Combustion waves

Detonation. In a detonation, the flame front travels as a shock wave, followed closely by a combustion wave, which releases the energy to sustain the shock wave. The detonation front travels with a velocity greater than the speed of sound in the unreacted medium. 

Another important concept is that of the critical ignition volume. During the propagation of the combustion wave, the flame volume cannot continually grow beyond a critical value without an additional supply of energy. The condition that controls the critical volume for ignition is reached when the rate of increase of flame volume is less than the rate of increase of volume of the combustion products. In this condition a positive exchange of heat between the flame and the fresh mixture is achieved. 

A unified statistical model for premixed turbulent combustion and its subsequent application to predict the speed of propagation and the stmcture of plane turbulent combustion waves is available (29—32). 

Design of explosion suppression systems is clearly complex, since the effectiveness of an explosion suppression system is dependent on a large number of parameters. One Hypothesis of suppression system design identifies a limiting combustion wave adiabatic flame temperature, below which combustion reactions are not sustained. Suppression is thus attained, provided that sufficient thermal quenching results in depression of the combustion wave temperature below this critical value. This hypothesis identifies the need to deliver greater than a critical mass of suppressant into the enveloping fireball to effect suppression (see Fig. 26-43). 

Chemical explosions are uniform or propagating explosions. An explosion in a vessel tends to be a uniform explosion, while an explosion in a long pipe is a propagating explosion. Explosions are deflagrations or detonations. In a deflagration, the burn is relatively slow, for hydrocarbon air mixtures the deflagration velocity is of the order of 1 m/s. In contrast, a detonation flame shock front is followed closely by a combustion wave that releases energy to sustain the shock wave. A... 

Detonation explosions are similar to combustion explosions and are exothermic reactions that proceed into the unreacted material at a velocity much greater than the speed of sound in an unreacted material and are accompanied by a flame front shock wave in the material followed closely by a combustion wave that releases the energy and sustains the shock wave at extremely high pressure [39] [40]. In hydrocarbons, the velocity can reach 6,000-9,000 ft/sec. 

Piane Combustion Wave. See Combustion Wave Propagation in Vol 3, C433-R Ref J.H. Burgoyne F. Weinberg, A Method of Analysis of a plane Combustion Wave , 4th SyropGombstn, Williams Wilkins, Baltimore (1953), 294-302. ... 

G.H. Markstein. Instability phenomena in combustion waves. Proceedings of the Combustion Institute, 4 44-59, 1952. 

In the 1950s, the more descriptive schlieren records of the interactions between pressure waves and deflagration fronts were obtained [16-18], and Oppenheim [9] introduced the hypothesis of the "explosion in the explosion" (of the detonating mixture) occurring in the regime of accelerating flame to explain the sudden change in the velocity of the combustion wave observed in the experiments. 

These nitridation reactions are also characterized by very high values of activation energy. As a result, the nitridation front represents a strongly nonlinear combustion wave which may be extremely thin and corrugated. 

Figure 6.12. Self-propagating high-temperature synthesis of NiAl (Fan etal. 2001). The temperature- time profile observed during the passage of a combustion wave is shown.

FIGURE 4.1 Combustion wave fixed in the laboratory frame. 

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. 

In the introduction to this chapter a combustion wave was considered to be propagating in a tube. When the cold premixed gases flow in a direction opposite to the wave propagation and travel at a velocity equal to the propagation velocity (i.e., the laminar flame speed), the wave (flame) becomes stationary with respect to the containing tube. Such a flame would possess only neutral stability, and its actual position would drift [1], If the velocity of the unbumed mixture is increased, the flame will leave the tube and, in most cases, fix itself... 

Explosion is a term that corresponds to rapid heat release (or pressure rise). An explosive gas or gas mixture is one that will permit rapid energy release, as compared to most steady, low-temperature reactions. Certain gas mixtures (fuel and oxidizer) will not propagate a burning zone or combustion wave. These gas mixtures are said to be outside the flammability limits of the explosive gas. 

Depending upon whether the combustion wave is a deflagration or detonation, there are limits of flammability or detonation. 

In general, the combustion wave is considered as a deflagration only, although the detonation wave is another class of the combustion wave. The... 

When an explosive gas mixture is placed in a tube having one or both ends open, a combustion wave can propagate when the tube is ignited at an open end. This wave attains a steady velocity and does not accelerate to a detonation wave. 

One of the problems in combustors that utilize premixed flames is the attainment of stable performance over an extended range of operation (turndown ratio). The condition, at which the combustion wave is driven back causing the flame to be extinguished when the flow velocity exceeds the burning velocity everywhere in the flow field, is of particular interest to this study. The physical mechanisms responsible for the blow-out limits and flame stabilization of jet flames is still a topic of extensive research [1, 2]. The flame stabilization technique discussed in this paper is aimed to control the velocity gradient in the region close to... 

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