Effect of spatial heterogeneity on near-limit propagation of a pressure-dependent detonation

The effect of introducing a spatial heterogeneity into an explosive medium is studied computationally by examining the detonation velocity near the limit to propagation in a thin explosive layer. The explosive system studied is an ideal gas with a single exothermic reaction governed by a pressure-dependent reaction rate (pn) with a pressure exponent of n=3. A pressure-dependent reaction rate, rather than the usual exponential dependence of reaction on temperature of Arrhenius kinetics, is used so that the detonation wave is not affected by the extreme temperature sensitivity associated with Arrhenius kinetics and can be modelled with simple, analytical techniques, and thus the effect of introducing heterogeneity can be clearly identified. The two-dimensional slab of explosive is bounded by a layer of inert gas with the same thermodynamic properties as the explosive. The heterogeneity is introduced into the explosive via a large-amplitude, two-dimensional sinusoidal ripple in density in the initialization of the simulation, while maintaining a constant pressure. The computational simulations are initialized with a ZND solution for the ideal CJ detonation, and the detonation is allowed to propagate into the explosive layer. The simulations show that the detonation in the heterogeneous medium exhibits a cellular-like structure of complex shock interactions. The detonation is able to propagate into a significantly thinner layer of explosive and can exhibit a greater velocity than the corresponding homogeneous case. A parametric study of varying the wavelength of the sinusoid shows the existence of an optimal size of heterogeneity at which the effect is the greatest corresponding to a wavelength that is approximately 10 to 50times the half reaction zone length of the ideal CJ detonation. As the wavelength is decreased to the size of the reaction zone length, the behavior reverts back to the homogeneous case. Wavelengths of heterogeneity significantly larger than the optimal case are deleterious to the propagation of the detonation wave.