Laser ignition of methane-air mixtures at high pressures and optical diagnostics

Laser-induced ignition of methane-air mixtures of varied composition was investigated experimentally using nanosecond pulses generated by Q-switched Nd:YAG lasers (wavelength 1064 nm, 532 nm and 355 nm) at initial pressures up to 4 MPa. The minimum focal spot diameter was found to be about 20 μm for effective ignition, independent of the laser wavelength, indicating that small impurity particles provide the seeds for laser plasma generation. The minimum laser pulse energy needed for ignition ranged from 2-15 mJ decreasing reciprocally with initial pressure and with fuel equivalence ratio φ in a mixing regime of φ=0.91 to φ=0.56. Corresponding threshold intensities ranged from 10¹⁰ to 10¹¹ W/cm². In this way, evidence for a non-resonant breakdown mechanism was established. Optical in-situ diagnosis of water vapor concentration covering the whole timespan of the combustion process in a stationary high pressure vessel with four optical windows was performed involving linear absorption measurements over the entire spectral absorption linewidth by rapidly tuned diode laser radiation at 2.55 μm. Additionally, planar laser-induced fluorescence was measured in a time-resolving fashion yielding 3-dimensional determination of the OH concentrations during the process. To the knowledge of the authors, these are the first results on laser-induced ignition under laboratory conditions well above atmospheric pressure being relevant for several technical combustion systems.