Initiation of detonation by steady planar incident shock waves

Abstract

The initiation of detonation by planar shocks has been studied in a vertical shock tube in which a removable diaphragm allows a shock, generated in a buffer section, to be transmitted, without any reflection at the interface, into the gas mixture under test. Streak schlieren photography confirms that a quasi-steady shock reaction complex is formed prior to the shock acceleration phase. The existence of a steady phase enables the induction delay time to be measured in a direct and unambiguous manner. It is also shown that microwave interferometry, in conjunction with pressure transducers, allows the delay time to be measured with reasonable precision. From the experimentally determined shock acceleration the locus of the exothermic reaction zone is calculated, and it is shown that the time coherence of energy release between particles entering the shock front at different times is a process that leads to the formation of reactive centers which are characteristic of mild ignition. Ignition delay data obtained by the incident shock method for oxyacetylene, diluted with nitrogen, are compared with those obtained by the well-established reflected shock technique. These results indicate that the incident shock method promises to be useful in high heat capacity systems for which the magnitude of the nonideality in the reflection process may be unacceptable.