Instability of lean H₂-O₂-CO₂ premixed flames on a flat burner

Abstract

Global warming and air pollution mainly caused by emissions of CO₂ and NOx are serious problems. We focus on a lean H₂-O₂ premixed flame because H₂-O₂ premixed flame emits no CO₂ and NOx. H₂-O₂ mixture also has dangerous properties such as large burning velocity, wide flammability range and so on. In order to suppress the burning velocity of H₂-O₂ premixed flame, we propose an addition of CO₂ to the mixture. However, a lean H₂-O₂-CO₂ premixed flame tends to become unstable and form a cellular flame front owing to its intrinsic instability. For control of the combustion, it is necessary to elucidate the instability of lean H₂-O₂-CO₂ premixed flames. In this study, we investigated the structure and behavior of lean H₂-O₂-CO₂ premixed flames on a flat burner by direct photographs, time series analysis of light emission and Rayleigh scattering, and understand the flame instability. As the results, it was observed that the cell width obtained from the direct photographs increased with decreasing of equivalence ratio. To investigate the fluctuation characteristics of the flames, we reconstructed attractors from the light emission intensity of the flames. The attractors had torus shapes which indicate that the flame behaviors are quasi-periodic. In the Rayleigh scattering, we obtained not only width but also depth of the cellular flame. The cell width and depth increased with decreasing of equivalence ratio. Furthermore, the width and depth became larger as the CO₂ concentration increases. These results showed that diffusive-thermal effect influenced the cellular structure.