Flame speed limiting value

In the so-called "wrinkled flame regime," the "turbulent flame speed" was expected to be controlled by a characteristic value of the turbulent fluctuations of velocity u rather than by chemistry and molecular diffusivities. Shchelkin [2] was the first to propose the law St/Sl= (1 + A u /Si) ), where A is a universal constant and Sl the laminar flame velocity of propagation. For the other limiting regime, called "distributed combustion," Summerfield [4] inferred that if the turbulent diffusivity simply replaces the molecular one, then the turbulent flame speed is proportional to the laminar flame speed but multiplied by the square root of the turbulence Reynolds number Re. ... 

There is, of course, a chemical effect in carbon monoxide flames. This point was mentioned in the discussion of carbon monoxide explosion limits. Studies have shown that CO flame velocities increase appreciably when small amounts of hydrogen, hydrogen-containing fuels, or water are added. For 45% CO in air, the flame velocity passes through a maximum after approximately 5% by volume of water has been added. At this point, the flame velocity is 2.1 times the value with 0.7% H20 added. After the 5% maximum is attained a dilution effect begins to cause a decrease in flame speed. The effect and the maximum arise because a sufficient steady-state concentration of OH radicals must be established for the most effective explosive condition. 

When the flow velocity is increased to a value greater than the flame speed, the flame becomes conical in shape. The greater the flow velocity, the smaller is the cone angle of the flame. This angle decreases so that the velocity component of the flow normal to the flame is equal to the flame speed. However, near the burner rim the flow velocity is lower than that in the center of the tube at some point in this area the flame speed and flow velocity equalize and the flame is anchored by this point. The flame is quite close to the burner rim and its actual speed is controlled by heat and radical loss to the wall. As the flow velocity is increased, the flame edge moves further from the burner, losses to the rim decrease and the flame speed increases so that another stabilization point is reached. When the flow is such that the flame edge moves far from the rim, outside air is entrained, a lean mixture is diluted, the flame speed drops, and the flame reaches its blowoff limit. 

Current research on control of combustion is focussed not only to reduce combustion-induced pressure oscillations and instability but also to improve combustion performance. Attention is being paid to increased flame speed and improved flame lift-off limits. Flame speeds ranging from laminar to 3.5 times laminar values have been examined, using a Countercurrent Swirl Combustor... 

Consideration of the curves shown in fig. 20 show s that the flame-speeds of mixtures of methane and an steadily rise to maximum values as the percentage of the combustible gas is raised from its lower limit of 5-6 to about 10 per cent. Further addition of methane reduces the speed until the flame is extinguished just beyond the upper limit value. 

The higher and lower limit speeds tend to approach the same value of 20 cm. per second for all the gases. It is interesting to note that in every case, except that of methane, the maximum flame speed occurs with a mixture containing more of the combustible gas than is required for complete combustion. 

Vapor pressure, psiat 100°F Latent heat of evaporatbn Gross heating value Net heating value Autoignitbn temperature Adiabatb flame temperature Stoichbmetric flame speed Electrical conductivity at 46°F Vapor density versus air Flash pobt Flammability limits Stoichbmetric air-fuel ratb... 

The comparison of physical properties of biogases with various origins and natural gas are depicted in Table 2.6. Biogases have lower calorific value, flame speed and flammability limits compared with natural gas, mainly because of COj presence (Porpatham et al. 2008). 

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