Gas turbine and Jet Propulsion


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Gas turbine and Jet Propulsion

  1. 1. GAS TURBINE  A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between.  The basic operation of the gas turbine is similar to that of the steam power plant except that air is used instead of water. Fresh atmospheric air flows through a compressor that brings it to higher pressure. Energy is then added by spraying fuel into the air and igniting it so the combustion generates a high-temperature flow. This high-temperature high-pressure gas enters a turbine, where it expands down to the exhaust pressure, producing a shaft work output in the process.  The turbine shaft work is used to drive the compressor and other devices such as an electric generator that may be coupled to the shaft. The energy that is not used for shaft work comes out in the exhaust gases, so these have either a high temperature or a high velocity.  The purpose of the gas turbine determines the design so that the most desirable energy form is maximized.  Gas turbines are used to power aircraft, trains, ships, electrical generators, or even tanks.
  2. 2. THEORY OF OPERATION  Gases passing through an ideal gas turbine undergo three thermodynamic processes. These are isentropic compression, isobaric (constant pressure) combustion and isentropic expansion. Together, these make up the Brayton cycle.  In a practical gas turbine, gases are first accelerated in either a centrifugal or axial compressor. These gases are then slowed using a diverging nozzle known as a diffuser; these processes increase the pressure and temperature of the flow. In an ideal system, this is isentropic. However, in practice, energy is lost to heat, due to friction and turbulence. Gases then pass from the diffuser to a combustion chamber, or similar device, where heat is added. , this occurs at constant pressure (isobaric heat addition).  As there is no change in pressure, the specific volume of the gases increases. In practical situations this process is usually accompanied by a slight loss in pressure, due to friction. Finally, this larger volume of gases is expanded and accelerated by nozzle guide vanes before energy is extracted by a turbine.
  3. 3.  Mechanically, gas turbines can be considerably less complex than internal combustion piston engines. Simple turbines might have one moving part: the shaft/compressor/turbine/alternative-rotor assembly (see image below), not counting the fuel system. However, the required precision manufacturing for components and temperature resistant alloys necessary for high efficiency often make the construction of a simple turbine more complicated than piston engines.
  4. 4. TYPES OF GAS TURBINES 1)Turbojet Engines-  Air breathing jet engines are gas turbines optimized to produce thrust from the exhaust gases, or from ducted fans connected to the gas turbines. Jet engines that produce thrust from the direct impulse of exhaust gases are often called turbojets, whereas those that generate thrust with the addition of a ducted fan are often called turbofans or (rarely) fan-jets.  Gas turbines are also used in many liquid propellant rockets, the gas turbines are used to power a turbo pump to permit the use of lightweight, low pressure tanks, which saves considerable dry mass.
  5. 5. DESIGN  Air intake- Preceding the compressor is the air intake (or inlet). It is designed to be as efficient as possible at recovering the ram pressure of the air stream tube approaching the intake. The air leaving the intake then enters the compressor.  Compressor- The compressor is driven by the turbine. The compressor rotates at a very high speed, adding energy to the airflow and at the same time squeezing (compressing) it into a smaller space. Compressing the air increases its pressure and temperature.  Combustion chamber- The burning process in the combustor is significantly different from that in a piston engine. In a piston engine the burning gases are confined to a small volume and, as the fuel burns, the pressure increases dramatically. In a turbojet the air and fuel mixture passes unconfined through the combustion chamber. As the mixture burns its temperature increases dramatically, but the pressure actually decreases a few percent.  Turbine- Hot gases leaving the combustor are allowed to expand through the turbine. Turbines are usually made up of metals which can resist the high temperature, and frequently have built-in cooling channels.
  6. 6. 2)Turboprop Engines- A turboprop engine is a type of turbine engine which drives an external aircraft propeller using a reduction gear. Turboprop engines are generally used on small subsonic aircraft, but some large military and civil aircraft, such as the Airbus A400M, Lockheed L-188 Electra and Tupolev Tu-95, have also used turboprop power.
  7. 7. TECHNOLOGICALASPECTS  Thrust in a turboprop is sacrificed in favor of shaft power, which is obtained by extracting additional power (up to that necessary to drive the compressor) from turbine expansion. While the power turbine may be integral with the gas generator section, many turboprops today feature a free power turbine on a separate coaxial shaft. This enables the propeller to rotate freely, independent of compressor speed.  Owing to the additional expansion in the turbine system, the residual energy in the exhaust jet is low. Consequently, the exhaust jet produces (typically) less than 10% of the total thrust.  Propellers are not efficient when the tips reach or exceed supersonic speeds. For this reason, a reduction gearbox is placed in the drive line between the power turbine and the propeller to allow the turbine to operate at its most efficient speed. The gearbox is part of the engine and contains the parts necessary to operate a constant speed propeller.
