Power Plant Engineering

GAS TURBINE POWER PLANT 275

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.

Fig. 9.8. Combustion Chamber with Upstream Injection with Bluff-body Flame Holder.

Fuel Primary

Zone

Air from

Compressor

Swirl Type

Flame Holder Secondary Zone

Tertiary

Zone

To

Tu r b i n e

Fig. 9.9. Combustion Chamber with Downstream Injection and Swirl Holder.
The common methods of flame stabilization used in practice are bluff body method and swirl
flow method. Two types of combustion chambers using bluff body and swirl for flame stabilization are
shown in Fig. 9.8 and Fig. 9.9. The major difference between two is the use of different methods to
create pilot zone for flame stabilization.

Nearly 15 to 20% of the total air is passed around the jet of fuel providing rich mixture in the
primary zone. This mixture burns continuously in the primary (pilot) zone and produces high tempera-
ture gases. About 30% of the total air is supplied in the secondary zone through the annuals around the
flame tube to complete the combustion. The secondary air must be admitted at right points in the com-
bustion chamber otherwise the cold injected air may chill the flame locally thereby reducing the rate of