Concentration Limits of Flame Propagation

Keywords Concentration limits Critical gap Critical tube diameter Flame 

Up-to-date safety engineering puts forth the challenge of determining ignition limits not only at normal temperature and pressure conditions (298 K and 0.1 MPa), but also for storage conditions at higher temperatures and pressures. 

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The concentration limits of flame propagation in large vessels, when thermal losses due to heat transfer are small, are explained by Ya.B. from the standpoint of thermal losses by radiation. The flame velocity at the concentration limits also turned out to differ significantly from zero. 

Experiment shows that near the concentration limits of flame propagation extremely interesting features of propagation phenomena are observed in a... 

It follows from experiment that at a certain optimum composition of the mixture the flame propagation rate is maximal and decreases both towards a lean and rich mixture. At a certain limiting composition, the flame extinguishes. This limiting composition is defined by the content of the combustible or by an appropriate value specifying the concentration limits of flame propagation. Experiments disclose that beyond the concentration limit the flame extinguishes at a certain final value Uq. 

Because internal combustion engines can work using hydrogen + hydrocarbon mixtures (including natural gas), some measurements of concentration limits of flame propagation in multi-component combustible gas mixtures have been performed. Information on the methodology used to obtain the data can be found in [17, 22-24]. Some useful information from the aforementioned publications is given below. 

V.A. Bunev, Determination of the concentration limits of flame propagation at elevated temperatures. Combust. Explos. Shock Waves 8(1), 67-69 (1972)... 

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. 

The preliminary results obtained show that the initiation limits for polydispersed mixtures and stability of flame propagation strongly depend on inhomogeneity of particles (droplets) concentration distribution typical for the majority of practical cases wherein the ignition and combustion of polydispersed mixtures take place. Thus to ensure stable ignition and combustion characteristics... 

These values expressed in percent by volume of fuel vapor in air are the ranges of concentration over which a particular vapor or gas mixture with air will burn when ignited. If a mixture within its explosive range of concentrations is ignited, flame propagation will occur. This range will be indicated by lei for lower explosive limit or uel for upper explosive limit. The values given, unless other wise indicated, are for normal conditions of temperature and pressure. 

The so-called diffusion theories of flame propagation, as exemplified by the work of Tanford and Pease 38), emphasize the transport of mass, in that concentration of an active radical is assumed to be the rate-controlling property. Its use seems to be fairly limited in that only a few specific reactions have been successfully studied with this theory. What is more interesting, however, is that this theory forms the counterpart to the thermal theory. These two extreme views bracket the actual case, and their study allows a consideration of each of two of the basic flame mechanisms, unencumbered by the other. Actual deflagration depends on both the transport of heat and the transport of mass, and a successful theory should contain both phenomena. 

Table 4 Dependence of the Limiting Concentration of Fuel on the Direction of Flame Propagation in Mixtures with a Deficiency of Fuel...

Many factors enter into the experimental determination of flammable limits of gas mixtures, including the diameter and length of the tube or vessel used for the test, the temperature and pressure of the gases, and the direction of flame propagation—upward or downward. For these and other reasons, great care must be used in the application of the data. In monitoring closed spaces where small amounts of gases enter the atmosphere, often the maximum concentration of the combustible gas is limited to one-fifth of the concentration of the gas at the lower limit of flammability of the gas-air mixture. 

These tests cover the methods of determination of the minimum temperature at which vapors in equilibrium with liquid solvent are sufficiently concentrated to form flammable mixtures with air at atmospheric pressure and concentration limits of chemicals. Flammable (explosive) limits are the percent levels, volume by volume, of a flammable vapor or gas mixed in air between which the propagation of a flame or an explosion will occur upon the... 

The concentration limits of flammabihty are determined using another method. The method is limited to atmospheric pressure and temperature of 150 C. Equipment is similar to that used in the previous method. A uniform mixture of vapor and air is ignited and flame propagation from ignition source is noted. The concenlration of flammable components is varied until a composition is found which is capable to propagate flame. 

In some other reactions, the more reactive radicals (H, OH, CHb") are replaced by the less active Br radicals. If Br meets H in the presence of a neutral molecule (third body), HBr is regenerated. It has been found by spectroscopy that the introduction of halogen-containing inhibitors into the flame clearly reduces the concentration of H, OH, and HCO radicals, whereas there is an increase in the content of the diradicals C2" and soot. As the concentration of inhibitor is increased, the flame temperature decreases. Small additions of halogen inhibitors (on the order of a few mol%) can reduce the rate of flame propagation up to 10-fold and have a marked effect on the ignition limits. On the other hand, halogens accelerate the formation of soot in the flame. 

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