laminar counterflow diffusion flame of methane/N2/O2 were studied. Overall CO2 resulted in a decrease in flame temperature whether on the fuel side or on the oxidizer side, with the negative effect being more obvious on the latter side. The prominent effects of CO2 on the flame temperature were derived from its thermal properties on the fuel side and its radiative properties on the oxidizer side. The results also highlighted the differences in the four effects of CO2 on the position of the flame front on different sides. In addition, an analysis of OH and H radicals and the heat release rate of the main reactions illustrated how CO2 affects the flame temperature.

A comprehensive numerical investigation of the uncoupled chemical, thermal, and transport effects of CO2 on the temperature of CH4/O2 counterflow diffusion flame under high pressure up to 5 atm was conducted. Three pairs of artificial species were introduced to distinguish the chemical effect, thermal effect, and the transport effect of CO2 on the flame temperature. The numerical results showed that both the chemical effect and the thermal effect of the CO2 dilution in the oxidizer side can decrease the flame temperature significantly, while the transport effect of CO2 can only slightly increase the flame temperature and can even be ignored. The reduction value of the temperature caused by the chemical effect of CO2 grows linearly, while that caused by the thermal effect increases exponentially. The RPchem and RPthermal are defined to explain the temperature reduction percentage due to the chemical effect and the thermal effect of CO2 in the total temperature reduction caused by CO2 dilution, respectively. The RPchem decreases with the increase of the pressure, the strain rate, and the CO2 dilution ratio, while the RPthermal behaves in the opposite manner. In the above conditions, the chemical effect plays a dominant role on the flame temperature reduction.

Extinction limits and flame microstructures of CH4 versus air (N2/O2) counterflow diffusion flames with additional CO2 to oxidizer stream under various pressure conditions were numerically investigated by a proposed decoupled strategy. The chemical and thermal effects of CO2 addition were isolated from the synergistic effect on extinction limits and flame microstructures. The results show that the extinction limits were decreased due to the CO2 addition and extended caused by the enhanced pressure, respectively. The changes of flame microstructures from far away from extinction to near extinction mainly are reflected in a narrower combustion zone and thinner flame thickness, while there is no visible displacement of the flame front location. Quantitative analyses manifest the thermal effect of CO2 addition on extinction limit is greater than chemical effect, and elevated pressure can enhance both thermal and chemical effects. An interesting phenomenon occurred between the thermal and chemical effects resulting from increasing CO2 mole fraction in an oxidizer on extinction limit; the former monotonically increase along with CO2 mole fraction, while the latter represents a kind of approximate quadratic-parabola tend along with CO2 mole fraction. The distributions of the production and consumption rate of hydroxyl (OH) radicals were presented to clarify the contribution of the kinetic term in the asymptotic solution to the extinction limits with increasing CO2 molar fraction and various pressures.