Combustion flame structure

An analysis has allowed the authors of Refs. 52 53) to find out a number of interesting features of flame spread over a material surface and investigate the combustion flame structure in the gas and condensed phases. However, the equations obtained contain too many unknown parameters which may be determined only by way of complicated experiments. For this reason the theory in its present form can be used only for rough estimates (e.g. for flame spread limits). 

The chronology of the most remarkable contributions to combustion in the early stages of its development is as follows. In 1815, Sir Humphry Davy developed the miner s safety lamp. In 1826, Michael Faraday gave a series of lectures and wrote The Chemical History of Candle. In 1855, Robert Bunsen developed his premixed gas burner and measured flame temperatures and flame speed. Francois-Ernest Mallard and Emile Le Chatelier studied flame propagation and proposed the first flame structure theory in 1883. At the same time, the first evidence of detonation was discovered in 1879-1881 by Marcellin Berthelot and Paul Vieille this was immediately confirmed in 1881 by Mallard and Le Chatelier. In 1899-1905, David Chapman and Emile Jouguet developed the theory of deflagration and detonation and calculated the speed of detonation. In 1900, Paul Vieille provided the physical explanation of detonation... 

Kasper, T.S. et al., Ethanol flame structure investigated by molecular beam mass spectrometry. Combust. Flame, 150,220,2007. 

This thermal flame structure indicates local heat flow from the flame tip to the adjacent combustion gases in... 

Chao, B.H., Egolfopoulos, F.N., and Law, C.K., Structure and propagation of premixed flame in nozzle-generated counterflow. Combust. Flame, 109,620,1997. 

S. H. Chung and C. K. Law, An integral analysis of the structure and propagation of stretched premixed flames. Combust. Flame 72 325-336,1988. 

T. Echekki and J. H. Chen, Structure and propagation of methanol-air triple flame. Combust. Flame 114 231-245, 1998. 

Y. Huang and V. Yang. Bifurcation of flame structure in a lean-premixed swirl-stabilized combustor Transition from stable to unstable flame. Combust. Flame, 136(3) 383-389, 2004. 

Seshadri, K. and Peters, N., The inner structure of methane-air flames. Combust. Flame 81 96 1990. 

B. Renou, M. Boukhalfa, D. Puechberty, and M. Trinite 2000, Local flame structure of freely propagating premixed turbulent flames at various Lewis number. Combust. Flame 123 107-115. 

B. Renou, A. Mura, E. Samson, and M. Boukhalfa 2002, Characterization of the local flame structure and the flame surface density for freely propagating premixed flames at various Lewis number. Combust. Sci. Technol. 174 143-179. 

Y.C. Chen, N. Peters, G.A. Schneemarm, N. Wruck, U. Renz, and M.S. Mansour 1996, The detailed flame structure of highly stretched turbulent premixed methane-air flames. Combust. Flame 107 223-244. 

Y.C. Chen and R. Bilger 2002, Experimental investigation of three dimensional flame-front structure in premixed turbulent combustion. Part I Hydrocarbon-air Bunsen flames. Combust. Flame 131 400-435. 

J. Abraham, RA. Williams, and RV. Bracco 1985, A discussion of turbulent flame structure in premixed charge, SAE Paper 850343, in Engine Combustion Analysis New Approaches, p. 156. 

Dally, B. B., Masri, A. R., Barlow, R. S., and Fiechtner, G. J., Instantaneous and mean compositional structure of bluff-body stabilized nonpremixed flames. Combust. Flame, 114, 119, 1998. 

Smith, J.R., The influence of turbulence on flame structure in and engine, in Flows in Internal Combustion Engines, T. Uzkan, Editor, ASME New York, 1982, pp. 67-72. 

Example of the failure of the cellular structure at the passage from solid walls to porous walls in the detonation of C2H2 + 2.5O2 mixture at 2.6kPa initial pressure. (Reprinted from Radulescu, M.I. and Lee, Combust. Flame, 131,29,2002. With permission.)... 

F. Pintgen, C.A. Eckett, J.M. Austin, and J.E. Shepherd, Direct observations of reaction zone structure in propagating detonations. Combust. Flame, 133, 211-229, 2003. 

N. Tsuboi, K. Eto, and A.K. Hayashi, Detailed structure of spinning detonation in a circular tube. Combust. Flame, 149, 144-161, 2007. 

To examine the effect of turbulence on flames, and hence the mass consumption rate of the fuel mixture, it is best to first recall the tacit assumption that in laminar flames the flow conditions alter neither the chemical mechanism nor the associated chemical energy release rate. Now one must acknowledge that, in many flow configurations, there can be an interaction between the character of the flow and the reaction chemistry. When a flow becomes turbulent, there are fluctuating components of velocity, temperature, density, pressure, and concentration. The degree to which such components affect the chemical reactions, heat release rate, and flame structure in a combustion system depends upon the relative characteristic times associated with each of these individual parameters. In a general sense, if the characteristic time (r0) of the chemical reaction is much shorter than a characteristic time (rm) associated with the fluid-mechanical fluctuations, the chemistry is essentially unaffected by the flow field. But if the contra condition (rc > rm) is true, the fluid mechanics could influence the chemical reaction rate, energy release rates, and flame structure. 

There are many different aspects to the field of turbulent reacting flows. Consider, for example, the effect of turbulence on the rate of an exothermic reaction typical of those occurring in a turbulent flow reactor. Here, the fluctuating temperatures and concentrations could affect the chemical reaction and heat release rates. Then, there is the situation in which combustion products are rapidly mixed with reactants in a time much shorter than the chemical reaction time. (This latter example is the so-called stirred reactor, which will be discussed in more detail in the next section.) In both of these examples, no flame structure is considered to exist. 

Since diffusion rates vary with pressure and the rate of overall combustion reactions varies approximately with the pressure squared, at very low pressures the flame formed will exhibit premixed combustion characteristics even though the fuel and oxidizer may be separate concentric gaseous streams. Figure 6.1 details how the flame structure varies with pressure for such a configuration where the fuel is a simple higher-order hydrocarbon [1], Normally, the concentric fuel-oxidizer configuration is typical of diffusion flame processes. 

Peters, N., and F. A. Williams. 1987. The asymptotic structure of stoichiometric methane-air flames. Combustion Flame 68 185-207. 

Hewson, J. C., and F.A. Williams. 1998. Rate-ratio asymptotic analysis of methane-air diffusion-flame structure for predicting production of oxides of nitrogen. Combustion Flame 117 441-76. 

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