Extinction diffusion-flame

The flow structures of lean limit methane and propane flames are compared in Figures 3.1.2 and 3.1.3. The structure depends on the Lewis number for the deficient reactant. A stretched lean limit methane flame (Lepreferential diffusion, giving it a higher burning intensity. Hence, the flame extinction limit is extended. On the other hand, for a stretched lean limit propane flame (Le>l), the same effect reduces the burning intensity, which can... 

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. 

We will try to generalize these effects for suppression and will adopt a temperature criterion for the extinction of a diffusion flame. Clearly at extinction, chemical kinetic effects become important and the reaction quenches . The heat losses for the specific chemical dynamics of the reaction become too great. This can be qualitatively explained in terms of Equation (9.12) ... 

Let us reconsider the critical flame temperature criterion for extinction. Williams [25], in a review of flame extinction, reports the theoretical adiabatic flame temperatures for different fuels in counter-flow diffusion flame experiments. These temperatures decreased with the strain rate (ua0/x), and ranged from 1700 to 2300 K. However, experimental measured temperatures in the literature tended to be much lower (e.g. Williams [25] reports 1650 K for methane, 1880 K for iso-octane and 1500 K for methylmethracrylate and heptane). He concludes that 1500 50 K can represent an approximate extinction temperature for many carbon-hydrogen-oxygen fuels burning in oxygen-nitrogen mixtures without chemical inhibitors . 

Figure 9.21 Adiabatic flame temperatures at extinction for premixed and diffusion flames (from Macek [26])...

A demonstration of the similarity of extinction in premixed and diffusion flames... 

The control volume analysis of the premixed flame of Section 4.5.4 can be used together with the analysis here in Section 9.9 for the diffusion flame to relate the two processes. We assume the kinetics is the same for each and given as in Equation (9.102). Since we are interested in extinction, it is reasonable to assume the heat loss from the flame to be by radiation from an optically thin flame of absorption coefficient, k ... 

Let us examine the 1300 °C criterion as a condition of flame extinction in diffusive burning to see how these critical conditions depend on oxygen and external radiation. After all, these two parameters - oxygen and radiation - are the two significant variables that make most solids bum. We will look to some data for anchoring this application. 

Modeling of extinction in turbulent diffusion flames by the velocity-dissipation-composition PDF method. Combustion and Flame 100, 211-220. 

FIGURE 8.19 Peak extinction coefficient versus equivalence ratio of fuel/oxygen stream mixture in propane and ethene opposed-jet diffusion flames. /w is the fuel injection parameter. The greater the extinction coefficient, the greater the soot mass. From Ref. [93]. 

One of the earliest detailed diagnostic efforts on sooting of diffusion flames was that of Wagner et al. [86-88], who made laser scattering and extinction measurements, profile determinations of velocity by LDV, and temperature measurements by thermocouples on a Wolfhard-Parker burner using ethene as the fuel. Their results show quite clearly that soot particles are generated near the reaction zone and are convected farther toward the center of the fuel stream as they travel up the flame. The particle number densities and generation rates decline with distance from the flame zone. The soot formation rate appeared to... 

Trevino, C., and F. A. Williams. 1988. Asymptotic analysis of the structure and extinction of methane-air diffusion flames. In Dynamics of reactive systems. Part I Flames. Eds. A. L. Kuhl, J. R. Bowen, J.-C. Leyer, and A. A. Borisov. AlAA progress in astronautics and aeronautics ser. Washington, DC American Institute of Aeronautics and Astronautics 113 129-65. 

Chelliah, H.K., and F. A. Williams. 1990. Aspects of the structure and extinction of diffusion flames in methane-oxygen-nitrogen systems. Combustion Flame 80 17-48. 

Rightley, M. L., and F.A. Williams. 1997. Structures of CO diffusion flames near extinction. Combustion Science Technology 125 181-200. 

Seshadri, K., and N. Peters. 1988. Asymptotic structure and extinction of methane-air diffusion flames. Combustion Flame 73 23-44. 

Beginning with the innovative work of Tsuji and Yamaoka [409,411], various counter-flow diffusion flames have been used experimentally both to determine extinction limits and flame structure [409]. In the Tsuji burner (see Fig. 17.5) fuel issues from a porous cylinder into an oncoming air stream. Along the stagnation streamline the flow may be modeled as a one-dimensional boundary-value problem with the strain rate specified as a parameter [104], In this formulation complex chemistry and transport is easily incorporated into the model. The chemistry largely takes place within a thin flame zone around the location of the stoichiometric mixture, within the boundary layer that forms around the cylinder. 

G. Dixon-Lewis, T. David, P.H. Gaskell, S. Fukutani, H. Jinno, J.A. Miller, R.J. Kee, M.D. Smooke, N. Peters, E. Effelsberg, J. Wamatz, and F. Behrendt. Calculation of the Structure and Extinction Limit of a Methane-Air Counterflow Diffusion Flame in the Forward Region of a Porous Cylinder. Proc. Combust. Inst., 20 1893-1904, 1984. 

G. Dixon-Lewis and M. Missaghi. Structure and Extinction Limits of Counterflow Diffusion Flames of Hydrogen-Nitrogen Mixtures in Air. Proc. Combust. Inst., 22 1461-1470,1988. 

J.S. Kistler, C.J. Sung, T.G. Kreutz, C.K. Law, and M. Nishioka. Extinction of Counterflow Diffusion Flames under Velocity Oscillations. Proc. Combust. Inst., 26 113— 120,1996. 

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