Visualization of Detonation Propagation in a Round Tube Equipped with Repeating Orifice Plates
This study used self-luminous high-speed photography to visualize quasi-detonation propagation and deflagration-to-detonation transition (DDT) in a transparent round tube equipped with repeating orifice plates. Experiments were conducted in a combustion channel consisting of a 3.16 m square channel with a 7.6 cm by 7.6 cm cross-section connected to a 1.55 m cylindrical channel filled with orifice plates. Rectangular ‘fence-type’ obstacles were installed on the top and bottom of the square channel with a 3.8 cm opening between them. Two sets of orifice plates with different diameters, d, representing different blockage ratios (BR) were tested (d=5.33 cm for 50% BR and 3.81 cm for 75% BR orifice plates). Stoichiometric hydrogen-oxygen mixtures at initial pressures of 4–60 kPa were ignited at one end of the combustion channel. Average propagation velocities were derived from shock-time-of-arrival measurements using pressure transducers in the square channel and high-speed video filmed through the round tube. As established in literature, a substantially higher orifice-diameter-to-detonation-cell-size ratio (d/λ) was observed for the 75% BR tests (d/λ=14) than the 50% BR tests (d/λ=1.4) and Peraldi’s d/λ≥1 criterion for the DDT limit. Video footage revealed that this difference may be attributed to a difference in near-limit detonation initiation modes: at 75% BR, detonation initiation occurred on the orifice face after shock reflection, whereas at 50% BR, detonation initiation occurred on the tube wall between orifice plates. Synchronized high-speed video and soot foils allowed for the classification of quasi-detonation propagation modes. At 50% BR, as initial pressure increased, fast-flame propagation transitioned to single-hot-spot wall ignition detonation to multi-hot-spot wall ignition detonation, and finally to continuous detonation propagation. A similar progression was observed at 75% BR, except that obstacle face ignition detonations occurred between the fast-flame and single-hot-spot regimes, and continuous detonations were not observed as tube strength concerns limited the maximum initial test pressure. For a continuous detonation, a higher average velocity was observed in the round tube than the square channel, which can be attributed to the effect of wave curvature during detonation diffraction past an obstacle (2D diffraction in the square channel compared to 3D diffraction in the round tube).