Nanduri et al. [51] reported the existence of 2-D propagating cellular flames in counter- flow configuration at Da beyond 1-D radiation-induced extinction limit, DaE,R. In this section, therefore, we investigate the dynamics of flame cells which survive beyondDaE,R by performing 2-D simulations of opposed tubular flames with the C-shaped and asym- metric IC’s. Propagating flame cells can be generated as follows; two IC’s are generated from the 1-D steady solution at DaE,P = 6.27×107 and then, 2-D simulations with the IC’s are performed at Da beyond DaE,R as well as DaE,P.
Figure 3.8 shows the temporal evolution of edge flames originated from the C-shaped IC at Da = 6.0 ×108 beyond DaE,R = 9.17 ×107. The initial C-shaped flame shrinks almost immediately after the start of the simulation due to excessive radiative heat loss.
Meanwhile, two relatively-strong reaction fronts develop at both ends of the flame and a diffusion flame in the middle becomes weak as shown in Fig. 3.8a. Then, the flame splits into two edge flames which propagate towards each other (see Figs. 3.8b and c). Finally, they collide each other, leading to total extinction (see Figs. 3.8d). If an edge flame exists beyond DaE,R somehow as in Fig. 3.8, it loses its diffusion flame tail due to excessive radiative heat loss enough to extinguish the corresponding 1-D steady flame such that only its flame edge can survive, at which the reaction enhanced by both the focusing effect of fuel with small LeF and large Da compensates for the excessive radiative heat loss. In addition, an edge flame has propagating characteristics by nature such that the edge flames developed from a cellular flame keep propagating toward unburned mixture with positive flame speed.
The dynamics of propagating edge flames is quantified by evaluating the propagation speed of the flame cells. The propagation speed of the edge flames is obtained by evalu- ating a displacement speed which measures the velocity of a scalar isocontour (e.g. tem- perature and fuel/oxidizer mass fractions) relative to local flow velocity. In this chapter, the azimuthal flow velocity is zero and hence, the displacement speed directly represents
Figure 3.8: Temporal evolution of flame cells originated from the C-shaped IC at Da = 6.0 ×108 with I = 10−8: (a) t = 1, (b) 3, (c) 9, and (d) = 12.
the propagation speed of the edge flames along the azimuthal direction. In the context of constant density, the displacement speed, Sd, can be defined by [89–92, 94, 97, 100]:
Sd =SdR+Sd,rD +Sd,θD = 1
|∇T|
ω+ 1 r
∂
∂r
r∂T
∂r
+ 1 r2
∂2T
∂θ2
, (3.9)
where SdR, Sd,rD, and Sd,θD represent the net heat release or the sum of heat release and radiative heat loss,r−andθ−directional diffusion components ofSd, respectively. In this form, therefore, each component of the displacement speed is a measure of its correspond- ing contribution to the edge flame propagation. Note that in our previous study [100], it was found thatSdexhibits positive values when an edge flame is generated or propagates towards unburned mixture while negative Sd appears at locations where local extinction occurs.
Figure 3.9 shows the temperature isocontours of the edge flames and the corresponding 1-D profiles of the displacement speed and its components along the maximum heat release plane at different times. As discussed above, the reaction fronts in Fig. 3.9a have positive Sd while the diffusion flame in the middle exhibits negative Sd due to relatively-strong r−directional diffusion, which verifies that edge flames start to form at two flame edges and local flame quenching starts to occur in between. After that, two propagating edge flames exhibit large positiveSdat the front and large negativeSdat the rear (see Figs. 3.9b and c), which implies that the edge flames is composed of a thin relatively-strong reaction front and a relatively-weak reaction tail. At last, they are extinguished by the head-on collision, manifested in negativeSd throughout the flames (see Fig. 3.9d).
Figure 3.9: Temperature isocontours (top) and the corresponding 1-D profiles of the displacement speed,Sd, (left) and its components (right) along the maximum heat release plane (bottom) from two edge flames at Da = 6.0×108 withI = 10−8: (a) t = 1, (b) 3, (c) 9, and (d) 12.
Figure 3.10: Temporal evolution of the tubular flames with the asymmetric IC at (a) Da
= 5.0 ×108 and (b) 6.0 ×108 with I = 10−8.
To verify the existence of a rotating edge flame, we performed additional 2-D sim- ulations with the asymmetric IC. Figure 3.10 shows different snapshots of edge flames originated from the asymmetric IC at Da = 5.0 ×108 and 6.0 ×108. At Da = 5.0
×108, similar to the result with the C-shaped IC, two edge flames develop first from the asymmetric IC and total extinction occurs by their head-on collision. At Da = 6.0
×108, however, a relatively-weak reaction front fails to develop into an edge flame while a relatively-strong reaction front survives and keep rotating with constant propagation speed.
To further identify the characteristics of the rotating flame cell, the maximum flame temperature as a function of Da from the asymmetric IC are shown in Fig. 3.11. The displacement speed, Sd, and its components of the rotating flame cell are also shown in the figure. When Da is relatively low (DaC,C < Da ≤ 1.0×108) and the consequent radiative heat loss is not high enough, a tubular flame with two edges develops first even from the asymmetric IC, subsequently evolving into a stationary cellular flame, similar to that in Fig. 3.7. WhenDais relatively large (1.0×108 < Da≤5.7×108), two flame cells develop from the asymmetric IC and total extinction occurs by their head-on collision as shown in Fig. 3.10a. However, when Da is large enough (5.7 ×108 < Da ≤ 1.5 ×109),
Da
T
maxS
dand com pone nt s
107 108 109
0.4 0.8 1.2
0 50 I = 1.0×10-8 100
DaC,C DaE,R
Sd SRd SDd,r SDd,θ 1-D
2-D
Stationary cellular flame
A rotating flame cell
Extinction by collision
Figure 3.11: The maximum flame temperature,Tmax, as a function of Damk¨ohler number, Da, from the asymmetric IC for I = 10−8 together with the displacement speed and its components of rotating flame cells.
only a relatively-strong flame cell survives and keeps rotating as shown in Fig. 3.10b.
When Da > 1.5 ×109, however, the surviving flame cell becomes unstable due to large radiative heat loss and consequently, a small flame cell falls apart from it. Ultimately, the total extinction occurs by the head-on collision of two flame cells similar to that in Fig. 3.10a.
To elucidate the propagating characteristics of the rotating flame cells at high Da, we evaluated their steady displacement speed and its components as shown in Fig. 3.11.
Note that they were measured at the maximum net heat release point using Eq. 3.9.
It is readily observed from the figure that as Da increases from 6.0 ×108 to 1.5 ×109, Sd increases from 7.244 to 9.260 due to the increase of SdR. In this regime, the size of the rotating flame cell decreases with Da and as such, the focusing effect of fuel with small LeF is enhanced, which increases both of the maximum local heat release and the consequentSdR. However,Tmaxis slightly decreased from 0.862 to 0.819 by large radiative heat loss despite the increase of the maximum local heat release. These results imply that the rotating flame cell can survive such large radiative heat loss by reducing its size and consequently increasing its local heat release and flame speed.