Flame speed/velocity

In the reaction 2one, an increase in the intensity of the turbulence is related to the turbulent flame speed. It has been proposed that flame-generated turbulence results from shear forces within the burning gas (1,28). The existence of flame-generated turbulence is not, however, universally accepted, and in unconfined flames direct measurements of velocity indicate that there is no flame-generated turbulence (1,2). 

Vapor Cloud Explosion (VCE) Explosive oxidation of a vapor cloud in a non-confined space (not in vessels, buildings, etc.). The flame speed may accelerate to high velocities and produce significant blast overpressure. Vapor cloud explosions in plant areas with dense equipment layouts may show acceleration in flame speed and intensification of blast. 

Flashback into a pre-mix duct occurs when the local flame speed is faster than the velocity of the fuel/air mixture leaving the duct. 

The conseqnence is that the rate of prodnction of volnme of hnrned prodncts is greater dne to the density decrease resnlting from the reaction. As the prodncts expand this canses the nnhnrned mixtnre to move as well. The flame is then seen to move forward with a higher apparent velocity, Vf, the snm of the mean nnhnrned gas velocity, u, and the tnrhnlent hnrning velocity, S. Vf is called the flame velocity (flame speed). 

Pipe diameter has an effect on flame propagadon. It is minimal in the range of L/D from 1 to —50. In this secdon of the pipe, the flame velocity is not affected by the diameter. Beyond an L/D of 50, flame speed increases with pipe diameter. 

Flame speed is proportional to the fnndamental bnrning velocity that is, as the fnndamental bnrning velocity increases, the flame speed increases. 

In this case ignition occnrs on the downstream side of the flame arrester. If the velocity of the gas is less than the flame speed, the flame stabilizes on the flame arrester and continnes to heat it nntil the gas flow is stopped or the flame is qnenched by other means. Continned heating conld canse ignition of the gas on the opposite side of the flame arrester. 

If the velocity of the gas is snfficiently greater than die flame speed, the flame is swept ont of the dnct, or it stabilizes at the exit of the dnct. 

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. 

Flow Controlled Aperture An aperture designed to produce flow velocities which exceed the local flame speed of the flammable mixture, thus preventing flame transmission in the reverse direction. 

To overcome this problem, they proposed a working-fluid heat-addition model. This model implies that the gas dynamics are not computed on the basis of real values for heat of combustion and specific heat ratio of the combustion products, but on the basis of effective values. Effective values for the heat addition and product specific heat ratios were determined for six different stoichiometric fuel-air mixtures. Using this numerical model, Luckritz (1977) and Strehlow et al. (1979) systematically registered the properties of blast generated by spherical, constant-velocity deflagrations over a large range of flame speeds. 

Overpressure within a vapor cloud is dependent upon outflow velocity, orifice diameter, and laminar flame speed expressed in the following semi-empirical relation ... 

The output from each case produces a wealth of information, including distribution of pressure, combustion products, rates of combustion, velocity components, etc. The results of each case will be summarized by presenting the pressure time histories at the eight locations that were presented in Figure 2 together with the flame speed along some selected directions. Some contour plots will also be presented. 

Flame speed The speed of a flame burning through a flammable mixture of gas and air measured relative to a fixed observer, that is, the sum of the burning and translational velocities of the unbumed gases. 

Vapor cloud explosion The explosion resulting from the ignition of a cloud of flammable vapor, gas, or mist in which flame speeds accelerate to sufficiently high velocities to produce significant overpressure. 

The factors tliat affect miconfined I apor cloud explosions me not well understood. In a model developed by William, it is assmned tliat ignition occurs at a point source, tliat tlie flame front travels out from tlie core at a flame speed S, and tliat the pressure waves produced by the flame generate a weak shock wave tliat travels ahead of tlie flame with a time-dependent velocity. Tlie equation for the flame speed for spherical systems is... 

The properties of natural gas are dominated by those of methane, notably a low maximum flame speed of 0.33 m/s. This strongly influences burner design, which must ensure that the mixture velocity is sufficiently low to prevent blow-off. Light-back , on the contrary, is very unlikely with such a low flame speed. 

A sample PIV image and the corresponding two-dimensional velocity map. The axial velocity along with distance from nozzle exit is plotted accordingly. This minimum point is defined as the reference flame speed. At this reference point, the linearity of the radial velocity profile is illustrated. 

Relation between flame speed Vj and maximum tangential velocity in an axially decaying vortex flow in a tube for various mixtures (tube diameter 31mm, the mean axial velocity 3m/s). (From Ishizuka, S., Combust. Flame, 82,176,1990.)... 

Figure 4.2.13 shows the variation of the flame speed with the maximum tangential velocity obtained with vortex ring combustion in the same mixture atmosphere [29]. The cylinder diameter was 100 mm and various lean, stoichiometric, and rich methane/ air and propane/air mixtures were examined. The diameter of the propagating flame was also determined and the ratio of the flame diameter to the core diameter was also plotted against the maximum tangential velocity. 

In all the mixtures, the flame speed increased almost linearly with an increase in the maximum tangential velocity. The value of the slope in the Vf-y(,max plane was almost unity for the stoichiometric mixtures, however, the slope became smaller for the lean and rich mixtures. The flame to the core diameter ratio decreased with the increasing Vg The ratio was around unity in the stoichiometric mixtures, while it was smaller than unity in the lean and rich mixtures. 

---