Flame Stabilization

 In order to accomplish commercial combustion, the supply velocity of the reactant mixture is desired to be extremely high; it is not unusual for this velocity to be as high as 10 X the maximum possible laminar flame speed of a given mixture.  Experience shows that the flame is blown away when the supply velocity exceeds the flame speed

 The maximum supply velocity with which fresh mixture may be brought to the flame front without blowing it away is known as blow ‑ off velocity  This important limiting velocity depends on the nature of the fuel and oxidant, their ratio, mixture temperature, combustion chamber pressure, turbulence, burner geometry, burner wall roughness, temperature, etc.

(a) Stability of a Bunsen Flame  Figure 8.8 indicated the mechanism which determines the shape of a Bunsen flame. This mechanism can explain the phenomena of blow ‑ off and flash ‑ back.  Figure 8.26 shows flame speed and normal component of the supply velocity u s in four different situations. Only the region close to the wall is shown since it is here that the flame is anchored to the burner.

 When u 0 > u s, the flame propagates into the barrel to bring about flash ‑ back. When the flow is very strong, u s > u 0, the flame blows ‑ off. The critical criterion for blow ‑ off arises when the u 0 and u s profiles are tangential  The velocity gradient (at the barrel wall near its mouth) has a definite effect on the flame stability; if it is too small, the flame flashes back; if it is too large, blow ‑ off occurs  In fact, measurements show the critical velocity gradient correlates well with the fuel oxidant ratio as qualitatively shown in Figure 8.27.

(b) Stabilization of Flames  The velocities encountered in modern propulsion engines and power plant burners are so high that the flame has to be stabilized by some artificial means.  Considering blow ‑ off as a situation arising to allow the residence time of the reactions to proceed to ignition, one may devise various possible flame stabilizers. Velocity is slowed down, flow remains high.  Three types of flame stabilizers are extensively known. These are: stabilization by pilot flames, by bluff bodies, and by recirculation

(b.1) By Pilot Flames  Suppose a pilot flame (as hot inert gas in Fig 8.28) is held adjacent to the cold reactant mixture flow issuing in the form of a high velocity jet.  Heat and mass are transferred across the boundary of the two streams by diffusion and mixing.  The reaction rate in the cold reactant mixture is thus enhanced shown in Figure 8.28 (Marble and Adamson).  Blow ‑ off would occur if the flow rate > the reaction rate in reactant mixture. Limiting blow-off velocity for a sustained flame is .

 The mass flow rate through the jet of area A is A  u s gm/sec.  is flame speed eigenvalue, which is functions of Lewis number and the shape of reaction rate-temp plot. Theoretically it is equal to slightly < ½. K is the mean conductivity of the mixture, C is the specific heat, V is the volume of jet.

(b.2) By Bluff Bodies  When a blunt body is placed in a high velocity reactant stream, the flow is greatly slowed down at the forward stagnation point (see Figure 8.29) to give ample opportunity for reactions to proceed to ignition.  One major disadvantage of solid bluff bodies is the drag they exert on the flow and the resultant loss of thrust.

 Campbell overcomes this drawback by employing an opposing gaseous jet in the reactant stream. It evolves as the opposing jet. The stream is slowed down and the flame is anchored as schematically shown in Figure 8.30  The blow ‑ off velocity is increased by increasing the injection pressure of the opposing jet and by increasing the temperature of the opposing jet gas.

(b.3) By Recirculation  When the solid bluff body discussed above is of finite length in the direction of flow, the pressure distribution prevents the high velocity flow from keeping attached to the solid surface.  Increasing pressure separates the boundary layer and causes eddy shedding in the "wake."  Under sufficiently fast flow conditions a (symmetric) recirculation pattern of flow is established behind the blunt body as shown in Figure The recirculation zone provides a station where reactions can take place.