Premixed and diffusion flames

Diffusion flames can best be described as the combustion state controlled by mixing phenomena—that is, the diffusion of fuel into oxidizer, or vice versa—until some flammable mixture ratio is reached. According to the flow state of the individual diffusing species, the situation may be either laminar or turbulent. It will be shown later that gaseous diffusion flames exist, that liquid burning proceeds by a diffusion mechanism, and that the combustion of solids and some solid propellants falls in this category as well. 

Any reactants capable of forming combustion products are composed of a mixture of oxidizer and fuel components, and the flame is produced by the reaction of the mixture. Two types of flames can be formed when the mixture bums a premixed flame or a diffusion flame.[i-2] A premixed flame is formed by the combustion of the two components when they are premixed prior to burning in the combustion zone. The intermingled oxidizer and fuel component molecules in the premixed reactants then react homogeneously. The temperature and the concentration of the products increase uniformly in the combustion zone. 

When the oxidizer and fuel components are physically separated and allowed to diffuse into each other in the combustion zone, a diffusion flame is formed. Since the molecular distributions of the oxidizer and fuel components are not uniform, the temperature and combustion products are also not uniformly distributed in the combustion zone. Thus, the rate of the reaction generating the combustion products is low when compared to that in a premixed flame because an additional dif-fusional process is needed to form the diffusion flame. 

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Laminar Versus Turbulent Flames. Premixed and diffusion flames can be either laminar or turbulent gaseous flames. Laminar flames are those in which the gas flow is well behaved in the sense that the flow is unchanging in time at a given point (steady) and smooth without sudden disturbances. Laminar flow is often associated with slow flow from small diameter tubular burners. Turbulent flames are associated with highly time dependent flow patterns, often random, and are often associated with high velocity flows from large diameter tubular burners. Either type of flow—laminar or turbulent—can occur with both premixed and diffusion flames. 

Beyler, C., Flammability limits of premixed and diffusion flames, in The SFPE Handbook of Fire Protection Engineering, 2nd edn (eds P.J. Di Nenno et al.). Section 2, Chapter 9, National Fire Protection Association, Quiney, Massachusetts, 1995, p. 2-154. 

Figure 9.21 Adiabatic flame temperatures at extinction for premixed and diffusion flames (from Macek [26])...

A demonstration of the similarity of extinction in premixed and diffusion flames... 

M.D. Rumminger, D. Reineldt, V. Babushok, and G.T. Linteris. Numerical Study of the Inhibition of Premixed and Diffusion Flames by Iron Pentacarbonyl. Combust. Flame, 116 207-219,1999. 

CJ. Sun, C.J. Sung, D.L. Zhu, and C.K. Law. Response of Counterflow Premixed and Diffusion Flames to Strain Rate Variations at Reduced and Elevated Pressures. Proc. Combust. Inst., 26 1111-1120,1996. 

As noted earlier, waste destruction occur as a consequence of premixed and diffusion flames. In premixed flames, the fuel and oxidizer are mixed at the molecular level, and the relative amounts of the reactants are described by the equivalence ratio (4>), defined below ... 

The formation and destruction of polycyclic aromatic hydrocarbons (pah) in flames have been extensively investigated during the last 20 years. Although many questions remain to be answered, some important steps have been elucidated. Profiles of PAH concentration in premixed and diffusion flames indicate that formation of PAH occurs very early in the process and is followed by PAH destruction either by burn-out or through formation of soot particles. 

Reactive flows are present in a variety of applications, such as premixed and diffusion flames, flow in porous media with and without dissolution, and precipitation of minerals. These flows frequently include a large number of chemical reactions, increasing the computational work to analyze them. 

Rumminger, M. Reinelt, D. Babushok, V. Linteris, G. (1999). Numerical study of the inhibition of premixed and diffusion flames by iron pentacarbonyl. Combustion and Flame, Vol. 116, pp. 207-219, ISSN 0010-2180... 

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