Influence of interface conditions on laser diode ignition of pyrotechnic mixtures: application to the design of an ignition device

Abstract

This paper treats of numerical modelling which simulates the laser ignition of pyrotechnic mixtures. The computation zone is divided into two fields. The first is used to take account of the heat loss with the outside. It can represent an optical fibre or a sapphire protective porthole. The second field represents the reactive tablet which absorbs the laser diode's beam. A specific feature of the model is that it incorporates a thermal contact resistance R c between the two computation fields. Through knowledge of the thermal, optical and kinetic properties, this code makes it possible to compute the ignition conditions. The latter are defined by the energy E 50 and the time t i of ignition of any pyrotechnic mixture and for various ignition systems. This work was validated in the case of an ignition system consisting of a laser diode with an optical lens re-focussing system. The reactive tablet contains 62% by mass of iron and 38% by mass of KClO 4. Its porosity is 25.8%. After an evaluation of the laser's coefficient of absorption, the variations of the ignition parameters E 50 and t i are studied as a function of the thermal contact resistance R c . Temperature profiles are obtained as a function of time and for various values of the thermal contact resistance R c . More fundamental observations are made concerning the position of the hot spot corresponding to priming. From this study, which concerns the heat exchange between the two media, several practical conclusions are given concerning the design of an ignition device. By evaluation of the thermal contact resistance R c , comparison with test results becomes possible and the results of the computations are in reasonable agreement with the test measurements.