Burning velocity definition

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). 

If, in an infinite plane flame, the flame is regarded as stationary and a particular flow tube of gas is considered, the area of the flame enclosed by the tube does not depend on how the term flame surface or wave surface in which the area is measured is defined. The areas of all parallel surfaces are the same, whatever property (particularly temperature) is chosen to define the surface and these areas are all equal to each other and to that of the inner surface of the luminous part of the flame. The definition is more difficult in any other geometric system. Consider, for example, an experiment in which gas is supplied at the center of a sphere and flows radially outward in a laminar manner to a stationary spherical flame. The inward movement of the flame is balanced by the outward flow of gas. The experiment takes place in an infinite volume at constant pressure. The area of the surface of the wave will depend on where the surface is located. The area of the sphere for which T = 500°C will be less than that of one for which T = 1500°C. So if the burning velocity is defined as the volume of unbumed gas consumed per second divided by the surface area of the flame, the result obtained will depend on the particular surface selected. The only quantity that does remain constant in this system is the product u,fj,An where ur is the velocity of flow at the radius r, where the surface area is An and the gas density is (>,. This product equals mr, the mass flowing through the layer at r per unit time, and must be constant for all values of r. Thus, u, varies with r the distance from the center in the manner shown in Fig. 4.14. 

If an attempt is made to define burning velocity strictly for such a system, it is found that no definition free from all possible objections can be formulated. Moreover, it is impossible to construct a definition that will, of necessity,... 

Effective Over-all Chemical Reaction Orders. Values of the effective over-all reaction order a have been obtained by many investigators. The published data are not comprehensive enough to permit any definite correlations to be made between reaction orders, pressure, temperature, and mixture ratios (26). One or more of the three basic types of flame measurements are used in determining reaction orders, these being flame thickness, burning velocity, and quenching distance. Reaction order data are available in the more recent literature for the following mixtures, obtained by the indicated method for various pressures, temperatures, and mixture ratios. 

In all these laminar-flame experiments, the combustion wave propagates at a definite velocity that empirically depends on the pressure, temperature, and composition of the initial combustible mixture. Our primary objective in this chapter is to predict this burning velocity. In Section 5.1.2 a simple physical picture of the deflagration wave is presented, leading to a crude estimate of the burning velocity. The discussion, which is similar to that of Landau and Lifshitz [3], illustrates the essential mechanism involved. 

If an attempt is made to define burning velocity strictly for such a system, it is found that no definition free from all possible objections can be formulated. Moreover, it is impossible to construct a definition that will, of necessity, determine the same value as that found in an experiment using a plane flame. The essential difficulties are as follow. (1) Over no range of r values does the linear velocity of the gas have even an approximately constant value and (2) in this ideal system, the temperature varies continuously from the center of the sphere outward and approaches the flame surface asymptotically as r approaches infinity. So no spherical surface can be considered to have a significance greater than any other. 

R.M. Fristrom, Definition of burning velocity and a geometric interpretation of the effects of flame curvature. Phys. Fluids 8(2), 273-280 (1965)... 

The wave travels at some definite velocity (burning vel) against the unbumed mixture. Velocity depends on ihe composition of the mixture being zero at the limits and at a max at some intermediate composition (eg 9.5% for firedamp). There are three types of flame ... 

The volume of the mixture which burns in unit time on 1 m2 of the flame is equal to u by the definition of the normal velocity its specific heat is equal to... 

The experimentally determined values of parameters Ki and K2 allow the Pigford model to be used to examine the extent of the spin-up zone on the disk surface. Before this can be done, however, an unambiguous mathematical definition for the spin-up zone is required. This was provided by Burns et al. as the location at which the radial velocity stops accelerating and starts following the decelerating profile described by the synchronized flow model as shown in Eq. (3). ... 

Parametric investigations of turbulent combustion are based on the definition of a reduced flame radius 7 l (0 that corresponds to the slope of the pressure - t time curve. When the chemical reaction is localized in narrow flamelets (see. Borgi diagram. Fig. 1.4), the spherical volume of radius 7 l (0 is close to the balanced turbulent combustion products volume. For this case, the pressure ratios P = P t) obtained experimentally during the time of burning are used for calculating / l (0-The turbulent flame velocity 5t (often designated as Ut) can be denoted by... 

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