Planar Detonation Wave Initiation in Large-Aspect-Ratio Channels

P LANAR detonation waves are often created in narrow rectangular channels to more easily visualize the cellular structure of gaseous detonation waves [1–6]. Typically, these detonation waves are initiated from a single high-energy source requiring several kilojoules of input energy, such as a strong spark or exploding wire that creates a spherically or cylindrically expanding wave. This wave is then diffracted until its radius of curvature is significantly larger than the channel height, at which point it is approximated as planar. Often a planar wave is desired in an insensitive mixture that is not able to support this diffraction process. In this situation, the wave can be initiated and diffracted in a more sensitive gas that is separated from the insensitive mixture by a thin diaphragm. Once the wave has diffracted to an approximately planar shape, it ruptures the diaphragm and enters the insensitive gas as a planar wave. Typically the diffraction process requires tens of channel heights for the wave to diffract to an approximately planar shape. In cases where a high energy source is required for initiation, an overdriven wave is often created that requires several more channel heights to relax to a stable velocity. In large channels, the distance required to generate a planar wave can grow impractically long for facilities with limited space. In this study, two initiator designs are presented that are able to form planar detonations with low input energy in large-aspect-ratio channels over distances corresponding to only a few channel heights. The initiators use a single spark and an array of small channels to shape the detonation wave. The first design, referred to as the static initiator, is simple to construct as it consists of straight channels which connect at right angles. However, it is only able to create planar waves using mixtures that can reliably detonate in its smallwidth channels. An improved design, referred to as the dynamic initiator, is capable of detonating insensitive mixtures using an oxyacetylene gas slug injected into the initiator shortly before ignition, but is more complex to construct. The two versions are presented next, including an overview of their design and operation. Design drawings of each initiator are available elsewhere [7]. Finally, photographs and pressure traces of the resulting planar waves generated by each device are shown.