  Influences of reaction zone thickening and local extinctions on the burning
rate of extremely turbulent hydrogen-enriched methane-air flames are
investigated using simultaneous planar laser-induced fluorescence of
formaldehyde molecule and hydroxyl radical as well as separate stereoscopic
particle image velocimetry techniques. Karlovitz numbers upto 76 are examined.
It is shown that, by increasing the turbulence intensity, the preheat and
reaction zone thicknesses can increase to values that are, respectively, 6.3
and 4.9 of the corresponding laminar flames. Broadening of these zones for
intensely turbulent hydrogen-enriched methane-air flames is shown
experimentally for the first time. Broadening of the reaction zone suggests
that the flamelet assumption used for development of the burning rate
formulations may not hold. Thus, a new formulation, which does not utilize the
flamelet assumption, is developed and used to calculate the burning rate of the
tested flames. It is shown that, at small turbulence intensities, the burning
rate values follow those of the local consumption speed, which is developed in
the literature based on the flamelet assumption. However, at large turbulence
intensities, the estimated burning rate features large values, and the ratio of
this parameter to the local consumption speed is consistent with the ratio of
the global and local consumption speeds reported in the literature. It is shown
that the ratio of the normalized burning rate to the normalized local
consumption speed is correlated with the broadening of reaction zone,
suggesting that the disparity between the values of the burning rate and local
consumption speed is linked to the reaction zone thickening. It is shown,
although the flame thickening increase the burning rate, local extinctions
decrease this parameter leading to the bending behavior reported in the
literature.
