Ignition of methane–air mixtures by laser heated small particles

Methane–air mixtures ignited by CW laser-heated targets on optical fiber tips: Comparison of targets, optical fibers, and ignition delays

“…Dubaniewicz et al (2000) observed methane-air ignition delays increased with decreasing power, using coal and Fe 3 O 4 targets on large core optical fiber tips. Ignition delays also increased with larger targets.…”

“…The 8% optimum concentration is similar to that used for mine apparatus approval testing with electrical sparks (MSHA, 1995). Other researchers observed lower optimum methane-air concentrations using cw lasers (Dubaniewicz et al, 2000;Hawksworth, 2000;Welzel et al, 2000). Perhaps ignition occurs at 8% most easily with the short duration sparks because diffusion effects described by Hertzberg (1989) don't have enough time to create stoichiometric concentrations with leaner mixtures before the ignition source is removed.…”

“…Schenk et al (2001) found that a toner consisting mostly of ferrite powder ignited certain other flammable atmospheres most easily, but not at powers below that obtained with the (FeMn) 2 O 3 targets. Dubaniewicz et al (2000) found Fe 3 O 4 targets produced slightly lower igniting powers than coal targets in methane-air mixtures, using a non-flammable grease to attach the targets to large core optical fibers. Larger coal particles required higher laser powers to ignite the methane-air, suggesting the heat of combustion from the coal was not a significant contributor to ignition compared to the heat generated by the laser.…”

“…Several researchers reported lean methane-air mixtures, ranging from about 5% to 6% methane-air, produced the lowest igniting powers with cw lasers (Dubaniewicz, Cashdollar, Green, & Chaiken, 2000;Hawksworth, 2000;Welzel et al, 2000). One exception was Zhang et al (1992), who reported 8% as the most easily ignited methane-air concentration.…”

“…Schenk et al (2001) did not observe ignition of methane-air using powers up to 370 mW with any target. Prior research (Dubaniewicz et al, 2000) suggests heat of combustion contributions from the small coal particle should be relatively small compared to the heat generated by the laser, and should not have such a significant impact on ignition delay. Excessive laser overshoot could produce apparently lower igniting powers, shorter ignition delays, and richer …”