Flame extinction

One of the most challenging aspects of modeling turbulent combustion is the accurate prediction of finite-rate chemistry effects. In highly turbulent flames, the local transport rates for the removal of combustion radicals and heat may be comparable to or larger than the production rates of radicals and heat from combustion reactions. As a result, the chemistry cannot keep up with the transport and the flame is quenched. To illustrate these finite-rate chemistry effects, we compare temperature measurements in two piloted, partially premixed CH4/air (1/3 by vol.) jet flames with different turbulence levels. Figure 7.2.4 shows scatter plots of temperature as a function of mixture fraction for a fully burning flame (Flame C) and a flame with significant local extinction (Flame F) at a downstream location of xld = 15 [16]. These scatter plots provide a qualitative indication of the probability of local extinction, which is characterized... 

Many polymer films, eg, polyethylene and polyacrylonitrile, are permeable to carbon tetrachloride vapor (1). Carbon tetrachloride vapor affects the explosion limits of several gaseous mixtures, eg, air-hydrogen and air-methane. The extinctive effect that carbon tetrachloride has on a flame, mainly because of its cooling action, is derived from its high thermal capacity (2). 

The fraction of black-body radiation actually emitted by flames is called emissivity. Emissivity is determined first by adsorption of radiation by combustion products (including soot) in flames and second by radiation wavelength. These factors make emissivity modeling complicated. By assuming that a fire radiates as a gray body, in other words, that extinction coefficients of the radiation adsorption are independent of the wavelength, a fire s emissivity can be written as... 

There is good experimental evidence for this mechanism of antimony-halogen synergism, most significantly that flame extinction time is a minimum for polymers containing antimony and halogen in the mole ratio 1 3. 

Flammability Limits Ignition of a Flammable Mixture and Limit Flame Extinction... 

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... 

History of upward flame propagation and extinction in lean limit methane/air mixture. Square 5 cm x 5 cm vertical tube. Green color frames indicate PIV flow images. Red color represents direct photography of propagating flame. Extinction starts just after frame c. Framing rate... 

Experiments confirm this mechanism. It was observed that before the extinction events set in, the speed of a limit flame propagating downward falls and the flame partially loses contact with the walls (Figure 3.1.14). In a square tube, local extinction starts in the corners, where heat loss to the walls is expected... 

Evolution of flow velocity field in flame coordinates during extinction of upward propagating lean limit methane flame. Frames selected from Figure 3.1.12. 

Strehlow R.A., Noe K.A., and Wherley B.L., The effect of gravity on premixed flame propagation and extinction in a vertical standard flammability tube, Proc. Combust. Inst., 21 1899-1908,1986. 

Sung C.J. and Law C.K., Extinction mechanisms of nearlimit premixed flames and extended limits of flammability, Proc. Combust. Inst., 26 865-873,1996. 

Shoshin Y. and Jarosinski ]., On extinction mechanism of lean limit methane-air flame in a standard flammability tube, paper accepted for publication in the 32nd Proceedings of the Combustion Institute, 2009. 

---