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Detonation of Gaseous Mixtures
Published in Kenneth M. Bryden, Kenneth W. Ragland, Song-Charng Kong, Combustion Engineering, 2022
Kenneth M. Bryden, Kenneth W. Ragland, Song-Charng Kong
Consider a combustible mixture of gaseous fuel and air in a long tube. The mixture is ignited at the closed end of the tube. A flame forms and begins to propagate along the tube at laminar flame speed. The propagating flame gradually loses its smooth shape and becomes wrinkled. As a result of the increase in effective flame surface, the flame accelerates with respect to the unburned gas. The wrinkled, fluctuating flame front generates turbulence and weak pressure pulses that run ahead of the flame front and gradually preheat the gas ahead of the flame, causing the flame to speed up. High-speed Schlieren photography (a light-scattering technique sensitive to density gradients) of the transition from a flame (subsonic combustion–deflagration) to a detonation (supersonic combustion) shows that as the flame accelerates, the pressure pulses become stronger, coalesce, and further preheat the gas ahead of the flame. Eventually, a pocket of gas ahead of the flame reaches its autoignition temperature and produces a local explosion. The rapidly expanding gases produce a shock wave that interacts with the walls, sending a forward propagating shock that rapidly ignites the fuel ahead and a backward moving shock that dies out. The forward moving shock–combustion complex is a detonation, and the rearward moving shock is called a retonation (Figure 8.1). This process is called the deflagration-to-detonation transition (DDT). Note that the velocity can be obtained from the slope of the various lines indicated in Figure 8.1.
Initiation Systems
Published in Per-Anders Persson, Roger Holmberg, Jaimin Lee, Rock Blasting and Explosives Engineering, 2018
Per-Anders Persson, Roger Holmberg, Jaimin Lee
The customers will not notice any difference when they examine the exterior of the new detonator. But the interior is totally different. The sensitive lead azide is replaced by an initiation element (Figure 5.5). This element consists of a steel shell (1), a sealing cup (2), PETN charges (3), and a delay charge (4). The PETN charges are characterized by different qualities and densities in order to achieve the stipulated function. By dividing the secondary explosive PETN into charges with various qualities, it is possible to control the transit from deflagration to detonation. The transition is termed Deflagration-to-Detonation Transition (DDT). An excellent recent article by McAfee, Asay, and Bdzil [1993] provides a lucid explanation of many of the complex phenomena which are parts of the DDT process.
Ignition and Propagation
Published in John A. Conkling, Christopher J. Mocella, Chemistry of Pyrotechnics, 2019
John A. Conkling, Christopher J. Mocella
If the conditions are suitable, a deflagrating pyrotechnic material can increase in burn rate depending on various factors previously discussed: the nature of the chemicals and mixture, gas and heat production, and overall confinement. If the pressure reaches a critical point, the deflagration mechanism can “transition” into a full detonation shock (pressure) wave moving at 1,000 m/s or more. This is known as a deflagration-to-detonation-transition, or DDT. In this case, the reaction rate rapidly increases as the process undergoes DDT, quickly increasing the instantaneous energy output.
Flame Acceleration and DDT in a Channel with Continuous Triangular Obstacles: Effect of Blockage Ratio
Published in Combustion Science and Technology, 2022
Xiaoxi Li, Jizhou Dong, Kaiqiang Jin, Qiangling Duan, Jinhua Sun, Huahua Xiao
Deflagration-to-detonation transition (DDT) caused by the flame acceleration in a channel is a complex process including spontaneous acceleration of a subsonic flame ignited by a weak ignition, to a high-speed deflagration and finally transition to a detonation (Ciccarelli and Dorofeev 2008; Oran, Chamberlain, and Pekalski 2020). The study of flame acceleration and DDT is both fundamentally important and practically relevant, primarily in connection with explosion safety and detonation-based propulsion (Ciccarelli and Dorofeev 2008; Li et al. 2021; Oran, Chamberlain, and Pekalski 2020; Zhang et al. 2020). The physics in this nonlinear and stochastic process involves various factors such as boundary layer, flame instabilities, shock-flame interaction, and turbulence (Ciccarelli and Dorofeev 2008; Khokhlov and Oran 1999; Valiev et al. 2010; Xiao and Oran 2019; Zhang et al. 2021). The underlying mechanism of DDT has not been fully understood.
Two-Dimensional Numerical Simulation of Detonation Transition with Multi-Step Reaction Model: Effects of Obstacle Height
Published in Combustion Science and Technology, 2019
Ayu Ago, Nobuyuki Tsuboi, Edyta Dzieminska, A. Koichi Hayashi
Transition to detonation (Deflagration-to-Detonation Transition: DDT) is known to occur in a pipe filled with premixed gas followed by propagating a flame for a while. Detonation is a phenomenon that can cause a piping explosion accident due to its own characteristics (Naitoh et al., 2012). In addition, due to large release of energy detonation is considered as an alternative combustion in a propulsion device such as Pulse Detonation Engines and Rotating Detonation Engines. Therefore, it is necessary to understand DDT mechanism and its initiation and transition conditions. It is known that a tube with obstacles promotes DDT (Shchelkin, 1940). Research has been conducted both experimentally (Dorofeev, 2008; Ishihara et al., 2017; Kuznetsov et al., 2002) and numerically (Gaathaug et al., 2012; Ogawa et al., 2011; Wang and Wen, 2016). However, it is not yet well understood how does a flame accelerates and shortens detonation transition time with different obstacle installation and initial conditions.