Gas Turbine Technology

Summary

Gas turbine technology covers the design, manufacture, operation, and maintenance of rotary conversion engines that generate power from the energy of the hot, pressurized gas they create. Key components are air and fuel intake systems, compressor, combustor, the gas turbine itself, and an output shaft in those gas turbines not designed to provide thrust alone as jet engines. Primary applications of gas turbines are jet engines and generators for industrial power and electric utilities. Sea and land vehicles can also be propelled by gas turbines.

Definition and Basic Principles

Gas turbine technology concerns itself with all aspects of designing, building, running, and servicing gas turbines, which power jet engines for airplanes and represent the heart of many contemporary power plants, among other uses. Strictly speaking, the gas turbine itself is only one part of a complex engine assembly commonly given this name.

A gas turbine employs the physical fact that thermal energy can be converted into mechanical energy. Its basic principle is often called the Brayton cycle, named after George Brayton, the American engineer who developed it in 1872. The Brayton cycle involves the compression of air, its heating by fuel combustion, and the release of the hot gas stream to expand and drive a turbine. The turbine creates both power for air compression and free power. The free power gained can be used as thrust in traditional jet engines or as mechanical power driving another unit, such as an electric generator, pump, or compressor.

The gas turbine is closely related to the steam turbine. The gas turbine got its name from the fact that it operates with air in gaseous form. A wide variety of fuels can be used to heat the gaseous air.

Background and History

The oldest known reference to an apparatus utilizing the physical principles of a gas turbine is a design by Leonardo da Vinci from 1550 for a hot-air-powered roasting spit. In 1791, British inventor John Barber obtained a patent for the design of a combined gas and steam turbine. However, the lack of suitable materials to withstand the heat and pressure needed for a working gas turbine impeded any practical applications for a long time.

In 1872, German engineer Franz Stolze designed a gas turbine, but driving its compressor used more energy than the turbine generated. In 1903, Norwegian inventor Aegidius Elling built the first gas turbine with a surplus of power. The idea to use gas turbines to build jet engines was pioneered by British aviation engineer and pilot Sir Frank Whittle in 1930. At the same time, German physicist Hans von Ohain developed a jet engine independently. On August 27, 1939, the German Heinkel He 178 became the first flying jet airplane. That year, the first power plant using a gas turbine became operational in Switzerland. Since then, the twin use of gas turbine technology either to propel jet airplanes or serve as a source for generating power on the ground, particularly electricity, has been subject to many technological advances.

How It Works

Brayton Cycle. A gas turbine engine is a device that converts fuel energy via the compression and subsequent expansion of air. Its working process is commonly called a Brayton cycle. The Brayton cycle begins with the compression of air in gaseous form, for which energy is expended. For this reason, a gas turbine assembly needs an air intake and a compressor where air is pressurized. Next, the heat from fuel combustion adds more energy to the compressed air. Fuel is burned, most commonly, internally in the engine's combustor. However, in a variation called the Ericsson cycle, the fuel is burned externally, and the generated heat is relayed to the compressed air. The third step of the cycle comes with releasing the heated gas stream through nozzles into the gas turbine proper. The gas expands as it loses some of its pressure and cools off. The energy released by gas expansion is captured by the gas turbine, which is driven in a rotary fashion as its blades are turned by the exiting hot gas stream. In the last step, the expanding air releases its leftover heat into the atmosphere.

Power Generation. As the hot gas stream flows along stator vanes to hit the airfoil-shaped rotor blades arranged on a disk inside the gas turbine, it drives the blades in rotary fashion, creating mechanical power. This power is captured by one or more output shafts, called spools. There are two uses for this power. The first is to drive the compressor of the gas turbine assembly, feeding power back to the first step of operations. Any free power remaining can be used to drive external loads or to provide thrust for jet engines, either directly through exhaust or by driving a fan or propeller.

The engineering challenge in gas turbine technology is to gain as much free power as possible. Attention has been focused on materials used inside the gas turbine, looking for those that can withstand the most heat and pressure and arranging the individual components of the gas turbine to optimize its output. The metal of single-crystal cast alloy turbine blades can withstand temperatures up to 1,950 degrees Fahrenheit (1,065 degrees Celsius). Ingenious air-cooling systems enable these blades to deal with gas as hot as 2,900 degrees Fahrenheit (1,600 degrees Celsius). Top compressors can achieve a 40:1 ratio. At the same time, designing gas turbines with multiple shafts to drive low- and high-pressure compressors or adding a power turbine behind the first turbine used only to gather sufficient power for the compressor has also improved efficiency. While early gas turbines used between 66 and 75 percent of the power they generated from fuel to drive their own compressor, leaving only 25 to 34 percent of free power, contemporary gas turbines for industrial power can achieve up to 65 percent of free power.

