The Adaptable Gas Turbine

Technologue The Adaptable Gas Turbine Lee S. Langston urbines have been around for a is also called a combustion turbine, a tur- Tlong time—windmills and water Whether creating boshaft engine and sometimes a gas tur- wheels are early examples. The name bine engine. For aviation applications it comes from the Latin turbo, meaning is usually called a jet engine, and vari- vortex, and thus the defining property electricity or moving ous other names (depending on the of a turbine is that a fluid or gas turns particular aviation configuration or the blades of a rotor, which is attached planes, this engine application) such as jet turbine engine, to a shaft that can perform useful work. turbojet, turbofan, fanjet and turboprop Hydrocarbon-fueled turbines, how- continues to inspire or prop jet (if it is used to drive a pro- ever, are one of the youngest energy peller). The compressor-combustor- conversion devices: Their first use in innovation turbine part of the gas turbine is com- either generating electricity or power- monly called the gas generator. ing jet aircraft flight took place in 1939. Through the efforts of many thousands and develop energy conversion devices. In Flight of engineers in the intervening 70 years An example of this conversion is trans- In an aircraft gas turbine, all of the tur- or so, such gas turbines have come to forming heat (say, from the combus- bine power is used to drive the com- dominate aircraft propulsion and, with tion of a hydrocarbon fuel) into motive pressor (which may also have an as- their now-unmatched thermal efficiency power (such as a jet powered airplane) sociated fan or propeller). The gas flow and low cost, are the superstars of elec- or electricity. Devices that perform this leaving the turbine is then accelerated tric power plants. With energy a central transformation are called prime movers. to the atmosphere through an exhaust concern in modern society, gas turbine The major modern-day prime mov- nozzle to provide thrust or propulsion technology continues to be innovative. ers convert heat supplied by nuclear power. Gas turbine or jet engine thrust or chemical reactions into useful forms power is equal to the momentum in- Power Conversion of energy. The gas turbine, co-invented crease in the mass flow from engine Much of my efforts as a mechanical en- by Hans von Ohain, Frank Whittle and inlet to exit, multiplied by the flight gineer, both in industry and academia, the engineers at the Swiss firm Brown, velocity. The actual thrust force pro- have been guided by the first law of Boveri & Cie, succeeded the steam en- duced in the engine (and pulling the thermodynamics (stated in the principle gine, realized in 1769 by Thomas New- plane forward) is the summation of of the conservation of energy): Energy is comen and James Watt; the spark igni- all the axial components of pressure neither created nor destroyed, but can tion engine of Nikolaus Otto from 1876; forces on the internal surfaces of the be changed in form. The “changed in the compression ignition engine of Ru- engine exposed to the gas path flow. form” part of the law is what many me- dolf Diesel from 1884 and the steam tur- A jet engine can be small enough chanical engineers do, as they research bine of Charles Parsons from 1897. to be handheld and produce a few The name gas turbine is somewhat pounds of thrust (1 pound of thrust is misleading, for it implies a simple tur- equivalent to 4.45 newtons of force) to Lee S. Langston is a professor emeritus of mechanical bine that uses gas as a working fluid. be used on model airplanes or military engineering at the University of Connecticut. He received a Ph.D. in 1964 from Stanford University. Actually, a gas turbine has a compressor drones. (The retired Swiss pilot Yves He was with Pratt & Whitney Aircraft as a research to draw in and compress gas (usually Rossy, nicknamed “Jetman,” attached engineer working on fuel cells, heat pipes and jet en- air), a combustor (or burner) to add com- four such small jet engines—each pro- gines from 1964 to 1977. He is a Life Fellow of the bustive fuel (usually a hydrocarbon liq- ducing 50 pounds of thrust or about American Society of Mechanical Engineers (ASME), uid or gas) to heat the compressed gas, 223 newtons—to a back-mounted has served as editor of ASME’s Journal of Engineer- and a turbine (or expander) to extract wing and flew across the English ing for Gas Turbines and Power and has been a power from the hot gas flow with its Channel in 2008 and over the Grand member of the Board of Directors of the ASME In- rotation of the turbine blades. Canyon in 2011.) On modern commer- ternational Gas Turbine