Propagation of Supersonic Combustion Waves
Abstract:
The propagation of a supersonic combustion wave in an obstructed channel is investigated using high-speed schlieren photography. Experiments were carried out in a 25.4 mm wide by 76.2 mm tall channel with equally spaced 19.1 mm tall fence-type obstacles mounted on the top and bottom surfaces. Tests were carried out with stoichiometric hydrogen–oxygen mixtures over an initial pressure range of 8 kPa - 30 kPa. The high-speed video shows that combustion wave propagation in the quasi-detonation propagation regime is very complex involving coupled local explosions, not all of which involve detonation initiation. For the most reactive mixtures (h/λ> 6.3) a sustained detonation propagates through the channel with some weakening due to diffraction around obstacles. For less reactive mixtures (6.3 > h/λ> 2) detonation propagation is intermittent, where the detonation fails due to diffraction around the obstacles and then is re-initiated at the channel centreline following the collision of two transverse shock waves. The transverse shock wave originates from a detonation that forms immediately following shock reflection off the top and bottom obstacle faces. Detonation initiation occurs at the corner, where the channel wall and obstacle meet, as a result of shock focusing and propagates into the unburned gas ahead of the flame. For mixtures where h/λ< 2, the wave propagation is governed by the outcome of the shock reflection off the obstacle pair (top and bottom). If shock reflection off the obstacle pair initiates an explosion at the obstacle face, the collision of the resulting two transverse shock waves produces a local explosion at the channel centreline after each obstacle resulting in an average lead shock velocity roughly equal to the isobaric speed of sound of the products. If shock reflection does not result in ignition of the gas at the obstacle face, the resulting propagation is in the choked flame regime.