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Abstract The objective of this work is to examine the structure of lean turbulent premixed CH4/air (air-flames) and CH4/O2/CO2 (oxy-flames) in a swirl-stabilized combustor at the large (macro) and small (micro) scales, and to explain why the latter burns more intensely despite its lower laminar burning velocity. Measurements of the instantaneous flame front and flow field using OH-PLIF and PIV, respectively, are analyzed. The CO2 dilution in the oxy-flame was adjusted to achieve the same adiabatic temperature for both flames while keeping the equivalence ratio (=0.65) and Reynolds number (=20,000) the same. Results show that at the large scale, the overall length of the oxy-flame is notably shorter than that of the air-flame. We use a strained flame model with detailed kinetics to show that the strained consumption speed of oxy-flames is lower than that of air-flames, mainly due to the chemical role of CO2, and hence laminar flame properties cannot explain the difference between the two flames in the turbulent case. Instead, the turbulent dynamics properties such as the small scale wrinkling of the flame, its surface area density and radius of curvature are obtained from the data and used to explain the observed trends. Measurements show that the flame surface area density of oxy-flames is higher because more flame segments with smaller radius of curvature appear. This is particularly noticeable downstream, and is consistent with the Lewis number effects, i.e., the response for the flame to thermo-diffusive instability. The local average burning intensity, being high under oxy-combustion conditions, explain the shorter overall average flame length.

Abstract We compare the conditions leading to the stabilization of turbulent methane air and oxy-flames in the outer recirculation zone (ORZ) of a lean premixed acoustically decoupled swirl combustor. The appearance of a flame in the ORZ is an important flame macrostructure transition that was previously shown to be associated with the onset of thermo-acoustic instability under acoustically coupled conditions. We find that, when similar bulk flow conditions are imposed in the ORZ, the transition is governed by the extinction strain rate and can occur at different adiabatic flame temperature and unstretched laminar burning velocity. First, we show that an important non-dimensional parameter characterizing the flow in the ORZ, that is the Strouhal number associated with the azimuthal ORZ spinning frequency, is independent of the Reynolds number and has the same constant value in air and oxy-combustion ( S t = f O R Z . D i n U i n , b u l k ≈ 0.12 ). This has the important implication that the inlet velocity is a more relevant parameter choice than the inlet Reynolds number in order to maintain similar flow conditions in the ORZ. Next, by comparing the extinction strain rates – computed at the measured ORZ temperature – we show the existence of a single correlation between the inverse of the ORZ spinning frequency (taken as a characteristic ORZ flow time) and the inverse of the extinction strain rate (taken as a characteristic flame time) valid for both air and oxy flames and delimiting the regions of existence of different flame macrostructures.