  8. 8. 3)Turbofans Engine- The turbofan engine is a jet engine with a large fan at the front. The fan sucks in air and most of the air flows around the outside of the engine, which make it operate quietly and provides more thrust at low speeds. Nowadays, most airliners are powered by turbofan engines. Compared to the turbojet, the turbofan engine has many advantages. In a turbojet all the air passes through the compressor, combustion chamber, and turbine. In a turbofan engine only a proportion of the incoming air goes into the gas generator. The rest of the air is directly ejected out of the engine, or mixed with the gas generator exhaust to produce a "hot" jet. The aim of this system is to increase the thrust without increasing fuel consumption. It achieves this by increasing the total mass of air that passes through the engine and reduces the velocity within the same total energy supply.
  9. 9. TYPES OF TURBOFANS-  Low-bypass turbofan  Afterburning turbofan  High-bypass turbofan Low Bypass Turbofan High Bypass Turbofan
  10. 10. 4)Turboshafts Engine- The Turbo-shaft engine is another form of gas-turbine engine, which is widely used in helicopters. It operates like a turboprop system. However, it does not have a propeller but drives the helicopter rotor instead. The turbo-shaft engine is designed to keep the speed of the helicopter rotor independent from the rotating speed of the gas generator. It allows the rotating speed of the rotor to remain constant even when the rotating speed of the generator is varied to adjust the amount of power it produces.
  11. 11. 5)Ramjets- The Ramjet engine is the simplest jet engine. It has no moving parts. It is essentially a turbojet engine without the rotating machinery inside the engine. So its compression ratio depends wholly on its forward speed. Because of this fact, it can not produce static thrust and it produces very little thrust, when the speed is below the speed of the sound. Consequently, a ramjet vehicle cannot take off by itself. So, other means, such as another aircraft may be needed to help it to take off. This engine is used in guided-missile system, and space vehicles.
  12. 12. DESIGN  A ramjet is designed around its inlet. An object moving at high speed through air generates a high pressure region upstream. A ramjet uses this high pressure in front of the engine to force air through the tube, where it is heated by combusting some of it with fuel. It is then passed through a nozzle to accelerate it to supersonic speeds. This acceleration gives the ramjet forward thrust.  Inlet- Ramjets try to exploit the very high dynamic pressure within the air approaching the intake lip. An efficient intake will recover much of the free stream stagnation pressure, which is used to support the combustion and expansion process in the nozzle.  Combustor- As with other jet engines, the combustor's job is to create hot air, by burning a fuel with the air at essentially constant pressure.  Nozzles- The propelling nozzle is a critical part of a ramjet design, since it accelerates exhaust flow to produce thrust. acceleration is typically achieved via a convergent-divergent nozzle.
  13. 13. CLASSIFICATION Gas turbine are mainly divided into two group- I .Constant pressure combustion gas turbine : i) Open cycle, ii) Closed cycle II. Constant volume combustion gas turbine : In almost all the field open cycle gas turbine plants are used. Closed cycle plants were introduced at one stage because of their ability to burn cheap fuel.
  14. 14. ADVANTAGES AND DISADVANTAGES OF CLOSED CYCLE OVER OPEN CYCLE  Advantages of closed cycle: i) Higher thermal efficiency ii) Reduced size iii) No contamination iv) Improved heat transmission v) Improved part load h vi) Lesser fluid friction vii) No loss of working medium viii) Greater output and ix) Inexpensive fuel.  Disadvantages of closed cycle: i) Complexity ii) Large amount of cooling water is required. This limits its use of stationary installation or marine use iii) Dependent system iv) The wt of the system pre kW developed is high comparatively, not economical for moving vehicles v) Requires the use of a very large air heater.
  15. 15. JET PROPULSION  Jet propulsion is thrust produced by passing a jet of matter (typically air or water) in the opposite direction to the direction of motion. By conservation of momentum, the moving body is propelled in the opposite direction to the jet.  A number of animals, including cephalopods, sea hares, arthropods, and fish have convergently evolved jet propulsion mechanisms. This is most commonly used in the jet engine, but is also the means of propulsion utilized by NASA to power various space craft.
  16. 16. JET PROPULSION IN ANIMALS  Jet propulsion in cephalopods is produced by water being exhaled through a siphon, which typically narrows to a small opening to produce the maximum exhalent velocity. The water passes through the gills prior to exhalation, fulfilling the dual purpose of respiration and locomotion. Sea hares (gastropod mollusks) employ a similar means of jet propulsion, but without the sophisticated neurological machinery of cephalopods they navigate somewhat more clumsily.  Some teleost fish have also developed jet propulsion, passing water through the gills to supplement fin-driven motion.  In some dragonfly larvae, jet propulsion is achieved by the expulsion of water from a specialized cavity through the anus. Given the small size of the organism, a great speed is achieved.  Scallops and cardiids, siphonophores, tunicates (such as salps), and some jellyfish also employ jet propulsion. The most efficient jet-propelled organisms are the salps,which use an order of magnitude less energy (per kilogram per meter) than squid.