Institute (IGTI). For the past Because the origin of the gas turbine cial jet aircraft, gas turbines are typi- 10 years he has written an annual review of the gas lies in both the electric power field and cally in the range of 30,000 pounds turbine industry for IGTI and ASME’s Mechani- cal Engineering Magazine. Address: University of aviation, there has been a profusion of of thrust (or 136,000 newtons), with Connecticut, Department of Mechanical Engineering, other names for the gas turbine. For the largest currently at about 100,000 191 Auditorium Road, U-3139, Storrs, CT 06269- land and marine applications the gas pounds of thrust (445,000 newtons) on 3139. Email: [email protected] turbine moniker is most common, but it Boeing’s long-range 777 airplanes. 264 American Scientist, Volume 101 The jet engine shown in the first figure is a turbofan engine, with a larger-diameter compressor-mounted fan. Thrust is generated by air passing through the fan alone (called bypass air) and through the gas generator itself. The combination of mechanisms greatly increases the fuel efficiency of the engine. With a large frontal area to pull in a higher mass of air (with the trade-off that the configuration does engender higher aerodynamic drag forces at cruis- ing flight velocities), the turbofan engine generates peak thrust at takeoff speeds. It is therefore most suitable for commercial aircraft, which need most of their lift to get off the ground, not to maneuver once in the air. In contrast, a turbojet does not have a fan and generates all of its thrust from air that passes through A cutaway illustration of a geared turbofan jet engine shows (from left to right) the fan and the gas generator. Turbojets have small- fan casing, gear box, low-pressure compressor, high-pressure compressor, annular combustor, high-pressure turbine and low-pressure turbine. The fan’s diameter ranges from 56 to 81 er frontal areas (and thus lower drag at inches, depending on the level of thrust the model of engine produces, which ranges from high flight velocities) and generate peak 15,000 to 30,000 pounds of thrust. (Image courtesy of Pratt & Whitney.) thrusts at high speeds, making them most suitable for fighter aircraft that travel at much higher velocities than Why Turbines? mechanical life is long and the corre- commercial craft. The gas turbine has some design ad- sponding maintenance cost is relatively vantages over other power systems. It is low. However, during its early develop- Grounded Power capable of producing large amounts of ment, the deceptive simplicity of the gas In nonaviation gas turbines, only part useful power for a relatively small size turbine caused problems, until aspects of the turbine power is used to drive and weight. Because motion of all its of its fluid mechanics, heat transfer and the compressor. The remainder is used major components involves pure rota- combustion were better understood. In as output shaft power to turn an energy tion (there is, for instance, no reciprocat- the words of Edward Taylor, the first conversion device, such as an electrical ing motion as in a piston engine), its director of the MIT Gas Turbine Lab- generator, or to compress natural gas in a pipeline so it can be transported. jet engine Shaft power land-based gas turbines can get very large (with an output as high as 375 megawatts, enough to power about 300,000 homes). The unit exhaust shown in the third figure is called an industrial or frame machine. It is con- structed for ruggedness and long life, so weight is not a major factor as it is with a jet engine. Typically frame ma- nozzle thrust power chines are designed conservatively but air in compressor fuel combustion turbine have made use of technical advances in jet engine development when it has power turbine shaft power made sense to do so. Lighter-weight gas turbines derived from jet engines and used for nonaviation applications are called aeroderiva- tive gas turbines. Aeroderivatives are used to drive natural gas pipeline exhaust compressors, power ships and produce electric power. They are used particularly to provide peaking and gas turbine intermediate power for electric utili- A jet engine (top) and a land-based gas turbine (bottom) function in the same manner, but ties, because they can start up quickly. with different end products. In both, air is taken in and compressed, then fuel is added and Peaking power supplements a utility’s combusted to heat the air. The heated air then turns the rotors of a turbine. In a jet engine, the normal output during high-demand hot exhaust is expelled through a narrower nozzle to create thrust.

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