As the name suggests, gas turbine engine compressors provide the compression part of the gas turbine engine thermodynamic cycle. There are three basic categories of gas turbine engine compressor: axial compressor, centrifugal compressor and mixed flow compressor. A fourth, unusual, type is the free-piston gas generator, which combines the functions of compressor and combustion chamber in one unit.
Most high-compression jet engine use axial compressors for their high efficiency. In the axial compressor the air flows parallel to the axis of rotation. Axial compressor are made to be multi-staged. A stage consists of a row of rotating blades called the rotor, which are connected to the central shaft and a row of stationary or fixed blades called stator. In axial flow compressor, the air flows from stage to stage. The role of the rotor blades is to accelerate the incoming air for increasing the kinetic energy of the air. Then the fluid is decelerated across the stators and as a consequence the kinetic energy is converted into the pressure rise by the stators. Through the compressor, the flow area decreases and the blades get smaller and smaller from stage to stage and this compensates for the increase of air pressure and density, creating a constant axial velocity. In a axial compressors, there may be up to 20 stages between the air inlet and compressor outlet.
The very first jet engines used centrifugal compressors and they are still used on small turbojets and turbo shaft engines.
The key component that makes a compressor centrifugal is the centrifugal impeller which contains a rotating set of blades. In a centrifugal compressor the airflow goes perpendicular to the axis of rotation. When the air passes through the rotating impeller it experiences a centrifugal force. Air is pushed toward the center and this radial movement of air results in a pressure rise and the generation of kinetic energy. Then the air passes through another key component of centrifugal compressor called diffuser section which responsibility is to convert the kinetic energy of the air into pressure rise by gradually slowing the air velocity. The diffuser is a fixed or static component that escorts the air flow after leaving the impeller. The impeller and the diffuser contributes about 65% and 35% of the total pressure produced in the centrifugal compressor respectively.
A typical, single-stage, centrifugal compressor can increase the pressure by a factor of 4. A similar single stage axial compressors can only increase the pressure by a factor of 1.2 but axial compressors have an advantage over centrifugal compressors because of their ability to have multiple stages. In the multi-staged compressors, the pressure is multiplied from row to row which can increase the pressure by a factor 40. It is much more difficult to produce an efficient multistage centrifugal compressor because the flow has to be ducted back to the axis at each stage therefore most high-compression jet engines incorporate multi-staged axial compressors. But if only a moderate amount of compression is required a centrifugal compressor is much simpler and efficient to use.
In the future, mixed flow compressors may feature at the small end of the market, particularly in turbofans, where the relatively large diameter across the diffuser of a centrifugal compressor is a significant disadvantage.
At high overall pressure ratios, the compression system is usually split into two units; a low-pressure (LP) compressor mounted on one shaft followed by a high-pressure (HP) compressor mounted on the HP shaft and driven by its own (HP) turbine. On civil turbofans, the first stage of the LP compressor is often a single-stage fan. If the HP compressor pressure ratio exceeds about 4.5:1, then the unit will probably have variable geometry (i.e. variable stators) on the first few stages, to make the surge line on the compressor map more shallow, to accommodate the shallow working line.
On their large civil turbofans, Rolls-Royce split the compression system into three: a fan, an intermediate-pressure (IP) compressor and an HP compressor, each unit being driven by its own turbine unit. The RB199 military turbofan also has this arrangement.
A free-piston gas generator is a free-piston engine whose exhaust is used to power a gas turbine. It combines the functions of compressor and combustion chamber in one unit. These machines were quite widely used in the period 1930–1960 but then fell out of favour. [1]
A jet engine is a type of reaction engine, discharging a fast-moving jet of heated gas that generates thrust by jet propulsion. While this broad definition may include rocket, water jet, and hybrid propulsion, the term jet engine typically refers to an internal combustion air-breathing jet engine such as a turbojet, turbofan, ramjet, or pulse jet. In general, jet engines are internal combustion engines.
A pump is a device that moves fluids, or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy.
A turbine is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced can be used for generating electrical power when combined with a generator. A turbine is a turbomachine with at least one moving part called a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades so that they move and impart rotational energy to the rotor. Early turbine examples are windmills and waterwheels.
A turbopump is a propellant pump with two main components: a rotodynamic pump and a driving gas turbine, usually both mounted on the same shaft, or sometimes geared together. They were initially developed in Germany in the early 1940s. The purpose of a turbopump is to produce a high-pressure fluid for feeding a combustion chamber or other use. While other use cases exist, they are most commonly found in liquid rocket engines.
The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a combination of the preceding generation engine technology of the turbojet, and a reference to the additional fan stage added. It consists of a gas turbine engine which achieves mechanical energy from combustion, and a ducted fan that uses the mechanical energy from the gas turbine to force air rearwards. Thus, whereas all the air taken in by a turbojet passes through the combustion chamber and turbines, in a turbofan some of that air bypasses these components. A turbofan thus can be thought of as a turbojet being used to drive a ducted fan, with both of these contributing to the thrust.
