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

Experimental and numerical laminar syngas combustion
Heat radiation of the burned gases to the chamber walls are modeled neglecting the
radiation of the particles. This assumption is reasonable due to the high purity of the
tested mixtures. During combustion, heat radiation can have origin on unburned gases,
which contains: H 2 , CO, CH 4 , CO 2 , N 2 and O 2 , and also on the burned gases, which
mainly contains: CO 2 , H 2 O and N 2 , being irrelevant the remaining combustion products.
Only molecules that have a non null dipolar moment are susceptible to emit thermal
radiation (Boust, 2006). Therefore only CO 2 , H 2 O and CH 4 are considered. In practical
terms, the radiation of the unburned gases heated by compression is insignificant
comparatively with the burned gases, which temperature is 6-7 times higher. Thus, it is
assumed that only CO 2 and H 2 O radiates significantly.
tel-00623090, version 1 - 13 Sep 2011
The radiation heat transfer is modeled by the Stefan’s law considering the burned
gases as a grey body with uniform temperature T g and ε the apparent grey-body
emissivity calculated from the contributions of H 2 O and CO 2 . As the spectral
emissivities of these species are similar, the emissivity variation term Δε is included.
ε = ε + ε −Δ ε
(4.25)
CO2 H2O
At the end of combustion, only burned gases are inside the chamber. Then, the net
superficial radiative flow Q ,r received by the wall, with absorption factor α, from the
burned gases is given by the Stefan’s constant.
4 4
( )
Q = αεσ T − T
(4.26)
r g w
When the sphere of burned gases (radius r) does not occupy the entire chamber
(radius R), the sphere surface ratio gives the radiative flow.
2
4π
r
Qr( r)
= Q
2 r
(4.27)
4π
R
The emissivity of H 2 O and CO 2 as well as the variation term are calculated using the
correlation of Leckner, (1972). This correlation reproduces the gases temperature
influence, the partial pressure of each species and the length of the average radius.
4.2.2 Calculation procedure
In the multi-zone model, flame propagation is seen as the consecutive consumption of
unburned mixture within the zones with an equal mass distribution between the zones
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