Air Intakes
the air intake of a gas turbine engine is either built into the frame itself, if the engine is mounted in the airframe. It is designed to provide a turbulence free supply of air to the first stage compressor of the engine, with the minimum energy loss occurring through the inlet.
For it to work satisfactorily, the air must reach the compressor at a uniform pressure, distributed evenly across the whole of the face of the first stage.
Pitot Type Intake
The simplest form of intake is a single entrance circular cross section. ‘pitot’ type. The air normally flows directly through the intake when it is fitted to wing mounted engines.
the air intake of a gas turbine engine is either built into the frame itself, if the engine is mounted in the airframe. It is designed to provide a turbulence free supply of air to the first stage compressor of the engine, with the minimum energy loss occurring through the inlet.
For it to work satisfactorily, the air must reach the compressor at a uniform pressure, distributed evenly across the whole of the face of the first stage.
Pitot Type Intake
The simplest form of intake is a single entrance circular cross section. ‘pitot’ type. The air normally flows directly through the intake when it is fitted to wing mounted engines.
However, it can for an ‘S’ shaped duct when used on engines which are mounted at the bottom of the aircraft fin.
The pitot type of intake maximises the use of ram effect on the air, due to the aircraft forward speed, and suffers only a minimum loss of ram pressure as the aircraft attitude changes.
When the engine is running on a stationary aircraft, the pressure at the inlet of the compressor is below ambient pressure. This is due to the engine compressors is increasing the velocity of the airflow through the intake.
Subsonic intake – usually takes me the form of a divergent duct. While the aircraft is moving at a reasonable speed, the divergent takes place, a reduction of the airflow velocity between the lip of the intake and the inlet of the compressor, and secondly, an increase of air pressure at the inlet of the compressor. However, while the engine is running on a stationary aircraft, the pressure at the inlet of the compressor is under ambient pressure. This is due to the engine compressor is increasing the velocity of the airflow through the intake.
Secondary air intake doors –
Are required on certain aircrafts, to allow a supplmenatarty airflow to reach the compressor face during the high power operation when the aircraft is either stationery or at low airspeeds.
Supersonic (diffuser)intakes require a type on intake as the engine cannot handle supersonic airflow.
Compressors
To increase the efficient of a gas turbine engine, the air being fed to it has to be compressed, before fuel added, and its burnt in the combustion chambers.
Pros and cons of centrifugal compressor
Pros
it is more robust, easier and cheaper to manufacture than the axial flow.
Cons
axial flow compressor (40:1) can take higher compression ratios than centrifugal. (12:1)
axial flow generates more thrust
The pitot type of intake maximises the use of ram effect on the air, due to the aircraft forward speed, and suffers only a minimum loss of ram pressure as the aircraft attitude changes.
When the engine is running on a stationary aircraft, the pressure at the inlet of the compressor is below ambient pressure. This is due to the engine compressors is increasing the velocity of the airflow through the intake.
Subsonic intake – usually takes me the form of a divergent duct. While the aircraft is moving at a reasonable speed, the divergent takes place, a reduction of the airflow velocity between the lip of the intake and the inlet of the compressor, and secondly, an increase of air pressure at the inlet of the compressor. However, while the engine is running on a stationary aircraft, the pressure at the inlet of the compressor is under ambient pressure. This is due to the engine compressor is increasing the velocity of the airflow through the intake.
Secondary air intake doors –
Are required on certain aircrafts, to allow a supplmenatarty airflow to reach the compressor face during the high power operation when the aircraft is either stationery or at low airspeeds.
Supersonic (diffuser)intakes require a type on intake as the engine cannot handle supersonic airflow.
Compressors
To increase the efficient of a gas turbine engine, the air being fed to it has to be compressed, before fuel added, and its burnt in the combustion chambers.
Pros and cons of centrifugal compressor
Pros
it is more robust, easier and cheaper to manufacture than the axial flow.
Cons
axial flow compressor (40:1) can take higher compression ratios than centrifugal. (12:1)
axial flow generates more thrust
Principles of Centrifugal Compressor
As the divergence between the impeller blades, the pressure and velocity of the air increase as it flow outwards between them. The air leaves the tip of the impeller and passes into the diffuser section. The diffuser section is a system of stationery divergent ducts which are designed to convert the kinetic energy of the air-stream, into potential energy. Also, the compression ratio of a single centrifugal compressor can be in the ratio of 4:1, which as a result means that the outlet pressure of the compressor would be four times greater than its inlet pressure. it is not considered possible having two centrifugal compressors/stages (12:1) |
Principles of Axial Flow Compressor
Stage one is row of rotor blades, of airfoil section, which are fastened to a disk, followed by one row of stator vanes. The stator vanes are fastened to the compressor outer casing and the spaces between the rotor blades and the stator vanes form divergent passages. Within the stator vanes, the air pressure is increased by the conversion of the kinetic energy into pressure energy. The pressure rises at each stage at a ratio 1:1 or 1.2:1. To achieve compression ratios demanded by powerful engines, many rotor stages should be fitted into one shaft, which is driven by its turbine. (output pressure is (91P.S.I) So therefore a number of compressor rotor stages on a single shaft, which is driven by a turbine is often called a spool. |
Air Flow Control
Variable inlet guide vanes - are fitted to engines which are prone to compressor stall at low R.P.M or during engine acceleration or deceleration. They're fitted in front of the first rotor stage as shown in the picture. At high compression the VIGVs reduce the swirl imparted to the airflow, thereby maintaining the correct angle of attack of the air flowing over the rotor blades. Variable stator vanes - to minimize airflow problems that may occur after the first rotor stage, some engines fit stator vanes. Compressor Bleed Air taken out of the compressor section of a gas turbine engine to prevent compressor stall and operate cabin pressurization and air-conditioning systems. Air may also be bled from the compressor for boundary-layer control. Multi spool compressors - Designed into two separate sections, High and low pressure. In more powerful engines, there's an intermediate pressure. |