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

Chapter 4
A very good agreement between the correlation and the experimental burning velocity
is found for every initial pressure test runs performed. Notice that for fluidized bed
syngas the lower initial pressure with successful ignition was 0.8 bar. The maximum
error of the burning velocity correlation is 8% for updraft syngas, 9% for downdraft
syngas and 5% for fluidized bed syngas. These errors are perfectly reasonable and the
discrepancy between syngas compositions errors has to do with experimental burning
velocity scattering.
4.2. Multi-zone spherical combustion
tel-00623090, version 1 - 13 Sep 2011
In an engine cycle, the heat losses could be endorsed 25% to wall-flame interaction
and 75% to the wall-burned gases interaction (Boust, 2006). Therefore, robust heat
transfer models of wall-flame interaction should be employed. A multi-zone numerical
heat transfer simulation code developed at the Laboratoire de Combustion et
Détonique for methane-air mixtures by Boust, (2006) is adapted herein to syngas-air
mixtures. The code allows simulating the combustion of homogeneous premixed gas
mixtures within constant volume spherical chamber centrally ignited. In spherical
combustion conditions, the model could also be used for predicting the quenching
distance.
4.2.1 Mathematical model
4.2.1.1 Flame propagation
The flame is considered a perfect sphere without thickness. The propagation of the
flame is imposed by its burning velocity that determines the thickness of the zone to
burn during a predefined time Δt. The combustion is supposed isobaric, being the
burning velocity S u given by the empirical correlation of Metghalchi and Keck, (1980)
expressed by the Eqs. 4.16 - 4.18.
The criterion that defines the end of combustion is the flame quenching distance. The
quenching distance defines how close the flame approaches the wall and plays a
definitive role on the wall-flame interaction as shown by Boust, (2006). The quenching
implementation in the computational code consists of stopping the propagation of the
flame when it approaches the wall in an equal distance to the frontal quenching
distance δ q . In order to estimate the quenching distance, the correlation of Westbrook
et al., (1981) is used:
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