Flame stabilization in high-velocity streams

The values of laminar flame speeds for hydrocarbon fuels in air are rarely greater than 45cm/s. Hydrogen is unique in its flame velocity, which approaches 240cm/s. If one could attribute a turbulent flame speed to hydrocarbon mixtures, it would be at most a few hundred centimeters per second. However, in many practical devices, such as ramjet and turbojet combustors in which high volumetric heat release rates are necessary, the flow velocities of the fuel-air mixture are of the order of 50m/s. Furthermore, for such velocities, the boundary layers are too thin in comparison to the quenching distance for stabilization to occur by the same means as that obtained in Bunsen burners. Thus, some other means for stabilization is necessary. In practice, stabilization 

FIGURE 4.52 Stabilization methods for high-velocity streams (a) vee gutter, (b) rodoz sphere, (c) step or sudden expansion, and (d) opposed jet (after Strehlow, Combustion Fundamentals, McGraw-Hill, New York, 1985). 

Recirculation of combustion products can be obtained by several means (1) by inserting solid obstacles in the stream, as in ramjet technology (bluff-body stabilization) (2) by directing part of the flow or one of the flow constituents, usually air, opposed or normal to the main stream, as in gas turbine combustion chambers (aerodynamic stabilization), or (3) by using a step in the wall enclosure (step stabilization), as in the so-called dump combustors. These modes of stabilization are depicted in Fig. 4.52. Complete reviews of flame stabilization of premixed turbulent gases appear in Refs. [66, 67], 

The types of obstacles used in stabilization of flames in high-speed flows could be rods, vee gutters, toroids, disks, strips, etc. But in choosing the 

Turbulent heat and Percentage of mass transfer across combustion completed 

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In the discussion of premixed turbulent flames, the case of infinitely fast mixing of reactants and products was introduced. Generally this concept is referred to as a stirred reactor. Many investigators have employed stirred reactor theory not only to describe turbulent flame phenomena, but also to determine overall reaction kinetic rates [23] and to understand stabilization in high-velocity streams [62], Stirred reactor theory is also important from a practical point of view because it predicts the maximum energy release rate possible in a fixed volume at a particular pressure. 

Williams, G.C., H.C. Hottel, and A. C. Scurlock. 1949. Flame stabilization and propagation in high-velocity gas streams. 3rd Symposium on Combustion, Flame and Explosion Phenomena Proceedings. Baltimore The Williams and Wilkins Co. 21-40. 

Scarlock, A. C., Flame Stabilization and Propagation in High Velocity Gas Streams, ... 

Utilizing a forced-draft fan, the burner has a gas head arranged to mix the fuel and air in a blast tube which controls the stability and shape of the flame. Gas exits from nozzles or holes in the head and is mixed partly in the high-velocity air stream and partly allowed to exit into an area downstream of a bluff body. Behind the bluff body, a relatively quiescent zone forms which provides a means for flame stability. Many configurations exist, but the most... 

Let us now consider continuous flows of premixed combustible gases and address the question of conditions necessary to retain a flame in the system [2]. This question is of practical significance for many power-production devices. To achieve high power densities, gas velocities in combustors exceed flame velocities, and so means must be found to stabilize flames against blowout, a condition at which the flames are transported through the exit of the burner so that combustion ceases. There are two main classes of stabilization techniques, stabilization by fluid streams and stabilization by solid elements. Although other stabilization methods may be envisioned, such as continuous or intermittent deposition of radiant or electrical energy, in the vast majority of practical continuous-flow systems, stabilization is obtained by techniques that fall within one of the two main classes. Stabilization by solid elements will be discussed first then stabilization by fluid streams will be considered. ... 

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