Deflagration to Detonation Transition from a Venting Pipe

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Abstract
A study has been made of the venting of a fast flame (100–800ms−1) in a round ignition pipe (140 mm 1 D.) into a semispherical vessel containing a mixture of the same composition and initial pressure. Under certain conditions the flame jet emerging from the pipe interacts with the remains of an unburned starting ring vortex set into motion by the accelerating flame front, and the subsequent intense combustion leads to the initiation of an unconfined detonation inside the vessel. A parametric investigation of this phenomenon in nitrogen-diluted (29.4−68.4%) stoichiometric mixtures of propane/oxygen has been carried out for a range of exit orifice diameters (50, 101 and 140mm) under 0.4 and 1 aim initial pressure. A small number of atmospheric pressure experiments have also been conducted with nitrogen-diluted (33.3−57.1%) stoichiometric methane/oxygen mixtures. A criterion, based on the notion of a critical Damköhler number and minimum energy release requirement, has been suggested for the initiation of unconfined detonation by a venting jet flame. All our experimental results for a given initial pressure and fuel may be collapsed to a single curve, separating conditions leading to detonation from those which do not. The critical orifice diameter below which a transition to unconfined detonation does not occur reduces for a venting fast flame by comparison with a steady detonation wave.

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