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Home , Steam turbine

Technologue

The Adaptable Gas

Lee S. Langston

urbines have been around for a is also called a combustion turbine, a tur- Tlong time—windmills and Whether creating boshaft 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 , and vari- vortex, and thus the defining property 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 . , , fanjet and Hydrocarbon-fueled , how- continues to inspire or prop jet (if it is used to drive a pro- ever, are one of the youngest energy peller). The -combustor- conversion devices: Their first use in innovation turbine part of the 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 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 continues to be innovative. ers convert heat supplied by . 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 en- duced in the engine (and pulling the (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 ; 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 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 —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 en- gine. 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 commer- cial 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. have small- fan casing, box, low-pressure compressor, high-pressure compressor, annular combus- tor, 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 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 . 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 nonavia- tion applications are called aeroderiva- tive gas turbines. Aeroderivatives are used to drive natural gas pipeline exhaust , power and pro- duce 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. In a land-based gas turbine, periods, such as summer air condition- the turning rotors spin the shaft on a power turbine to create electricity. The hot exhaust can be ing in major cities. put to additional uses in creating heat or power to increase efficiency. www.americanscientist.org 2013 July–August 265 was not part of aircraft flight. Before jet engines, an aviation piston engine manufacturer could expect to sell 20 to 30 times the original cost of the engines in aftermarket parts. With the advent of the jet engine, this aftermarket fig- ure dropped to three to five times the original cost (an important reduction that made air travel affordable and reli- able, and airlines profitable, although engine manufacturers have had to alter their business models). In recent years, technology and market demands have resulted in even longer lasting engine components, dropping the aftermarket figure to increasingly lower levels. A well-managed airline will try to keep a jet-powered plane in the air as much as 18 hours a day, 365 days a year. If well maintained, the airline expects the engines to remain in service and on the wing for 15,000 to 30,000 hours of operation, depending on the number of A 375-megawatt gas turbine is at the heart of 578-megawatt combined-cycle power plant in takeoffs and landings experienced by Irsching, Germany. The gas turbine alone weighs 489 tons. The combined-cycle power plant the plane. After this period, the jet en- has reached a thermal efficiency of 60.75 percent, which probably makes it the most efficient gine will be taken off and overhauled, ever operated. (Photograph courtesy of AG.) usually with replacement of parts that experience heating, such as the com- oratory, early gas turbine compressor or sorghum). Some recent work in South bustor and turbine. (Currently the in- designs foundered on a rock, and the Africa on a type of flight shutdown rate of a jet engine is rock was stall. Stall is the sudden block- called a pebble bed reactor (which uses ten- less than 1 per 100,000 flight hours. In age and even reversal of engine flow, nis ball–sized spheres of graphite em- other words, on average, an engine fails caused when fluid separated away from bedded with fissile material) provided in flight once every 30 years.) the compressor surfaces instead helium gas to power a type of turbine Aircraft jet engines make up about of flowing evenly over them. Taylor that has a closed cycle, meaning it uses a 25 percent of the cost of the airplane. paraphrased P. T. Barnum’s words to gas preheated by an external source that In 2011 the worldwide aviation gas tur- describe two kinds of stall: You can op- is recirculated through the system. bine market amounted to $32 billion, of erate a compressor so it stalls all of the An additional advantage of gas tur- which $27 billion was for commercial blades some of the time (called surge) or bines is that the usual working fluid aircraft, with the remainder for mili- some of the blades all of the time (called is atmospheric air, and the machine tary applications. Currently there are rotating stall). It took much early re- does not require liquid cooling—an about 19,400 airplanes in the world- search and development to avoid such important consideration in many parts wide air transport fleet. Both major stall conditions. of the world, where cooling water is in airplane manufacturers, Boeing in the Although a gas turbine must be short supply. United States and Airbus in Europe, started by some external means (a In the early days of its develop- project that there will be 34,000 aircraft small external motor or other source, ment, one of the major disadvantages in world fleets by 2030. such as another gas turbine), it can be of the gas turbine was its lower effi- This promising market is stimulat- brought up to full load (peak output) ciency (hence higher fuel usage) when ing jet engine development for com- conditions in minutes, in contrast with compared to other engines and steam mercial airlines, with an emphasis a steam turbine plant whose startup turbine power plants. However, over on fuel economy. Currently, 40 to 60 time is measured in hours. the past 70 years, continuous engineer- percent of airline operating expenses Gas turbines can also use a variety of ing development has pushed the ther- are jet fuel costs. The Pratt & Whitney fuels. Natural gas is commonly used in mal efficiency (18 percent for the 1939 turbofan engine shown in the first fig- land-based gas turbines, whereas light Brown Boveri gas turbine) to present ure is currently being developed for distillate (or kerosene-like) oils power air- levels of about 45 percent for simple new, single-aisle, 90- to 200-passenger craft jet engines and marine gas turbines. cycle operation. Efficiencies can reach aircraft. This engine has a hub-mount- Diesel oil or specially treated residual oils over 60 percent for combined-cycle op- ed gearing system that drives the (such as biodiesel) can also be used, as erations, where exhaust gases are put front mounted fan at lower speeds, well as combustible gases (such as meth- to additional use. permitting as much as 16 percent less ane) derived from blast furnaces, refiner- fuel consumption and much reduced ies, landfills, sewage and gasification of Taking Off engine noise. Later, the geared-fan solid fuels such as , wood chips and It is now hard to remember when the technology may be applied to higher- bagasse (the crushed stalks of sugarcane aviation gas turbine—the jet engine— thrust engines for larger airplanes.

