UBC Theses and Dissertations
Hydrogen-enriched methane-air combustion at intense turbulence Mohammadnejad Daryani, Sajjad
Internal structure and burning velocity of hydrogen-enriched methane-air turbulent premixed flames are investigated experimentally using planar laser-induced fluorescence of formaldehyde molecule and hydroxyl radical as well as stereoscopic particle image velocimetry techniques. A Bunsen burner is utilized to complete experiments at intense turbulence intensities (Karlovitz numbers up to 76.0). 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. Turbulent flow characteristics of the flames suggest that the potential penetration of eddies into the preheat and reaction zones is the underlying reason for the broadening of these zones. Broadening of the reaction zone, which is studied in this thesis, suggests that the flamelet assumption used in formulating the local consumption speed in past studies may not hold. Thus, a new formulation, which does not utilize the flamelet assumption, is developed and used for estimating the burning velocity of premixed flames. It is shown that, at small turbulence intensities, the values of the estimated burning velocity follow those of the local consumption speed. However, at large turbulence intensities and consistent with the results in the literature, the local consumption speed underpredicts the values of the burning velocity. It is shown that the amount of this underprediction is correlated with the broadening of the reaction zone, suggesting that the non-flamelet behavior contributes to this underprediction. It is shown, although the flame thickening increases the burning velocity, extinctions decrease this parameter, which may cause a characterized behavior, referred to as bending. The amount of this bending is shown to be related to the inverse of the Damköhler number. Using this, a mathematical formulation that allows for the estimation of the burning velocity is developed. It is shown all of the above are influenced by the air entrainment, referred to as lack of back support. Specifically, this influences the flames structure and the burning velocity by diluting the reactants, changing the effective fuel-air equivalence ratio, and altering both the effective turbulence intensity and the effective normalized integral length scale.
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