1978-02-01

Jet Ignition of an Ultra-Lean Mixture 780637

The paper reports a preliminary study of jets of active radicals used as igniters for lean mixtures. The jets were generated either by combustion or by electric discharge. Experiments were performed in a cylindrical steel vessel, 9 cm in diameter and 9 cm long, filled initially with either air or an ultra-lean (equivalence ratio: 0.5) methane-air mixture at atmospheric pressure and room temperature. Observations were made by schlieren photography, using a sub-microsecond spark discharge in air as a point light source.
The gasdynamic properties of the jets were shown to be primarily governed by their initial velocity, while the particular process by which they were formed played, in this respect, a secondary role. The jets of radicals invariably appeared as turbulent plumes which were embedded in blast waves headed by hemispherical shock fronts.
The most interesting properties of jet ignition are: (a) a controllable depth of penetration, so that combustion can be started at any desired location within the charge, (b) zonal pre-turbulization enhancing the combustion process, and (c) a wide dispersion resulting in multi-point ignition, so that combustion is initiated throughout a relatively large segment of the medium rather than in the form of a small laminar flame kernel, as it does in the conventional case of unconfined spark discharge.

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