Laser ignition system

LASER IGNITION SYSTEM

Lewis and von Elbe's data on minimum spark ignition energies in gases have been the standard for many years, however, these data still do not agree with the most detailed computational models available. With this motivation, their classic experiment was revisited using laser ignition sources, with an emphasis on better characterization of the ignition source and its effect on minimum ignition energy. The apparatus consisted of a laser ignition source operating either as a Q-switched nanosecond or a pulse mode-locked picosecond laser. For CH4-air mixtures of varying stoichiometry the minimum ignition energy was bracketed through repeated trials at varying laser spark energies. Laser spark kernel sizes were quantified by imaging the visible emission of these sparks. Results showed that the laser ignition experiments are consistent with Lewis and von Elbe's measurements for lean and rich mixtures, however, for near-stoichiometric mixtures, the laser ignition values were higher. These results are interpreted in the context of the size of the energy deposition region.
To exemplify the magnitude of the discrepancy between experimental and numerical results, Lewis and von Elbe reported an experimental value of MIE of 0.4 mJ for a stoichiometric CH4 -air mixture at 1 atm while the computational prediction of Sloane and Ronney using detailed chemical, hydrodynamic, and transport models, is 0.10 mJ. Also, Lewis and von Elbe infer that the minimum flame kernel diameter is about 2 mm, based on the spark gap providing the lowest MIE, whereas Sloane and Ronney predict 0.6 mm. Also, while Lewis and von Elbe clearly recognized the importance of heat losses to the spark electrodes, they did not report the diameter of the electrodes they used. These discrepancies between experiments and computations may possibly be due to an inadequate characterization of the ignition source, given the limitations of the instruments available at the time of Lewis and von Elbe's experiments. Alternatively, aspects of the physics of energy deposition, not present in even the most detailed models available to date, may play a role in ignition efficiency. For example, the MIE values for laser ignition of H2-air mixtures measured by Syage et al exceed the electric discharge measurements by a factor of 6 at stoichiometry. Toward the lean and rich limits, the discrepancy narrows but does not disappear. Consequently, the goal of this work is to employ modern lasers and optical instruments to produce ignition sources, characterize these sources and compare measured ignition parameters to previous experiments and computational models.
The laser-based techniques to be employed here has several likely advantages over the classical minimum ignition energy experiments. First, the actual energy deposited in the gas, rather than just the energy stored in a capacitor bank, is measured. Second, there are no electrodes to act as heat sinks. Third, the distribution of energy within the ignition region is readily characterized by optical means. For these reasons, it is believed that this system can provide a more accurate and complete characterization of the ignition event. If the mechanism of energy deposition has bearing on MIE, then one might observe different MIE values by different ignition sources. A collection of results would form a base of knowledge that would assist in developing more refined models of flame ignition and may provide insight into the long-standing discrepancies between electric spark experiments and computational predictions.

Laser ignition system

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