Presented by:
JAGADEESH D.L

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Gas turbine
Introduction
A gas turbine is an engine where fuel is continuously burnt with compressed air to produce a stream of hot, fast moving gas. This gas stream is used to power the compressor that supplies the air to the engine as well as providing excess energy that may be used to do other work.
The engine consists of three main parts. The compressor, the combustor and the turbine.
The compressor usually sits at the front of the engine. There are two main types of compressor, the centrifugal compressor and the axial compressor. The compressor will draw in air and compress it before it is fed into the combustion chamber. In both types the compressor rotates and it is driven by a shaft that passes through the middle of the engine and is attached to the turbine.
The combustor is where fuel is added to the compressed air and burnt to produce high velocity exhaust gas. Down the middle of the combustor runs the flame tube. The flame tube has a series of holes in it to allow in the compressed air. It is inside the flame tube that fuel is injected and burnt. There will be one or more igniters that project into the flame tube to start the mixture burning. Air and fuel are continually being added into the combustor once the engine is running. Combustion will continue without the use of the igniters once the engine has been started. The combustor and flame tube must be very carefully designed to allow combustion to take place efficiently and reliably. This is especially difficult given the large amount of fast moving air being supplied by the compressor. The holes in the flame tube must be carefully sized and positioned. Smaller holes around where the fuel is added provide the correct mixture to burn. This is called the primary zone. Holes further down the flame tube allow in extra air to complete the combustion. This is the secondary zone. A final set of hole just before the entry to the turbine allow the remainder of the air to mix with the hot gases to cool them before they hit the turbine. This final zone is known as the dilution zone. The exhaust gas is fed from the end of the flame tube into the turbine.
The turbine extracts energy from the exhaust gas. The turbine can, like the compressor, be centrifugal or axial. In each type the fast moving exhaust gas is used to spin the turbine. Since the turbine is attached to the same shaft as the compressor at the front of the engine the turbine and compressor will turn together. The turbine may extract just enough energy to turn the compressor. The rest of the exhaust gas is left to exit the rear of the engine to provide thrust as in a pure jet engine. Or extra turbine stages may be used to turn other shafts to power other machinery such as the rotors of a helicopter, the propellers of a ship or electrical generators in power stations.
Cold air is drawn in from the left into the compressor (blue). The compressed air (light blue) then goes into the combustor. From the outside of the combustor the air goes through holes (purple) into the flame tube (yellow). Fuel is injected (green) into the flame tube and ignited. The igniters are not show here. The hot exhaust gas flows from the end of the flame tube past the turbine (red) rotating it as it passes. From there the exhaust exits the engine. The turbine is connected via a shaft (black) to the compressor. Hence as the turbine rotates the compressor rotates with it drawing in more air to continue the cycle.
Obviously this is only a simplified view of a gas turbine engine and a working engine is much more complicated.
History
1500: The "Chimney Jack" was drawn by Leonardo da Vinci which was turning a roasting spit. Hot air from a fire rose through a series of fans which connect and turn the roasting spit.
1629: Jets of steam rotated a turbine that then rotated driven machinery allowed a stamping mill to be developed by Giovanni Branca.
1678: Ferdinand Verbeist built a model carriage relying on a steam jet for power.
1791: A basic turbine engine was patented with all the same elements as today's modern gas turbines. The turbine was designed to power a horseless carriage.
1872: The first true gas turbine engine was designed by Dr F. Stolze, but the engine never ran under its own power.
1897: A steam turbine for propelling a ship was patented by Sir Charles Parsons. This principle of propulsion is still of some use.
1903: A Norwegian, Ægidius Elling, was able to build the first gas turbine that was able to produce more power than needed to run its own components, which was considered an achievement in a time when knowledge about aerodynamics was limited. Using rotary compressors and turbines it produced 11 hp (massive for those days). His work was later used by Sir Frank Whittle.
1914: The first application for a gas turbine engine was filed by Charles Curtis.
1918: One of the leading gas turbine manufacturers of today, General Electric, started their gas turbine division.
1920. The then current gas flow through passages was developed by Dr A. A. Griffith to a turbine theory with gas flow past airfoils.
1930. Sir Frank Whittle patented the design for a gas turbine for jet propulsion. His work on gas propulsion relied on the work from all those who had previously worked in the same field and he has himself stated that his invention would be hard to achieve without the works of Ægidius Elling. The first successful use of his engine was in April 1937.
1934. Raúl Pateras de Pescara patented the free-piston engine as a gas generator for gas turbines.
1936. Hans von Ohain and Max Hahn in Germany developed their own patented engine design at the same time that Sir Frank Whittle was developing his design in England.
