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

Experimental and numerical laminar syngas combustion
In order to quantify the thermal losses of different mixtures special care must be taken
in the heat flux integration. The integration time should capture the entire phenomenon.
The cooling time is possible to control by establishing a certain cooling period.
However, the combustion time changes with the fuel, initial pressure and equivalence
ratio, which calculation is not straightforward. Therefore, the comparison of heat flux
estimative already shown in figures 4.41-4.49 is made based on the heat flux peaks
(Table 4.4).
Table 4.4 – Heat Flux peaks for stoichiometric syngas-air and methane-air mixtures.
tel-00623090, version 1 - 13 Sep 2011
Qw (kW/m 2 ) Updraft Downdraft Methane (Boust, 2006)
1.0 bar, φ=0.8 216 kW/m 2 350 kW/m 2 -
1.0 bar, φ=1.0 368 kW/m 2 402 kW/m 2 649 kW/m 2
1.0 bar, φ=1.2 220 kW/m 2 313 kW/m 2 -
5.0 bar, φ=1.0 777 kW/m 2 949 kW/m 2 (*) 1622 kW/m 2
(*) linear interpolation.
From Table 4.4 some conclusions can be drawn:
- The heat flux increases with initial pressure increase for all cases. The amount
(volume and energy) of the explosive mixture inside the vessel increases with
initial pressure increase.
- The heat flux increases with the heat value of the fuel-air mixture. In this case,
one should also take into account the air-fuel ratio.
- The equivalence ratio has the influence of decreasing the heat flux compared to
stoichiometric mixtures and follows the behavior of pressure peak.
The thickness of wall quench layers is a primary source of unburned fuels (Saeed and
Stone, 2004). Thus, this combustion characteristic is very important and was predicted
for stoichiometric updraft and downdraft syngas compositions by the Westbrook
criterion defined by a zero flame stretch (figure 4.51). This is practically the case when
the flame reaches the wall given the large curvature radius. The Westbrook criterion is
valid in pressure range 1-40 bar and burning velocity correlation used in the code
shows to be valid up to 33 bar, thus one impose this limit to quenching distance
estimation.
136