The primary function of the compressor is to supply enough air to satisfy the requirements of the combustion section. Specifically, the compressor increases the pressure of the airflow from the air inlet duct and directs it to the burners in the quantity and at the pressures required.
A secondary function is to supply compressor bleed air to operate various components throughout the engine and aircraft
Because the compressor section receives energy from the turbine via the drive shaft, it is not a closed system. Therefore, the increase in velocity will not result in a decrease in pressure. In fact, both velocity and pressure will increase in the compressor section and therefore, total pressure increases.
Axial-Flow Compressors- The term axial-flow applies to the axial (straight line) flow of air through the compressor section of the engine. An axial-flow compressor has two main elements: Rotor blades and stator vanes. Rotor blades are rotating, airfoil- shaped blades, while stator vanes are stationary airfoil-shaped blades. Each rotor and stator pair forms a stage
The rotors are driven by the turbine at high speeds (near 15,000 RPM), which increases the velocity and pressure of the incoming airflow, thus, increasing total pressure. This high velocity airflow is then pushed through the stator vanes which act like diffusers (the airflow velocity decreases with a proportional increase in pressure). The airflow then passes on to the next rotor/stator stage and the process continues.
The compressor is setup so that the airflow velocity remains fairly constant from inlet to exit. As the airflow pressure increases through each compressor stage, the air is compressed. If the air is compressed and its volume is not decreased, its velocity will decrease excessively toward the rear stages of the compressor and the stall area will be approached.Although the pressure increase per stage is not as great as in a centrifugal flowcompressor, the efficient use of multiple stages can produce very high overall compression ratios. Current axial flow compressors have efficiencies near 90 percent and compression ratios approaching 15:1. Several high performance engines have compression ratios near 25:1. Remember that typical centrifugal compressors can only attain compression ratios between 6:1 and 7:1.
The low-pressure compressor is located at the front of the compressor section. It will provide the initial pressure increase to airflow arriving from the inlet. As such, this compressor spool must spin slow enough to provide an initial pressure increase without creating an excessive velocity increase.
The high-pressure compressor is located after the low-pressure compressor, and provides a further increase to airflow pressure. This compressor is turned by the high- pressure turbine. Located forward of the low-pressure turbine, the high-pressure turbine will receive more energy from the combustion section. Therefore, it will turn the high- pressure compressor at a faster rate.
The high-pressure spool is turned at higher speeds by the high-pressure turbine, both because it is smaller and lighter weight, and because the high-pressure turbine is located directly after the burner chamber. This higher speed helps to produce a vacuum, which eases the transition from the low to the high pressure compressor.
A basic law of aerodynamics states that the speed of sound increases as the air temperature increases. Since the air temperature is increased through the compression phase, the high-pressure compressor can attain higher speeds without exceeding the speed of sound (Mach). Also, the blades of the high-pressure compressor are shorter than those of the low-pressure compressor, and can turn faster before exceeding their limiting Mach number.
When a dual-axial compressor is used in an engine, higher compression ratios can be attained with minimum total compressor weight and frontal area. Usually, the rear compressor rotor is speed-governed by the engine fuel control and is the rotor to which the engine starter is connected.
Advantages of Axial Flow Compressors:
1.High peak efficiencies
2. Small frontal area reduces drag
3. Straight through-flow, allowing for high
ram efficiency
4. Combustion efficiency is better than
centrifugal compressors
5. With the dual/twin/split spool, starting flexibility is greater and it has improved high-altitude performance
Disadvantages
1.At low inlet speed, airflow will decrease in the compressor, creating a high angle of attack on the rotor blades that could lead to a compressor stall.
2.High-speed aircraft may experience an inlet air temperature of 250 degrees F. because of ram effect. These high compressor inlet air temperatures cause low compression ratios (due to air density changes) and will also reduce the air supply to the rear of the compressor
Axial Flow Compressor
3. Good efficiencies only possible over a narrow rotational-speed
4. Difficulty of manufacture and high cost
5. High starting power requirements
