Flammable limits, flame propagation

The autoignition temperature is the minimum temperature required for self-sustained combustion in the absence of an external ignition source. The value depends on specified test conditions. Tht flammable (explosive) limits specify the range of concentration of the vapor in air (in percent by volume) for which a flame can propagate. Below the lower flammable limit, the gas mixture is too lean to burn above the flammable limit, the mixture is too rich. Additional compounds can be found in National Fire Protection Association, National Fire Protection Handbook, 14th ed., 1991. 

The lower flammable limit (LEL) or lower explosive limit (LEL) is the minimum concentration of vapor in air below which a flame is not propagated when an ignition source is present (61—64). Below this concentration, the mixture is considered too lean to bum. The lower flammable limit and the flash point of a flammable Hquid are closely related by the Hquid s vapor pressure characteristics. 

LEL (lower explosive, OR FLAMMABLE, LIMIT) The minimum eoneentration of a gas, vapour, mist or dust in air at a given pressure and temperature that will propagate a flame when exposed to an effieient ignition souree. Generally expressed as % by volume for gases and vapours, and as mg/m for mists or dusts. 

Let s now mm attention back to the flammability limit itself. When small increments of a combustible gas are successively mixed with air, a concentration is finally attained in which a flame will propagate if a source of ignition is present. This is referred to as the Lower Flammable Limit (LFL or lower explosion limit) of the gas in air. As further increments of the gas are added, a higher concentration of flammable gas in air will finally be attained in which... 

Flammable limits The minimum and maximum concentrations of a gas or vapor in air which can be ignited and sustain a self-propagating flame. 

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. 

Flammable Limits The minimum and maximum concentration of fuel vapor or gas in a fuel vapor or gas/gaseous oxidant mixture (usually expressed in percent hy volume) defining the concentration range (flammable or explosive range) over which propagation of flame will occur on contact with an ignition source. See also Lower Flammable Limit and Upper Flammable Limit. 

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 effect of natural gravity on flammability limits has been known for a long time. The difference between flammability limits for downward and upward flame propagation was first observed by White [26], for hydrogen/air mixtures. Subsequently, similar effects were also found for other mixtures. For propane flames, the lean flammability limit for both downward and upward propagation was observed to be = 0.53. The rich limits were = 1.64 for downward and = 2.62 for upward propagation. Such wide gap between the flammability limits for rich mixtures is explained in... 

For propane flames, fhe quenching distance (for downward propagating flames) is limited by the distance between the walls of about 10 mm. In larger channels, the flame is quenched at the flammability limits. 

Jarosinski, J., Podfilipski, J., and Fodemski, T., Properties of flames propagating in propane-air mixtures near flammability and quenching limits. Combust. Sci. Tech., 174 167, 2002. 

The creation of a steady flame hole was previously carried out by Fiou et al. [36]. In their experiments, a steady-annular premixed edge flame was formed by diluting the inner mixture below the flammability limit, for both methane/air and propane/air mixtures. They found that a stable flame hole was established when the outer mixture composition was near stoichiometry. Their focus, however, was on the premixed flame interaction, rather than on the edge-flame formation, extinction, or propagation. 

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