Experimental studies using ethylene/nitrogen/oxygen mixtures were conducted of the transition of a detonation from a pre-detonator to a tube of larger cross sectional area in which a significant area change occurs. The purpose of the work was to further understanding of the processes that allow establishing a detonation in the thrust tube of a hydrocarbon-fueled pulse detonation engine, using a pre-detonator operating only with air under dynamic fill conditions. Two sets of experiments were performed to explore specifically the use of overdriven detonation waves to initiate a thrust tube detonation using compact pre-detonators. Two important results were clear from these studies. The region containing obstacles in the pre-detonator must only be long enough to accelerate the deflagration wave up to the Chapman–Jouguet deflagration velocity and should terminate at this point. In addition, the distance between the last obstacle and the entrance of the transition section should be chosen so that an overdriven detonation wave enters the transition section. Although some intermittency in successful detonation transitioning was noticed for the shortest pre-detonator studied, the results clearly show that efficiently generating an overdriven detonation wave in the pre-detonator is a key parameter for obtaining a thrust tube detonation transition.

The results from a series of detonation experiments conducted to characterize the deflagration-to-detonation transition (DDT) process for ethylene-air mixtures in a 44-mm-square, 1.65-m-long tube are described. Experiments were conducted for both single-shot detonations involving quiescent mixtures as well as multicycle detonations involving dynamic fill. For the experiments, high-frequency pressure and flame emission measurements were made to obtain the compression wave and flame speeds, respectively. In addition, schlieren and hydroxyl-radical/planar-laser-induced-fluorescence (OH-PLIF) imaging were applied to investigate the interactions between the shock-wave and combustion phenomena during both deflagration and detonation. For ethylene-air mixtures, strategically placed obstacles were necessary to achieve DDT. The effect of the presence of obstacles on flame acceleration was systematically investigated by changing the obstacle configuration. The parametric study of obstacle blockage ratio, spacing between obstacles, and length of the obstacle configuration indicated that for successful detonations the obstacle needs to accelerate the flame to a minimum flame speed of roughly half the Chapman-Jouguet detonation velocity. Differences in the flame and compression wave velocities demonstrated the development of a coupled feedback mechanism as the wave propagated along the tube. A series of simultaneous schlieren and OH-PLIF images showed that the obstacle plays a major role in generating small/large-scale turbulence that enhances flame acceleration. Localized explosions of pockets of unburned mixture further enhanced the shock-wave strength to continuously increase the flame speed. The results of this experimental study support the importance of obstacles as a means to enhance DDT and provide a potential solution for practical pulse-detonation-engine applications.

This work advances the understanding of the conditions that lead to the initiation of a detonation in the thrust tube of a hydrocarbon fueled Pulse Detonation Engine (PDE), using a compact pre-detonator operating under dynamic fill conditions using only air as the oxidizer. Whenever a significant area change occurs, successful detonation transition from the smaller to the larger tube depends on the smaller cross sectional dimension of the former, hereinafter referred to as the critical dimension, on the area ratio of the two tubes, and on the detonation state. It is seen that for a compact, low-weight pre-detonator with a critical dimension approaching the detonability limit of one cell width, an overdriven detonation sufficiently lessens the diffraction that occurs at the area expansion such that a thrust tube detonation can be initiated. Due to the small critical dimension to cell width ratio, a Chapman-Jouguet detonation propagating in the same mixture would uncouple at the area expansion and decelerate to a deflagration. Experiments performed in a rectangular PDE tube equipped with an optically accessible transition section show that direct thrust tube detonation ignition results from locally overdriven detonations initiated either by the collision of a transverse shock and a transverse detonation, or by two transverse detonations occurring at multiple locations within the area expansion or within the thrust tube immediately downstream from the exit of the transition section.

A series of single-shot detonation experiments was conducted to characterize the deflagration to detonation transition (DDT) process for ethylene-air mixtures in a 45 mm square tube that was 1.65 m in length. Pressure and flame emission measurements were made to obtain the compression wave and flame speeds, while OH PLIF and Schlieren imaging were applied to investigate the combustion phenomena during both deflagration and detonation. The effect of the presence of obstacles on flame acceleration was systematically investigated by changing the obstacle configuration. A parametric study of obstacle blockage ratio, spacing between obstacles, and length of the obstacle section demonstrated that the obstacle must accelerate the flame to a minimum flame speed of roughly half the Chapman-Jouguet (C-J) detonation velocity in order for the wave to transition to a detonation. Differences in the flame and compression wave velocities demonstrated the development of a coupling mechanism as the wave propagated along the tube. A series of simultaneous Schlieren and OH PLEF images showed that the obstacle plays a major role in generating large-scale turbulence to enhance flame acceleration. Localized explosions of pockets of unburned mixture further enhanced the shock wave strength to continuously increase the flame speed. The results of this study support the importance of obstacles as a means to enhance DDT and provide a potential solution for practical pulse detonation engine applications.