Bluff-body flames

F.H. Wright and E.E. Zukowsky 1962, Flame spreading from bluff-body flame holders, Proc. Combust. Inst. 8 933-943. 

FIGURE 4.55 Flame-spreading interaction behind multiple bluff-body flame stabilizers. 

From these correlations it would be natural to expect that the maximum blowoff velocity as a function of air-fuel ratio would occur at the stoichiometric mixture ratio. For premixed gaseous fuel-air systems, the maxima do occur at this mixture ratio, as shown in Fig. 4.56. However, in real systems liquid fuels are injected upstream of the bluff-body flame holder in order to allow for mixing. Results [60] for such liquid injection systems show that the maximum... 

Bluff-body flame stabilization in nonpremixed and partially premixed gaseous systems is complicated by the mixing of fuel and oxidizer. In addition to the aspects considered above, it is necessary to control the fuel distribution in the burner. 

Thus, theoretical modeling of bluff-body flame stabilization cannot yet compete with the experimental approaches. Partially, it is due to the fundamental problems relevant to the turbulent combustion theory. The underestimation of the role of theory is also due to the lack of systematic studies based on existing models. 

Figure 12.3 shows some computational examples of nonreactive and reactive turbulent flows in a combustor with the bluff-body flame holder. The size of the combustor in Fig. 12.3 is 35 x 8 cm. The characteristic height and length of the bluff body is H = 2 cm. The left boundary is set as inlet, right boundary as outlet, and the upper and lower boundaries as rigid walls. 

Davies, T. W., and J. M. Beer. 1971. Flow in the wake of bluff-body flame stabilizers. 13th Symposium (International) on Combustion Proceedings. Pittsburgh, PA The Combustion Institute. 631-38. 

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... 

Limitations on temperatures of solid materials often cause the methods of stabilization by solid elements, discussed so far, to be impractical. In most applications of stabilization by solid elements the flame is attached in the wake behind the element, so that the solid is not fully exposed to the flame temperature. Representative examples are bluff-body flame stabilizers, such as the stabilizing rods or plates placed normal to the flow in ramjets and afterburners, which were mentioned in Sections 5.1.1 and 10.3.5. A distinctive feature of bluff-body flame stabilization is the presence of a recirculation zone behind the body. Unlike the alternate vortices shed from bluff bodies in cold flow over the Reynolds-number range of practical interest, a well-defined vortex, steady in the mean, is observed to exist just downstream from the stabilizer when combustion occurs. This is a toroidal vortex for an axisymmetric stabilizer or a pair of identical counterrotating line vortices for rodlike stabilizers. The reason for the drastic change in the... 

Sandia National Laboratories TNF Workshop website. Bluff body flames (2015). www.sandia.gov/TNF/bluffbod.html... 

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