Conventional combustion chambers are generally of optimized rating for
take-off or near take-off operation. This signifies that, in the
primary zone of the combustion chamber, a fraction of the air flow of
the compressor is introduced so that, with the injected fuel, the
fuel-air mixture in this zone would be essentially stoichiometric in
turbo engines. Under these conditions, due to the levels of
temperature and high pressures, as complete as possible a combustion is
obtained, combustion yields greater than 0.99 are attained, the speeds
of the chemical reaction being optimum for these stoichimoetric
mixtures.
In addition, the pressures and temperatures at the outlet of the
compressor are lower; the result is that the chamber, with the partial
charge is very much maladjusted and that the slow speed combustion
efficiency rarely goes beyond 0.93. The combustion is, therefore, very
incomplete, which means much higher concentrations of carbon monoxide
and unburnt residues at the exhaust than under normal operation. The
proportions of the pollutants are all the higher, the lower the total
yield of the combustion.
The fresh gas/burnt gas mixture must also be
advantageous because it contributes to the increase in the temperature
of the carburized mixture and, therefore, aids in its atomization and
consequently permits an improvement in the speed of the chemical
reaction. In conventionally allowing this contact of the carburized
mixture with the high temperature gas from the combustion it is
desirable to arrange for a recirculation of the latter by searching
for a convenient turbulence level.
All of these solutions, which allow an
improvement in the combustion yield have, however, a maximum
efficiency only for values sufficient for the pressures and temperatures
of the air at the chamber inlet.
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