Hydrocarbon-air flames

We have shown that planar premixed hydrocarbon-air flames are unstable over a range of wavelengths, typically from several centimeter to a few millimeter. The only exception is for slow flames propagating downward. 

A plof of fhe real part of the relative heat release response for three Lewis numbers is shown in Figure 5.1.10. This plot was calculated for a reduced activation energy y3 = 10 and a burnf gas femperafure of 1800 K, represen-fative of a lean hydrocarbon-air flame. Note fhaf fhe order of magnitude of fhe relative response of fhe flame is only a little more than unity. This is a relatively weak response. For example, a sound pressure level of 120 dB corresponds to a relative pressure oscillation p /p = 2 X10 so fhe fluctuation in the heat release rate will be of fhe same order of magnifude. 

We can now estimate the order of magnitude of the acoustic growth rate expected for this mechanism. Consider a lean hydrocarbon-air flame. Si 0.3 m/s. The ratio of the specific heats of the mixture is C /C = 1.4. 

The recombination zone falls into the burned gas or post-flame zone. Although recombination reactions are very exothermic, the radicals recombining have such low concentrations that the temperature profile does not reflect this phase of the overall flame system. Specific descriptions of hydrocarbon-air flames are shown later in this chapter. 

Prompt NO mechanisms In dealing with the presentation of prompt NO mechanisms, much can be learned by considering the historical development of the concept of prompt NO. With the development of the Zeldovich mechanism, many investigators followed the concept that in premixed flame systems, NO would form only in the post-flame or burned gas zone. Thus, it was thought possible to experimentally determine thermal NO formation rates and, from these rates, to find the rate constant of Eq. (8.49) by measurement of the NO concentration profiles in the post-flame zone. Such measurements can be performed readily on flat flame burners. Of course, in order to make these determinations, it is necessary to know the O atom concentrations. Since hydrocarbon-air flames were always considered, the nitrogen concentration was always in large excess. As discussed in the preceding subsection, the O atom concentration was taken as the equilibrium concentration at the flame temperature and all other reactions were assumed very fast compared to the Zeldovich mechanism. 

As discussed in the introduction to Chapter 4, the existence of C2 and CH in hydrocarbon-air flames is well established. Methylene (CH2) arises in most combustion systems by OH and H attack on the methyl radical (CH3). Similar attack on CH2 creates CH. 

FIGURE 8.12 Effect of excess air on the formation of S03 in a hydrocarbon-air flame (after Barrett etal. [45]). 

The H + O2 competition is responsible for several important aspects of combustion phenomena. For example, the second explosion limit for hydrogen-oxygen mixtures is explained by the competition between H + O2 branching and termination (Section 13.2.6). The observed reduction in hydrocarbon-air flame speeds with increased pressure between 1 and 10 atm is caused by the branching-termination competition. For a given temperature, as the pressure increases, the concentration of [M] increases, which favors the termination reaction. Thus the chain branching competes less favorably for a greater portion of the flame, which diminishes the flame speed [427]. 

Figure 11. Effect of Reynolds number on ratio of turbulent to laminar burning velocity for hydrocarbon-air flames. Constant density and viscosity (8)...

The general range of hydrocarbon-air premixed flame speeds falls around 40 cm/s. Using a value of thermal diffusivity evaluated at a mean temperature of 1300 K, one can estimate to be close to 0.1 cm. Thus, hydrocarbon-air flames have a characteristic length of the order of 1 mm. The characteristic time is ( /5l ). and for these flames this value is estimated to be of the order of a few milliseconds. If one assumes that the overall activation energy of the hydrocarbon-air process is... 

The color of a hydrocarbon-air diffusion flame is distinctively different from that of its premixed counterpart. Whereas a premixed hydrocarbon-air flame is... 

Sivathan, Y. R. and Faeth, G. M., Generalized state relationships for scalar properties in non-premixed hydrocarbon/air flames, Combust. Flame, 82, 211-230, 1990. 

Chemi-ionization is an extreme case of chemi-excitation and has been observed in many reacting systems, although the term was first used to explain the high ion concentrations observed in hydrocarbon/air flames. For a general review of chemi-ionization, see Fontijn. ... 

For most hydrocarbon-air flames, the properties of nitrogen dominate, and the heat capacity of the mixture is very close to that of nitrogen, whose value changes from 1000 to 1300J/kgK for temperatures between 300 and 3000 K [20]. 

A. A. Westenberg. "Kinetics of NO and CO in lean, premixed hydrocarbon-air flames (Reaction kinetics of NO and CO formation in lean premixed hydrocarbon-air flames)," Combustion Sci. and Technol., 4, 59-64, 1971. 

Rybitskaya, L Shmakov, A. Korobeinichev, O. (2007). Propagation Velocity of Hydrocarbon-Air Flames Containing Organophosphorus Compeunds at Atmospheric Pressure, Combustion, Explosion, and Shock Waves, Vol. 43, No. 3, pp. 253-257, ISSN 0010-5082... 

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