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Detonation direct initiation

Direct initiation of detonation can be achieved by condensed explosives, explosion of another gas mixture, electrical wire explosion, high-voltage sparks, and laser sparks.

The conditions are characterized by the critical (minimal) energy E^*_n required to produce a shock wave going over to a self-sustained detonation; n = 1, 2, 3 denotes planar, cylindrical, and spherical waves respectively. The rapid release of a spatially concentrated source of energy results in the production of a decaying blast wave in the surrounding gas. A very strong gradient in temperature and pressure occurs behind the blast wave and serves to quench the chemical reaction. On the other hand, the very high temperatures produced behind the blast induce very rapid chemical reaction. The net effect of quenching due to strong gradient and accelerated reaction due to strong blast waves result in either initiation or failure depending on the strength of the source.

A heuristic limiting condition proposed by Zel'dovich (ZeldovichYaB:1956) is that at least one reaction time must have elapsed before the instantaneous Mach number reaches the CJ value for initiation to be successful. Combining this simple criterion with strong blast wave (similarity) theory results in a quantitative prediction for the critical energy (LeeJHS:1966). In a spherical geometry, the result is:

E_{c} =I\left(\gamma \right)\rho _{0} D_{CJ} \Delta ^{3},

where \Delta is the induction length and I\left(\gamma \right) is a constant of the order of 1 determined by blast wave theory.

This expression can be used together with detailed kinetic computations of the reaction zone length to estimate the critical initiation energy for specific fuel-oxidizer mixtures. Atkinson (AtkinsonR:1980) showed satisfactory agreement between their kinetic computations and experimental data for hydrogen-air initiation.

Zeldovich Ya.B., Kogarko S.M. and Simonov N.N. (1956) An experimental investigation of spherical detonation of gases. Soviet Physics - Technical Physics, 1:1689-1713.(BibTeX)
Lee J.H.S, Lee B.H.K. and Knystautas R. (1966) Direct initiation of cylindrical gaseous detonations. Physics of Fluids, 9:221-222.(BibTeX)
Atkinson R., Bull D.C. and Shuff P.J. (1980) Initiation of spherical detonation in hydrogen-air. Combustion and Flame, 39:287-300.(BibTeX)

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