Flame fronts premixed, laminar

P. Clavin. D)mamic behaviour of premixed flame fronts in laminar and turbulent flows. Progress in Energy and Combustion Science, 11 1-59,1985. 

Total ion concentration profiles were measured through the flame front of laminar, premixed, low pressure acetylene-oxygen flames at equivalence ratios, <(), from... 

The above approaches to tabulation, whilst mostly applied in the simulation of combustion problems, have a general foundation that would be relevant to many kinetic systems. However, a special class of tabulation methods has been developed for flame simulations. If a fast exothermic reaction takes place between two components (e.g. a fuel and an oxidiser) of a gaseous system, then flames are observed. In premixed flames the fuel and the oxidiser are premixed before combustion takes place, whilst in non-premixed (diffusion) flames, the fuel and the oxidiser diffuse into each other, and the flame occurs at the boundary or flame front. Premixed and non-premixed flames are two extreme cases, but in many practical flames, continuous states between these two extremes will exist. Flames can be classified as laminar or turbulent according to the characteristics of the flow. Flames are special types of reaction—diffusion systems, characterised by high spatial gradients in temperature and species concentrations, and consequently reaction rates will have a high spatial variability. 

To analy2e premixed turbulent flames theoretically, two processes should be considered (/) the effects of combustion on the turbulence, and (2) the effects of turbulence on the average chemical reaction rates. In a turbulent flame, the peak time-averaged reaction rate can be orders of magnitude smaller than the corresponding rates in a laminar flame. The reason for this is the existence of turbulence-induced fluctuations in composition, temperature, density, and heat release rate within the flame, which are caused by large eddy stmctures and wrinkled laminar flame fronts. 

Premixed turbulent combustion regime diagram proposed by Chen and Bilger. Two intermediate regimes are delineated between distributed flame front and wrinkled laminar flamelets. (Reprinted from Chen, Y.C. and Bilger, R., Combust. Flame, 131, 400, 2002. With permission. Figure 9, p. 411, copyright Elsevier editions.)... 

The characteristics of a reactive gas (a premixed gas) are dependent not only on the type of reactants, pressure, and temperature, but also on the flow conditions. When the flame front of a combustion wave is flat and one-dimensional in shape, the flame is said to be a laminar flame. When the flame front is composed of a large number of eddies, which are three-dimensional in shape, the flame is said to be a turbulent flame. In contrast to a laminar flame, the combustion wave of a turbulent flame is no longer one-dimensional and the reaction surface of the combustion wave is significantly increased by the eddies induced by the dynamics of the fluid flow. 

The nature of intermediates formed in diffusion flames is similar to the premixed ones, albeit differences in the contacting pattern. In Fig. 11, the species concentration profiles in a laminar ethylene diffusion flame front are presented. The fuel and oxygen diffuse toward each other undergoing virtual annihilation within the flame zone concomitant with the establishment of a peak temperature of about 1600°C. Because premixed systems provide a better control of combustor temperature, and many practical combustion devices operate under diffusion limited conditions, considerable effort has been expended to ensure the rapid mixing of fuel and oxygen in combustion chambers and approach premixed conditions. 

In premixed combustion, the characteristic laminar speed, Uf, is the laminar flame propagation speed and the characteristic length scale, Lp, corresponds to the flame thickness. The laminar flame regimes are confined to regions (1) and (2P), where in the latter the turbulence intensity, u, is less than the laminar flame speed, Mp. Region (1) is defined by Rct< and the flame front is essentially a thin planar... 

At first glance, a premixed laminar flame front looks very similar to the one-dimensional flow reactors discussed in elementary physical chemistry and chemical engineering. The question naturally arises, How do flames differ from such simple reaction systems The difference lies in the strong interaction between the chemical reactions and the transport processes of diffusion and thermal conduction. 

In premixed combustion, the velocity at which the flame front propagates (normal to itself) is called laminar burning velocity, 5l- This is a thermochemical transport property that depends on the equivalence ratio between fuel and air, the temperature of the unbumed mixture and the pressure. 

A laminar flame velocity is one of the fundamental characteristics of premixed combustible gas reactivity. It specifies an amount of mixture reacting across a unit flame front area per unit time. According to the classical definition, a laminar flame (combustion) velocity is the expansion rate of a flat one-dimensional flame front in the direction normal to the wave surface with respect to the unburned gas [1]. 

Nonpremixed edge flames (a) 2D mixing layer (From Kioni, P.N., Rogg, B., Bray, K.N.C., and Linan, A., Combust. Flame, 95, 276, 1993. With permission.), (b) laminar jet (From Chung, S.H. and Lee, B.J., Combust. Flame, 86, 62,1991.), (c) flame spread (From Miller, F.J., Easton, J.W., Marchese, A.J., and Ross, H.D., Proc. Combust. Inst., 29, 2561, 2002. With permission.), (d) autoignition front (From Vervisch, L. and Poinsot, T., Annu. Rev. Fluid Mech., 30, 655, 1998. With permission.), and (e) spiral flame in von Karman swirling flow (From Nayagam, V. and Williams, F.A., Combust. Sci. Tech., 176, 2125, 2004. With permission.). (LPF lean premixed flame, RPF rich premixed flame, DF diffusion flame). 

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