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 (phi), 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/H-2/air flames can be neglected, confirming the accuracy of the S-T data. Results with increasing hydrogen additions (delta = 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 delta = 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 phi, 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 ST data with various delta can be well approximated by a general correlation (S-T - S-L)/u' = 0.17Da(0.43), covering both corrugated flamelet and distributed regimes with very small data scatter, where Da is the turbulent Damkohler 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. (c) 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.