Abstract
Our interest in this chapter is to look inside the thermochemical reactors discussed in the previous chapter. Inside the reactors, the formation of visible flame is one obvious phenomenon. Flames are of two types; (a) premixed flames and (b) nonpremixed or diffusion flames. A Bunsen burner, shown in Fig. 8.1, is a very good example in which both types of flames are produced. In this burner, air and fuel are mixed in the mixing tube; and this premixed mixture burns forming a conical flame of a finite thickness (typically, blue in color). This is called the premixed flame. It is so called because the oxygen required for combustion is obtained mainly from air, which is mixed with the fuel. This premixed combustion releases a variety of species, stable and unstable. Principally, the carbon monoxide resulting from the fuel-rich combustion burns in the outer diffusion flame. The oxygen required for combustion in this part of the flame is obtained from the surrounding air by diffusion (or by entrainment). The overall flame shape is determined by the magnitude of the mixture velocity and its profile as it escapes the burner tube, coupled with the extent of heat losses from the tube wall.
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Notes
- 1.
Of course, here the reference is being made to average \(V_{u}\). In reality, near the tube/duct walls, the local velocity tends to be zero, and therefore, \(S_{l}\) may exceed the unburned velocity there. Methods for avoiding flashback are not discussed here.
- 2.
- 3.
Stoichiometric coefficient based on oxidant air is \(R_{st} =\) air/fuel ratio.
- 4.
For premixed flames, \(\delta \simeq \) 1 mm as will be found shortly.
- 5.
In industrial boilers, such flue gas recirculation is known to reduce \(NO_{x}\) formation by 50% to 80% [122].
- 6.
Unlike \(S_{l}\), which is a characteristic property of the fuel, \(S_{t}\) very much depends on the local conditions—that is, burner geometry, unburned conditions, mixture velocity, and the levels of turbulence.
- 7.
Experimental evaluations require time averaging of the flame area and this procedure introduces several uncertainties in the reported data.
- 8.
Length l is purely fictitious and is introduced to make progress with the analysis in Ref. [113]. Its estimate will be provided shortly.
- 9.
Strictly, \(St\, Pr^{m} = f/2\). However, for gases, \(Pr^{m} \simeq 1\).
- 10.
The reader can verify this from properties of air.
- 11.
The total time delay is a combination of physical time delay required for atomization, heating up and vaporization, and the chemical time delay, which is dominated by chemical kinetics.
- 12.
Chemical formula: \(\text {C}_{10}\text {H}_{20}\) to \(\text {C}_{15}\text {H}_{28}\); average formula: \(\text {C}_{12}\text {H}_{23}\).
- 13.
In practice, ignition delay studies are carried out in a continuous flow device for \(300< T_{i} < 1300\) K [11, 84] and in a shock tube for \(1200< T_{i} < 2500\) K [109, 121].
- 14.
More elaborate models of ignition delay consider multistep mechanisms with several species (see, for example, [42]).
- 15.
The miner’s safety lamp known as Davy’s lamp uses this principle for arresting flame propagation.
- 16.
Flame extinction can be brought about in several ways. One method is to blow a flame away from its reactants, as is done by hard blowing. Another way is to dilute or cool the flame temperature by adding water. It is also possible to reduce the reaction kinetic rates by adding suppressants, such as halogens. Yet another method, used specifically in oil well fires, is to blow out a flame by explosive charges so the flame is starved of oxygen. The addition of carbon dioxide extinguishes a flame through the same effect. Our interest here is to study flame extinction due to passage-size reduction.
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Date, A.W. (2020). Premixed Flames. In: Analytic Combustion. Springer, Singapore. https://doi.org/10.1007/978-981-15-1853-9_8
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DOI: https://doi.org/10.1007/978-981-15-1853-9_8
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