Theory of operation
Gas turbines are described thermodynamically by the Brayton cycle, in which air is compressed isentropically, combustion occurs at constant pressure, and expansion over the turbine occurs isentropically back to the starting pressure.
In practice, friction, and turbulence cause:
a) non-isentropic compression - for a given overall pressure ratio, the compressor delivery temperature is higher than ideal.
b) non-isentropic expansion - although the turbine temperature drop necessary to drive the compressor is unaffected, the associated pressure ratio is greater, which decreases the expansion available to provide useful work. available to provide useful work.
c) pressure losses in the air intake, combustor and exhaust - reduces the expansion
As with all cyclic heat engines, higher combustion temperature means greater efficiency. The limiting factor is the ability of the steel, nickel, ceramic, or other materials that make up the engine to withstand heat and pressure. Considerable engineering goes into keeping the turbine parts cool. Most turbines also try to recover exhaust heat, which otherwise is wasted energy. Recuperators are heat exchangers that pass exhaust heat to the compressed air, prior to combustion. Combined cycle designs pass waste heat to steam turbine systems. And combined heat and power (co-generation) uses waste heat for hot water production.
Mechanically, gas turbines can be considerably less complex than internal combustion piston engines. Simple turbines might have one moving part: the shaft/compressor/turbine/alternator-rotor assembly (see image above), not counting the fuel system.
More sophisticated turbines (such as those found in modern jet engines) may have multiple shafts (spools), hundreds of turbine blades, movable stator blades, and a vast system of complex piping, combustors and heat exchangers.
As a general rule, the smaller the engine the higher the rotation rate of the shaft(s) needs to be to maintain tip speed. Turbine blade tip speed determines the maximum pressure that can be gained, independent of the size of the engine. Jet engines operate around 10,000 rpm and micro turbines around 100,000 rpm.
Thrust bearings and journal bearings are a critical part of design. Traditionally, they have been hydrodynamic oil bearings, or oil-cooled ball bearings. This is giving way to foil bearings, which have been successfully used in micro turbines and auxiliary power
Jet engines
Jet engines are gas turbines optimized to produce thrust from the exhaust gases.
Auxiliary power units
Auxiliary power units (APUs) are small gas turbines designed for auxiliary power of larger machines, such as those inside an aircraft. They supply compressed air for aircraft ventilation (with an appropriate compressor design), start-up power for larger jet engines, and electrical and hydraulic power. These are not to be confused with the auxiliary propulsion units, also abbreviated APUs, aboard the gas-turbine-powered Oliver Hazard Perry-class guided-missile frigates. The Perrys' APUs are large electric motors that provide maneuvering help in close waters, or emergency backup if the gas turbines are not working.
Gas turbines for electrical power production
Industrial gas turbines range in size from truck-mounted mobile plants to enormous, complex systems. They can be particularly efficient — up to 60% — when waste heat from the gas turbine is recovered by a heat recovery steam generator to power a conventional steam turbine in a combined cycle configuration. They can also be run in a cogeneration configuration: the exhaust is used for space or water heating, or drives an absorption chiller for cooling or refrigeration. A cogeneration configuration can be over 90% efficient. The power turbines in the largest industrial gas turbines operate at 3,000 or 3,600 rpm to match the AC power grid frequency and to avoid the need for a reduction gearbox. Such engines require a dedicated enclosure.
Simple cycle gas turbines in the power industry require smaller capital investment than coal, nuclear or even combined cycle natural gas plants and can be designed to generate small or large amounts of power. Also, the actual construction process can take as little as several weeks to a few months, compared to years for base load power plants. Their other main advantage is the ability to be turned on and off within minutes, supplying power during peak demand. Since they are less efficient than combined cycle plants, they are usually used as peaking power plants, which operate anywhere from several hours per day to a couple dozen hours per year, depending on the electricity demand and the generating capacity of the region. In areas with a shortage of base load and load following power plant capacity, a gas turbine power plant may regularly operate during most hours of the day and even into the evening. A typical large simple cycle gas turbine may produce 100 to 300 megawatts of power and have 35 to 40% thermal efficiency. The most efficient turbines have reached 46% efficiency.
Turboshaft engines
Turboshaft engines are often used to drive compression trains (for example in gas pumping stations or natural gas liquefaction plants)and are used to power almost all modern helicopters. The first shaft bears the compressor and the high speed turbine (often referred to as “Gas Generator” or "N1"), while the second shaft bears the low speed turbine (or “Power Turbine” or "N2"). This arrangement is used to increase speed and power output flexibility.