Contributions of flame thickening and extinctions to a heat release rate marker of intensely turbulent premixed hydrogen-enriched methane-air flames

Abstract

Influences of the reaction zone thickening and extinctions on a heat release rate marker 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. The fuel-air equivalence ratio is set to 0.7, and the amount of hydrogen-enrichment varies from 0% to 70% by volume. Mean bulk flow velocities ranging from 5 to 35 m/s with Reynolds and Karlovitz numbers of 18 to 2729 and 0.1 to 76.0, respectively, 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, which were measured experimentally. 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 turbulent flame speed formulations may not hold. Thus, a new formulation, which does not utilize the flamelet assumption, is developed and used to calculate a heat release rate marker of the tested flames. It is shown that, at small turbulence intensities, the heat release rate marker 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 heat release rate marker 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 heat release rate marker to the normalized local consumption speed is correlated with the broadening of reaction zone, suggesting that the disparity between the values of the heat release rate marker and the local consumption speed is linked to the reaction zone thickening. It is shown, although the flame thickening increases the heat release rate marker, extinctions decrease this parameter, which may contribute to the characterized bending behavior. It is argued that the amount of bending is related to the inverse of the Damköhler number. Using this, a mathematical formulation that allows for estimation of the heat release rate marker is developed. The predictions of the proposed formulation agree well with the normalized global consumption speeds of past experimental investigations.