Effect of Blockage Ratio on the Deflagration-To-Detonation Transition in the Supersonic Jet Created by the Obstruction: A Study on Mixtures with Transverse Concentration Gradient

In pulse detonation engines (PDEs), it is very important to produce a suitable detonation where both the required initial energy and run-up distance to detonation are optimal. The deflagration-to-detonation transition (DDT) in channels fitted with a single high blockage ratio (BR) obstacle where the DDT occurs in the supersonic turbulent jet created by the obstruction can be considered as a solution. Unlike idealized simulations with perfectly premixed mixtures, real-world PDEs often experience inhomogeneous fuel-air mixing due to discrete fuel injection. Hence, this research has numerically studied DDT in a non-uniform mixture, mimicking realistic operating conditions. According to the results, varying the concentration gradient of fuel in the mixture and the BR of the obstacle reveals different DDT mechanisms. In the lean mixture, mechanisms governing the DDT are collision of the shock reflected from the duct walls with the flame and pressure wave focusing near the flame front. In the stoichiometric mixture, both the shock-flame interaction and the role of Mach stem formed near the lower wall are of particular importance. When the blockage ratio of an obstacle is below the 0.85 threshold, the run-up distance to detonation is not highly influenced by the decrease of blockage ratio. At average hydrogen concentrations of 22.5% and 30%, the thresholds are 0.85 and 0.9, respectively. Furthermore, the time of detonation initiation decreases with an increase in average hydrogen concentration for all the considered blockage ratios.