C. AXIAL FLOW COMPRESSORS

Where the discharge of the wheels of a centrifugal compressor leaves at right angles to the wheel, in an axial flow machine, the flow of gas is parallel to the rotor. Imagine a series of fans placed in front of each other. When they are operating, the flow of air from the first fan is fed into the second fan which further increases the flow of air. This is repeated through the series of fans tending to give a much increased air flow from the final fan.
The rotor of an Axial Flow compressor consists of a number of stages (or rows) of many ‘ROTOR’ (Rotating) blades fitted at an angle into the rotor body – similar to the blades of a fan. The blades in each row become smaller and narrower from stage to stage. This arrangement allows for the decrease in volume of the gas as its pressure increases from stage to stage. However, as the gas leaves each stage, it is moving at high velocity and in the opposite direction to the rotation of next stage. In the casing of the machine, rows of ‘STATOR’ (Static) blades are fitted in diaphragms placed at an opposite angle to the rotor blades between the rotor stages.

These Stator blades have two purposes;

1. To decrease the velocity of the gas – thereby increasing pressure (Bernoulli's Principle).
2. To redirect the flow of gas into the blades of the next stage.

This arrangement pushes the gas Axially along the length of the machine increasing pressure from stage to stage. The discharge pressure from the final stage will depend upon the number of stages in the compressor and/or the machine speed. The volume of gas compressed depends on the physical size of the compressor. This type of compressor is generally utilised in the air compressor section of a Combustion Gas Turbine.
The following pictures (Figures: 9, 10, & 11) show some internal arrangements of Axial Flow
Compressors.

Figure: 9 - Axial Flow Compressor – Horizontally Split – Open


Figure: 10 - Axial Flow Compressor Stator Blades


Figure: 11 - View of Axial Flow Compressor Internals


Figure: 12 Simplified Diagram of an Axial Flow Compressor