Flame blow out

It is well known that in a jet flame blow-out occurs if the air-fuel mixture flow rate is increased beyond a certain limit. Figure 18.3 shows the relationship between the blow-out velocity and the equivalence ratio for a premixed flame. The variation of blow-out velocity is observed for three different cases. First, the suction collar surrounding the burner is removed and the burner baseline performance obtained. Next, the effect of a suction collar itself without suction flow is documented. These experiments show that for the nozzle geometry studied, the free jet flame (without the presence of the collar) blows out at relatively low exit velocities, e.g., 2.15 m/s at T = 1.46, whereas for > 2 flame lift-off occurs. When the collar is present without the counterflow, the flame is anchored to the collar rim and blows out with the velocity of 8.5 m/s at T = 4. Figure 18.4a shows the photograph of the premixed flame anchored to the collar rim. The collar appears to have an effect similar to a bluff-body flame stabilizer. The third... 

The combustion stability was increased by porous inserts. The porous layers had enough heat capacity to ignite the spray in the event of flame blow-out due to short interruptions in fuel or air supplies. 

Consider Fig. 4.32, a graph of flame velocity 5L as a function of distance, for a wave inside a tube. In this case, the flame has entered the tube. The distance from the burner wall is called the penetration distance dv (half the quenching diameter dT). If iij is the mean velocity of the gas flow in the tube and the line labeled (7, is the graph of the velocity profile near the tube wall, the local flame velocity is not greater than the local gas velocity at any point therefore, any flame that finds itself inside the tube will then blow out of the tube. At a lower velocity u2, which is just tangent to the SL curve, a stable point is reached. Then u2 is the minimum mean velocity before flashback occurs. The line for the mean velocity % indicates a region where the flame speed is greater than the velocity in the tube represented by in this case,... 

The characteristics of the lifted flame are worthy of note as well. Indeed, there are limits similar to those of the seated flame [1], When a flame is in the lifted position, a dropback takes place when the gas velocity is reduced, and the flame takes up its normal position on the burner rim. When the gas velocity is increased instead, the flame will blow out. The instability requirements of both the seated and lifted flames are shown in Fig. 4.37. 

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... 

The effect of suction velocity on the flame stabilization can be observed in Fig. 18.5, where the suction mass flow rate is kept constant while varying T. For a given there is a limiting velocity ratio —U2/U1) below which the flame will blow out. For example, at = 1, the limiting velocity ratio is about 0.1. At higher values of F, this ratio decreases as shown in the figure. 

Figure 18.8 Variation of strain rate with equivalence ratio for a free jet flame 1 Ur = 0.0, 2 — Ur = 0.17 m/s, 3 — blow-out, and 4 — lift-out...

Broadwell, J. E., W. J. A. Dham, and M. G. Mungal. 1985. Blow-out of turbulent diffusion flames. 20th Symposium (International) on Combustion Proceedings. Pittsburgh, PA The Combustion Institute. 303-10. 

Feikema, D., R-H. Chen, and J. F. Driscoll. 1991. Blow-out of nonpremixed flames Maximum coaxial velocities achievable with and without swirl. Combustion Flame 86 347-58. 

Catalytic combustion has been commercially demonstrated to reduce NO.. emissions to below 3 ppm while keeping CO and UHC emissions below 10 ppm without the need for expensive exhaust clean-up systems. In addition, a catalytic combustor reduces typical DLN problems such as risk of blow-out and flame instability. Also, the economic advantage of primary methods including catalytic combustion as opposed to secondary clean-up measures (SCR and SCONOx) has recently been assessed [1]. 

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