Power Plant Engineering

 9.3.3 COMBUSTION CHAMBERS
The gas turbine is a continuous flow system; therefore, the combustion in the gas turbine differs
from the combustion in diesel engines. High rate of mass flow results in high velocities at various points
throughout the cycle (300 m/sec). One of the vital problems associated with the design of gas turbine
combustion system is to secure a steady and stable flame inside the combustion chamber. The gas tur-
bine combustion system has to function under certain different operating conditions which are not usu-
ally met with the combustion systems of diesel engines. A few of them are listed below:
1. Combustion in the gas turbine takes place in a continuous flow system and, therefore, the
advantage of high pressure and restricted volume available in diesel engine is lost. The chemical
reaction takes place relatively slowly thus requiring large residence time in the combustion
chamber in order to achieve complete combustion.

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2. The gas turbine requires about 100:1 air-fuel ratio by weight for the reasons mentioned ear-
lier. But the air-fuel ratio required for the combustion in diesel engine is approximately 15:1.
Therefore, it is impossible to ignite and maintain a continuous combustion with such weak
mixture. It is necessary to provide rich mixture fm ignition and continuous combustion, and
therefore, it is necessary to allow required air in the combustion zone and the remaining air
must be added after complete combustion to reduce the gas temperature before passing into
the turbine.
3. A pilot or recirculated zone should be created in the main flow to establish a stable flame that
helps to ignite the combustible mixture continuously.
4. A stable continuous flame can be maintained inside the combustion chamber when the stream
velocity and fuel burning velocity are equal. Unfortunately most of the fuels have low burn-
ing velocities of the order of a few meters per second, therefore, flame stabilization is not
possible unless some technique is employed to anchor the flame in the combustion chamber.