Laser Spark Ignition of Premixed Methane–Air Mixtures: Parameter Measurements and Determination of Key Factors for Ultimate Ignition Results

Abstract

In this study, we present an experimental investigation of the parameters of the laser spark ignition of premixed methane-air mixtures and the determination of the key factors for the ultimate ignition result. Ignition is achieved in a mesh honeycomb burner using the 1064 nm output of a neodymium-doped yttrium aluminum garnet (Nd : YAG) laser. All pertinent laser ignition parameters, including the minimum ignition energy, the ignition time and blow out time, and the effects that the variation of experimental conditions, such as the spark energy, ignition position, equivalence ratio (ER), and flow rate, have on these parameters have been addressed systematically. To identify the key factors for the ultimate result of laser ignition, several parameters of the ignition processes are measured simultaneously, with an emphasis given to the temporal behavior of the hydroxyl (OH) radicals in relation to the data regarding the spark energy and the local ER. A clear finding of the study is that successful ignition events are always related to higher OH radical photon emissions, considered to be proportional to the concentration level of the OH radicals present, thus indicating a direct link between the OH level at early times (on a microsecond scale) and the ultimate result of laser ignition. Two-dimensional correlation plots of the spark energy, local ER, and OH radical photon count at early times with the ultimate results of laser ignition indicate that the spark energy and local ER do not play a critical role in determining the success or failure of the ignition and that the OH concentration in the early time range is the key factor in determining the final fate of laser ignition. Finally, on the basis of the results obtained here and in the existing literature, some considerations of the mechanism of laser ignition are presented.

Robust and ultralow-energy-threshold ignition of a lean mixture by an ultrashort-pulsed laser in the filamentation regime

“…However, in the ns-LISI scheme, OH radicals mainly come from atomic recombination in the cooling process of hot plasma generated by breakdown. The stochastic nature of the breakdown around the threshold affects chain branching, leading to potential ignition failure 29 . Since there is no breakdown in ultrashort LI, the ignition result can be readily predetermined.…”

Laser ignition of methane jets in homogenous and isotropic turbulence

“…Minimum ignition energy (MIE) has been identified as an important parameter for ignition studies, which has a great dependence on mixture fraction and flow conditions around the ignition point. The influence of the mixture fraction on MIE has been studied extensively both in quiescent conditions [8][9][10] and laminar premixed flows [11,12]. The MIE was found to be the lowest at mixture fractions near stoichiometric and increase towards the lean and rich sides of stoichiometry.…”