Effects of bluff bodies on the propagation behaviors of gaseous detonation

Abstract

In this study, we report an experimental investigation of detonation propagation in a tube filled with bluff bodies. Experiments were carried out in a 6 m long rectangular cross-section (112 mm × 107 mm) tube. Firstly, the effect of an array of cube bodies (60 mm × 60 mm × 60 mm) on the detonation propagation characteristics were studied. Hydrogen, ethylene and acetylene mixed with air and hydrogen-oxygen diluted with argon were used as the test mixtures. Evenly spaced photodiodes were mounted on the top wall to recorded the optical signals, from which the detonation velocity could be determined. Soot foils were adopted in hydrogen-oxygen-argon mixture to record the evolution of the cellular structure. The results show that the flame accelerates rapidly in the obstructed tube. The critical conditions for deflagration to detonation transition (DDT) are found to be consistent with L/λ > 7, where L is the modified characteristic geometrical size for the tube with repeated cubes and λ is the detonation cell size. The long soot foils indicate that the detonation modes are influenced significantly by the bluff bodies. Near the limits, only some traces of transverse waves and shock waves can be observed, making the distinction between choked flame and quasi-detonation blurred. Secondly, the effect of a single bluff body on the detonation diffraction was investigated. Stoichiometric hydrogen-oxygen and those diluted with argon at sub-atmospheric pressures were used as the test mixtures. In the vicinity of the bluff body, a soot foil was used to record the detonation cellular structure evolution, from which the effect of the geometry of the bluff body on the diffraction and re-initiation processes was studied. Three transition regimes are observed: (1) a detonation propagates continuously over the bluff body; (2) the cellular structure initially fails and then recover due to the two symmetrical diffraction waves collision or interaction between shock waves and the bottom wall; (3) a detonation quenches and then re-initiate by flame acceleration. The re-initiation distance was found to be dependent on mixture sensitivity (chemical length scale) and the geometry of the bluff bodies (physical length scale).