Diffusion flame, extinction limits

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... 

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. 

Diffusion flaming combustion of polymeric materials may be compared with that of unmixed gases. Zeldovidi related the extinction limits of the diffusion flame with the reactant concentration (mass flow rate) in the chemical reaction zone and its cooling. At the extinction limit the intensity of diffusion combustion of unmixed gases is at its maximum corresponding to the combustion intensity of a premixed stoichiometric mixture of the same gases. 

A more detailed analysis has shown that near the extinction limit the reactant diffusion fluxes do not reach the reaction zone in a stoichiometric combination. A portion of the reactants is observed to bypass the high-temperature flame zone. However, if we consider the rate of material consumption in the reaction zone instead of its supply rate, the above statement will be valid for any diffusion reaction rate. As regards the determination of the extinction limits of diffusion combustion, it is very important that the equations describing the steady-state combustion are valid even in the flame extinction stage. 

Frey and T ien estimated theoretically the extinction limits of the diffusion flaming combustion for a thermally thin fuel according to oxygen concentration. 

The authors of Ref. generalized all the published polymer combustion limits from the viewpoint of the effect of different factors on the cooling of the reaction zone. At the extinction limit of diffusion combustion, the ratio of heat losses from the front edge of the combustion zone to the total heat generation due to the chemical reaction must be proportional to RT, /E here, Tj is the flame temperature at the extinction limit and E the gas-phase reaction activation energy... 

Macek 4,) believes that near the extreme point in the ignition region the chemical reaction responsible for combustion changes its direction and stoichiometry. As a consequence, the diffusion flame temperature at the extinction limit is higher than the flame temperature in the case of pre-mixed gases of the some composition, and both are smaller than the temperature corresponding to the complete combustion of the fuel. 

Experiments under subcritical conditions appear to be most promising in this respect. As an example, we may cite a number of works in which different relationships at the extinction limit were used for the determination of the effective activation energy and preexponential factor of the gas-phase combustion reaction. In particular, Krishnamurthy 87) calculated the kinetic parameters of the gas-phase combustion of PMMA from the relationship between the combustion rate and the oxygen pressure and concentration at the extinction limit (Eg = 88 kJ /mol k0 = 3x 1012 cm3/mol s). Other authors 76,94) did the same by analyzing the relationship between the extinguishing oxidant flow velocity and oxidant concentration, with the help of an opposed flow diffusion flame (OFDF) apparatus. A similar relationship between flow velocity and oxidant temperature was suggested, since preheating of the oxidant was found to immediately affect the flame temperature. For PMMA, PE and polyoxymethylene (POM) Eg = 98.5, 140 and 121 kJ/mol, respectively, were reported. 

The theoretical analysis of extinction of the diffusion flame of combustible liquids with flat flame geometry which may be established with the OFDF apparatus 97> was extended to polymer flames of similar geometry 98). Heat losses by radiation from the polymer surface were taken into account in estimates of the flame temperature at the extinction limit. The kinetic parameters of the gas-phase reaction (Eg, kJ/mol and kg, cm3 mol-1 s 1) have been determined at Yox = 0.16 — 0.20 for PMMA, POM, PE and PS, respectively 167.2 and 1.2xl016 183.9 and 2.6xl016 167.2 and 2.2 x 1015 234.0 and 1.7 x 1017. From a comparison with other data it follows that, as Yox increases, the effective energy of the gas-phase reaction decreases. 

Laminar flame speed is one of the fundamental properties characterizing the global combustion rate of a fuel/ oxidizer mixture. Therefore, it frequently serves as the reference quantity in the study of the phenomena involving premixed flames, such as flammability limits, flame stabilization, blowoff, blowout, extinction, and turbulent combustion. Furthermore, it contains the information on the reaction mechanism in the high-temperature regime, in the presence of diffusive transport. Hence, at the global level, laminar flame-speed data have been widely used to validate a proposed chemical reaction mechanism. 

---