Effect of Deflagration-to-Detonation Transition on Pulse Detonation Engine Impulse

A detonation tube was built to study the deflagration-to-detonation transition (DDT) process and the impulse generated when combustion products exhaust into the atmosphere. The reactants used were stoichiometric ethylene and oxygen mixture with varying amounts of nitrogen present as diluent. The effects of varying the initial pressure from 30 kPa to 100 kPa were studied, as were the effects of varying the diluent concentration from 0% to 73.8% of the total mixture. Measurements were carried out with the tube free of obstacles and with three different obstacle configurations. Each obstacle configuration had a blockage ratio of 0.43. It was found that the inclusion of obstacles dramatically lowered the DDT times and distances as compared to the no obstacle configuration. The obstacles were found to be particularly effective at inducing DDT in mixtures with low pressures and with high amounts of diluent. At the lowest pressures tested (30 kPa), obstacles reduced the DDT time and distance to approximately 12.5% of the no obstacle configuration values. The obstacles also allowed DDT to occur in mixture compositions of up to 60% diluent, while DDT was not achieved with more than 30% diluent in the no obstacle configuration. A ballistic pendulum arrangement was utilized, enabling direct measurement of the impulse by measuring the tube's deflection. Additional means of impulse comparison consisted of integrating the pressure over the front wall of the tube. Impulse measurements were then compared with a theoretical model and were found to fit well cases that did not contain internal obstacles. The inclusion of obstacles allowed DDT to occur in mixtures with high amounts of diluent where DDT was not observed to occur in the cases without obstacles. Roughly 100% more impulse was produced in the obstacle configurations as compared to the no obstacle configuration under these conditions. In instances where DDT occurred in the no obstacle configuration, the use of obstacle configurations lowered the impulse produced by an average of 25%. For cases where no obstacles were used and DDT occurred, the pressure derived impulses (pressure impulse) and impulses determined from the ballistic pendulum (ballistic impulses) are similar. For cases were obstacle configurations were tested, pressure impulses were more than 100% higher on average than ballistic impulses. This difference exists because the pressure model neglects drag due to the obstacle configurations.