Pr Philippe Gillard
Abstract: An experimental study of laser ignition of Zr/KClO4, Zr/PbCrO4, Fe/KClO4, Fe/KNO3, B/KClO4 and B/KNO3 mixtures is presented in this paper. The ignition system is composed of two lenses which focus the laser beam on the tablet. A sapphire porthole, juxtaposed with the pyrotechnic composition, protects the optical unit but causes losses of heat which result in an increase of the energy threshold.
This experimental set-up was used to determine the ignition sensitivity of five mixtures. A particular behaviour of B/KNO3 was observed. Experimental results pointed out an increase of the energy of ignition when the laser power grows. The other mixtures exhibit a conventional behaviour of the trend of ignition threshold E50 according to the power P of the laser beam: a continuous decrease of the value of E50 is observed when P is rising.
Some parameters linked to the experimental device also have a great influence. Among them the power density seems to be predominant but the thermal conductivity of the sapphire windows also plays an important role. It is possible to find an optimum value of the laser diameter, for a given power density. This experimental and parametric study have shown that, on such devices, it is possible to find optimal conditions of the ignition of various pyrotechnic mixtures.
A modelling, based on a progressive absorption of the laser beam inside a reactive pyrotechnic composition, was developed. It takes account of the energy exchanges between the ignition system and the pellet. Two interface parameters make it possible to optimize calculations. One relates to the heat transfer on the level of the porthole/tablet interface, the other relates to the laser absorption of energetic materials. By combining these two terms, it has been possible to corroborate the experimental thresholds on five different pyrotechnic mixtures.
To evaluate the interface parameters, whose implication in initiation is critical, several methods are exposed. One of them proposes a numerical calculation based on a random and automatic installation of the grains. The result gives Gaussian variables. This method enables simulation of the statistical tests of Bruceton and Langlie which are used in pyrotechnics.
The numerical results show that the sensitivity of the mixtures depends primarily on the propagation of heat towards the porthole and the interior of the tablet, as well as on the in-depth absorption of the laser in the pellet.
Keywords: Laser diode, ignition, pyrotechnic mixtures, numerical modelling
Ref: JPyro, Issue 26, 2007, pp53-59
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