Flame acceleration

A radial force on the pipe wall ahead of the deflagration wave. There is a varying pressnre between the aconstic wave and the flame front where the pressnre bnilds from near atmospheric pressnre, Pi (step change at the wave front) to eight times Pi (or higher) at the flame front. The pressnre ratios depend on the flame acceleration. There is no snch effect with a detonation. 

Theoretical research is then discussed. Most theoretical research has concentrated on blast generation as a function of flame speed. Models of flame-acceleration processes and subsequent pressure generation (CFD-codes) are described as well, but in less detail. 

Obstacles introduced in unconfined cylindrical bubbles resulted only in local flame acceleration. Pressures measured at some distance from the cylindrical bubble were, in general, two to three times the pressure measured in the absence of obstacles. 

In all of these tests, flame acceleration was minimal or absent. Acceleration, when it occurred, was entirely due to intrinsic flame instability, for example, hydrodynamic instability (Istratov and Librovich 1969) or instability due to selective diffusion (Markstein 1964). To investigate whether the flame would accelerate when allowed to propagate over greater distances, tests were carried out in an open-sided test apparatus 45 m long (Harris and Wickens 1989). Flame acceleration was found to be no greater than in the balloon experiments (Table 4.1a). 

The introduction of obstacles within unconfined vapor clouds produced flame acceleration. On a small scale, an array of vertical obstacles mounted on a single plate (60 X 60 cm) resulted in flame accelerations within the array (Van Wingerden and Zeeuwen 1983). Maximum flame speeds of 52 m/s for acetylene-air were found, versus 21 m/s in the absence of obstacles, over 30 cm of flame propagation. 

The introduction of obstacles results in some flame acceleration, especially for the more reactive fuels. This effect is especially strong if the flame surface is distorted by the presence of obstacles over its entire surface, such as were present in the experiments of Dorge et al. (1976) and Harrison and Eyre (1986, 1987). The more reactive the fuel, the more effect obstacles seem to have on flame acceleration (Harris and Wickens 1989). 

Flame acceleration was minimal after ignition of dispersed fuel-air clouds under unconhned conditions in the absence of obstacles. 

As previously demonstrated, the introduction of obstacles and partial confinement results in some flame acceleration (Zeeuwen et al. 1983). 

Experiments in tubes are not directly applicable to vapor cloud explosions. An overview of research in tubes is, however, included for historical reasons. An understanding of flame-acceleration mechanisms evolved from these experiments because this mechanism is very effective in tubes. 

Chapman and Wheeler (1926, 1927) conducted early flame-propagation experiments in tubes. They observed continuous flame acceleration and substantial increases in acceleration in tubes with internal obstructions (Table 4.4). These early findings were subsequently confirmed by many others, including Dorge et al. (1979,... 

Chan et al. (1983) studied flame propagation in an obstructed channel whose degree of confinement could be varied by adjustment of exposure of the perforations in its top. Its dimensions were 1.22 m long and 127 x 203 mm in cross section. Results showed that reducing top confinement greatly reduced flame acceleration. When the channel s top confinement was reduced to 10%, the maximum flame speed produced for methane-air mixtures dropped from 120 m/s to 30 m/s. 

Flame acceleration does not generate extremely high overpressures. That is, numerical simulation of an explosion process with a steady flame speed equal to the highest flame speed observed results in a conservative estimate of its blast effects. 

Chan, C., J. H. S. Lee, I. O. Moen, and P. Thibault. 1980. Turbulent flame acceleration and pressure development in tubes. Proceedings of the First Specialists Meeting of the Combustion Institute, Bordeaux, France, pp. 479-484. 

Hjertager, B. H., K. Fuhre, S. J. Parker, and J. R. Bakke. 1984. Flame acceleration of propane-air in a large-scale obstructed tube. Progress in Astronautics and Aeronautics. 94 504-522. AIAA Inc., New York. 

Hjertager, B. H., K. Fuhre, and M. Bjorkhaug. 1988a. Concentration effects on flame acceleration by obstacles in large-scale methane-air and propane-air explosions. Comb. Sci. Tech., 62 239-256. 

Kjaldman, L., and R. Huhtanen. 1985. Simulation of flame acceleration in unconfined vapor cloud explosions. Research Report No. 357. Technical Research Centre of Finland. 

Moen, I. O., M. Donato, R. Knystautas, and J. H. Lee. 1980a. Flame acceleration due to turbulence produced by obstacles. Combust. Flame. 39 21-32. 

Sherman, M. P., S. R. Tiezsen, W. B. Bendick, W. Fisk, and M. Carcassi. 1985. The effect of transverse venting on flame acceleration and transition to detonation in a large channel. Paper presented at the 10th Int. Coll, on Dynamics of Explosions and Reactive Systems. Berkeley, California. 

Marx, K. D., J. H. S. Lee, and J. C. Cummings. 1985. Modeling of flame acceleration in tubes with obstacles. Proc. of 11th IMACS World Congress on Simulation and Scientific Computation. 5 13-16. 

The DDT can be observed in a variety of situations, including flame propagation in smoofh fubes or channels, flame acceleration caused by repealed obstacles, and jet ignition. The processes leading to detonation can be classified into two categories ... 

Figure 8.4.5 presents the streak, direct photograph illustrating the stages of transition to detonation after a weak ignition and flame acceleration phase. Four main regions may be identified ... 

The DDT process in short tubes may occur at shorter distances than in long tubes owing to mixture precompression and flame interaction with the pressure waves reflected from the far end. This effect, together with surface roughness, plays a key role in the flame acceleration process. 

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] ... 

Kuznetsov, M. et al.. Effect of boundary layer on flame acceleration and DDT, Proceedings of the 20th International Colloquium on the Dynamics of Explosions and Reactive Systems on CD, Montreal, 2005. 

V.N. Gamezo, T. Ogawa, and E.S. Oran, Flame acceleration and ddt in channels with obstacles Effect of obstacle spacing. Combust. Flame, published online July 2008. 

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