Flames velocity

In some cases, it is impractical to use a plenum chamber under the constriction plate. This condition arises when a flammable or explosive mixture of gases is being introduced to the reactor. One solution is to pipe the gases to a multitude of individual gas inlets in the floor of the reactor. In this way it may be possible to maintain the gas velocities in the pipes above the flame velocity or to reduce the volume of gas in each pipe to the point at which an explosion can be safely contained. Another solution is to provide separate inlets for the different gases and depend on mixing in the fluidized bed. The inlets should be fairly close to one another, as lateral gas mixing in fluidized beds is poor. 

Fuel Minimum ignition temp., K/ F Calculated flame temperature, K/ F Flammability limits, % fuel gas by volume in air Maximum flame velocity, ni7s and ft/s % theoretical air for max. flame velocity... 

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. 

Quenching Diameter, Quenching Length, and Flame Velocity... 

Figure 4.5. Flame velocity, peak overpressure, and overpressure duration in gas cloud explosions following vessels bursts (Giesbrecht et al. 1981).

In elongated confined vessels, with one end closed and the opposite end open or removable, when an explosion begins at or near the closed end, the rapid movement of the flame front caused by the high volume from combustion wall cause displacement of the unburnt mixture ahead of it. Apparently this characteristic is independent of the nature of the combustible material [54], and the velocity can reach 80%-90% of the flame velocity, in part due to the high turbulence generated in the unburnt mixtures. 

Si is the laminar flame velocity, the function Z(co) is the heat response function Equation 5.1.16, whose real part is plotted in Figure 5.1.10. The function f(r, giJ is a dimensionless acoustic structure factor that depends only on the resonant frequency, a , the relative position, r, of the flame, and the density ratio Pb/Po-... 

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. ... 

Flame velocity versus fuel concentration for H2/air mixtures in the 10 m long tubes of 5, 15, and 30 cm internal diameter with obstacles (orifice plates) BR = 1 - d /D - blockage ratio, where d is the orifice diameter and D is the tube diameter. (From Lee, J.H., Advances in Chemical Reaction Dynamics, Rentzepis, P.M. and CapeUos, C., Eds., 246,1986.)... 

If the mixture (or a dust cloud) is confined, even if only by surface irregularities or local partial obstructions, significant pressure effects can occur. Fuel-air mixtures near to stoicheiometric composition and closely confined will develop pressures of several bar within milliseconds, and material damage will be severe. Unconfined vapour explosions of large dimensions may involve higher flame velocities and significant pressure effects, as shown in the Flixborough disaster. 

Flame velocity. Hydrogen has a faster flame velocity (1.85 m/s) than other fuels (gasoline vapor—0.42 m/s methane—0.38 m/s). 

In both of these experimental arrangements, for a given mixture, there is a unique duct velocity (vu) that matches the burning velocity. In the Spalding burner, this is the adiabatic burning velocity (or the true, S U). If vu > Su the condition is not stable and the flame will blow off or move away from the exit of the duct until a reduced upstream velocity matches Su. If vu <, S U, the flame will propagate into the duct at a speed where the flame velocity is, S U vu. This phenomenon of upstream propagation is known as... 

Consider the ignition of a flammable mixture at the closed end of a horizontal circular tube of cross-sectional area A. The tube has a smaller opening at the other end of area A0. The process is depicted in the figure below. The flame velocity in the tube is designated as vf. It is distinct from the ideal burning velocity, Su, which is constant in this adiabatic case. The following assumptions apply ... 

Solve the energy equation and discuss the behavior of the flame velocity in the tube. (There is a limit to the speed since the exit velocity cannot exceed the speed of sound, i.e. choked flow.)... 

The amount of explosion overpressure is determined by the flame speed of the explosion. Flame speed is a function of the turbulence created within the vapor cloud that is released and the level of fuel mixture within the combustible limits. Maximum flame velocities in test conditions are usually obtained in mixtures that contain slightly more fuel than is required for stoichiometric combustion. Turbulence is created by the confinement and congestion within the particular area. Modem open air explosion theories suggest that all onshore hydrocarbon process plants have enough congestion and confinement to produce vapor cloud explosions. Certainly confinement and congestion are available on most offshore production platforms to some degree. 

In oil and gas facilities, these effects can be generally related to flame velocity, where this velocity is below 100 m/s (300 ft./s), damage is considered unlikely (Note This is generally within the limits of confinement normally found in offshore facilities). The size of a vapor cloud or plume in which such velocities can occur has been experimentally investigated at the Christian Michelsen Institute (CMI, Norway). The experiments demonstrated that flames need a "run-up" distance of approximately 5.5 meters (18 ft.) to reach damaging speeds. Therefore vapor clouds with dimensions less than this may not cause substantial damage. This is a much over-simplification of the factors and variables involved, but does assume the WCCE of congestion, confinement and gas concentrations. 

A patented water injection system has been devised for extinguishing oil and gas well fires in case of a blowout. The "Blowout Suppression System" (BOSS) consist of finely atomized water injected to the fluid stream of a gas and oil mixture before it exits a release point. The added water lowers the flame temperature and flame velocities thereby reducing the flame stability. In the case where the flame cannot be completely dissipated, the fire intensity is noticeably deceased, preserving structural integrity and allowing manual intervention activities. A precaution in the use of such a device is that, if a gas release fire is suppressed but the flow is not immediately isolated, a gas cloud may develop and exploded that would be more destructive that the pre-existing fire condition. 

The flame velocity—also called the burning velocity, normal combustion velocity, or laminar flame speed—is more precisely defined as the velocity at which unbumed gases move through the combustion wave in the direction normal to the wave surface. 

Later, there were improvements in the thermal theories. Probably the most significant of these is the theory proposed by Zeldovich and Frank-Kamenetskii. Because their derivation was presented in detail by Semenov [4], it is commonly called the Semenov theory. These authors included the diffusion of molecules as well as heat, but did not include the diffusion of free radicals or atoms. As a result, their approach emphasized a thermal mechanism and was widely used in correlations of experimental flame velocities. As in the... 

These theories fostered a great deal of experimental research to determine the effect of temperature and pressure on the flame velocity and thus to verify which of the theories were correct. In the thermal theory, the higher the ambient temperature, the higher is the final temperature and therefore the faster is the reaction rate and flame velocity. Similarly, in the diffusion theory, the higher the temperature, the greater is the dissociation, the greater is the concentration of radicals to diffuse back, and therefore the faster is the velocity. Consequently, data obtained from temperature and pressure effects did not give conclusive results. 

Some evidence appeared to support the diffusion concept, since it seemed to best explain the effect of H20 on the experimental flame velocities of CO—02. As described in the previous chapter, it is known that at high temperatures water provides the source of hydroxyl radicals to facilitate rapid reaction of CO and 02. 

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