Initiation of Gaseous Detonation

Abstract

Most fuel-oxygen gas mixtures can be detonated. If a detonating mixture is diluted with an inert gas such as nitrogen, there then exists a particular oxygen to nitrogen ratio below which the mixture can no longer be detonated. If this oxygen to nitrogen ratio is less than about 0.25 (composition of air), the fuel will also detonate when mixed with air. If a mixture detonates, the detonation states can be predicted quite adequately for most practical purposes by the classical Chapman-Jouguet theory. However, no exact quantitative theory currently exists whereby one can predict, a priori, whether a given fuel-air mixture can detonate, and if so, what the detonability limits are. Neither can one predict whether a flame can accelerate to a detonation in this mixture, or whether the detonation can be initiated directly via a powerful explosive charge. The practical significance of the above questions is self-evident, and considerable effort has been directed in recent years toward answering these questions in connection with vapor cloud explosions, accidental. or otherwise. Our qualitative understanding of the detonation phenomena is almost complete. The aim of this article is to report primarily on the initiation aspect and to demonstrate the central role it plays in the overall detonation phenomena. The article is not meant to be an extensive review of the current literature. It expresses my personal view of how the various pieces of the problem are now fitting together. Generally speaking, there are two modes of initiation: a slow mode where the detonation is formed via an accelerating flame and a fast mode where the detonation is formed instantaneously when a sufficiently powerful igniter is used. The slow mode is usually referred to as the transition from deflagration to detonation. Turbulence and interactions between pressure waves and flame are the principle flame-acceleration mechanisms that generate the critical states for the onset of detonation. In general, the ignition source plays no role in the transition processes. On the other hand, the ignition source plays the dominant role in the fast mode of initiation. The blast wave generated by the igniter energy produces the necessary critical states for the onset of detonation. The fast mode is referred to as direct