Methods of re-initiation and critical conditions for a planar detonation transforming to a cylindrical detonation within a confined volume

The conditions necessary for the successful transformation of a planar detonation into a diverging, cylindrical detonation after diffraction into a confined volume have been investigated for four different fuel–oxygen mixtures over a wide range of gap sizes. The test mixtures were H2 + 0.5O2, C2H2 + 2.5O2, C2H2 + 4O2 and C2H4 + 3O2, and the range of gap size (w) varied from 1 to 45.5 mm. This range of mixtures and gap sizes plus the higher fidelity owing to pressure measurements along the endwall allowed greater precision when interpreting the results than what was seen in earlier studies, leading to the identification of a wider range of conditions for the successful transformation of the detonation than what was heretofore assumed. The spontaneous re-initiation and continuous reflected re-initiation mechanisms previously observed by Murray and Lee [8] were confirmed, but the present study improves upon the prior one through the conclusive observation that successful propagation of detonations after multiple reflections from the front and back walls of the confined volume is also possible. When compared with literature data, the current results are in fair agreement when only spontaneous and single-reflected re-initiation modes are considered, but when multiple reflections are included, the go/no-go boundary for successful transmission is much wider than found from the limited literature data available. The demarcation boundary was found to fit the relationship w/λ = 0.14w + 0.24, with λ being the cell size in the axial tube. In addition to significantly expanding the database on the axial-to-cylindrical detonation transition in a confined volume and obtaining a new correlation for the demarcation condition, the present paper also shows that there is an optimum distance from the exit of the detonation tube to the endwall, approximately equal to one tube diameter, which promotes successful transmission.