Normal Burning Rate

The flame of a combustible mixture is capable of propagating towards the unburnt gas. In practice, there can be flames in a closed confinement of the initially immobile gas and also flames in a gas flow entering at a certain velocity into the combustion zone. A certain propagation rate which is always that of the flame front relative to the unburnt gas is characteristic of both cases. 

The basic quantitative flame characteristic is the so called normal or fundamental burning rate Uq, representing the relative rate of the flame front displacement perpendicular to the flame front surface, defined by 

The normal burning rate, which is a function of the chemical and thermodynamic properties of the combustible, can always be measured by some technique. For a review of such techniques see e.g. [497]. 

The measured normal burning rate values range from several centimetres to several metres per second. An example of a very slow-burning mixture is that of 6% methane and 94% air. Its normal burning rate is 5 cm/s. A very fast-burning 

The basic factor responsible for the flame propagation is the chemical reaction which represents the source providing the therihal and chemical energies sustaining combustion. The decisive role of the chemical reaction in the mechanism of flame propagation was first suggested by Payman and Wheeler [363]. 

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When BlkPdr is used as a pressed grain, surface normal burning rates can be defined. These studies continue as an important tool in examining the complexities of the burning mechanism. These and other properties of various BlkPdrs have been compared (Ref 72), as has the temp dependence of the burning rates. The enormous volume of published data deals, however, only with conventional, mill process, powder, while the corresponding data on the jet mill powder await the completion of ongoing studies... 

The normal burning rate of gases is generally some metres per second. In this type of combustion, no excess pressure is generated since a continuous input of the combustible gas mixture and output of the combustion products is possible. 

However, even in this simplified case, the calculation of the normal burning rate is only possible by numerical integration of the thermal conductivity and diffusion equations. For this reason, before the advent of computers, which permit a strict numerical solution of the problem for any complex chemical combustion mechanism (provided the rate constants and diffusion coefficients are known with sufficient accuracy), attempts were made to obtain an analytical solution under certain allwances and thus to restrict the set of differential equations to only one. At present, these attempts are mostly of historical interest though the illustrative nature of the analytical expressions for the normal combustion rate as a function of parameters characteristic of molecular and chemical kinetic properties of combustibles gives these attempts a certain advantage over numerical solutions. 

Long-burning formulations for infrared tracking flares typically comprise burn rate modifiers that extend the burning time. For this purpose, barium stearate has been proposed. Figure 10.20 shows the effect of stearate content on both normalized burn rate and spectral efficiency. Whereas the burn rate drops to levels of 20% of the unaffected composition shown below, the spectral efficiency climbs up to levels of 140% [28]. 

Binder (vrt%) Softening agent (wt%) PTFE (wt%) Mg (wt%) Modifier (wt%) Normalized burn rate (mm ) Radiant intensity (Wsr- ) Spectral efficiency Og- sr-i)... 

The use of ultra-fine AP to increase the burning rates of double-base propints has been described previously. This technology has also been used with hydrocarbon binders, but dispersion and high viscosity have presented mixing problems. Fine AP can be conveniently dispersed by slurrying first with a surfactant and an organic liq such as 1,1,2-trichloro-1,2,2-trifluoroethane or normal hexane in a process described by Alley (Ref 58a). Sodium sulfonate... 

A material burning in an enclosure will depart from its burning rate in normal air due to thermal effects of the enclosure and the oxygen concentration that controls flame heating. Chapter 9 illustrated these effects in which Equation (9.73) describes steady burning in the form ... 

Equation (5-5) assumes that the burning rate is constant. More detailed pool burning geometry models are available (Spouge, 1999). Circular pools are normally assumed where dikes lead to rectangular shapes, an equivalent diameter is used in the calculation. 

Charcoal is used as a fuel and burning rate stabiliser while dextrin (starch gum) forms a viscous dispersion in water such that it can be added to the pyrotechnic ingredients to form a slurry that will adhere to the wires when dipped. It is normally necessary to dip the wires several times in order to build up a satisfactory coating before drying in a current of warm air. 

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