Combustion instability flame fronts

What are the mechanisms by which slow, laminar combustion can be transformed into an intense, blast-generating process This transformation is most strongly influenced by turbulence, and secondarily by combustion instabilities. A laminar-flame front propagating into a turbulent mixture is strongly affected by the turbulence. Low-intensity turbulence will only wrinkle the flame front and enlarge its surface area. With increasing turbulence intensity, the flame front loses its more-or-less smooth, laminar character and breaks up into a combustion zone. In an intensely turbulent mixture, combustion takes place in an extended zone in which... 

Ishizuka, S., Miyasaka, K., and Law, C.K., Effects of heat loss, preferential diffusion, and flame stretch on flame-front instability and extinction of propane/air mixtures. Combust. Flame, 45,293,1982. 

The growth rate of the instability depends on the relative geometry of the flame front and the combustion chamber. Here, we give the results for the simple geometry of a flame propagating from the open to the... 

Transition to detonation caused by instabilities near the flame front, the flame interactions with a shock wave, another flame or a wall, or the explosion of a previously quenched pocket of combustible gas... 

The fundamentals of combustion instability are presented by G. Searby in Chapter 5.1 and phenomena examined by him fall into two categories instability of flame fronts and thermo-acoustic instabilities. Each category can be subdivided further, and these are discussed. 

Thermal expansion of a gas in a curved flame front leads to the formation of gasdynamic vorticity in the combustion products and is the cause of a flame instability discovered by L. D. Landau, and also by G. Darriet (France), in 1944. It turned out, however, that this instability was very reluctant to exhibit itself in experiments The first explanation of such a phenomenon—using the example of a spherical flame—was given by A. G. Istratov and V. B. Librovich. Ya.B. and his coauthors [34] proposed a method for calculating rapid combustion in a tube containing an elongated flame... 

The instability caused by HD and DD phenomena is often observed in lab-scale experiments with hydrogen + air mixtures. Photographs of a spherical flame in mixtures containing 10% and 50% hydrogen [2, 34] denote a smooth combustion front in the rich mixture and a cellular front in the lean mixture. In closed combustion chambers the perturbation of the flame front increases due to the impact of pressure waves reflected from the walls [35, 36]. In quick-burning mixtures a hedgehog -like structure of the front appears when the flame approaches the walls. 

The instability of boundaries between gases of various densities is considerable in confined vessels, tubes and ducts. Flame acceleration due to Rayleigh -Taylor instability is explained by the fact that when the pressure wave crosses the flame front from the side of the combustion products (their density is less than that in the fresh mixture), the amplitude of all the flame front irregularities grows rapidly and the flame surface area increases. The extreme H2 + air mixture flame acceleration resulting from turbulence was mentioned earlier. In confined vessels, flame acceleration up to the detonation velocity is often caused by flame front instabilities when it interacts with compression waves. 

Such diffusion-driven instabilities have been observed earlier in combustion systems. As early as 1892, Smithells reported the observation of cellular flames in fuel-rich mixtures [40]. An example is shown in figure A3.14.14. These were explained theoretically by Sivashinsky in terms of a thermodifflisive mechanism [41]. The key feature here involves the role played by the Lewis number, Le, the ratio of the thermal to mass diffusivity. If Le <1, which may arise with fuel-rich flame, for which H-atoms are the relevant species, of relatively low thermal conductivity (due to the high hydrocarbon content), a planar flame is unstable to spatial perturbations along the front. This mechanism has also been shown to operate for simple one-off chemical wave fronts, such as the iodate-arsenite system [42] and for various pH-driven fronts [43], if... 

The most efficient flame accelerating mechanism is development of large-scale combustion front curvature [16]. The front curvature is developed either by multiple interactions between a turbulent combustion front and pressure perturbations (Taylor instability [17]) or the flame interaction with large-scale obstacles located periodically. 

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