28
Figure 22: Comparison of extinction limits predicted by 2D and 3D simulations with 3
different channel heights (2.85 cm, 5 cm, and 10 cm).
The difference in taller combustor (10 cm) is due to different heat loss rate of the flame
located in different height. Local flames along Z-direction are subject to different
temperature gradients, and the closer the flame to the insulation, the larger the
temperature gradient. Therefore, the flame close to the insulation is easier to be
extinguished than the flame close to the symmetry plane. Since 2D model is not capable
to reflect the difference of local heat loss in Z-direction, the taller the combustor, the
larger the discrepancy between 2D and 3D predictions. Also, when the heat release is
smaller (i.e. low Re), the heat loss effect becomes more significant. Therefore, larger
discrepancy can be found at lower Re (Fig. 22).
This explanation can be supported by examining the flame structure in 2D and 3D
models. Figure 23 shows reaction contours (Re = 110, at limits) for 2D models with 3
different height’s out-of-plane heat loss setting. Their reaction structures look very
similar at limit conditions. Figure 24 shows their corresponding 3D version in both slices
0.15
0.2
0.25
0.3
0.35
100 1000
3D 2.85 cm
2D 2.85 cm
3D 5 cm
2D 5 cm
3D 10 cm
2D 10 cm
Equivalence Ratio
Re