Effects of H₂ or CO₂ addition, equivalence ratio, and turbulent straining on turbulent burning velocities for lean premixed methane combustion

Using hydrogen or carbon dioxide as an additive, we investigate the bending effect of turbulent burning velocities (S_T / S_L) over a wide range of turbulent intensities (u^′ / S_L) up to 40 for lean premixed methane combustion at various equivalence ratios (φ{symbol}), where S_L is the laminar burning velocity. Experiments are carried out in a cruciform burner, in which a sizable downward-propagating premixed CH₄/diluent/air flame interacts with intense isotropic turbulence in the central region without influences of ignition and unwanted turbulence from walls. Simultaneous measurements using the pressure transducer and pairs of ion-probe sensors at various positions of the burner show that effects of gas velocities and pressure rise due to turbulent combustion on S_T of lean CH₄/H₂/air flames can be neglected, confirming the accuracy of the S_T data. Results with increasing hydrogen additions (δ = 10, 20, and 30% in volume) show that the bending of S_T / S_L vs u^′ / S_L plots is diminished when compared to data with δ = 0, revealing that high reactivity and diffusivity of hydrogen additives help the reaction zone remaining thin even at high u^′ / S_L. In contrast, the bending effect is strongly promoted when CO₂ is added due to radiation heat losses. This leads to lower values of S_T / S_L at fixed u^′ / S_L and φ{symbol}, where the slope n can change signs from positive to negative at sufficiently large u^′ / S_L, suggesting that the reaction zone is no longer thin. All S_T data with various δ can be well approximated by a general correlation (S_T - S_L) / u^′ = 0.17 Da^0.43, covering both corrugated flamelet and distributed regimes with very small data scatter, where Da is the turbulent Damköhler number. These results are useful in better understanding how turbulence and diluents can influence the canonical structures of turbulent premixed flames and thus turbulent burning rates.