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

Experimental and numerical laminar syngas combustion
this is endorsed to no spherical propagation as shown above by schilieren flame
images.
Tests runs with initial pressures of 0.5, 2.0, 5.0, and in some cases 6.0 bar were also
performed in order to increase the burning velocity range of pressures. At initial
pressure of 7.0 bar and room temperature (293 K) all typical syngas-air mixtures fail to
ignite.
4.1.2.2 Burning velocity
tel-00623090, version 1 - 13 Sep 2011
Figures 4.29 - 4.31 shows stretched burning velocity for typical syngas-air mixtures for
various equivalence ratios using Eq. (4.14). At this stage all the range of points are
shown to emphasize the inaccuracy of the burning velocity calculation due to the
pressure derivative fluctuation in the early stage of flame propagation, where the
pressure does not change to much. After this stage a fast increase in pressure makes
the burning velocity to increase rapidly up to a stable value. Afterwards, a nearly linear
increase in burning velocity is observed. In some cases there is a sudden increase in
burning velocity, which according to Saeed and Stone, (2004) is attributed to the
development of cellular flame. In the final stage burning velocity start to decrease due
to inflection in the pressure curve which gives place to lower pressure derivatives. The
highest pressure derivative defines the upper limit of the inquiry region.
Workers such as Gülder (1984), Metghalchi and Keck, (1982), and Ryan and Lestz
(1980) who used the constant-volume method for the determination of the burning
velocities have assumed that the flame front is smooth, with no cellular or wrinkling
flames. However, cellular flames can be formed under certain conditions, and in the
present study cellularity was found for syngas flames. When cellularity triggers, the
increase in the surface area leads to a sudden increase in the burning velocity and any
inclusion of these data points would lead to higher burning velocity predictions. This is
due to the transformation of the smooth spherical flame front to the polyhedral flame
structures which increase the surface area of the flame front, thereby invalidating the
smooth flame assumption of Eq. (4.14). Therefore, in this work the sudden increase of
burning velocity was removed from the inquiry region.
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