266 American Scientist, Volume 101 Although military jet engines repre- sent a smaller segment of the gas tur- bine market, the technology developed gas turbine generator electricity there has historically resulted in ben- efits for commercial aviation. The new U.S. F135 Joint Strike Fighter engine, at 40,000 pounds of thrust, is a case in heat point. It powers three variants of air- condensor craft: An Air Force fighter that takes off recovery steam conventionally, a carrier-based Navy cooling generator water jet and a short takeoff/vertical landing exhaust aircraft for the Marines. Temperatures in the Joint Strike Fighter engine run 3,600 degrees Fahr- enheit (1,982 degrees Celsius). How do steam turbine generator electricity the cobalt-nickel alloy turbine survive such running conditions? The vanes and blades are cooled to some eight-tenths to nine-tenths of their al- A combined-cycle gas turbine power plant takes the exhaust from the gas turbine, typically at loy melting temperatures (2,200 to temperatures around 1,000 degrees Fahrenheit, and uses it to produce steam that is fed into a steam 2,600 degrees Fahrenheit). Each high- turbine. Spent steam from the steam turbine is fed into a condensor so water can be recovered and temperature turbine airfoil is formed recirculated in the system. Both the gas turbine and the steam turbine produce electricity, increas- from an elaborate casting to accommo- ing the overall efficiency of converting fuel into power for the combined plant to about 58 percent. date the intricate internal passages and surface hole patterns necessary to chan- A good efficiency value for modern Such increasing need for power is nel and direct cooling air (bled from the gas turbines is 40 percent, whereas a being fulfilled by gas turbines, in both compressor) within and over its exter- steam turbine at typical combined-cycle flight propulsion and electrical gen- nal surfaces. An error in hole location or conditions is about 30 percent. Using eration. One can safely predict that the cooling air pressure ratios could cause the first law of thermodynamics and gas turbine will increase its role as a airfoil gas path inhalation rather than the definition of thermal efficiency, the prime mover, as engineers continue to cooling exhalation, which at such high combined efficiency of the two is about improve its performance and find new temperatures would be catastrophic. 58 percent, greater than either of the uses for it. The cooling design is based on some individual devices alone. 30 years of research and unequivocally The heart of the combined-cycle Bibliography pushes forward the state-of-the-art of plant (or more accurately, the combined Bathie, W. W. 1996. Fundamentals of Gas Tur- turbine performance and durability. power plant, because the thermody- bines, 2nd edition. New York: John Wiley namic cycles aren’t combined) is the gas & Sons. Working Together turbine with its gas exhaust tempera- Conner, M. 2001. Hans von Ohain: Elegance in Flight. Reston, VA: American Institute of In the past 30 years, advances in non- ture, typically at about 1,000 degrees Aeronautics and Astronautics. aviation technology have almost dou- Fahrenheit (or 538 degrees Celsius), suf- Golley, J. 1987. Whittle: The True Story. Wash- bled the thermal efficiency of new gas ficient to produce steam to power the ington, DC: Smithsonian Institution Press. turbine electric power plants. In 2011, steam turbine. The Siemens 375 mega- Horlock, J. H. 1992. Combined Power Plants. the worldwide market for nonaviation watt gas turbine shown in the third fig- Oxford, England: Pergamon Press. gas turbines came in at $16 billion, most ure is the center of a new 578-megawatt Langston, L. S. 2013. Not so simple . of it for new electrical plants. Modern combined-cycle gas turbine plant in Mechanical Engineering Magazine Janu- gas turbine combined-cycle power Irsching, Germany. On May 19, 2011, ary:46–51. Langston, L. S. 2012. Breaking the barrier. Me- plants produce electric power at levels Siemens announced it had reached chanical Engineering Magazine May:33–37. as high as half a gigawatt, with ther- a thermal efficiency of 60.75 percent, Langston, L. S. 2008. Pebbles making waves. mal efficiencies that now exceed the which probably makes it the most ef- Mechanical Engineering Magazine Febru- 60 percent mark—almost twice what ficient heat engine ever operated. ary:34–38. I learned about as an undergraduate Langston, L. S. 2007. Fahrenheit 3,600. Me- mechanical engineering student. More Power chanical Engineering Magazine April:34–37. A combined-cycle gas turbine power “I sell here, sir, what all the world de- Langston, L. S. 2004. Turbines, Gas. Encyclope- plant uses a gas turbine (usually fueled sires to have—POWER.” These were the dia of Energy, Volume 6. San Diego: Elsevier, pp. 221–230. by natural gas) to drive an electrical words of early British industrialist Mat- Taylor, E. S. 1970. Evolution of the jet engine. generator. The hot exhaust is then used thew Boulton to James Boswell, quoted Astronautics & Aeronautics 8:64–72. to produce steam in a heat exchanger in Boswell’s 1791 book The Life of Samuel Van der Linden, Septimus. The World’s First (called a heat recovery steam generator) Johnson. Boulton and his partner, Scot- Industrial Gas Turbine Set at Neuchatel to supply a steam turbine whose use- tish engineer James Watt, manufactured (1939): An International Historic Mechani- ful work output provides the means to the first steam engines. Their firm went cal Engineering Landmark, September 2, 1988. New York: The American Society of generate more electricity. (If the steam is out of business long ago, but the world’s Mechanical Engineers. http://files.asme. used instead to heat buildings, the unit need for power has multiplied many org/ASMEORG/Communities/History/ would be called a plant.) times over since Boulton met Boswell. Landmarks/5604.pdf www.americanscientist.org 2013 July–August 267