Turbulent burning velocity

Keywords Explosion suppression Flame surface Optical diagnostics Turbulent burning velocity Turbulent pulsations... 

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). 

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. 

Shy, S.S., Jang, R.H., and Tang, C.Y., Simulation of turbulent burning velocities using aqueous autocatalytic reactions in a near-homogeneous turbulence. Combust. Flame, 105, 54, 1996. 

Abdel-Gayed,R.G., Bradley, D.,andLawes, M., Turbulent burning velocities A general correlation in terms of straining rates, Proc. R. Soc. Lond. A, 414, 389,1987. 

Shy, S.S., Lin, W.J., and Peng, K.Z., High-intensity turbulent premixed combustion General correlations of turbulent burning velocities in a new cruciform burner, Proc. Combust. Inst., 28, 561, 2000. 

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. 

FUatyev, S.A., J.F. Driscoll, C.D. Carter, J.M. Donbar, Measured properties of turbulent premixed flames for model assessment, including burning velocities, stretch rates, and surface densities. Combust Flame, 2005. 141(1-2) 1-21. 

The fact that the fuel/air ratio is spatially constant in HCSI engines, at least within a reasonably close approximation, allows substantial simplifications in combustion models. The burn rate or fuel consumption rate dm /dt is expressed as a function of flame surface area the density of the unburnt fuel/air mixture Pu, the laminar burning velocity Sl, and the fluctuations of velocities, i.e., E as a measure of turbulence, u. ... 

Embedded in such models, in which variations were developed [12] are further detailed. The laminar burning velocity is expressed as a function of fuel type, fuel/ air ratio, level of exhaust gas recirculation, pressure, temperature, etc. Furthermore, submodels have been developed to describe the impact of engine speed, port-flow control systems, in-cylinder gross-flow motion (i.e., swirl, tumble, squish), and turbulent fluctuations u. Thus, with a wider knowledge base of the parametric impact of external variables, successful modeling of... 

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]). 

A related term is flame speed. Flame speed is the speed with which a flame appears to move relative to a stationary observer. The flame speed can be much larger than the burning velocity due to expansion ofthe combustion products, instability and turbulent deformation ofthe flame. Flame speeds of 30-300 ft/sec (9-90 m/sec) are commonly observed for hydrocarbon-air mixtures. A gas phase detonation occurs when the flame speed exceeds the speed of sound in the burning vapor air mixture. 

Gieras, M., R. Klemens, and P. Wolanski. 1996. Evaluation of turbulent burning velocity for dust mixtures. 7th Colloquium (International) on Dust Explosions Proceedings. Bergen, Norway. 535-51. 

Abdel-Gayed, R. G., K. J. AI-Khishali, D. Bradley, and M. Laws. 1984. Turbulent burning velocities and flame straining in explosions. Proc. Royal Society London A 391 393-414. 

Bray, K. N. C. 1990. Studies of the turbulent burning velocity. Proc. Royal Society London A 431 315-35. 

Flame thickness Burning velocity Burning velocity Quenching distance Quenching distance Homogeneous turbulent reactor... 

If the unburned gas is in a condition of turbulent flow, the flame no longer appears as a well-defined stationary wave in the gas, as it does in laminar streams. A turbulent Bunsen flame, for instance, appears to the eye or in a time-exposed photograph as a brush-like region of indefinite extent, quite thin near the base and becoming wider toward the top (Figure 9). In short, a turbulent flame has a very complicated structure compared to a laminar flame and does not provide the observer with a ready frame of reference for the measurement of burning velocity. The reason is of course the turbulent fluctuations of velocity superimposed on the main flow. 

The point is that no unambiguous turbulent burning velocity can be measured at present. However, it is perfectly possible to determine burning rates with reference to arbitrarily chosen surfaces in time-exposed photographs these rates must serve at present to characterize turbulent flames. 

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). 

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) ... 

Figure 10. Effect of fuel concentration on turbulent burning velocity of ethene-air mixtures at various Reynolds numbers. Constant density and viscosity %-inch burner (8)...

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... 

Inasmuch as the trends in turbulent burning velocity with fuel concentration and type and with initial temperature parallel those shown by laminar flames, it is clear that the differences between turbulent and laminar flames originate in the nature of... 

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