Fast-flame-to-detonation transition in a round tube: comparison with a square channel

Abstract

The deflagration-to-detonation transition (DDT) downstream of a 76%-blockage-ratio perforated plate was investigated in a 7.6-cm-inner-diameter polycarbonate round tube. Tests were primarily conducted in stoichiometric propane–oxygen mixtures with and without argon dilution at initial pressures of 4.2–35 kPa, and the results were compared to experiments conducted in a square channel of identical transverse dimension and with a nearly identical perforated plate. Simultaneous high-speed side- and end-view photography identified the precise location of detonation onset, and soot-coated foils provided a record of the shock dynamics preceding the onset of detonation, and the subsequent detonation cell structure. Prompt detonation initiation was independent of channel geometry, occurring at identical conditions as in the square channel. At lower initial pressures, detonation was initiated either at the channel centerline following a near-symmetric shock implosion, or subsequently following shock reflection of the near-symmetric expanding shock at the tube wall. For the “stable” argon-diluted mixtures, more reactive conditions were required for DDT in the round tube than in the square channel, demonstrating that for regular-cell structure mixtures detonation initiation is promoted by shock focusing in the channel corners. For the “unstable” \({\textrm{C}}_{3}{\textrm{H}}_{8}+5{\textrm{O}}_{2}\) mixture, there was little difference in the DDT limit for the round and square channels, indicating that detonation initiation driven by hot-spot formation following transverse wave collisions is dominant, i.e., as effective as shock focusing in the corner.