Correlating minimum ignition criteria

Abstract

Potential hazards associated with the use of optical fibres carrying laser beams in flammable atmospheres have prompted a series of ignition studies. Investigations are described in which diverse target materials are exposed to radiation from various lasers in a variety of flammable mixtures. A distinctive feature of these systems is that ignition by a laser beam expanding from a fine optical fibre, interacting with particles of varying size and position, can, with minor adjustments to the geometry, give rise to wide variations in irradiated area and duration. This contrasts with the more customary circumstances in which the criterion is either a minimum ignition energy (short time, small dimensions) or an ignition temperature (long times, large dimensions). It is shown that the laser ignition criteria of a minimum power flux at large areas and minimum igniting power for small areas may be interpreted in terms of an igniting energy increasing linearly with time and area, but tending to constant values for dimensions smaller than the quenching distance. A general theory is developed, correlating the various ignition criteria with each other and with fundamental combustion parameters. The hazard may be related to corresponding safety considerations for the dissipation of electrical energy in flammable atmospheres by taking into account the absorptivity/emissivity characteristics of each particular laser wavelength/target combination.