Rich premixed flame

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). 

Liquid fuel 1 Fuel-rich premixed flame... 

Blevins and Gore [14, 15] found that low-stretch-rate partially premixed flames involve multiple peaks in the profiles of intermediate hydrocarbon species. In particular, the CH species existing between the premixed and the diffusion flame part of the partially premixed flames were observed to react with NO and create an intermediate NO consumption zone. DuPont et al. [16] for low-stretch-rate flames also found the double peaks of intermediate hydrocarbon species and the NO consumption zone. However, Tanoff et al. [17] used the CH peak to characterize the location of the rich premixed flame and the OH peak to characterize the location of the diffusion flame. The NO concentration profiles showed that the peak NO mole fractions first increased and then decreased with increasing levels of partial premixing. However, the emission index of NO was not reported. 

The oxidation of soot in fuel-rich premixed flames proceeds through hydroxyl attack, with one atom of carbon removed by each OH radical hitting the surface of the soot. The absolute level and composition dependence of the disappearance rate of soot show that OH is the important species. The oxidation was followed experimentally by laser light scattering from soot particles generated in one flame and passed directly into the gas supply of a larger hydrogen/air flame controlled at 1700-2200 K. 

Because of the relatively low temperatures in the fuel-air mixing zone near the flame lift-off, the experimentally observed soot formation cannot be explained in terms of fuel pyrolysis. To account for the soot observed in this zone, Dec and Coy [13] hypothesize that there is a standing fuel-rich premixed flame just upstream from the liquid phase, where the fuel vapor and air mixture reaches an equivalence ratio between two and five. Such a fuel-rich combustion zone is ideal for soot production, because the combustion products contain an abundance of excess fuel... 

Among the various selection considerations are specific combustion characteristics of different fuels. One of the combustion characteristics of gaseous fuels is their flammability limit. The flammability limit refers to the mixture proportions of fuel and air that will sustain a premixed flame when there is either limited or excess air available. If there is a large amount of fuel mixed with a small amount of air, then there is a limiting ratio of fuel to air at which the mixture will no longer sustain a flame. This limit is called the rich flammability limit. If there is a small amount of fuel mixed with excess air, then there is a limiting ratio of the two at which the flame will not propagate.This limit is called the lean flammability limit. Different fuels have different flammability limits and these must be identified for each fuel. 

Comparison of the C2 radical mole fraction profile measured by LIF and CRDS in a flat, premixed fuel-rich propene flame at 50mbar. 

Hansen, N. et al.. Initial steps of aromatic ring formation in a laminar premixed fuel-rich cyclopentene flame, /. Phys. Chem. A, 111, 4081,2007. 

Yang, B. et al.. Identification of combustion intermediates in isomeric fuel-rich premixed butanol-oxygen flames at low pressure. Combust. Flame, 148, 198, 2007. 

It is presumed that the global-quenching criteria of premixed flames can be characterized by turbulent shaining (effect of Ka), equivalence ratio (effect of 4>), and heat-loss effects. Based on these aforemenhoned data, it is obvious that the lean methane flames (Le < 1) are much more difficult to be quenched globally by turbulence than the rich methane flames (Le > 1). This may be explained by the premixed flame shucture proposed by Peters [13], for which the premixed flame consisted of a chemically inert preheat zone, a chemically reacting inner layer, and an oxidation layer. Rich methane flames have only the inert preheat layer and the inner layer without the oxidation layers, while the lean methane flames have all the three layers. Since the behavior of the inner layer is responsible for the fuel consumption that... 

A number of theoretical (5), (19-23). experimental (24-28) and computational (2), (23), (29-32). studies of premixed flames in a stagnation point flow have appeared recently in the literature. In many of these papers it was found that the Lewis number of the deficient reactant played an important role in the behavior of the flames near extinction. In particular, in the absence of downstream heat loss, it was shown that extinction of strained premixed laminar flames can be accomplished via one of the following two mechanisms. If the Lewis number (the ratio of the thermal diffusivity to the mass diffusivity) of the deficient reactant is greater than a critical value, Lee > 1 then extinction can be achieved by flame stretch alone. In such flames (e.g., rich methane-air and lean propane-air flames) extinction occurs at a finite distance from the plane of symmetry. However, if the Lewis number of the deficient reactant is less than this value (e.g., lean hydrogen-air and lean methane-air flames), then extinction occurs from a combination of flame stretch and incomplete chemical reaction. Based upon these results we anticipate that the Lewis number of hydrogen will play an important role in the extinction process. 

The relative importance of these three mechanisms in NO formation and the total amount of prompt NO formed depend on conditions in the combustor. Acceleration of NO formation by nonequilibrium radical concentrations appears to be more important in non-premixed flames, in stirred reactors for lean conditions, and in low-pressure premixed flames, accounting for up to 80% of the total NO formation. Prompt NO formation by the hydrocarbon radical-molecular nitrogen mechanism is dominant in fuel-rich premixed hydrocarbon combustion and in hydrocarbon diffusion flames, accounting for greater than 50% of the total NO formation. Nitric oxide formation by the N20 mechanism increases in importance as the fuel-air ratio decreases, as the burned gas temperature decreases, or as pressure increases. The N20 mechanism is most important under conditions where the total NO formation rate is relatively low [1],... 

Within the frame of the present first series of experiments it was almost always oxygen which was injected into supercritical water-methane mixtures. There were several reasons for this first choice. One of these was the desire, to study rich flames and their possible products first. Often the water to methane mole fraction ratio was 0.7 to 0.3. But mixtures down to a methane mole fraction of 0.1 were also used. It was possible, however, to inject oxygen and methane simultaneously into the supercritical water and produce a flame. Not possible was the production of true premixed flames. After a retraction of the thin inner nozzle capillary of the burner (see Fig. 1 b) the two gases could be mixed just below the reaction cell, but the flame reaction proceeded from the nozzle tip in the cell back towards this mixing point immediately. 

Partially premixed flames are formed when a rich mixture of fuel and oxidizer is injected into an oxidizer stream. Below a certain value of the equivalence ratio of the rich mixture, a flame structure involving a premixed flame in the vicinity of a diffusion flame exists. Several experimental studies of NO emission properties of partially premixed laminar [1-4] and turbulent [5-10] flames have been reported in the literature. The results from the most recent studies indicate that using an optimum level of partial premixing can reduce NO emissions. 

This methodology results in excellent predictions of NO mole fractions for partially premixed flames. The discrepancies for diffusion flame are on the fuel-rich side and therefore related to reactions that occur in the absence of O2. 

DuPont, V., M. Pourkashanian, A. P. Richardson, A. Williams, and M. J. Scott. 1996. The importance of prompt-NO formation and of NO reconversion in strained laminar binary rich partially premixed flames. In Transport phenomena in combustion. Washington, DC Taylor Francis 1 263-74. 

The AN particles incorporated into GAP-AN pyrolants form a molten layer on the burning surface and decompose to form oxidizer fragments. The fuel-rich gas produced by the decomposition of GAP interdiffuses with these oxidizer fragments on and above the burning surface and produces a premixed flame. A luminous flameis formed above the burning surface. 

A series of fuel rich H2/O2/N2 premixed flames were burned at atmospheric pressure on a 2 cm dia. Padley-Sugden (7) burner constructed of bundled sections of stainless steel hypodermic... 

Crittenden, B.D., and Long, R., "Formation of Polycyclic Aromatics in Rich, Premixed Acetylene and Ethylene Flames" Comb, and Flame, 20, 359-368, 1973. 

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