Flame propagation detonation transition

Moen, I.O., Sulmistras, A., Hjertager, B.H. and Bakke, J.R., "Turbulent Flame Propagation and Transition to Detonation in Large Fuel-Air-Clouds," Proceedings of the Twenty-First Symposium fInternational on Cumbustion. The Combustion Institute,... 

Overdriven Detonation The unstahle condition that exists during a defla-gration-to-detonation transition (DDT) before a state of stable detonation is reached. Transition occurs over the length of a few pipe diameters and propagation velocities of up to 2000 m/s have been measured for hydrocarbons in air. This is greater than the speed of sound as measured at the flame front. Overdriven detonations are typically accompanied by side-on pressure ratios (at the pipe wall) in the range 50-100. A severe test for detonation flame arresters is to adjust the run-up distance so the DDT occurs at the flame arrester, subjecting the device to the overdriven detonation impulse. 

In 1957, a flame propagating in a long tube under conditions resulting in a deflagration to detonation transition (DDT) was given the name "tulip" by Salamandra et al. [7]. This term was subsequently commonly applied in detonation studies to describe this typical shape [8,9]. Figure 5.3.2 shows a few... 

While it is observed that the velocity of flame propagation in a combustion wave is of the order of magnitude of 10 to 500 cm/sec, the velocity of detonation waves is of the order of 3 X 10 cm/sec, and the transition between them seems to include velocities not attainable as stationary waves. ... 

We summarize a number of simulations aimed at deciphering some of the basic effects which arise from the interaction of chemical kinetics and fluid dynamics in the ignition and propagation of detonations in gas phase materials. The studies presented have used one- and two-dimensional numerical models which couple a description of the fluid dynamics to descriptions of the detailed chemical kinetics and physical diffusion processes. We briefly describe, in order of complexity, a) chemical-acoustic coupling, b) hot spot formation, ignition and the shock-to-detonation transition, c) kinetic factors in detonation cell sizes, and d) flame acceleration and the transition to turbulence. 

First we described some of the properties occurring in propagating detonations, for which the structure is highly dependent on the chemical kinetic-fluid dynamic interactions. Finally, in the last section we discussed some processes and mechanisms involved in the transition to turbulence, which is important for flames. 

The experiments from [32] and also from [33] reveal that practical recommendations depend on the geometric dimensions of the facility. In [33] the duct had a 0.195 x 0.144 m cross-section and a 2.44 m length was used instead of 2.44 x 1.8 m cross section duct. The experiments in the small-scale facility showed that mixtures were shown to be safe from the combustion to detonation transition with up to 20% H2 content in a smooth wall duct and up to 17% H2 in a duct with obstacles. It is easy to see the difference in data when compared with the large-scale experiments. Quasi-detonation combustion regimes depend on facility design features and the initial pressure it distinguishes them from normal flame propagation and detonation. 

Further flame velocity growth may be caused by a change in the main mechanism of flame propagation, which occurs at DDT transition. The self-ignition of the compressed mixture behind the shock front becomes a key process continuing combustion wave propagation. The detonation wave velocity depends on the mixture composition and the initial conditions. Typical values of the detonation velocity are 1,000-2,000 m/s. 

Distance of deflagration to-detonation transition or pre- detonation distance (FDD) Distance from the flame source to the place of detonation onset. FDD depends on an explosive volume configuration, type of fuel-oxidizer mixture, presence of flow turbulence sources in the path of the flame propagation. 

From the practical point of view, the most important aspects of the accelerated flame phenomenon are with respect to the steady-state propagation of very highspeed flames, transition to detonation, and propagation of sub-CJ detonations (quasi-detonations). 

For very rough tubes, the flame acceleration is much more rapid as shown in the previous section. Transition to detonation is also clearly marked by a local explosion and abrupt change in the wave speed. The wall roughness controls the propagation of the wave by providing [5] ... 

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