Measurements of minimum ignition energy in premixed laminar methane/air flow by using laser induced spark

Reducing engine pollutant emissions and fuel consumption is an important challenge. Lean-burning engines are a promising development; however, such engines require high-energy ignition systems for typical working conditions (equivalence ratio, Φ < 0.7). Laser-induced ignition is envisaged as a way to obtain high-energy ignition as a result of progress that has been made in laser beam technology in terms of stability, size, and energy. This study investigated the minimum energy necessary to ignite a laminar premixed methane air mixture experimentally. A parametrical study was performed to characterize the effects of the flow velocity, equivalence ratio, and lens focal length on the minimum energy required for ignition. Experiments were conducted using a premixed laminar CH₄/air burner. Laser-induced breakdown was achieved by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser with an anti-reflection-coated lens. Mixture ignition and the early stages of flame propagation were studied using a high speed Schlieren technique. Despite the stochastic characteristic of the laser breakdown phenomena, good reproducibility in the minimum energy required for the ignition measurements was observed. The cases in which the CH₄/Air mixture flow ignites are defined as those with a laminar flame front propagation visible in the Schlieren images 10 ms after the energy deposition. The same minimum ignition energy (MIE) versus equivalence ratio (Φ) type of curves were obtained with a laser-induced spark and with a spark plug. Due to the threshold of energy required to obtain breakdown and the stochastic character of the energy absorption by the spark, a constant value was obtained (corresponding to the breakdown threshold) when the minimum ignition energy was lower than the breakdown threshold. As already noticed by several authors, MIE values higher than those observed using spark plugs were obtained. However, these differences tended to disappear at the lean and rich fuel limits.