Proceedings ArticleDOI

# Laser induced spark ignition of methane-oxygen mixtures

25 Sep 1991-

AbstractResults from an experimental study of laser induced spark ignition of methane-oxygen mixtures are presented. The experiments were conducted at atmospheric pressure and 296 K under laminar pre-mixed and turbulent-incompletely mixed conditions. A pulsed, frequency doubled Nd:YAG laser was used as the ignition source. Laser sparks with energies of 10 mJ and 40 mJ were used, as well as a conventional electrode spark with an effective energy of 6 mJ. Measurements were made of the flame kernel radius as a function of time using pulsed laser shadowgraphy. The initial size of the spark ignited flame kernel was found to correlate reasonably well with breakdown energy as predicted by the Taylor spherical blast wave model. The subsequent growth rate of the flame kernel was found to increase with time from a value less than to a value greater than the adiabatic, unstretched laminar growth rate. This behavior was attributed to the combined effects of flame stretch and an apparent wrinkling of the flame surface due to the extremely rapid acceleration of the flame. The very large laminar flame speed of methane-oxygen mixtures appears to be the dominant factor affecting the growth rate of spark ignited flame kernels, with the mode of ignition having a small effect. The effect of incomplete fuel-oxidizer mixing was found to have a significant effect on the growth rate, one which was greater than could simply be accounted for by the effect of local variations in the equivalence ratio on the local flame speed.

Topics: Flame speed (71%), Laminar flame speed (68%), Ignition system (58%)

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01 Jan 1988
Abstract: Recent advances in the understanding of the structure, propagation, and extinction of laminar flames under the influence of stretch, as manifested by the existence of flame curvature, flow nonuniformity, and flame motion, are reviewed. The emphasis is on premixed flames because of the richness and subtlety of the phenomena involved. The review distinguishes the influences of the tangential and normal velocity gradients on the flame response, both at the hydrodynamic scale and within the flame structure, and emphasizes the importance of the preferential diffusion nature of heat and mass transport, as well as the extent to which the flame can freely adjust its location in response to stretch in order to achieve complete reaction. It is then demonstrated that stretch has only minimal effect on an adiabatic, unrestrained, diffusionally-neutral flame with complete reaction in that the temperature, propagation rate, and thickness of the flame are invariant to stretch, and that stretch alone cannot extinguish such a flame. In the presence of preferential diffusion and/or when the flame movement is restrained, the response of the flame to stretch becomes more sensitive and extinction is also possible. The concept of flame stretch is applied to interpret such practical flame phenomena as flame stabilization and flame-front instability, determination of laminar flame speeds and flammability limits, concentration and temperature modifications in flame chemistry, and modeling of turbulent flames. The properties of stretched diffusion flames are then briefly discussed. The review closes with suggestions for further research.

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206 citations

Journal ArticleDOI
Abstract: The structure and propagation of stretched premixed flames with preferential diffusion are analyzed using an approximate integral approach which yields algebraic relations for the characteristic scales, overall conservation requirements, and balances and continuity between the various scalar properties and their transport rates. Specific flame configurations studied include the one-dimensional steadily propagating planar flame, the steady curved flame in nonuniform flow, and the outwardly/inwardly propagating spherical flame. Linearized results agree with those obtained from asymptotic analysis assuming small stretch. The present work yields enhanced insight into the dominant and coupled physical processes governing these complex flame phenomena, and demonstrates the utility of the integral analysis in theoretical combustion studies.

135 citations

Proceedings ArticleDOI
01 Feb 1978
Abstract: Preliminary test results are presented for a spark ignition engine which uses a focused, laser beam and conventional spark ignition as ignition sources For a steady running, single cylinder engine with minimum spark for best torque (MBT) spark timing and fixed throttle position, laser ignition provides improved engine performance and efficiency, extension of the lean limit of operation by five air/fuel ratios, and increased nitric oxide production Carbon monoxide and hydrocarbon emissions are essentially the same With the laser, the spark location was found to have little effect on performance except when it was moved near the combustion chamber wall The minimum laser pulse energy required for steady engine operation seems to be dictated by the minimum energy required to achieve breakdown of the laser pulse in air at the same pressure Raising the spark energy above this minimum level is desirable to produce a steady running engine The use of exhaust gas recirculation (EGR) was found to be effective in reducing nitric oxide levels The nitric-oxide specific fuel consumption tradeoff with 16% EGR showed the laser to be superior to the standard ignition system

124 citations