Premixed flat flame

In this section we consider problems in which there is convective and diffusive transport in one spatial dimension, as well as elementary chemical reaction. The computational solution of such problems requires attention to discretization on a mesh network and solution algorithms. For steady-state situations the computational problem is one of solving a boundary-value problem. In chemically reacting flow problems it is not uncommon to have steep reaction fronts, such as in a flame. In such a case it is important to provide adequate mesh resolution within the front. Adaptive mesh schemes are used to accomplish this objective. 

Modeling diffusive transport requires appropriate constitutive relationships, such as Fourier s law for heat conduction or Fick s law for species diffusion. It is important to 

This section concentrates on laminar premixed flames, which serve to illustrate many attributes of steady-state one-dimensional reacting systems. The governing equations themselves can be written directly from the more general systems derived in Chapter 3. Referring to the cylindrical-coordinate summary in Section 3.12.2, and retaining only the axial components, the one-dimensional flame equations reduce immediately to 

Despite the fact that steady-state solutions are the principal concern here, the transient terms are retained to facilitate the hybrid solution algorithm as discussed in Chapter 15 [159]. Alternative formulations for the diffusive mass flux, jkiZ, were introduced briefly in Section 3.5.2, and are discussed in more depth later in this chapter. 

For a strictly one-dimensional steady flow, the continuity equation can be replaced by 

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Dishnct fuel-specific reaction chemistry is also seen in premixed flat flames of the four butanols. Figure 2.9 shows PIE curves for m/z = 72 (C4H8O). The species pool is quite different, with butanal present in the 1-butanol flame, 2-methyl propanal in the i-butanol flame, and 2-butanone in both the 2-butanol and the f-butanol... 

Fig. 1.1 Illustration of a premixed flat-flame burner and an opposed-flow diffusion flame.

Perhaps the most studied laboratory flame is the premixed flat flame. As illustrated in the left-hand panel of Fig. 1.1, a steady flame is established above a porous burner face. Such flames are used widely in combustion laboratories, where a variety of optical and probe-based diagnostics are used to measure species and temperature profiles. Models play an essential role in assisting the interpretation of the data. In addition to the premixed flat... 

As mentioned in the previous section, laminar, premixed, flat flames are used widely in the study of combustion chemistry. The left-hand panel of Fig. 1.1 shows a typical burner setup. The flames themselves are accessible to an array of physical and optical diagnostics, and the computational models can incorporate the details of elementary chemical reactions. 

The objective of this problem is to explore the multicomponent diffusive species transport in a chemically reacting flow. Figure 3.18 illustrates the temperature, velocity, and mole-fraction profiles within a laminar, premixed flat flame. These profiles are also represented in an accompanying spreadsheet (premixed h2. air-flame. xls). 

Fig. 16.8 Illustration of a premixed flat-flame burner. Fuel and oxidizer are first premixed, and then flow through a porous burner face. A steady, one-dimensional flat flame is stabilized by heat transfer to the cooled burner face. The solutions shown here are for a methane-air flame, in which the air contains water vapor at 100% relative humidity. By plotting the temperature and selected species profiles, one can observe some of the complexities of flame structure.

In Fig. 14, atmospheric pressure premixed, flat flames of C2H3CI are shown under both fuel-rich and fuel-lean conditions. Under fuel-rich conditions, CHC flames exhibit similar luminosity as hydrocarbon flames. However, under excess air conditions, the flames of chlorinated hydrocarbons exhibit white luminosity, likely because of the radiative recombination of chlorine atoms (Fig. 14A). Flames of highly chlorinated hydrocarbons also exhibit soot formation even at stoichiometric conditions (Fig. 14B) owing to the suppression of oxidation reactions. 

A near-sooting laminar premixed flat flame was produced at a burner chamber pressure of 2.67 kPa (20 torr) with 52.1 normal cm s l (0.1 MPa, 298 K) of feed gas consisting of 29.5 mol%... 

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