Turbine Engine Compressor Sections: Basic theory and operation

Basic theory and operation By Joe Escobar Turbine engines power many of today's aircraft. The power that is generated by these engines relies on the expanding gas that is the result of combustion in the combustion section. In order to...


Some centrifugal-flow compressor sections also include auxiliary air intake doors (blow-in doors) as part of the plenum chamber. These doors provide air to the engine compartment during ground operation when air requirements for the engine exceed the airflow through the inlet ducts. The doors are held closed by spring action when the engine is not operating. During operation, the doors open automatically whenever engine compartment pressure drops below atmospheric pressure. During takeoff and flight, ram air pressure in the engine compartment aids the springs in holding the doors closed.

The diffuser of a centrifugal-flow compressor section is an annular chamber provided with a number of vanes that form a series of divergent passages into the manifold. The diffuser vanes direct the flow of air from the impeller to the manifold at an angle designed to retain the maximum amount of energy that is provided by the impeller. They also deliver the air to the manifold at a velocity and pressure satisfactory for use in the combustion chambers.

The compressor manifold diverts the airflow from the diffuser, which is an integral part of the manifold, into the combustion chambers. The manifold has one outlet port for each chamber so that the air is evenly divided. A compressor outlet elbow is bolted to each of the outlet ports. These air outlets are constructed in the form of ducts and are known by a variety of names, such as air outlet ducts, outlet elbows, or combustion chamber inlet ducts. These ducts perform a very important part of the diffusion process - they change the radial direction of the airflow to an axial direction, where the diffusion process is completed after the turn. To help the elbows perform this function in an efficient manner, turning vanes (cascade vanes) are sometimes fitted inside the elbows. These vanes reduce air pressure losses by presenting a smooth, turning surface.

There are inherent advantages and disadvantages to each type of compressor. Knowing this, some of today's engine manufacturers utilize the advantages of each type by using a combination of both in their compressor section. Here are some of the advantages and disadvantages of each type of compressor.

Advantages/disadvantages
Centrifugal flow compressor
Advantages:

  • Light weight
  • High-pressure rise per stage
  • Simplicity of manufacture, thus low cost
  • Low weight

Disadvantages:

  • Large frontal area for given airflow
  • More than two stages are not practical because of losses in turns between stages

Axial flow compressor
Advantages:

  • The ability to handle large volumes of airflow and a high-pressure ratio
  • Small frontal area for given airflow
  • Straight-through flow, allowing high ram efficiency

Disadvantages:

  • More susceptible to foreign object damage
  • Expensive to manufacture
  • Very heavy in comparison to the centrifugal compressor with the same compression ratio

Bleed air
The compressed high temperature air that is produced by the compressor section can be bled off and used for various functions. Bleed air can be taken from any of the various pressure stages of the compressor section. The location of the bleed air port is dependent on the pressure or temperature required for a particular job. Bleed air ports are small openings in the compressor case at the associated compressor stage. So varying degrees of pressure or temperature are achieved by tapping into the appropriate stage. Often air is bled from the final stage since this is where the pressure and temperature are at their highest.

Some applications for bleed air include:

  • Cabin pressurization, heating, and cooling
  • De-icing
  • Pneumatic starting of engines
  • Auxiliary drive units
  • Control-booster servosystems
  • Power for running instruments

Sometimes it is necessary to cool the bleed air coming from the engine as in the case of cabin pressurization. In these cases some type of refrigeration unit or heat exchanger is used to cool the air down.

Engine compressors have numerous uses. They are a vital part of a turbine engine, providing the high-pressure, high-temperature air for combustion as well as bleed air for system operation. What type compressor is powering your engine?

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