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

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

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Experimental study of engine-like turbulent combustion -5.0 ms -3.75 ms -2.5 ms -1.25 ms TDC 1.25 ms 2.5 ms 3.75 ms 5.0 ms 6.25ms (a) -6.25 ms - 5.0 ms -3.75 ms -2.5 ms -1.25 ms tel-00623090, version 1 - 13 Sep 2011 TDC 1.25 ms 2.5 ms 3.75 ms 5.0 ms (b) -12.5 ms -11.25 ms -10.0 ms -8.75 ms - 7.5 ms -6.25 ms - 5.0 ms - 3.75 ms -2.5 ms -1.25 ms TDC 1.25 ms 2.5 ms 3.75 ms (c) Figure 5.23- Direct visualization of stoichiometric methane-air flame in a RCM for various Ignition timings. (a) 5 ms BTDC; (b) 7.5 ms BTDC; (c) 12.5 ms BTDC. Pictures of the initial phase of combustion show an initially quasi-spherical, relatively smooth flame kernel for syngas compositions and methane cases for the various ignition timings. The laminar behavior of the flame remains longer time as the ignition is made far from TDC. In the case of ignition timing at 12.5 ms BTDC, the flame kernel grows and experience flattening when piston is close to TDC position before the transition to turbulent combustion. Because the combustion continues in the expansion stroke, it is not possible to determine from direct flame visualizations of the combustion chamber the combustion duration. However, estimation about the rapidity of the combustion can be made from figures 5.21-5.23. It is observed that the time at which the flame occupies the entire chamber increases as the igniting timing increases for the whole fuels. This 164

Chapter 5 observation emphasis the fact of the lower peak pressures for ignition timings close to TDC. As seen on 3.2.5 turbulence intensity is higher close to TDC on the RCM, thus the increased turbulence in the unburned mixture at the time of combustion will increase the burning rate (Alla, 2002). Comparing the three fuels, it is observed that combustion is faster for methane, followed by downdraft syngas and finally by updraft syngas. This behavior is in agreement with the heat of reaction of the mixtures as well as with the laminar burning velocity determined on 4.1.2.3 for typical syngas compositions. 5.3 Conclusion tel-00623090, version 1 - 13 Sep 2011 An experimental approach to syngas engine-like conditions on a rapid compression machine is made. There is an opposite behavior of the in-cylinder pressure between single compression and compression-expansion strokes. The first is that one gets higher in-cylinder pressures on single compression event than for compression-expansion events, which emphasis the fact of the constant volume combustion to be the way of getting higher pressures. The second is that for single compression peak pressure decreases as the ignition delay increases. In opposite, for compression-expansion the peak pressure increases with the ignition delay increase. This opposite behaviour in relation to the ignition timing has to do with the deviation of the spark from TDC position that influences the extent of the combustion in the compression stroke and this extent has different consequences on peak pressure regarding to the number of strokes events. For single compression it reduces the constant volume combustion duration. For compression-expansion strokes it increases the combustion duration on the compression stroke where the heat released has the effect of generate pressure before expansion. In both experimental events, higher pressures are obtained with methane-air mixture followed by downdraft-syngas and lastly by updraft-syngas. These results could be endorsed to the heat of reaction of the fuels and air to fuel ratio under stoichiometric conditions, but also to burning velocity. Crossing the heat value with the air to fuel ratio conclusion could be drawn that the energy content inside the combustion chamber is in agreement, however not proportional with the obtained pressures. 165

Experimental study of engine-like turbulent <strong>combustion</strong><br />

-5.0 ms -3.75 ms -2.5 ms -1.25 ms TDC<br />

1.25 ms 2.5 ms 3.75 ms 5.0 ms 6.25ms<br />

(a)<br />

-6.25 ms - 5.0 ms -3.75 ms -2.5 ms -1.25 ms<br />

tel-00623090, version 1 - 13 Sep 2011<br />

TDC 1.25 ms 2.5 ms 3.75 ms 5.0 ms<br />

(b)<br />

-12.5 ms -11.25 ms -10.0 ms -8.75 ms - 7.5 ms<br />

-6.25 ms - 5.0 ms - 3.75 ms -2.5 ms -1.25 ms<br />

TDC 1.25 ms 2.5 ms 3.75 ms<br />

(c)<br />

Figure 5.23- Direct visualization of stoichiometric methane-air f<strong>la</strong>me in a RCM for various<br />

Ignition timings. (a) 5 ms BTDC; (b) 7.5 ms BTDC; (c) 12.5 ms BTDC.<br />

Pictures of the initial phase of <strong>combustion</strong> show an initially quasi-spherical, re<strong>la</strong>tively<br />

smooth f<strong>la</strong>me kernel for syngas compositions and methane cases for the various<br />

ignition timings. The <strong>la</strong>minar behavior of the f<strong>la</strong>me remains longer time as the ignition is<br />

ma<strong>de</strong> far from TDC. In the case of ignition timing at 12.5 ms BTDC, the f<strong>la</strong>me kernel<br />

grows and experience f<strong>la</strong>ttening when piston is close to TDC position before the<br />

transition to turbulent <strong>combustion</strong>.<br />

Because the <strong>combustion</strong> continues in the expansion stroke, it is not possible to<br />

<strong>de</strong>termine from direct f<strong>la</strong>me visualizations of the <strong>combustion</strong> chamber the <strong>combustion</strong><br />

duration. However, estimation about the rapidity of the <strong>combustion</strong> can be ma<strong>de</strong> from<br />

figures 5.21-5.23. It is observed that the time at which the f<strong>la</strong>me occupies the entire<br />

chamber increases as the igniting timing increases for the whole fuels. This<br />

164

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