Mechanism Of Detonation Initiation Research Articles

Initiation of cylindrical detonation by rapid energy deposition along a line

Initiation of gaseous detonation in hydrogen, ethylene, and acetylene-air mixtures by a line source of condensed explosives (detonating cord) is investigated experimentally. Initiation in the cylindrical geometry is important in validating a general theory of initiation and has particular significance to the interaction of high-velocity projectiles with combustible gas. The flowfields generated by the detonating cord resemble the flows observed around blunt hypersonic projectiles in combustible gas, with oblique detonation being observed in good agreement with theoretical predictions. When the energy of the detonating cord greatly exceeds the critical energy per unit length to initiate a cylindrical detonation, the blast wave generated by the cord decays monotonically to the Chapman-Jouguet (CJ) detonation speed. As the source energy approaches the critical energy predicted by theoretical models of initiation, the results become increasingly complex. In these cases, the blast wave and combustion front first decay to sub-CJ velocity, then exhibit oscillations before the final onset of detonation. The initiation of gaseous detonation is also no longer sequential along the length of the cord and is instead associated with discrete explosion centers. This phenomenon is similar to what has been previously observed in spherical initiation, suggesting a universal mechanism of detonation initiation.

On the Critical Conditions for the Initiation of a Detonation in a Nonuniformly Perturbed Reactive Fluid

The critical conditions for the initiation of a strong Zeldovich--von Neumann--Döring (ZND) detonation in a nonuniformly perturbed reactive fluid are derived using a high activation-energy asymptotic analysis. The initial disturbances are taken to have amplitudes of the order of the small inverse activation energy, with a characteristic wavelength of the order of an acoustic scale, and consist of initial temperature, concentration, pressure, velocity, or density perturbations. The detonation initiation mechanism is based on the work of Dold and Kapila, which describes how ignition of a reactive fluid leads to the generation of a supersonic, shockless, weak detonation. A transition from the weak detonation to a ZND detonation occurs if the initial disturbance is sufficiently strong to induce a gradient of thermal ignition times that cause the weak detonation to slow to the Chapman--Jouguet detonation velocity. Underpinning the whole process is the ability to calculate the path of the weak detonation. In Dold and Kapila's theory this has to be done numerically. Here, the path of the weak detonation, and thus the precise location and time of a transition to strong detonation, is derived analytically through a long-wavelength analysis of the induction zone of the explosion by assuming the initial disturbances vary slowly on the characteristic acoustic scale. Comparisons of the analytically derived results with exact numerical solutions of the induction zone evolution, the thermal runaway, and the weak detonation path, arising from initial disturbances which vary explicitly on the acoustic scale, are shown to be excellent. Moreover, this analysis provides a theoretically based understanding of how each type of initial disturbance, be it an entropy disturbance involving initial temperature nonuniformities or an acoustic disturbance involving initial velocity and pressure fluctuations, influence both the location and the time of the initial point of thermal runaway in the fluid, and the subsequent generation and the speed of the weak detonation. In combination with the analysis of Dold and Kapila, the theory contained herein gives a completely analytical description of how a ZND detonation is generated from an initially perturbed reactive fluid. An extension of the analysis to determine the path of a weak detonation arising from an initial nonuniformity in a three-dimensional geometry is also presented. Finally, as a by-product of the present analysis, a rational derivation of Zeldovich's notion of a constant volume spontaneous flame is given; moreover, it results in a significant improvement on Zeldovich's results.

Critical Initiation Conditions for Gaseous Diverging Spherical Detonations

The diverging spherical detonation wave in gaseous explosives is obtained either with a point source of explosion of energy E or through the transmission of a plane detonation from a cylindrical tube of diameter d into a large volume. The mechanism of detonation initiation in both cases is based on the shock to detonation transition.The experimental critical conditions lead to an initiation criterion for detonation resulting from the competition between the expansion behind the leading shock wave on one hand and the shock-induced chemical heat release on the other. Whatever the type of ignition source, the detonation is obtained when the radius of curvature of the wave overcomes a particular critical value R c whose size includes a constant and large number of cell width λ CJ (R c ? 20 λ CJ ) and then can be considered as intrinsic to the detonative mixture used. λ CJ , which is the mean size of the cellular structure of a CJ detonation, is proportional to the global chemical induction length L i , calculated in the ZND scheme, by also a large factor (generally more than 10). Two other criteria define the critical initiation energy Ec and the critical tube diameter dc for obtaining detonation with respect to this intrinsic length R c .

The behavior of detonation waves at concentration gradients

Two detonation tubes were used to study detonation propagation in concentration gradients. Each tube, 22 × 10 mm and 50 mm diameter in cross section, respectively, was fitted with slide valves that with the tubes mounted vertically allowed known concentration gradients to form by diffusion. The peak-transmitted shock strength when a detonation is incident on an abrupt planar interface with an inert gas was found to agree reasonably well with the simple Paterson-Glass model. A significant improvement was found in the predicted values when allowance was made for the Taylor expansion and energy and momentum losses to the tube walls. Under certain conditions a secondary pressure pulse was observed behind the transmitted shock; the origin of this pulse is the explosion of the unreacted gas between the transmitted shock and the unburned gas. When incident on gradients of fuel concentration, the transmitted detonation wave velocity and cell size adjusted rapidly, to values appropriate to the local gas composition and wall boundary layer losses. Finally, it was shown that the mechanism of detonation initiation in a weaker mixture by an incident detonation propagated across a concentration gradient differs in one important respect from that in a homogeneous mixture in that a secondary shock is formed as an intermediate stage.

Question of initiation of lead azide detonation in a prepunchthrough electric field

The authors present experimental results proving the nonthermal nature of the detonation initiation mechanism for lead azide. They conclude that the nature of experimental dependencies represented in their research permits the assertion that initiation of lead azide detonation occurs by the mechanism of shock ionization that develops in the strong field domains. The nature of the strong field domain is asociated either with the formation of domains of near-contact depletion by majority charge carriers or by capture of charge carriers by defects in the crystalline structure. In either case, electrically active structure is formed.

Direct initiation of spherical detonation by a hot turbulent gas jet

An experimental study to establish the basic mechanisms of transition from deflagration to spherical detonation downstream of flow obstructions has been performed. The flow obstructions consist of circular and rectangular orfice plates, multiple rectangular orifice plates and circular hole perforated plates. In order to keep the apparatus reasonably small and also to facilitate photographic observations at low initial pressures, acetylene-oxygen mixtures at equimolar composition were used throughout. The principal diagnotics were stereoscopic streak and framing schlieren photography. The apparatus consists of a spherical flame chamber connected to a cylindrical detonation chamber via an orifice plate mounted in the port-hole connecting the two chambers. The mechanism of detonation initiation is found to be due to the intense mixing of the hot combustion products with the unburned gases. Large scale eddies are found to be essential for detonation. These eddies provide the main mechanism of entrainment, and are also partially responsible for providing energy to maintain the intensity of the finer scale turbulence. When wire screens are used in conjunction with the orifice plates, the formation of detonation is facilitated, indicating that the fine scale turbulence which is produced plays a role in rapid mixing of the entrained gases inside the large scale eddies. Under critical conditions, when the initiation results from one single eddy, it appears that shock wave amplification by coherent energy release (SWACER) inside an eddy, where a gradient field of induction time due to entrainment is present, is most probable. This suggests that the minimum size of the required eddies must be at least of the order of the detonation kernel (i.e., transverse wave spacing) of the mixture. The present experimental findings that the obstructions which produce eddies of size of the order of transverse wave spacing lead readily to detonation initiation support this view.