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

Experimental and numerical laminar syngas combustion
across the flame, increasing with increasing density ratio, σ. Thus σ is probably the
most sensitive parameter controlling the onset of hydrodynamic instability. Next to σ,
the flame thickness, δ, is also expected to have a strong influence on the hydrodynamic
instability, for two reasons. First, it measures the influence of curvature which, being
positive for the outwardly propagating spherical flame, has a stabilizing effect on the
cellular development. The thinner the flame, the weaker is the influence of curvature
and consequently the stronger is the destabilizing propensity. The second influence is
that it controls the intensity of the baroclinic torque developed over a slightly wrinkled
flame surface, which depends on the density gradient across the flame and the
pressure gradient along the flame (Sun et al., 1999).
tel-00623090, version 1 - 13 Sep 2011
For the development of the diffusional-thermal instability, an appropriate parameter
representing the effect of non-equidiffusion is the flame Lewis number, L e [Markstein,
(1951); Matalon and Matkowsky, (1982); Law and Sung, (2000)]. It is well established
and understood theoretically that unstretched flames are diffusionally unstable (stable)
for L e that are smaller (greater) than a value slightly less than unity. However, the
calculation of effective Lewis number for a multi-component fuel mixture is not as
straightforward as for pure fuel-air mixtures and is a subject that is out of the scope of
the present work.
4.1.1.2 Flame Radius
Fig. 4.16 gives the variations of flame radius versus the time for syngas-air mixtures.
The study shows that flame expands spherically after the ignition, and the flame radius
will increase rapidly in the subsequent process. There are a quasi-linear correlation
between flame radius and time for all syngas cases. The higher gradient of radius-time
curves is obtained for stoichiometric syngas-air mixtures. The lean (φ=0.8) and rich
(φ=1.2) syngas-air mixtures have similar gradients in the updraft case. This difference
increases for the downdraft syngas case. Fluidized syngas has the lowest radius-time
curve gradients. Information for rich mixture of fluidized syngas is missing due to
ignition difficulties.
According to the Markstein view the gradient of the radius-time curve reflects the
stretching effectiveness of flame. For an unstable flame, the gradient of the radius-time
curve will decrease with the flame expansion, while for a stable flame; the gradient of
the radius-time curve will increase with the flame expansion. The quasi-linear
100