Burning velocity of flames

Jahn [407] has measured the burning velocities of flames at atmospheric pressure formed from a range of CO/O2/N2 and CO/O2/CO2 mixtures, containing a little water vapour or hydrogen. The data are reproduced by Lewis and von Elbe [4]. They refer to an average burning velocity over the surface of the inner cone of a Bunsen type flame. The difference between the flames with H2 and CO2 as diluent was small, and probably reflects the differences in transport properties. Fiock and Roder [408] used the soap bubble technique (for details see e.g. Fristrom and... 

The maximum velocity at the axis is twice the average, whereas the velocity at the wall is zero. The effect of the burner wall is to cool the flame locally and decrease the burning velocity of the mixture. This results in flame stabilization. However, if the heat-transfer processes (conduction, convection, and radiation) involved in cooling the flame are somehow impeded, the rate of heat loss is decreased and the local reduction in burning velocity may no longer take place. This could result in upstream propagation of the flame. 

Heat is produced by chemical reaction in a reaction zone. The heat is transported, mainly by conduction and molecular diffusion, ahead of the reaction zone into a preheating zone in which the mixture is heated, that is, preconditioned for reaction. Since molecular diffusion is a relatively slow process, laminar flame propagation is slow. Table 3.1 gives an overview of laminar burning velocities of some of the most common hydrocarbons and hydrogen. 

The volume-source method is not only useful in a spherical approach, but can also be used in more arbitrary geometries, where it is possible to express the volume source strength in a product of burning velocity and flame surface area ... 

This concept can be generalized for more arbitrarily shaped clouds, provided that a reasonable estimate can be made of combustion process development in terms of burning velocity and flame surface area. According to Strehlow (1981), a conservative estimate of source strength is made by... 

In contrast to the lean propane flame, the burning intensity of the lean limit methane flame increases for the leading point. Preferential diffusion supplies the tip of this flame with an additional amoxmt of the deficient methane. Combustion of leaner mixture leads to some extension of the flammability limits. This is accompanied by reduced laminar burning velocity, increased flame surface area (compare surface of limit methane... 

Stone, R., Clarke, A., and Beckwith, P, Correlations for the laminar-burning velocity of methane/diluent/air mixtures obtained in free-fall experiments. Combust. Flame, 114, 546, 1998. 

S.S. Shy, S.I. Yang, W.J. Lin, and R.C. Su 2005, Turbulent burning velocities of premixed CH4/diluent/air flames in intense isotropic turbulence with consideration of radiation losses. Combust. Flame 143 106-118. 

H. Kobayashi, T. Tamura, K. Maruta, T. Niioka, and RA. Williams 1996, Burning velocity of turbulent premixed flames in a high pressure environment, Proc. Combust. Inst. 26 389-396. 

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. 

Cummings[37] measured the burning velocity of a mixture of NO and H2 in a burner flame over a wide pressure range, and found it to be independent of pressure (about 0.56 ms ) between 0.1 MPa and 4.0 MPa. However, Strauss and Edsel found that, at a mixture ratio of 1 1, the burning velocity increased from 0.56 m s to 0.81 m s at 5.2 MPa. 

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,... 

SympCombstn (1953), pp 321—28 (Flame Propagation The Influence of Pressure on the Burning Velocities of Flat Flames)... 

Use GRI-Mech (GRIM30. mec) and a laminar premixed flame code to calculate the burning velocity of a methane-air mixture at 1.0 atm. Repeat the calculation, replacing the nitrogen in the combustion air with helium. Compare flame speeds and adiabatic flame temperatures. 

Flow of Suspended Particles. Small particles suspended in the combustible stream have been used for the study of Bunsen flames. Andersen and Fein 2P) use strobo-scopically illuminated particle tracks for the determination of normal burning velocities and flame temperatures. Flame studies using similar techniques are reported by Fristrom, Avery, Prescott, and Mattuck (3P). Wolfhard and Parker 10P) have made temperature measurements of flames containing incandescent particles. The acceleration of flow through a flame front causes particles greater than about 2 microns to lag. Thus, the particles may not follow the flow streamlines. Gilbert, Davis, and Altman (4P) discuss the corrections which must be applied to obtain accurate results. 

Turbulent burning velocities of various mixtures, as measured from Bunsen flames, are directly proportional to the laminar burning velocities of the same mixtures. For example, the following equation applies to propane, ethene, and acetylene, over the Reynolds number range 3000 to 40,000 (8) ... 

In conclusion of this section it should be noted that the main dimensionless parameters of the problem considered are the ratio of the propagation flame velocity along the channel axis to the normal velocity of flame (parameter e = U/un — 1), and the degree of thermal gas expansion (a = T0/Tb where T0 and Tb are the initial and burned gas temperatures). 

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