for efficient airpumping and higher compression ratios, this type of
compressor is very complex and very expensive to manufacture. For
these reasons the axialflow design finds its greatest application
where required efficiency and output override the considerations of
cost, simplicity, and flexibility of operation. However, due to
modern technology, the cost of the small axialflow compressor, used
in Army aircraft, is coming down.
c. Axialcentrifugalflow compressor. The axialcentrifugal
flow compressor, also called the dual compressor, is a combination of
the two types, using the same operating characteristics. Figure 1.15
shows the compressor used in the T53 turbine engine. Most of the gas
turbine engines used in Army aircraft are of the dual compressor
design. Usually it consists of a five or sevenstage axialflow
compressor and one centrifugalflow compressor. The dual compressors
are mounted on the same shaft and turn in the same direction and at
the same speed. The centrifugal compressor is mounted aft of the
axial compressor. The axial compressor contains numerous airfoil
shaped blades and vanes that accomplish the task of moving the air
mass into the combustor at an elevated pressure.
As the air is drawn into the engine, its direction of flow
is changed by the inlet guide vanes. The angle of entry is
established to ensure that the air flow onto the rotating compressor
blades is within the stallfree (angle of attack) range. Air
pressure or velocity is not changed as a result of this action. As
the air passes from the trailing edge of the inlet guide vanes, its
direction of flow is changed due to the rotational effect of the
compressor. This change of airflow direction is similar to the
action that takes place when a car is driven during a rain or snow
storm. The rain or snow falling in a vertical direction strikes the
windshield at an angle due to the horizontal velocity of the car.
In conjunction with the change of airflow direction, the
velocity of the air is increased. Passing through the rotating
compressor blades, the velocity is decreased, and a gain in pressure
is obtained. When leaving the trailing edge of the compressor
blades, the velocity of the air mass is again increased by the
rotational effect of the compressor. The angle of entry on to the
stationary stator vanes results from this rotational effect as it did
on the airflow onto the compressor.
Passing through the stationary stator vanes the air
velocity is again decreased resulting in an increase in pressure.
The combined action of the rotor blades and stator vanes results in an