Burning velocity data

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). 

In this paper, we present some of our more recent results where we have developed the global kinetics for some ternary fuel systems CH4 - H2 - CO and natural gas - H2O - CO and have correlated these flame data and other plug flow data with burning velocity data. 

The characteristic that has been called the low-velocity regime (M2) would have been observed at a much earlier stage if burn-out data had existed for experiments covering a very wide range of mass velocities with the other system parameters fixed. Even today there is no really good direct example of... 

Optimized y Values and rms Errors for Eqs. (23) and (24) Applied to High-Velocity Regime Burn-Out Data from Table I... 

Fig. 38. Test of an equivalent-diameter hypothesis using annulus burn-out data for water [from Barnett (B4)]. The continuous lines are evaluated from the correlation of round tube data given in Section VIII, A for the same pressure, mass velocity, and length as the annulus data, and for the diameters shown.

Law, C.K., A compilation of experimental data on laminar burning velocities. Reduced Kinetic Mechanisms for Application in Combustion, Eds. N. Peters and B. Rogg, Springer-Verlag, Heidelberg, Germany, pp. 15-26, 1993. 

FIGURE 4.47 General trend of experimental turbulent burning velocity (5X/SL) data as a function of turbulent intensity (U ISL) for Rj = 1000 (from Ronney [39]). 

Values of n are available for numerous gases and gas mixtures. Burning velocities have been measured on burner flames, and flame temperatures, Tb, can be computed thermodynamically. It is thus possible to put Equation 5 to a test by comparing experimental values of minimum spark-ignition energies with values calculated from data on quenching distances, burning velocities, heat conductivities, and flame temperatures. 

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. 

CHEMICAL VARIABLES. In general, turbulent burning velocity rises to a maximum rich of stoichiometric, and then declines, in a manner similar to that of laminar flames. Figure 10 shows the typical behavior (8). The maximum does not shift with changing Reynolds number some data do show a slight shift to richer mixtures at higher turbulence levels 78). 

PHYSICAL VARIABLES. There has not yet been any adequate study of the effects of pressure, so only changes in temperature and in the nature of the turbulent flow can be considered. The small amount of data available shows that turbulent burning velocity changes with temperature in very nearly the same way as laminar burning velocity. For instance, the values of Heiligenstaedt (12) for coke oven gas-air mixtures, over the range 10° to 400° C., can be correlated within 10% by the Reynolds number of the flow when plotted as Ut/Ul Similarly, Delbourg (12) found for town gas-air flames... 

Qb (for blow-off). For gradients less than gf, for example, line 1, the burning velocity is somewhere greater than the flow velocity, so the flame will flash back for gradients greater than g6, for example, line 2, the flow velocity is everywhere greater than the burning velocity, so the flame must blow off. Stability data for both laminar and turbulent flow may be correlated by gf and gb this is reasonable because in either case there is a laminar sublayer at the burner wall (23). 

Aside from this, the data on burning velocities seem to be in almost quantitative accord with the conduction equation (XIV. 10.23) when adapted to flames in finite systems such as cylinders and spheres. The velocity of flame propagation in tubes is complicated by the viscous drag exerted by the walls on the flowing gas, together with the heat losses at the walls. The resulting Poiseuille type of flow tends to make the flame fronts parabolic in these systems. 

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... 

Lewis and von Elbe have ignited CO/O2 mixtures in the centre of spherical reaction vessels, and have obtained data on the rate of propagation of the flame and the rate of pressure change in the vessel. From such results the burning velocity S, can be calculated [4], according to the thin flame approximation, from eqn. (120),... 

The decomposition of ozone has been of great interest to those concerned with combustion, because of the apparent simplicity of the reaction and the fact that there is only one product gas, oxygen. Lewis and von Elbe (13) developed a theory of flame propagation in ozone-oxygen mixtures on the basis of their burning velocity studies. They (13) derived high-temperature specific heat values for oxygen from their explosion data. 

Harris, M. E., Grumer, J., Von Elbe, G., and Lewis, B., Burning velocities, quenching, and stability data on nonturbulent flames of methane and propane with oxygen and nitrogen, in Third Symposium on Combustion, Flame and Explosion Phenomena, Williams Wilkins, Baltimore, 1949, 80. 

As noted earlier (Section 4.2), burning velocities in oxygen are considerably greater than those in air. Similarly, since the flame temperature increases as the oxygen content of the atmosphere increases, the amount of heat fed back to a liquid pool [Equation (4.51)] also increases. Accordingly, the liquid burning rate increases, but reliable quantitative data are available at present only for fires in air. 

---