Experimental investigation and comparison of flame acceleration, hot spot ignition, and initiation of detonation in curved and straight channels

Abstract

An experimental investigation of the deflagration to detonation transition (DDT) in four types of channels with rectangular cross sections and various boundary conditions is presented. The experimental channels included curved and straight channels with closed and open outlets. Experimental tests were performed using a stoichiometric mixture of ethylene and oxygen with initial pressures of 10–60 kPa. Flame shape was observed by a high-speed camera. Local velocity at high spatial resolution was calculated as the ratio of the distance between the flame fronts in sequential frames to the time interval between them. The amplification of a shock wave ahead of the flame and evolution of a series of shock waves after hot spot ignition were monitored using pressure transducers. In addition, a soot-foil technique was used to confirm the exact location of the onset of the overdriven detonation wave. The experiments showed that for a curved channel, three flame shapes (spherical, tulip, and tongue) were observed before hot spot ignition. Notably, the flame was tongue-shaped for about half the time required for the DDT. A funnel of unreacted gas formed between the tongue-shaped flame and the outer wall, at which hot spots were most frequently triggered. The locations of hot spots strongly depended on initial pressure in curved channels with closed outlets. When the pressure was relatively low (p0 < 22 kPa), hot spots always ignited at the intersection of the tongue-shaped flame and the outer wall (the root of the tongue-shaped flame), while relatively high initial pressure (p0 > 33 kPa) caused the hot spots to explode at the surface of the outer wall near the head of the tongue-shaped flame. At intermediate pressures, i.e., 22 ≤ p0 ≤ 33, hot spots ignited at both locations. The above results were very reproducible. Finally, the DDT for closed and open outlets in curved channels were compared with two types of straight channels, and it was found that the time required from ignition to onset of an overdriven detonation in the four types were, in order of magnitude, open curved, closed curved, open straight, and closed straight channels.