Abstract

The transmission of detonation waves, propagating in a homogeneous, gaseous, reactive medium, from a tube into an unconfined space is well known to succeed or fail based on the tube diameter. Below a certain diameter, the detonation fails to transition into the unconfined space, while for a large enough geometry, the transition succeeds. This critical diameter is well correlated to the incoming detonation cell size. For common undiluted hydrocarbon mixtures with a strong degree of transverse instability, the ratio of critical tube diameter to cell size has been measured at Dc = 13λ. In this paper, stoichiometric acetylene-oxygen mixture at different initial pressures is detonated in a circular tube that transitions into an effectively unconfined space. The transition is observed with simultaneous schlieren photography and soot foil records to look at the role of transverse cellular instability. Three regimes of transition are observed: supercritical, where the cellular pattern is continuously connected from the donor tube to the larger space; subcritical, where the wave fails and the cellular pattern disappears; and a critical regime, where the wave initially fails, asymptoting to a weakly decoupled shock-reaction front regime, and exhibits a subsequent re-initiation in a critical zone of pre-shocked gas through the onset of an explosion bubble. A substantial amount of transverse instability remains even after the expansion wave reaches the central axis, sustaining the diffracted wave at a critical thermodynamic state for the re-initiation. The location of this critical zone is identified at about 22λ and a small obstacle is used to promote the generation of transverse waves and a re-initiation kernel. The re-initiation is effected by placing an obstacle in the critical region. The role of the resulting instability is also illustrated through a simple numerical simulation using an obstacle in the sub-critical regime to perturb the flow and promote the re-initiation.