No abstract

No abstract

The current status of the direct initiation problem, where a powerful source drives a blast wave into an explosive gaseous mixture to generate a Chapman-Jouguet (CJ) detonation, is critically assessed. The current theories that are most successful in estimating the critical energy required for initiation are semiempirical in nature, in that they involve an experimentally determined length-scale (typically cell size) to characterize the explosive mixture. The eventual analytic theory of initiation should be based exclusively on the constitutive properties of the explosive. To date, attempts at a comprehensive theory of initiation have invoked quenching of the reaction front by curvature or unsteadiness of the blast wave. Simple analytic models of initiation as well as numerical simulations and experiments, however, all indicate that initiation near the critical regime is the result of a reacceleration of the blast wave from a sub-CJ minimum. Hence, the criterion for initiation must take into account the amplification of the blast wave due to coherent coupling with the chemical energy release. The effect of 'hot spots' is also shown to have a pronounced effect in reducing the critical energy required for initiation. These results suggest directions that future investigations can pursue toward a rigorous theory of direct initiation.

In this paper, the critical energies required for direct initiation of spherical detonations in four gaseous fuels (C 2 H 2 , C 2 H 4 , C 3 H 8 and H 2 ) -oxygen mixtures at different initial pressures, equivalent ratios and with different amount of argon dilution are reported. Using these data, a scaling analysis is performed based on two main parameters of the problem: the explosion length R o that characterizes the blast wave and a characteristic chemical length that characterizes the detonation. For all the undiluted mixtures considered in this study, it is found that the relationship is closely given by R o ≈ 26λ, where λ is the characteristic detonation cell size of the explosive mixture. While for C 2 H 2 − 2.5O 2 mixtures highly diluted with argon, in which cellular instabilities are shown to play a minor role on the detonation propagation, the proportionality factor increases to 37.3, 47 and 54.8 for 50%, 65% and 70% argon dilution, respectively. Using the ZND induction length ∆ I as the characteristic chemical length scale for argon diluted or 'stable' mixtures, the explosion length is also found to scale adequately with R o ≈ 2320∆ I .

No abstract