Studies of Overdriven Detonation Wave Transition in a Gradual Area Expansion for PDE Applications

Abstract

This work advances the understanding of the conditions that lead to the initiation of a detonation in the thrust tube of a hydrocarbon fueled Pulse Detonation Engine (PDE), using a compact pre-detonator operating under dynamic fill conditions using only air as the oxidizer. Whenever a significant area change occurs, successful detonation transition from the smaller to the larger tube depends on the smaller cross sectional dimension of the former, hereinafter referred to as the critical dimension, on the area ratio of the two tubes, and on the detonation state. It is seen that for a compact, low-weight pre-detonator with a critical dimension approaching the detonability limit of one cell width, an overdriven detonation sufficiently lessens the diffraction that occurs at the area expansion such that a thrust tube detonation can be initiated. Due to the small critical dimension to cell width ratio, a Chapman-Jouguet detonation propagating in the same mixture would uncouple at the area expansion and decelerate to a deflagration. Experiments performed in a rectangular PDE tube equipped with an optically accessible transition section show that direct thrust tube detonation ignition results from locally overdriven detonations initiated either by the collision of a transverse shock and a transverse detonation, or by two transverse detonations occurring at multiple locations within the area expansion or within the thrust tube immediately downstream from the exit of the transition section.