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 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 refer to 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, velocity and volume of the gas flow. This is directed through a diffuser (nozzle) over the turbine's blades, spinning the turbine and powering the compressor. Energy is extracted in the form of shaft power, compressed air and thrust, in any combination, and used to power aircraft, trains, ships and generators.
Theory of operationGas 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.
As with all cyclic heat engines, higher combustion temperature means greater efficiency. The limiting factor is the ability of the steel, 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 may have multiple shafts (spools), hundreds of turbine blades, movable stator blades, and a vast system of complex piping, combustors and heat exchangers. The largest gas turbines operate at 3,000 or 3,600 rpm to match the AC power grid. They require a dedicated building. Smaller turbines, with fewer compressor/turbine stages, spin faster. Jet engines operate around 10,000 rpm and microturbines 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 hydrodynamic foil bearings, which have become common place in microturbines and APU’s (auxiliary power units.) Jet enginesSee jet engine page. Gas turbines for electrical power production Powerplant gas turbines range in size from truck-mounted mobile plants to enormous, complex systems. They can be particularly efficient—up to 60 percent—when waste heat from the gas turbine is recovered by a conventional steam turbine in a combined cycle. 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 powerplant turbines typically have axial compressors and axial turbines with 30 to 40 stages (see image at left). They approach 40 percent thermal efficiency.
Microturbines Microturbines are becoming wide spread for distributed power and generator applications. They range from handheld units producing less than a kilowatt to power station units producing megawatts. Also known as:
Part of their success is due to advances in electronics, which allow unattended operation and interfacing with the commercial power grid. Electronic power switching technology eliminates the need for the generator to be synchronized with the power grid. This allows, for example, the generator to be integrated with the turbine shaft, and to double as the starter motor. 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. Micro turbine 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 micro turbine efficiencies are 20 to 35 percent. When in a combined heat and power system, overall efficiencies of greater than 90 percent may be achieved. Auxiliary power unitsAPUs 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 vehiclesGas turbines are used on ships, locomotives, helicopters, and in the M1 Abrams and T-80 tanks. 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 140 km/h, at a turbine speed of 50,000 rpm. The car ran on petrol, paraffin or diesel oil, but fuel consumption problems proved insurmountable for a production car. It is currently on display at the London Science Museum. 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 107.8 mph (173 km) and had a top speed of 142 mph (229 km). American car manufacturer Chrysler demonstrated several prototype gas turbine-powered cars from the early 1950s through the early 1980s. A history of Chrysler turbine cars (http://www.aardvark.co.nz/pjet/chrysler.shtml) In 1993 General Motors introduced the first commercial gas turbine powered hybrid vehicle—as a limited production run of the EV-1. A Williams International 40 kW turbine drove an alternator which powered the battery-electric powertrain. The turbine design included a recuperator. Amateur gas turbinesA 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. There are several small businesses that produce gas turbines for model planes. Advances in technologyGas turbine technology has steadily advanced since its inception and continues to evolve; research is active in producing ever smaller gas turbines. Computer design, specifically CFD and finite element analysis along with material advances, has allowed higher compression ratios and temperatures, more efficient combustion, better cooling of engine parts and reduced emissions. Additionally, compliant foil bearings were commercially introduced to gas turbines in the 1990s. They can withstand over a hundred thousand start/stop cycles and eliminated the need for an oil system. On another front, 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 do not require an oil system and can run unattended for months on end. Naval useGas turbines are used in many naval vessels, where they are valued for their high power-to-weight ratio and their ships' resulting acceleration and ability to get underway quickly. The first gas-turbine-powered naval vessels were the U.S. Coast Guard's Hamilton-class High Endurance Cutters. Since then, they have powered the U.S. Navy's Perry-class frigates, Spruance-class and Arleigh Burke-class destroyers, and Ticonderoga-class guided missile cruisers. USS Makin Island, a modified Wasp-class amphibious assault ship, is to be the Navy's first amphib powered by gas turbines. See alsoExternal links
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