Multi-point laser-induced ignition of air-methane mixtures by a high peak-power passively Q-switched Nd:YAG/Cr4+:YAG laser

Abstract

We report on multi-point laser ignition of air-methane mixtures in a static, constant-volume spherical chamber. A fiber-coupled diode-pumped, monolithic, passively Q-switched Nd:YAG/Cr4+:YAG composite ceramic laser with four-output beams, each beam containing laser pulses with 3.8 mJ energy at 0.9 ns duration, was built for the experiments. The ignition was done in one to four points, in a compact area located close to the wall of the combustion chamber. In a first set of experiments, ignition was investigated for air-methane mixtures with relative equivalence ratio λ ranging from 0.94 (rich mixtures) to 1.7 (lean mixtures) at 1 bar initial pressure. Next, the ignition of lean air-methane mixtures was studied for high initial pressures up to 8 bar. Analysis of pressure history diagrams indicates increases (within a few percent) of the maximum pressure when ignition was done in four points in comparison with ignition at one location. It is concluded that the main advantage of multi-point ignition, for this configuration, is a decrease (around 10%) in combustion time, leading to a faster combustion. Also, it has been observed that multi-point ignition widens the ignition range of λ, for both lean and rich air-methane mixtures.