Flame propagation limits

Mayer, E., A theory of flame propagation limits due to heat loss. Combust. Flame, 1 438,1957. 

When thermal losses are taken into account the combustion temperature depends on the flame propagation velocity. Consideration of this dependence, together with the Arrhenius dependence of the flame velocity on the combustion temperature, leads to a theory of the flame propagation limit at the limit the decrease in the combustion temperature is RT2/E and the normal flame velocity falls by a factor of /e compared to the value in the absence of thermal losses. 

Besides the equilibrium radiation considered in this article, the concentration limits can also be affected by chemiluminescence which arises if, in the combustion, chemical compounds form with a non-equilibrium energy distribution along the degrees of freedom of molecules or an atom. The chemiluminescence itself cannot lead to the appearance of a flame propagation limit if only one specific energy fraction is emitted. However, forced luminescence in an optical resonator (in combustion lasers) can lead to quenching of the flame. 

It is interesting to compare the result with the theory of the flame propagation limit in gases which was developed by the author [14], In the case of... 

Fire Properties Associated with Flame Propagation Limiting Oxygen Index... 

FLAME PROPAGATION LIMITING OXYGEN INDEX AND FIRE PROPAGATION INDEX... 

Addition of chemically active compounds to flame, which are able to change the flame velocity, the flame propagation limits and the other macrokinetic parameters, seems to be the most effective way to control combustion. Of sp>ecial interest are chemically active inhibitors producing a noticeable effect on flame at low concentrations, which do not change the flame stoichiometry. Thousands of elementary reactions involving hundreds of sprecies proceed in hydrocarbon flame. However, the key reactions are those involving atoms and free radicals with their reaction rates being much faster than those of the other reactions. The inhibitors mainly interact and affect the above processes. 

S.M. Kogarko, A.G. Lyamin, O.E. Popov, A.Y. Kusharin, A.V. Dubrovin, Determination of flame propagation limits in stoichiometric oxyhydrogen mixtures with steam. In Hydrogen Behaviour and Control and Related Containment Loading Aspects. IAEA-TC-476.6, Vienna, 1984, pp. 37 1... 

The experimental technique applied allows determination of the micro-droplets effect on the flame propagation limit in a hydrogen + air + saturated steam mixture. The mixture was considered incombustible when the photodiodes did not record radiation within 0.4 s after the ignition time. 

In a heated 5-cm diameter and 1.8 m-length vertical tube, the flame propagation limits (lower ignition) have been measured at 295, 373 and 473 K temperatures and 0.1 MPa pressure [8]. In accordance with the obtained data, the amount of N2 necessary for complete suppression of the combustion at 295 K is equal to 87% and increases up to 92% at 473 K. The range of the obtained temperature limit data has been extended to 523 K, and the pressure limit data range to 2 MPa [12]. For the test [12] a heated vessel of about 50-L volume has been used. 

Let us consider flame propagation limitations caused by aperture sizes, tube diameters, or gap dimensions through which the flame cannot propagate. These issues are important for safe engineering. 

A result close to the aforementioned one can be obtained without time-consuming calculations like in [27]. Specific characteristics of the change in combustible gas mixture flame propagation limits are well known. Let us assume that such a mixture contains n compounds, with the concentration of each individual substance - its lower (or upper) flame propagation limit - ( i. Following the Le Chateher s law [58] ... 

Flammable limits are important as they indicate the range of concentrations within which a comhnstion reaction may occnr. If a concentration of a fnel-oxidant mixtnre can be maintained below the LFL or above the UFL, then there is no possibility of flame propagation. Fignre 3-10 (page 32) is a typical flammability diagram with the flammable zone between the LFL and the UFL indicated. 

With respect to blast effects, Rosenblatt and Hassig s (1986) conclusions are fully in line with those of Raju and Strehlow (1984). Except in a limited area at the cloud s edge, the blast peak overpressures are produced by the very first stage of flame propagation, during which the flame is spherical. 

Among the various selection considerations are specific combustion characteristics of different fuels. One of the combustion characteristics of gaseous fuels is their flammability limit. The flammability limit refers to the mixture proportions of fuel and air that will sustain a premixed flame when there is either limited or excess air available. If there is a large amount of fuel mixed with a small amount of air, then there is a limiting ratio of fuel to air at which the mixture will no longer sustain a flame. This limit is called the rich flammability limit. If there is a small amount of fuel mixed with excess air, then there is a limiting ratio of the two at which the flame will not propagate.This limit is called the lean flammability limit. Different fuels have different flammability limits and these must be identified for each fuel. 

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... 

Flow velocity field determined by PIV. Lean limit flames propagating upward in a standard cylindrical tube in methane/air and propane/ air mixtures, (a) Methane/air—laboratory coordinates, (b) propane/air—laboratory coordinates, (c) methane/air—flame coordinates, and (d) propane/air—flame coordinates. 

Streamlines of lean limit flames propagating upward in (a) methane/air and (b) propane/air mixtures (flame coordinates). 

Stretch rate as a function of distance, from the top leading point along the flame front, (a) for a lean limit methane flame and (b) lean limit propane flame propagating upward in a standard cylindrical tube. 

In the case of flame propagation in the lean limit methane/air mixture, the local laminar burning velocity at... 

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... 

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