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Effect of flow turbulence

Laminar flame propagation is intrinsically unstable. The nature of the flame instabilities is aerodynamic and possibly, dependent on the molecular weight of the fuel and the mixture composition, diffusional-thermal as well (MarksteinGH:1964). Flame instabilities give rise to flame-generated turbulence (SivashinskyGI:1979). In the most high-reactive fuel-oxygen mixtures such as stoichiometric hydrogen-oxygen and acetylene-oxygen these phenomena are the immediate cause of a run-up to detonation ((SokolikAS:1963) and (KogarkoSM:1965)). In relatively less-reactive fuel-air mixtures these phenomena seem to be controlled by the Huyghens principle owing to which waves tend to a plane geometry (KarlovitzB:1951).

Figure 1. Positive feedback triggered by boundary conditions,
the basic mechanism of a deflagra-tive gas explosion.

A further speed-up of the process is only possible under the appropriate boundary conditions. Rigid boundaries induce a structure in the expansion flow consisting of velocity gradients and turbulent mo-tion. When the flame contacts such a flow structure, the combustion rate is increased in several ways. The flame is stretched in the velocity gradients, thereby increasing its area and effective burning speed. Turbulence does not only speed up the transport processes of heat and species but, above all, turbulence increases the effective flame surface area i.e. the interface between flammable mixture and combustion products where the reaction takes place. Initially, when the turbulence is of low intensity, the eddies only wrinkle the flame surface and increase its effective area and burning speed. The conse-quence is a stronger expansion flow - flow velocities increase. Higher flow velocities go hand in hand with higher turbulence intensity levels. Under the influence of higher turbulence intensities, the flame front gradually looses its original smooth appearance. Its structure changes. Turbulent eddies tend to disintegrate the front leading to higher combustion rates. Higher combustion rates produce stronger expansion flow and higher intensity turbulence etc ...

Turbulence generative boundary conditions trigger a positive feedback in the process of flame propagation by which it develops more or less exponentially both in speed and pressure (CCPS, 1994).

The basic concept of a gas explosion can be summarised as a flame propagation process in a flamma-ble mixture, which is sped up by interaction with its self-induced expansion flow field.

During this development, the process of turbulent mixing between flammable mixture and combustion products largely determines the combustion rate. The combustion front manifests itself as an extended mixing zone in which the internal interface between flammable mixture and combustion products can become very large. Turbulence generative (boundary) conditions are required for the development of high flame speeds and blast in deflagrative gas explosions.

Markstein G.H. (1964) Nonsteady flame propagation. Pergamon Press.(BibTeX)
Sivashinsky G.I. (1979) On self-turbulization of a laminar flame. Acta Astronautica, 6:569-591.(BibTeX)
Sokolik A.S. (1963) Self-ignition flame and detonation gases. Technical Report NASA-TT-F-125, NASA.(BibTeX)
Kogarko S.M., Adushkin V.V. and Lyamin A.G. (1965) Investigation of spherical detonation of gas mixtures. Fizika Goreniya i Vzryva, 1:22-34.(BibTeX)
Karlovitz B., Denniston D.W. and Wells F.E. (1951) Investigation of turbulent flames. Journal of Chemical Physics, 19(5):541-547.(BibTeX)

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Page last modified on February 17, 2009, at 03:09 PM