A mechanic once told a non-aviation friend of his that a turbine engine has four stages of operation — suck, squeeze, bang, and blow. In this simple explanation, the compressor section is responsible for the squeeze. It provides the required volume of high-temperature, high-pressure air to the combustion section to satisfy the engine’s combustion requirements. It also provides bleed air for various aircraft systems. This article, based on AC65-12A, will take a brief look at the basic construction and operation of typical turbine engine compressor sections.
There are two basic types of compressors — axial flow and centrifugal flow. The difference between them is the way that the air flows through the compressor.
In an axial flow compressor, air is compressed while continuing its original direction of flow. From inlet to exit the air flows along an axial path and is compressed at a ratio of approximately 1.25 to 1.
An axial flow compressor has two basic elements — a rotor and a stator. The rotor has blades that are fixed on a spindle. These blades impel air rearward in the same way a propeller does. They are basically small airfoils. The rotor turns at a high speed and impels the air through a series of stages. A high velocity airflow is produced.
After the air is impelled by the rotor blades, it goes through the stator blades. The stator blades are fixed and act as diffusers at each stage. They partially convert high velocity air into high pressure. Each rotor/stator pair is a compressor stage.
Each consecutive compressor stage compresses the air even more. The number of stages is determined by the amount of air and total pressure rise required. The greater the number of stages, the higher the compression ratio.
In a centrifugal-flow engine, the compressor performs its job by picking up the entering air and accelerating it outwardly through centrifugal action. It basically consists of an impeller (rotor), a diffuser (stator), and a compressor manifold. The two main elements are the impeller and diffuser.
The impeller’s function is to pick up and accelerate the air outwardly to the diffuser. It may be either single entry or double entry. Both are similar in construction to a piston engine supercharger impeller. The double impeller is similar to two impellers back to back. However, because of the much greater combustion air requirements in turbojet engines, the impellers are larger than supercharger impellers.
The main differences between the two types of impellers are the size and the ducting arrangement. Double-entry types have a smaller diameter, but are usually operated at a higher rotational speed to ensure sufficient airflow. The single-entry impeller permits convenient ducting directly to the impeller eye (inducer vanes) as opposed to the more complicated ducting necessary to reach the rear side of the double-entry type. Although they are slightly more efficient in receiving, single-entry impellers need to be large in diameter to deliver the same amount of air as the double-entry type. Of course, this increases the overall diameter of the engine.
A plenum chamber is included in the ducting for double-entry compressor engines. This chamber is necessary because the air must enter the engine at almost right angles to the engine axis. Therefore, in order to give a positive flow the air must surround the engine compressor at a positive pressure before entering the compressor.
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.
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