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Fundamentals of high-energy spark ignition with lasers

Bradley, D., Sheppard, C.G.W., Suardjaja, I.M. and Woolley, R. (2004) Fundamentals of high-energy spark ignition with lasers. Combustion and Flame, 138 (1-2). pp. 55-77. ISSN 0010-2180

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Abstract

An experimental study of laser-induced spark ignition of flammable, gaseous premixtures is reported, with theoretical interpretations. Ignition was in an explosion bomb, equipped with four variable-speed fans that facilitated the study of quiescent and isotropic turbulent conditions. Good optical access enabled the progress of plasma fronts, shock waves, igniting kernels, and propagating flames to be recorded with high-speed schlieren photography. A focused beam from a Q-switched Nd:YAG laser initiated electrical breakdown, with plasma energies between 85 and 200 mJ. Probabilities of breakdown were found for air and isooctane-air mixtures over ranges of pressures and temperatures. Blast-wave theory applied to shock-wave trajectories enabled initial plasma conditions to be inferred. This suggested electron temperatures of over 10 5 K and very high pressures. Calculated values of the absorption coefficient for the laser beam energy show these plasma properties to be commensurate with the observed energy and size. The ensuing rarefaction wave creates toroidal rings at the leading and trailing edges of the plasma. The former decays more rapidly and a third lobe of the kernel is generated that moves towards the laser. In flammable mixtures this enhances the flame spread. Laminar flame speeds are overdriven by this gasdynamic effect, as well as by the high energy of the plasma, to such an extent that the flame speed decays from elevated values as the flame stretch decreases, contrary to the increases that occur with normal flames with positive Markstein numbers. The extent to which turbulence narrows the ignition limits is found experimentally. For mixtures close to the lean flammability limit, strong gasdynamic flows induced by laser ignition can stretch the flames to extinction and narrow the ignition limits. If a flame becomes established, eventually the third lobe disappears as the initial gas dynamic effects decay and are overwhelmed by the imposed flow fields. Nevertheless, the overdrive effects persist for some time and overdriven flames were observed in regimes where normal flames would have quenched.

Item Type: Article
Keywords: Lasers Shock waves Spark ignition Turbulence Combustion Explosions Fans Flame research Ignition Laminar flames Laser induced spark ignition Q switched lasers laser shock wave article calculation energy explosion hyperbarism optical rotation particle size photography plasma pressure priority journal temperature temperature sensitivity velocity
Institution: The University of Leeds, The University of Sheffield
Academic Units: The University of Leeds > Faculty of Engineering (Leeds) > School of Mechanical Engineering (Leeds)
The University of Sheffield > Faculty of Engineering (Sheffield) > Department of Mechanical Engineering (Sheffield)
Depositing User: Mr Christopher Hardwick
Date Deposited: 20 Aug 2009 18:07
Last Modified: 08 Feb 2013 17:08
Published Version: http://dx.doi.org/10.1016/j.combustflame.2004.04.0...
Status: Published
Publisher: Elsevier Science B.V., Amsterdam
Identification Number: 10.1016/j.combustflame.2004.04.002
URI: http://eprints.whiterose.ac.uk/id/eprint/9198

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