The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, and a turbine. The compressed air from the compressor is heated by burning fuel in the combustion chamber and then allowed to expand through the turbine. The turbine exhaust is then expanded in the propelling nozzle where it is accelerated to high speed to provide thrust. Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently into practical engines during the late 1930s.
An air compressor is a machine that takes ambient air from the surroundings and discharges it at a higher pressure. It is an application of a gas compressor and a pneumatic device that converts mechanical power into potential energy stored in compressed air, which has many uses. A common application is to compress air into a storage tank, for immediate or later use. When the delivery pressure reaches its set upper limit, the compressor is shut off, or the excess air is released through an overpressure valve. The compressed air is stored in the tank until it is needed. The pressure energy provided by the compressed air can be used for a variety of applications such as pneumatic tools as it is released. When tank pressure reaches its lower limit, the air compressor turns on again and re-pressurizes the tank. A compressor is different from a pump because it works on a gas, while pumps work on a liquid.
Centrifugal compressors, sometimes called impeller compressors or radial compressors, are a sub-class of dynamic axisymmetric work-absorbing turbomachinery.
A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor.
An impeller or impellor is a driven rotor used to increase the pressure and flow of a fluid. It is the opposite of a turbine, which extracts energy from, and reduces the pressure of, a flowing fluid.
An axial compressor is a gas compressor that can continuously pressurize gases. It is a rotating, airfoil-based compressor in which the gas or working fluid principally flows parallel to the axis of rotation, or axially. This differs from other rotating compressors such as centrifugal compressor, axi-centrifugal compressors and mixed-flow compressors where the fluid flow will include a "radial component" through the compressor.
Turbomachinery, in mechanical engineering, describes machines that transfer energy between a rotor and a fluid, including both turbines and compressors. While a turbine transfers energy from a fluid to a rotor, a compressor transfers energy from a rotor to a fluid.
Centrifugal pumps are used to transport fluids by the conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy typically comes from an engine or electric motor. They are a sub-class of dynamic axisymmetric work-absorbing turbomachinery. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute chamber (casing), from which it exits.
A compressor map is a chart which shows the performance of a turbomachinery compressor. This type of compressor is used in gas turbine engines, for supercharging reciprocating engines and for industrial processes, where it is known as a dynamic compressor. A map is created from compressor rig test results or predicted by a special computer program. Alternatively the map of a similar compressor can be suitably scaled. This article is an overview of compressor maps and their different applications and also has detailed explanations of maps for a fan and intermediate and high-pressure compressors from a three-shaft aero-engine as specific examples.
The Heinkel HeS 8 was an early jet engine designed by Hans von Ohain while working at Heinkel. It was the first jet engine to be financially supported by the RLM, bearing the official name 109-001. Had development continued it would have been known as the Heinkel 001, but it does not appear this was used in practice.
A centrifugal fan is a mechanical device for moving air or other gases in a direction at an angle to the incoming fluid. Centrifugal fans often contain a ducted housing to direct outgoing air in a specific direction or across a heat sink; such a fan is also called a blower, blower fan, or squirrel-cage fan. Tiny ones used in computers are sometimes called biscuit blowers. These fans move air from the rotating inlet of the fan to an outlet. They are typically used in ducted applications to either draw air through ductwork/heat exchanger, or push air through similar impellers. Compared to standard axial fans, they can provide similar air movement from a smaller fan package, and overcome higher resistance in air streams.
A fan is a powered machine used to create a show of air. A fan consists of a circling arrangement of vanes or blades, generally made of wood, plastic, or metal, which act on the air. The rotating assembly of blades and hub is known as an impeller, rotor, or runner. Usually, it is contained within some form of housing, or case. This may direct the airflow, or increase safety by preventing objects from contacting the fan blades. Most fans are powered by electric motors, but other sources of power may be used, including hydraulic motors, handcranks, and internal combustion engines.
Between 1936 and 1940 Alan Arnold Griffith designed a series of turbine engines that were built under the direction of Hayne Constant at the Royal Aircraft Establishment (RAE). The designs were advanced for the era, typically featuring a "two-spool" layout with high- and low-pressure compressors that individually had more stages than typical engines of the era. Although advanced, the engines were also difficult to build, and only the much simpler "Freda" design would ever see production, as the Metrovick F.2 and later the Armstrong Siddeley Sapphire. Much of the pioneering work would be later used in Rolls-Royce designs, starting with the hugely successful Rolls-Royce Avon.
This article briefly describes the components and systems found in jet engines.
The Power Jets WU was a series of three very different experimental jet engines produced and tested by Frank Whittle and his small team in the late 1930s.