Laser-induced ignition of methane-air mixtures by a four-beam, pulse-burst mode passively Q-switched Nd:YAG/Cr4+:YAG laser

Abstract

In this work we report on the characteristics of laser ignition of CH4-air mixtures, using multi-point (up to four points) ignition with single-pulse beams as well as with trains of two pulses. A compact Nd:YAG/Cr4+:YAG laser with four beams, each beam being operated in single-pulse or burst mode (with up to five pulses), each pulse with an energy of 3.8 mJ and 0.9 ns duration was employed in the experiments. A combustion chamber with spherical geometry was used to ignite CH4-air mixtures with various values of the air–fuel equivalence ratio, λ and at different initial pressures of the fuel mixtures. Analysis of pressure curves indicates that using multiple ignition points leads to an acceleration of the flame speed in the early kernel development phase. It has been observed that for an initial pressure of 1 bar in the combustion chamber, ignition at a point with two laser pulses decreases the duration of combustion by about 7 % compared to ignition at a point with a single laser pulse. For ignition in four points with one or two pulses, the combustion is 10 % and 20 % faster, respectively. The combustion efficiency is also analyzed, and we report that a multi-point laser ignition source leads to a slight increase in the efficiency of burning the fuel near the lean limit. The ignition limits, for both lean and rich CH4-air mixtures, have also been determined for initial pressures in the combustion chamber of up to 9 bars and each ignition method, and a discussion of these is presented.