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

Experimental and numerical laminar syngas combustion
4.1.1.3 Flame speed
Fig. 4.17 gives the stretched flame speed versus the flame stretch rate for three typical
syngas compositions. Removing the parts influenced by ignition energy and high
pressure at late stages of flame propagation. A linear correlation between the stretched
flame speed and the flame stretch rate is found. The unstretched flame speed is
obtained by extrapolating the line to κ= 0, while the gradient of the S n –κ curve gives the
value of the Markstein length.
tel-00623090, version 1 - 13 Sep 2011
The maximum value of syngas-air flame speeds is presented at the stoichiometric
equivalence ratio, while lean or rich mixtures decrease the flame speeds. Downdraft
syngas composition shows the highest flames speeds for all the equivalence ratios
considered. The stretched flame speed increases with the increase of flame stretch
rate for lean (φ=0.8) syngas-air mixtures. This behavior remains for stoichiometric and
rich (φ=1.2) mixtures in the case of updraft and downdraft compositions.
A remarkable decrease of the stretched flame speed versus the stretch rate is
demonstrated for stoichiometric fluidized composition. This might result from the
occurrence of incomplete combustion.
Positive gradients of S n –κ curves are derived for three equivalence ratios in the updraft
and downdraft syngas cases. A positive gradient of S n –κ curve is also shown by the
lean fluidized syngas. These correspond to negative values of Markstein number,
indicating the instability of flames.
On the other hand, positive values of Markstein number indicate stability of flames of
mixture combustion. These correspond to stoichiometric fluidized syngas.
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