Thermal Structure of Laminar Methane/Air Flames: Influence of H₂ Enrichment and Reactants Preheating

Abstract

The Rayleigh scattering technique has been applied to a V-shaped laminar flame in order to investigate the effect of reactant temperature on the thickness and thermal structure of an H₂-enriched laminar methane-air flame. A systematic comparison of experimental and numerical results obtained with the GRI3.0 chemical mechanism is provided. First, the effects of reactant temperature on the pure methane-air flame are presented. A decrease in flame thickness and higher temperature gradients in the flame front are observed, and the maximum temperature gradient is shifted toward lower progress variable values. The preheat zone is substantially modified by the reactant preheating. Globally, the numerical predictions are in good agreement with the experimental results and validate the measurements. In addition, the effects of H₂ enrichment on the thermal methane/air flame thickness are presented for a reactant temperature of 300 K. We show that the main role of H₂ enrichment is to increase the temperature gradient and to shift toward lower values of the thermal progress variable c the location of the maximum temperature gradient. The reactivity of the mixture is strongly increased. At constant equivalence ratio, the burnt gas temperature is not modified by the H₂ enrichment, and the analysis can be done either in c or T space. Finally, the effect of reactant preheating on H₂/CH₄/air flames (40% H₂ dilution in volume) is investigated. The flame thickness is found to be lowered when reactant temperature is increased, but unlike the preheated methane-air flame, the localization of the maximum temperature gradient is not found to be markedly shifted. The reactivity of the mixture is rather controlled by the H radicals produced by H₂ dilution than the reactant preheating.

Keywords:

• Hydrogen
• Laminar flame thickness
• Preheating
• Rayleigh scattering
• Thermal flame structure

Acknowledgments

In Memoriam. This work is dedicated to Yannick Lafay, who tragically died during its Ph.D. thesis. He was an excellent student and a dear friend. We would like to express here the high value of his work and his exceptional qualities that allowed him to bring this project to a successful conclusion.