Flame blowing

A peak velocity through the flare end (tip) of as much as 0.5 mach is generally considered a peak, short term. A more normal steady state velocity of 0.2 mach is for normal conditions and prevents flare/lift off [58]. Smokeless (with steam injection) flare should be sized for conditions of operating smokelessly, which means vapor flow plus steam flow [33c]. Pressure drops across the tip of the flare have been used satisfactorily up to 2 psi. It is important not to be too low and get flashback (without a molecular seal) or blowoff where the flame blow s off the tip (see Ref. 57), Figure 7-71. 

Theoretical studies are primarily concentrated on the treatment of flame blow-off phenomenon and the prediction of flame spreading rates. Dunskii [12] is apparently the first to put forward the phenomenological theory of flame stabilization. The theory is based on the characteristic residence and combustion times in adjoining elementary volumes of fresh mixture and combustion products in the recirculation zone. It was shown in [13] that the criteria of [1, 2, 5] reduce to Dunskii s criterion. Longwell et al. [14] suggested the theory of bluff-body stabilized flames assuming that the recirculation zone in the wake of the baffle is so intensely mixed that it becomes homogeneous. The combustion is described by a second-order rate equation for the reaction of fuel and air. 

The available criteria of flame blow-off do not provide complete understanding of the phenomenon. There is no evidence of the effect of multiple variables on flame stability mentioned above. 

As mentioned in section 12.1, Dunskii [12] was the first who put forward the phenomenological theory of flame stabilization. The theory is based on the characteristic residence time, L, and combustion time, tc, in adjoining elementary volumes of fresh mixture and combustion products in the recirculation zone behind the bluff body. Dunskii s condition for flame blow-off is U/tc = Mi, where Mi is the Mikhelson number close to unity (for example, for cone flame holder the measurements give Mi = 0.45 [36]). Residence time L is taken proportional to the flame holder size, H, and inversely proportional to the approach flow velocity, U, i.e., L = H/U. Combustion time is estimated as tc = at/Si, where... 

Basevich, V. Ya., and S. M. Kogarko. 1972. Comparison of limiting flame blow-off velocities for different types of stabilizers. Sov. J. Combustion, Explosion, Shock Waves 8(4) 582. 

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. 

If burning velocity exceeds gas velocity, the flame flashes back if the burning velocity is less than gas velocity, (he flame blows off into the atmosphere and becomes extinct. The flame establishes itself when the velocities are equal... 

Interestingly, the high cycle of heat release or simply the hot spot in this case closely follows the vortex development. The main difference from the previous case was that the inlet velocity was much lower, while the equivalence ratio was higher. In other words, the convective time scale wtis less and the chemical time scale was higher. This suggests that one of the important parameters to consider in identifying the proper location of controlled fuel injection is the Damkohler number. Near the flame blow-off limit, however, one would expect the hot spot to lag the vortices slightly. 

Investigations of the diffusion flame stabilization occurring at subsonic efflux of a round fuel stream in unperturbed air have proposed two mechanisms of flame blow-off. 

Note 12 Warning—Proper operating procedures are required for safety as well as for reliability of results. An explosion can result from flame blow-back unless the correct burner head and operating sequence are used. 

---