Laser ignition of premixed methane-air gas flow

Laser-ignition experiments were conducted on methane-air premixed-gas flow within a pipe. In the experiments, we varied the mean flow velocity with values of 15, 30, and 60 m/s, corresponding to the Reynolds-number range of 19,000–77,000, the absorbed laser energy with values of 7.5, 15, and 30 mJ, and the mole fraction of methane within the range of 5.4–7.2%, corresponding to the fuel-equivalence-ratio range of 0.54–0.74, to investigate the methane mole fraction yielding a 50% ignition success probability for each combination of the mean flow velocity and the absorbed laser energy. In order to analyze the experimental results from the viewpoint of turbulence, we first plotted the experimental conditions yielding a 50% ignition success probability onto the u ′ / S L − L i / δ L phase diagram commonly used in turbulent-combustion studies, where L i is the integral length scale of turbulence, u ′ is the rms of velocity fluctuations due to turbulence (turbulence intensity), δ L is the flame thickness of the laminar flame, and S L is the burning velocity of the laminar flame. As a result, the critical conditions for ignition for each absorbed energy aligned on a straight line corresponding to a constant value of the turbulent Damköhler number D a = ( L i / u ′ ) / ( δ L / S L ) . However, its value for which the critical conditions for ignition aligned differed for each different absorbed energy. Next, we plotted the critical conditions for ignition onto the u ′ / S L − R FK / δ L phase diagram replacing the turbulence integral length scale L i with the initial flame-kernel radius R FK . In this case, all experimental results aligned on a single line represented by a constant value of D a FK = ( R FK / u ′ ) / ( δ L / S L ) . Here, D a FK is the flame-kernel turbulent Damköhler number. We propose this flame-kernel turbulent Damköhler number as the dimensionless quantity governing the ignitability limit for turbulent premixed combustible mixtures.