Limiting oxidant concentration flame propagation

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 concentration of flammable vapor in air, oxygen, or other oxidant that will propagate flame on contact when provided with a source of ignition. The lower explosive limit (LEL) is the concentration below which a flame will not propagate the upper explosive limit (UEL) is the concentration above which a flame will not propagate. A change in temperature or pressure may vary the flammable limits. 

There exists an oxidant concentration below which the fuel-oxidant mixture is not capable of propagating flame. This oxidant concentration is called the limit-ing/minimum oxidant concentration (LOC/MOC). LOC is dependent on the type of inert gas used. 

Most powders and dusts found in industry can explode when mixed with the right amount of oxidant. For a dust cloud to explode dust must be explosible and airborne dust cloud must be present in an atmosphere capable of supporting combustion and in contact with an ignition source of sufficient energy size distribution of airborne particles must be capable of supporting flame propagation concentration of dust particles must be between the Lower Explosibility Limit (LEL), typically 20 to 100 g m, and Upper Explosibility Limit (UEL), typically in excess of 2000 g m. ... 

Elammability Limits The minimum and maximum concentrations of combustible material in a homogeneous mixture with gaseous oxidizer that will propagate a flame. 

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. 

Once ignition has occurred in a mixture of fuel and oxidizer, propagation will continue, provided the concentrations are sufficient and no disturbance results in excessive cooling. The zeroth-order rate model is assumed to represent the lean case. Substituting the selected properties into Equation (4.43), the net release and loss curves are plotted in Figure 4.22 as a function of the flame temperature. The initial temperature of the mixture is 25 °C and fuel mass fractions are 0.05 and 0.03, representative of stoichiometric and the lower limit respectively. At this lower limit, we should see that a steady solution is not possible, and the calculations should bear this out. The burning rate is evaluated at the flame temperature, and 6K is found from Equation (4.44) with Su at the flame temperature... 

Two limits of solvent flammability exist. The lower flammability limit is the minimum eon-centration of solvent vapor in oxidizing gas (air) that is capable of propagating a flame through a homogeneous mixture of the oxidizer and the solvent vapor. Below the lower flammability limit the mixture is too lean to bum or explode. The upper flammability limit is the maximum concentration of solvent vapor in an oxidizing gas (air) above which propagation of flame does not occur. Mixtures with solvent vapor concentrations above the upper flammability limit are too rich in solvent or too lean in oxidizer to bum or explode. 

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