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

Experimental set ups and diagnostics
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6
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(1) (2)
(3)
Pressure (bar)
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2
1
0
Shot 1
Shot 2
Shot 3
0 10 20 30 40 50 60 70 80 90 100
Time (ms)
Figure 3.8 - Pressure versus time in rectangular chamber
tel-00623090, version 1 - 13 Sep 2011
Three combustion stages can be observed from the pressure versus time curves
shown in the figures 3.7 and 3.8: (1) development of a flame kernel followed by flame
propagation at constant pressure; (2) flame development followed by rapid pressure
increase and (3) flame quenching followed by burned gases cooling.
The first stage, that has a changeable duration in accordance with the initial conditions
of the mixture, corresponds to the formation of the initial flame kernel. The ignition is
provoked by a spark of duration of approximately 5 ms and allows inflaming the mixture
creating a flame kernel that develops at sensibly constant pressure. During this phase,
flame development could be difficult. In one hand, the reduced flame radius makes the
curvature effect more significant. On the other hand, the electrodes can provoke the
quenching of the flame due to heat absorption. In the second stage, a stable flame is
established. It begins when the pressure starts to increase and ends when the
pressure reaches its maximum value. Fisson et al. (1994), shown that a well
established flame is obtained when the pressure reaches 1.5 times the initial pressure.
At the end of this stage, the flame approaches the wall magnifying the heat losses,
which can violate the hypothesis of adiabatic compression. For this reason, the
maximum pressure P max is lower than the adiabatic pressure P v which would be
obtained if the process were adiabatic throughout all this phase. The third and last
phase represents the end of combustion, where the flame reaches the wall nonuniformly.
On the double effect of the thermal losses and the deactivation of the free
radicals, the flame is quenched followed by the burned gases cooling.
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