Gas turbine
The world's first commercial, oil-free gas turbine (manufactured by Capstone). This machine has a single stage radial compressor and turbine, a recouperator, and foil bearings
A gas turbine is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between.
Gas turbine may also mean just the turbine element.
Energy is added to the gas stream in the combustor, where Air is mixed with fuel and ignited. Combustion increases the temperature and volume of the gas flow. This is directed through a nozzle over the turbine's blades, spinning the turbine and powering the compressor.
Energy is extracted in the form of shaft power, compressed air, or thrust. Such uses are for powering aircraft, trains, and generators.
Theory of operation
Gas turbines are described thermodynamically by the Brayton cycle. In this cycle the air is compressed isentropically, combustion occurs at constant pressure, and expansion over the turbine occurs isentropically back to the starting pressure.
As with all cyclic heat engines, a greater combustion temperature means greater efficiency. Material properties are the limiting factor for combustion temperature. Considerable engineering goes into keeping the turbine parts as cool as possible. 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 piston Internal combustion engines. Simple turbines might have one moving part (aside from the fuel system); the shaft/compressor/turbine/alternator-rotor assembly (see image above).
More sophisticated turbines may have multiple shafts (spools), hundreds turbine blades, movable stator blades, and a vast system of complex plumbing, combustors and heat exchangers.
The very largest gas turbines operate at 3000 or 3600RPM to match the AC power grid. They require a dedicated building for installation. Smaller turbines, with fewer compressor/turbine stages, need to spin faster. Jet engines operate around 10,000RPM and microturbines around 100,000 RPM.
Thrust 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 hydrodynamic foil bearings, which have become common place in microturbines and APU’s (auxiliary power units.)
Jet engines
See jet engine page.
GE H series power generation gas turbine. This 400MW unit has an efficiency of over 60% in [[combined cycle
Power plant gas turbines range in size from truck mounted mobile plants to enormous, complex systems.
They can be particularly efficient when waste heat from the gas turbine is recovered by a conventional steam turbine in a process known as a combined cycle. Efficiencies of 60% can be realized.
Gas turbines in the power industry require smaller capital investment than coal or nuclear and can be designed to generate small or large amounts of power. Their main advantage is the ability to be turned on and off within minutes, supplying power during peak demand. Large turbines may produce hundreds of megawatts.
Large power plant turbines typically have axial compressor and axial turbine designs with 30-40 stages (see image at left). They approach 40% efficiency.
Also Known as:
One advantage microturbine-alternators have over piston engines is their high power density (with respect to volume and weight). This is due to high rotation speed. The need for a recuperator, however, mitigates this advantage.
Microturbine designs usually consists of a single stage radial compressor, a single stage radial turbine and a recuperator. Recuperators are difficult to design and manufacture because they operate under high pressure and temperature differentials. Waste heat can be used for hot water production.
Typical microturbine efficiencies are 20-35%. When in a combined heat and power system, overall effiencies of up to 90+% may be achieved.
In 1950, designer F. R. Bell and Chief Engineer Maurice Wilks from British car manufacturers Rover unveiled the first car powered with a gas turbine engine. The two-seater JET1 had the engine positioned behind the seats, air intake grilles on either side of the car and exhaust outlets on the top of the tail.
During tests, the car reached top speeds of 140Km/h, at a turbine speed of 50,000 rpm.
The car ran on petrol, paraffin or diesel oil, but fuel consumption problems proved unsurmountable for a production car.
Rover and the BRM Formula One team joined forces to produce a gas turbine powered coupe, which entered the 1963 24 hours of Le Mans, driven by Graham Hill and Richie Ginther. It averaged 107.8mph (173km) and had a top speed of 142mph (229km).
In 1993 General Motors introduced the first commercial gas turbine powered hybrid vehicle- as a limited production test run version of the EV-1. A Williams International 40KW turbine drove an alternator which powered the battery/electric powertrain. The turbine design included a recuperator.
There are several small businesses that produce gas turbines for model planes.
Computer design, specifically CFD and FEA along with material advances have allowed higher compression ratios and temperatures, more efficient combustion, better cooling of engine parts and reduced emissions.
Compliant Foil bearings were commercially introduced to gas turbines in the 1990’s. They can withstand over a hundred thousand start/stop cycles and eliminated the need for an oil system.
Microelectronics and power switching technology have enabled commercially viable micro turbines for distributed and vehicle power. An excellent example is the Capstone line of micro turbines, which don't require an oil system and can run un-attended for months on end.
Research is active in producing ever smaller gas turbines.Gas turbines for electrical power production
Micro Turbines
Micro turbines are becoming wide spread for distributed power and generator applications. They range in size from sub-Kilowatt hand held units, to hundreds of Kilowatts and the size of a car.
Part of their success is due to advances in electronics, which allow un-attended operation and interfacing with the commercial power grid. Electronic power switching technology eliminates the need for a sycronized generator and gearing. This allows, for example, the generator to be integrated with the turbine shaft, and to double as the starter motor.Auxiliary power units
APU's are small gas turbines designed for auxiliary power of larger machines, usually aircraft. They are well suited for supplying compressed air for aircraft ventilation (with an appropriate compressor design), start-up power for larger jet engines, and electrical and hydraulic power.Gas turbines in vehicles
Gas turbines are used on ships, locomotives, helicopters, and in the M1 Abrams and T-80 tanks.Amateur gas turbines
A popular hobby is to construct a gas turbine from an automotive turbocharger. A combustion chamber is fabricated and plumbed between the compressor and turbine. Like many technology based hobbies, they tend to give rise to manufacturing businesses over time. See external links for resources.Advances in technology
Gas turbine technology has steadily advanced since its inception and continues evolve.