Etude de la combustion de gaz de synthèse issus d'un processus de ...

Experimental and numerical laminar syngas combustion
1.4
1.2
Su/S 0 u
1.0
0.8
φ
1.0
0.8
0.6
0.00 0.05 0.10 0.15 0.20 0.25 0.30
Karlovitz number
Figure 4.22 – Evolution of the laminar burning velocity versus Karlovitz number for fluidized bed
syngas-air mixture at different equivalence ratios and 1.0 bar.
The linear behavior of the normalized burning velocity with Karlovitz number supports
tel-00623090, version 1 - 13 Sep 2011
that the Markstein number is independent of the Karlovitz number as it was verified by
Aung et al. (1997). Also the generally low Karlovitz numbers obtained for all the typical
syngas compositions and equivalence ratios indicates small influence of stretch rate on
the syngas-air flames.
As it was pointed out by Aung at al., (1997) and Bradley et al., (1996) the Markstein
length is a fundamental property of premixed laminar flames and it is necessary to
measure it precisely. Table 4.2 shows Markstein lengths and Markstein numbers for
syngas-air mixture at various equivalence ratios. If Ma0, It is in the stable regime (Law,
2006). If Ma=0, the flame is neutrally stable, and S u = S 0 u
at all values of stretch rate.
Table 4.2– Markstein lengths and Markstein numbers versus equivalence ratio of syngas-air
mixtures at 1.0 bar and 293 K.
Eq.
Updraft Downdraft Fluidized bed
Ratio L b L u Ma L b L u Ma L b L u Ma
φ=0.6 -2×10 -3 -4.1×10 -4 -1.55 -9×10 -4 -1.9×10 -4 -1.4 n.d. n.d. n.d.
φ=0.8 -7×10 -4 -1.3×10 -4 -1.21 -4×10 -4 -7.4×10 -5 -0.84 -4×10 -4 -7.5×10 -5 -0.5
φ=1.0 -7×10 -4 -1.2×10 -4 -1.39 -5×10 -4 -8.7×10 -5 -1.17 8×10 -4 1.4×10 -4 1.18
φ=1.2 -5×10 -4 -8.9×10 -5 -0.90 -4×10 -4 -7.4×10 -5 -0.94 n.d. n.d. n.d.
Updraft and downdraft syngas shows to be in the preferential diffusion instability
regime as the Markstein number is negative for all cases. The flame can be seen to be
neutrally stable for fluidized bed syngas somewhat between φ=0.8 and φ =1.0 as the
Markstein number changes from a negative to a positive value.
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