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Text: What Is a Gas Turbine Engine?
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When you go to an airport and see the commercial jets there, you can't help but notice the huge engines that power them. Most commercial jets are powered by turbofan engines, and turbofans are one example of a general class of engines called gas turbine engines.
A gas turbine engine uses a pressurized gas to spin a turbine and compressor. Inside the engine, fuel is ignited to increase the gas’s temperature and pressure. This causes the gas to act on the turbine at a higher velocity. Gas turbine engines are used to power many aircraft and helicopters, boats and have recently been used in some battle tanks.
A turbine uses the energy of a flowing fluid to rotate a wheel. The concept of a turbine has existed for many years; windmills and water wheels are simple examples. Moving air passes over a windmill’s slanted blades, causing the wheel to spin. Turbines can also be used with steam, as in the case of many power plants. The turbine in a gas turbine engine, however, uses highly pressurized air to rotate.
Norwegian engineer Aegidius Elling was granted a patent for a gas turbine in 1884. His first turbine model that produced more power than it consumed was unveiled in 1903. Elling’s design locked a spinning air compressor to the turbine, a feature widely used today. He believed that if more heat-resistant materials could be found, the gas turbine engine could be used to power airplanes.
The main parts of a gas turbine engine are the compressor, the combustion area and the turbine. Air enters the compressor at normal pressure and then gets compressed. In the combustion area, some type of fuel is burned to increase the temperature and energy content of the air. The high-temperature, high-pressure gas is then forced to exit the engine, turning the turbine on its way out. A solid shaft connects the compressor and turbine—the rotation of the turbine is used to spin the compressor, which makes the engine as a whole more efficient.
The shaft that connects the compressor to the turbine may or may not be used to power additional devices. In a jet engine, the method of obtaining thrust is expelling the exhaust gas at a high velocity, which causes the aircraft to be pushed forward. In vehicles that are not powered by thrust, the spinning shaft can be used to do mechanical work.
A gas turbine engine has several advantages over the type of engine found in most automobiles. First of all, it has a better power-to-weight ratio. Gas turbine engines are also smaller than their automobile counterparts for a given amount of power. These reasons explain why many helicopters and airplanes use this type of engine.
Exercise 9. Translate the sentences with the adjectives in the comparative form. Remember! To compare things we use: much - намного; far - гораздо; a little – немного.
1. The more efficient the compressor, the higher the pressure generated for a given work input.
2. The more efficiently the turbine uses the expanding gas, the greater the output of the work for a given pressure drop in the gas.
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3. Because the turbojet engine is a heat engine, the higher the temperature of combustion the greater is the expansion of the gases.
4. Although the turbo-rocket engine is smaller and lighter than the turbo/ramjet, it has higher fuel consumption.
5. The centrifugal flow compressor is usually more robust than the axial compressor and is also easier to develop and manufacture.
6. The axial flow compressor consumes far more air than a centrifugal compressor of the same frontal area and can be designed to attain much higher pressure ratios.
7. The more the pressure ratio of a compressor is increased the more difficult it becomes to ensure that it will operate efficiently over the full speed range.
8. Engines operating at higher turbine inlet temperatures are thermally more efficient and have an improved power to weight ratio.
9. Bypass engines have a better propulsive efficiency and thus can have a smaller turbine for a given thrust.
Text: Types of Turbines
There are many different kinds of turbines:
You have probably heard of a steam turbine. Most power plants use coal, natural gas, oil or a nuclear reactor to create steam. The steam runs through a huge and very carefully designed multi-stage turbine to spin an output shaft that drives the plant's generator.
Hydroelectric dams use water turbines in the same way to generate power. The turbines used in a hydroelectric plant look completely different from a steam turbine because water is so much denser (and slower moving) than steam, but it is the same principle.
Wind turbines, also known as wind mills, use the wind as their motive force. A wind turbine looks nothing like a steam turbine or a water turbine because wind is slow moving and very light, but again, the principle is the same.
A gas turbine is an extension of the same concept. In a gas turbine, a pressurized gas spins the turbine. In all modern gas turbine engines, the engine produces its own pressurized gas, and it does this by burning something like propane, natural gas, kerosene or jet fuel. The heat that comes from burning the fuel expands air, and the high-speed rush of this hot air spins the turbine.
