Flame configuration

In view of the growing interest in combustion studies of higher hydrocarbons, various experiments have been conducted with counterflow twin-flame configuration... 

Sheu, W.J. and Sivashinsky, G.L, Nonplanar flame configurations in stagnation point flow. Combust. Flame, 84, 221,1991. 

Previous data have concerned rmconfined flame configurations driven by velocity perturbations. These cases are less dependent on the geometry because sound generation is not modified by reflection from boundaries. It is also easier to examine rmconfined flames with optical techniques. However, in many applications, combustion takes place in confined environments and sound radiation takes place from the combustor inlet or exhaust sections. The presence of bormdaries has two main effects ... 

Instantaneous images obtained in a turbulent premixed V-shaped flame configuration, methane and air in stoichiometric proportions. (Reproduced from Kobayashi, H., Tamura, T., Maruta, K., Niioka, T, and Williams, F. A., Proc. Combust. Inst., 26,389,1996. With permission. Figure 2, p. 291, copyright Combustion Institute.)... 

FIGURE 6.3 Two-dimensional Wolfhard-Parker fuel jet burner flame configuration (after Smyth et al. [2]). 

In the present analysis, the outer convective-diffusive zones flanking the reaction zone are treated in the Burke-Schumann limit with Lewis numbers unity. Lewis numbers different from unity are taken into account where reactions occur. These Lewis-number approximations are especially accurate for methane-air flames and would be appreciably poorer if hydrogen or higher hydrocarbons are the fuels. To achieve a formulation that is independent of the flame configuration, the mixture fraction is employed as the independent variable. The connection to physical coordinates is made through the so-called scalar dissipation rate. 

Fig. 1.2 Illustration of a stagnation-flame configuration for the deposition of a polycrystalline diamond film. The photograph of the flame itself shows a highly luminous flat flame just above the deposition surface.

Because thermal NO is the dominating source under conditions with high temperatures and excess air, it was once assumed that prompt NO formation is negligible in most practical applications. This assumption is hardly valid, however. Turbulent diffusion flames are the most common practical flame configuration. In these flames the reaction zone is typically somewhat fuel rich, providing favorable conditions for prompt NO formation. While the relative contributions of the two formation mechanisms is still in dispute, there is little doubt that prompt NO is an important source of NO in most practical gas-diffusion flames. 

A further consequence of the upstream diffusion to the burner face could be heterogeneous reaction at the burner. Such reaction is likely on metal faces that may have catalytic activity. In this case the mass balance as stated in Eq. 16.99 must be altered by the incorporation of the surface reaction rate. In addition to the burner face in a flame configuration, an analogous situation is encountered in a stagnation-flow chemical-vapor-deposition reactor (as illustrated in Fig. 17.1). Here again, as flow rates are decreased or pressure is lowered, the enhanced diffusion tends to promote species to diffuse upstream toward the inlet manifold. 

Fig. 17.6 Illustration of an opposed-nozzle diffusion-flame configuration.

Opdiff, simulates opposed-flow premixed or diffusion stagnation-flame configurations. 

For gaseous flames, the LES/FMDF can be implemented via two combustion models (1) a finite-rate, reduced-chemistry model for nonequilibrium flames and (2) a near-equilibrium model employing detailed kinetics. In (1), a system of nonlinear ordinary differential equations (ODEs) is solved together with the FMDF equation for all the scalars (mass fractions and enthalpy). Finite-rate chemistry effects are explicitly and exactly" included in this procedure since the chemistry is closed in the formulation. In (2). the LES/FMDF is employed in conjunction with the equilibrium fuel-oxidation model. This model is enacted via fiamelet simulations, which consider a laminar counterflow (opposed jet) flame configuration. At low strain rates, the flame is usually close to equilibrium. Thus, the thermochemical variables are determined completely by the mixture fraction variable. A fiamelet library is coupled with the LES/FMDF solver in which transport of the mixture fraction is considered. It is useful to emphasize here that the PDF of the mixture fraction is not assumed a priori (as done in almost all other flamelet-based models), but is calculated explicitly via the FMDF. The LES/FMDF/flamelet solver is computationally less expensive than that described in (1) thus, it can be used for more complex flow configurations. 

Problems with solvent flameout, hydrocarbon quenching and structure-response variations for different sulfur- and phosphorus-containing compounds with the single flame detector can be partially overcome using a dual-flame configuration. Figure 3.23... 

In an open flame configuration, the homogeneous reaction must occur to some extent because the gas stream passes through an open flame at a high temperature. Thus, the downstream catalytic reaction may involve different reactant species than if the gas stream had not passed through an open flame, making study of the catalytic oxidation itself difficult. An additional complication is the possibility of chemical reaction between the fuel used for the flame and the VOC in the inlet gas stream, which could result in a variety of products. 

Figure 2.16 compares magnitudes of gas-to-load radiation and gas-to-refractory-to-load radiation for a specific furnace/flame configuration. 

Fig. 22.5 Counter-flow spray diffusion flame configuration with a water spray in the oxidant stream...

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