Transition to detonation in a flame jet

Abstract

The results of an experimental and numerical investigation of flame-jet ignition of an acetyleneair cloud contained in a 2 m diameter plastic bag, 8 m long, are reported. The flame jet is producted by an acetyleneair flame in a 0.66 m diameter tube, 11 m long. The composition of the mixture was varied to obtain the critical composition for initiation of detonation in the cloud, with and without obstacles at the end of the tube. Numerical calculations were used to characterize the flame jet. Transition to detonation in the flame jet from an open tube was observed for acetyleneair mixtures with more than 7.9% C 2H 2. The mechanism of transition observed on the high-speed film records is analogous to that seen in similar laboratory experiments, with a localized explosion somewhere in the turbulent flame jet leading to the onset of detonation. Transition was facilitated by placing various obstacles at the exit of the tube. With an orifice plate with 50 mm diameter holes, initiation of detonation in a 7.0% C 2H 2 mixture occurs uniformly across the flame jet a short distance downstream of the plate. With a circular central obstacle in the tube exit, transition to detonation was observed in a 5.6% C 2H 2 mixture. From the results of this investigation, it is concluded that the phenomena of hot gas jet initiation of detonation seen in laboratory experiments with fueloxygen mixtures also occur in fuelair mixtures provided the jet is large enough. The size of the jet can be reduced significantly if obstacles and/or nearby physical boundaries are present to perturb the turbulent flame jet.