Fuel. Fuel for gas turbines is variable and ranges from the hydrocarbon product kerosene for jet engines to coal or natural gas for industrial gas turbines. The engineering challenges have been to optimize fuel efficiency and to lower emissions, particularly of nitrogen oxides.

Operation and Maintenance. Gas turbines require careful operation as they are very responsive, and malfunctions in either the compressor or turbine can happen in fractions of a second. The primary control systems are handled by computer and are hydromechanical and electrical. From start and stop and loading and unloading the gas turbine, operating controls cover speed, temperature, load, surges, and output. The control regimen ranges from sequencing to routine operation and protection control.

Gas turbines have some accessories to facilitate their operation. These include starting and ignition systems, lubrication and bearings, air-inlet cooling and injection systems for water, and steam or technical gases such as ammonia to control nitrogen oxide emissions.

To facilitate maintenance, gas turbines used for aircraft and those modeled after these have a modular design so that the individual components such as compressor, combustor, and turbine can be taken out of the assembly individually. Those gas turbines also allow a borescope inspection of their insides by an optical tube with a lens and eyepiece. Heavy industrial gas turbines are not designed for borescope inspection and must be dismantled for inspection and maintenance. Preventive maintenance is essential for gas turbines.

Applications and Products

Jet Engines. The first jet engines were designed to use the free power of gas turbines exclusively for thrust. This gave them a speed advantage over piston-engine aircraft but at the price of very low fuel economy. As a result, gas turbine technology developed more economical alternatives. For helicopters and smaller commercial airplanes, the turboprop system was developed. Here, the gas turbine uses its free power to drive the aircraft's propeller.

The turbofan jet engine is used in 90 percent of contemporary medium to large commercial aircraft. Efficiency is increased by adding a fan that acts like a ducted propeller and that is driven by the free power of the gas turbine. In contemporary high-bypass turbofan jet engines in use since the 1970s and continuously improved since, much of the air taken in bypasses the compressor and is directly propelled into the engine by the turbine-driven fan, up to a bypass ratio of 5:1. Only a smaller part of air is taken into a low- and then a high-pressure compressor, joined with fuel burned in the combustor and driving both a high- and low-pressure turbine. The net resulting thrust, primarily from the fan, is achieved with high fuel economy and relatively little noise.

Military aircraft fly at supersonic speed and use afterburners with their gas turbines. This is done by adding another combustor behind the turbine blades and before the exhaust nozzle, creating extra thrust at the expense of much fuel.

Aero-Derivative Gas Turbines, or "Aeros." Because of their relatively low weight, gas turbines based on jet engine design have been used on the ground for power generation and propulsion. Especially with the contemporary trend toward turbofan jet engines, an aero-derivative gas turbine that uses one or more spools, or output shafts, to provide a mechanical drive needs very little adaptation from air to ground use. There are also hybrid gas turbines that use an aero-derivative design but replace the jet engine's lighter roller and antifriction ball bearings with hydrodynamic bearings typical of the heavy industrial gas turbine.

With higher shaft speed but lower airflow through the turbine, aero-derivative turbines require less complex and shorter maintenance than other ground gas turbines. Many models, like GE Vernova's LM2500 aero-derivative gas turbine technology, can be installed in under two weeks by a twenty-man crew. They are often used in remote areas, where they are employed to drive pumps and compressors for pipelines. They have cold-start times of 5 minutes, and because of their quicker start, stop, and loading times, aero-derivative gas turbines are also used for flexible peak load power generation and ground propulsion. Additionally, they have significant fuel flexibility, operating on gas, distillate, hydrogen blends, and more. Mitsubishi Power, Siemens Energy, and General Electric typically dominate the turbine market, but GE Vernova and Baker-Hughes hold most of the aero-derivative gas turbine industry.

Heavy Industrial Gas Turbines. Gas turbines for use on the ground can be built more sturdily and larger than jet engines. These heavy units have been generally used for power generation or to drive heavy industrial pumps or compressors, with power generation increasingly important. Gas turbine technology has experimented with a variety of designs for these turbines. One decision is whether to place the output shaft (spool) at the “hot” end, where the gas stream exits the turbine, or at the “cold” end in front of the air intake. “Cold” end drives are easier to access for service and do not have to withstand the hot environment at the turbine end, but their position has to be carefully designed not to disturb the air intake. If the output shaft would cause turbulence or vortex in the air flowing into the compressor, this could lead to a surge, potentially destroying the whole engine.

