Burning Velocity and Flame Speed

Fundamental, laminar, and turbulent burning velocities describe three modes of flame propagation (see the Glossary for definitions). The fundamental burning velocity, S, is as its name implies, a fundamental property of a flammable mixture, and is a measure of how fast reactants are consumed and transformed into products of combustion. Fundamental burning velocity data for selected gases and vapors are listed in Appendix C of NFPA68 (1998). 

FIGURE 4-1. Concentration and temperature profiles through a premixed flame. 

FIGURE 4-2. Sketch of differences in the local direction (upper) and flame front topography (lower) between a laminar and turbulent flame. 

Overview of Combustion and Flame Propagation Phenomena Related to DDAs 

The conseqnence is that the rate of prodnction of volnme of hnrned prodncts is greater dne to the density decrease resnlting from the reaction. As the prodncts expand this canses the nnhnrned mixtnre to move as well. The flame is then seen to move forward with a higher apparent velocity, Vf, the snm of the mean nnhnrned gas velocity, u, and the tnrhnlent hnrning velocity, S. Vf is called the flame velocity (flame speed). 

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As in the soap bubble method, only fast flames can be used because the adiabatic compression of the unbumed gases must be measured in order to calculate the flame speed. Also, the gas into which the flame is moving is always changing consequently, both the burning velocity and flame speed vary throughout the explosion. These features make the treatment complicated and, to a considerable extent, uncertain. 

Recent efforts to distinguish between the terms burning velocity and flame speed on the basis of Eulerian and Lagrangian coordinate systems appear to introduce confusion. Therefore, the terms are used interchangeably here, as synonyms for such terms as deflagration velocity, wave speed, and propagation velocity. They all refer to velocities measured with respect to the gas ahead of the wave. 

If gas mixt has s low burning velocity, the flame will eventually travel at a lower constant speed, which is equal to the product of die burning velocity and the expansion ratio. Rapidly burning flames, however, will give rise to a shock wave which will accelerate the flame and may lead to detonation Ref. A. Everet G. Minkoff, Fuel 33,... 

Fig. 7.16. Computed loci of changes in turbulent burning velocity and related parameters during engine combustion at four different engine speeds. Ignition occurs at the lowest point on each curve. As engine speed increases, combustion moves away from the continuous laminar flame sheet regime. From [132].

A. Lipatnikov, J. Chomiak, Turbulent burning velocity and speed of developing, curved, and strained flames. Proc. Combust. Inst. 29, 2113-2121 (2002)... 

S has been approximated for flames stabili2ed by a steady uniform flow of unbumed gas from porous metal diaphragms or other flow straighteners. However, in practice, S is usually determined less directly from the speed and area of transient flames in tubes, closed vessels, soap bubbles blown with the mixture, and, most commonly, from the shape of steady Bunsen burner flames. The observed speed of a transient flame usually differs markedly from S. For example, it can be calculated that a flame spreads from a central ignition point in an unconfined explosive mixture such as a soap bubble at a speed of (p /in which the density ratio across the flame is typically 5—10. Usually, the expansion of the burning gas imparts a considerable velocity to the unbumed mixture, and the observed speed will be the sum of this velocity and S. ... 

In the reaction 2one, an increase in the intensity of the turbulence is related to the turbulent flame speed. It has been proposed that flame-generated turbulence results from shear forces within the burning gas (1,28). The existence of flame-generated turbulence is not, however, universally accepted, and in unconfined flames direct measurements of velocity indicate that there is no flame-generated turbulence (1,2). 

Flame Speed The speed of a flame front relative to a fixed reference point. Flame speed is dependent on turbulence, the equipment geometry, and the fundamental burning velocity. 

The only model ever published in the literature is poor. The fact, for instance, that burning speed is taken as proportional to wind speed implies that, under calm atmospheric conditions, burning velocities become improbably small, and flash-fire duration proportionately long. The effect of view factors, which change continuously during flame propagation, requires a numerical approach. 

Flame speed The speed of a flame burning through a flammable mixture of gas and air measured relative to a fixed observer, that is, the sum of the burning and translational velocities of the unbumed gases. 

In both of these experimental arrangements, for a given mixture, there is a unique duct velocity (vu) that matches the burning velocity. In the Spalding burner, this is the adiabatic burning velocity (or the true, S U). If vu > Su the condition is not stable and the flame will blow off or move away from the exit of the duct until a reduced upstream velocity matches Su. If vu <, S U, the flame will propagate into the duct at a speed where the flame velocity is, S U vu. This phenomenon of upstream propagation is known as... 

The burning velocity is not constant over the cone. The velocity near the tube wall is lower because of cooling by the walls. Thus, there are lower temperatures, which lead to lower reaction rates and, consequently, lower flame speeds. The top of the cone is crowded owing to the large energy release therefore, reaction rates are too high. 

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