Gas turbine engines are, theoretically, extremely simple. They have three parts:
1. Compressor - Compresses the incoming air to high pressure
2. Combustion area - Burns the fuel and produces high-pressure, high-velocity gas
3. Turbine - Extracts the energy from the high-pressure, high-velocity gas flowing from the combustion chamber.
Types of Turbine Engines:
a Turbine engines are classified according to the type of compressors they use
a There are three types of compressors-centrifugal flow, axial flow, and centrifugal-axial flow
a Compression of inlet air is achieved in a centrifugal flow engine by accelerating air outward perpendicular to the longitudinal axis of the machine
a The axial-flow engine compresses air by a series of rotating and stationary airfoils moving the air parallel to the longitudinal axis
a The centrifugal-axial flow design uses both kinds of compressors to achieve the desired compression
a The path the air takes through the engine and how power is produced determines the type of engine
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There are four types of aircraft turbine engines:
§ Turbojet
§ Turboprop
§ Turbofan
§ Turboshaft
Text: Engine Types
During the development of jet engine technology, several types of engines have been invented. Most of these types perform differently at varying airspeeds, which means that each type of engine has varying characteristics. The turbojet is the earliest jet engine and formed the base for the engines we use today. Besides the turbojet, turboprop engines were widely used and still powers many aircraft today. Nowadays, the turbofan engine is used most common by commercial aviation and makes use of a fan which drives air around bypass ducts. We'll discuss these types in more detail below.
Turbojet
The turbojet is the simplest jet engine in terms of construction. It is still widely used in military aviation since it allows aircraft to fly at speeds exceeding Mach 1. Although there are many types its basic design consists of an air intake, a gas generator unit and an exhaust nozzle. The gas generator in turn consists of the compressor, combustion chamber and turbine. In this design, the turbine drives only the compressor and various accessory systems; this leaves more energy to provide actual thrust than in other types. As said before, the turbojet is commonly used to provide power in order to fly supersonic where it fuel efficiency increases. Turboprop
The turboprop is similar to the turbojet, except that the turbine drives a propeller, as well, in a two-spool configuration. Contrary to the turbojet, the turboprop accelerates a large amount of air to a relatively low exhaust velocity. Because of this, the turboprop reaches a very high fuel-efficiency at the expense of airspeed. Because of this, the turboprop engine performs best in the 250 to 450 mph speed range. Additionally, the turboprop generates more noise than the turbojet.
Turbofan
Above 450 mph both the turbofan and turbojet is most widely used on commercial aircraft and business jets. The turbofan engine was designed in order to permit higher turbine temperatures without increasing gas velocity dramatically because this would decrease efficiency in subsonic flight. The turbofan makes use of a huge fan driven by the turbine commonly known as the N1. This fan directs air through bypass ducts, which lead air around the core engine to a separate exhaust nozzle. This method has proven to be very efficient at transonic airspeeds, making the turbofan ideal for modern commercial aircraft. The turbofan can be categorized by the ratio at which air moves through and around the core engine: Low bypass-ratio turbofans and high bypass-ratio turbofans. Almost every modern transport aircraft is powered by turbofan engines which contribute to high efficiency.
Turboshafts
The turboshaft is another form of gas-turbine engine that operates much like a turboprop system. It does not drive a propeller. Instead, it provides power for a helicopter rotor. The turboshaft engine is designed so that the speed of the helicopter rotor is independent of the rotating speed of the gas generator. This permits the rotor speed to be kept constant even when the speed of the generator is varied to modulate the amount of power produced.
Ramjets
The simplest jet engine – the ramjet – has no moving parts. The speed of the jet “rams” or forces air into the engine. It is essentially a turbojet in which rotating machinery has been omitted. Its application is restricted by the fact that its compression ratio depends wholly on forward speed. The ramjet develops no static thrust and very little thrust in general below the speed of sound. As a consequence, a ramjet vehicle requires some form of assisted takeoff, such as another aircraft. It has been used primarily in guided-missile systems. Space vehicles use this type of jet.
Exercise 10. Translate the nouns or adjectives formed from the verbs in the left column.
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| burner heat conversion acceleration operation diffuser production rise rotary movable expansion requirement supply assembly addition | to burn - зажигать to heat - нагревать to convert - преобразовать to accelerate - ускорять to operate - действовать to diffuse - распространять to produce - создавать to rise - увеличивать, поднимать to rotate - вращать to move - двигать to expand - расширять to require - требовать to supply - снабжать to assemble - собирать to add - добавлять |
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