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

Chapter 4
4.2.3.4 Quenching distance and heat flux estimations
After validating the numerical code for updraft and downdraft syngas compositions,
under various conditions of pressure and equivalence ratios, one can then use it to
predict the quenching distance and heat flux of syngas-air mixtures.
As flame propagates from the centre of the chamber, pressure increases, and
consequently the temperature also increases. This is the compression phase (1) in the
figure 4.50. The heat transfer from the unburned gases to the wall, manly by
conduction, but also marginally by radiation. The heat flux through the wall increases
up to the flame quenching.
tel-00623090, version 1 - 13 Sep 2011
During wall-flame interaction, the flame transfer in average one third of its thermal
power to the wall (Boust, 2006), which makes a heat flux peak to appear. The flame is
quenched at a finite distance to the wall, the quenching distance. It remains, therefore,
a thin layer of unburned gases between the burned gases and the wall. The instant of
the heat flux peak is less reproducible than its amplitude. In fact, such instant is
somewhere between the inflexion point of the pressure curve and the instant of
maximum pressure. During this phase (2), the wall-flame interaction is gradually
dispersed throughout the chamber, which explains the inflexion point in the pressure
curve. The existence of the phase (2) indicates that combustion is not strictly spherical.
After the peak of pressure, the combustion phase gives place to the cooling phase (3).
The wall heat losses are now only due to the burned gases heat source. The heat
transfer is made through the thin layer of unburned gases between the burned gases
and wall.
1.0
P
(1)
(2)
(3)
P (MPa) ; Qw (MW/m 2 )
0.8
0.6
0.4
0.2
Qw
--------- d2P/dt2
0.0
0 50 100 150 200 250 300
Time (ms)
Figure 4.50 – Combustion development in spherical chamber fluidized bed syngas.
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