Laser-induced spark ignition of CH4/air mixtures

Laser-induced spark ignition of CH4-air mixtures was experimentally investigated using a nanosecond pulse at 1064 nm from a Q-switched Nd-Yag laser. Laser irradiance in the order of 10(12) to 10(13) W/cm(2) was found to be sufficient to ignite a mixture having from 6.5 to 17% methane by volume (equivalence ratio, ER, from 0.66 to 1.95). The dependence of the breakdown threshold laser energy, E-thr, on the gas pressure was in agreement with the electron cascade theory. Depending on the laser energy, E-o, the spark absorption coefficient in the range from 0.1 to about 100 cm(-1) was calculated using the electron-ion inverse bremsstrahlung process. The minimum ignition energy was about one order of magnitude higher than the minimum ignition energy obtained by the electric spark ignition. It had its lowest value remaining at about 3 to 4 mJ for a mixture having 10 to 15% methane by volume (ER = 1.058 to 1.68) and it increased sharply toward the far-lean and the far-rich sides of the stoichiometry. The average length and radius of the spark for a stoichiometric or near-stoichiometric methane-air mixture were about 0.8 mm and 0.3 mm, respectively. For lean or rich methane-air mixtures, the average long axis of the spark size varied from about 0.8 to 2 mm, whereas for the short axis it varied from about 0.4 to 1.2 mm depending on the methane volume fraction. (C) 1999 by The Combustion Institute.