There are also design differences regarding the numbers of shafts (spools) and turbines within a contemporary heavy industrial gas turbine. The basic form has one output shaft rotating at the speed of the compressor and turbine. At the output, this speed can be geared up or down depending on the speed desired for the application the gas turbine is driving. This design is almost exclusively used for power generation. An alternative to minimize gear losses is to put a second, free-power turbine behind the first gas turbine driving the compressor. This means that the speed of the free-power turbine can be regulated independently of the turbine speed needed to drive the compressor, which makes it an attractive design when pumps or compressors are driven by the gas turbine. This design is only possible with a “hot” end configuration. Finally, there are gas turbine designs that use more than one shaft (spool). A dual spool split output shaft gas turbine, for example, employs three output shafts to operate independently with a high-pressure and low-pressure turbine-compressor assembly as well as a free-power turbine.

Because a single-cycle, stand-alone gas turbine has a fuel efficiency of as little as 17 percent, meaning 83 percent of the energy created is used for the compressor, engineers have combined gas turbines for power generation in cogeneration or combined-cycle power plants. In a cogeneration plant, the remaining heat that exits the gas turbine is used for industrial purposes, such as heating steam for a refinery. In a combined-cycle power plant, the heat from typically two gas turbines fuels a steam turbine. This can create fuel efficiencies ranging from 55 to 65 percent or more. These gas turbines typically create about 250 to 350 megawatts of electrical power each.

Marine and Tank Propulsion. Aero-derivative gas turbines are also used to propel ships, particularly military vessels. Military requirements for high speed outweigh the disadvantages of fuel and construction costs that make gas turbines too expensive for commercial ships. Gas turbines can also be used as tank engines, for example, in the American M1A1 Abrams tank or the Russian T-80. However, their high fuel use provides an engineering challenge, particularly at idle speed. The M1A1 tanks have been retrofitted with batteries for idling, and the Russian T-80 was replaced by the diesel-engine-powered T-90.

Turbochargers. Their low fuel efficiency makes gas turbines unsuitable for car propulsion. However, small gas turbines working as turbochargers are commonly added to increase the power of diesel car engines. The power from the turbine is used to compress the air taken in by the diesel engine, increasing its performance.

Careers and Course Work

Gas turbine technology has been key to two of the world's leading industries, power and aviation, so job demand in the field should remain very strong. Students interested in the field should take science courses in high school, particularly physics, mathematics, and computer science. An associate's degree in an engineering or science field (engineering or industrial technology) will provide a good entry point.

A bachelor of science in an engineering discipline, particularly mechanical, electrical, or computer engineering, is excellent preparation for an advanced job. A BS in physics or mathematics would point to a more theoretically informed career, perhaps in design. A bachelor of arts in environmental studies is also useful, as emission control is becoming a major part of gas turbine technology. A minor in any science is always beneficial.

If one's career focuses on advanced work, a master of science in mechanical, electrical, and computer engineering or environmental science and management could be chosen. For top scientific positions, a PhD in these disciplines and some postdoctoral work are advisable.

Because of the global nature of the field, students should maintain a general openness to work abroad or in somewhat remote locations, with the exception of those purely interested in design. The field can also be attractive to students with expertise in support functions, including those who have earned a BA in English, communications, economics, or biology. A master of business administration would serve as preparation for the business end of the field.

Social Context and Future Prospects

As more nations industrialize and global development continues, the demand for power and mobility, including air travel, is expected to increase. Especially with the key applications of jet engines and gas turbines for power generation, the field of gas turbine technology is likely to keep its great relevancy. The quest for more efficient gas turbines that combust their fuel with as little emissions as possible will continue to motivate major developments in the field. If gas turbines can become an ever-more efficient and low-emission power source, they have the potential, like fuel cells, to become part of the next generation of power sources. There is also much promise linked to micro gas turbines as a source of efficient, affordable, clean, and decentralized power.

The design challenge for jet engines is to reduce fuel consumption and noise while increasing power. Research is ongoing to employ micro gas turbines in electric hybrid car engines to lower overall carbon dioxide emissions from personal transport. This area remained a concern. In 2023, Rolls-Royce announced the testing phase of developing a small gas turbine engine intended to power hybrid-electric flights of small aircraft. This smaller technology is a step toward bringing these engines to hybrid vehicles. Still, micro gas turbine hybrid vehicles remained impractical for the average consumer. Tests proved the turbine was inefficient, expensive, and very loud. Research continually seeks to cut costs and make these engines a reality.

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