Adiabatic Flame Temperature for Controlling the Macrostructures and Stabilization Modes of Premixed Methane Flames in a Model Gas-Turbine Combustor

Premixed oxygen-enriched air-methane flames (CH₄/O₂/N₂) are compared with their oxy-methane counterparts (CH₄/O₂/CO₂) in the same model gas-turbine combustor and under identical conditions of oxygen fraction (OF = 21-70 vol %) and equivalence ratio (φ = 0.2-1.0). The flow rates of nonpreheated reactant gases were adjusted for each tested flame to sustain a common bulk velocity at burner throat throughout the whole study to maintain similar flow conditions and turbulence intensities for all isothermal flow fields. The flashback and blowout limits were quantified to identify the combustor stability maps within the OF-φ space. The adiabatic flame temperature (T_ad) was also mapped over the same test ranges for both N₂ and CO₂ flames. The effect of T_ad on flame macrostructure and stabilization mode was studied in detail by imaging selected flames. The following novel findings were found to apply to both N₂ and CO₂ flames at common inlet bulk velocity: Their stable combustion zones can both be characterized by T_ad only, although they have different T_ad maps. Combustion is thus governed mainly by the reaction kinetics (especially near the flashback limits) under similar cold flow conditions. Both N₂ and CO₂ flames undergo the same changes in macrostructure and stabilization mode as T_ad is increased from the blowout limits to the flashback ones. Stable flames of different φ and OF but the same T_ad have identical shapes, which shows the direct dependence of flame macrostructure and stabilization mode on T_ad under similar cold flow conditions. Both N₂ and CO₂ stability maps can be subdivided into subzones based on T_ad only, where each zone has a single prevailing flame macrostructure irrespective of φ and OF. This is yet another proof that T_ad is an excellent tool for predicting flame macrostructure at constant inlet bulk velocity. On the basis of these findings, this study recommends to design and operate future oxy-fuel gas-turbine combustors based on T_ad (and not OF or φ), particularly at high and medium loads away from blowout, following the existing common practice among manufacturers of conventional lean-premixed air-fuel gas turbines to quantify combustor performance in terms of T_ad.