Meta-stable Deflagration Waves

Experiments on deflagration to detonation transition (DDT) in a smooth tube indicate that a deflagration wave does not accelerate continuously to the final detonation velocity. Instead, the maximum deflagration velocity that is reached just prior to the abrupt onset of detonation corresponds to about half the CJ detonation velocity of the mixture ( 12VCJ). The duration of this quasi-steady 1 2VCJ state can be of the order of a few tube diameters but, under certain conditions, can also persist for tens of tube diameters before detonation onset occurs. This 12VCJ state is meta-stable, and the onset of detonation corresponds to the “explosive growth” of instabilities, manifested by the spontaneous appearance of cellular structure when the detonation is formed. This meta-stable state is also observed in the direct initiation of spherical detonations in the critical energy regime where the initiation blast decays to about 12VCJ prior to the onset of detonation [1]. The low velocity phase of a galloping detonation is also found to be similar to this meta-stable state. At the end of the low velocity phase, rapid amplification of the precursor shock leads to the formation of an overdriven detonation to start the next galloping cycle [2]. Even when the transverse waves of a selfsustained detonation are dampened out by acoustic absorbing walls, the detonation fails and becomes a 1 2VCJ meta-stable deflagration wave [3]. Therefore, it appears that the 1 2VCJ meta-stable deflagration state is a common feature in the phenomenon that leads to the abrupt formation of a detonation. Although this meta-stable regime has been observed in previous investigations of DDT, it has not been singled out for a more thorough study. In the present paper, results of an experimental study of this meta-stable deflagration regime are reported. A detonation is reflected from a perforated plate in order to bypass the irreproducible flame acceleration phase and to establish a meta-stable deflagration wave downstream. Since each individual hole is choked, the combustion products flow through the holes at the speed of sound (or about 12VCJ ). Therefore, the interface (between the transmitted combustion products and the unburned mixture) where ignition of a deflagration occurs is also initially at the 12VCJ meta-stable state. A meta-stable deflagration can thus be generated in a reproducible manner for varying initial conditions.