COMPRESSORS

A compressor is designed to supply a specific quantity of compressed air to combustion chamber with the least expenditure of the power produced by the turbine.

The amount of air passing through a compressor will depend upon:

 Air density

 Aircraft speed

 Engine r.p.m.

COMPRESSION PRINCIPLE

By increasing the velocity (and hence the kinetic energy) of the air flowing through the rotating part of the compressor, and then slowing down the air to the static portion (by diffusion), the kinetic energy is converted into pressure energy.

COMPRESSOR REQUIREMENTS

To perform maximum efficiency, the compressor must satisfy a number of requirements. The most important are:

 It must produce the desired pressure rise.

 Compression must be performed with minimum losses. Large losses, waste power taken from the turbine.

 Where possible, the tip speed of the rotating blades or impeller should be subsonic. A speed of Mach 0.9 at any point on the radius is normally the design maximum preferred.

COMPRESSOR TYPES

There are two types of compressor available for use in gas turbine engines. These are Axial Flow and Centrifugal. Either type, or a combination of both, may be used Both types are connected to, and driven by, the turbine via a drive shaft. A compressor comprises primarily a rotor and a stator.

1- THE AXIAL-FLOW COMPRESSOR

The axial-flow compressor has two main elements: a rotor and a stator. The rotor has blades fixed on a spindle. These blades impel air rearward in the same manner as a propeller because of their angle and airfoil contour. The rotor, turning at high speed, takes in air at the compressor inlet and impels it through a series of stages.

From inlet to exit, the air flows along an axial path and is compressed at a ratio of approximately 1.25:1 per stage. The action of the rotor increases the compression of the air at each stage and accelerates it rearward through several stages. With this increased velocity, energy is transferred from the compressor to the air in the form of velocity energy. The stator blades act as diffusers at each stage, partially converting high velocity to pressure.

Each consecutive pair of rotor and stator blades constitutes a pressure stage. The number of rows of blades (stages) is determined by the amount of air and total pressure rise required. Compressor pressure ratio increases with the number of compression stages. Most engines utilize up to 16 stages and

more. The function of the stator vanes is to receive air from the air inlet duct or from each preceding stage and increase the pressure of the air and deliver it to the next stage at the correct velocity and pressure.

2- CENTRIFUGAL-FLOW COMPRESSOR

The centrifugal-flow compressor consists of an impeller (rotor), a diffuser (stator), and a compressor manifold. Centrifugal compressors have a high pressure rise per stage

that can be around 8:1. The two main functional elements are the impeller and the diffuser. Although the diffuser is a separate unit and is placed inside and bolted to the manifold, the entire assembly (diffuser and manifold) is often referred to as the diffuser. The impeller, whose function is to pick up and accelerate the air outwardly to the diffuser.

The diffuser is an annular chamber provided with a number of vanes forming 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 imparted by the impeller. The compressor manifold diverts the flow of air from the diffuser, which is an integral part of the manifold, into the combustion chambers.

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