Centrifugal flow - Compressors in Aircraft:
The centrifugal impeller is rotated at high speed by the turbine and centrifugal action causes the air between the impeller vanes to accelerate radially outwards until it is thrown off at the tip into the diffuser. The radial movement of the air across the impeller, from eye to tip, causes a drop in air pressure at the eye and the faster the impeller is turning, the lower the pressure at the eye becomes. The low pressure existing at the eye of the revolving impeller induces a continuous flow of air through the engine intake and into the eye of the impeller. The air, in turn, is accelerated across the impeller and passed into the diffuser. The kinetic energy in the air is then converted to pressure energy ready to enter the combustion chamber. The action of the diffuser is illustrated in figure.
The final volume and mass airflow delivered by the centrifugal compressor is dependent on:
a Pressure ratiob Operating RPMc Diameter of the impeller
NOTE: This is assuming a constant air density at the inlet of the compressor.
The ratio of the inlet pressure to outlet pressure of the compressor is called pressure ratio. The higher the pressure of the air the more efficiently the thrust will be produced with a corresponding improvement to the fuel economy of the engine. The maximum pressure ratio normally obtainable from a single stage centrifugal compressor is approximately 5:1 and from a two stage, approximately 8:1.Design of the more modern centrifugal compressors sees them approaching pressure ratios of 15:1.
Diameter of Impeller
A large impeller will deliver a greater mass of air than a small impeller, however a large diameter compressor leads to an increase in the frontal area of the engine causing excess drag forces on the aircraft.