Early gas turbines employed centrifugal compressors, which are relatively simple and inexpensive. They are, however, limited to low pressure ratios and cannot match the efficiencies of modern axial-flow compressors. Accordingly, centrifugal compressors are used today primarily in small industrial units.
An axial-flow compressor is the reverse of a reaction turbine. The blade passages, which look like twisted, highly curved airfoils, must exert a tangential force on the fluid with the pressures on one side of the blade higher than on the other. For subsonic flow, an increase in pressure requires the flow area to also increase, thus reducing the flow velocity between the blade passages and diffusing the flow. A row of compressor blades must be viewed as a set of closely spaced, highly curved airfoil shapes with which airflow strongly interacts. There will not only be a rise in pressure along the blades but a variation between them as well. Flow friction, leakage, wakes produced by the previous sets of blades, and secondary circulation or swirl flows all contribute to losses in a real unit. Tests of stationary blade assemblies, known as cascades, can be performed in special wind tunnels, but actual blade arrangements in a rotating assembly require special test setups or rigs.
Blades must be designed not only to have the correct aerodynamic shape but also to be light and not prone to critical vibrations. Recent advances in compressor (and turbine) blade design have been aided by extensive computer programs.
While moderately large expansion-pressure ratios can be achieved in a reaction-turbine stage, only relatively small pressure increases can be handled by a compressor stage—typically pressure ratios per stage of 1.35 or 1.4 to 1 in a modern design. Thus, compressors require more stages than turbines. If higher stage pressure ratios are attempted, the flow will tend to separate from the blades, leading to turbulence, reduced pressure rise, and a “stalling” of the compressor with a concurrent loss of engine power. Unfortunately, compressors are most efficient close to this so-called surge condition, where small disturbances can disrupt operation. It remains a major challenge to the designer to maintain high efficiency without stalling the compressor.
As the air is compressed, its volume decreases. Thus the annular passage area should also decrease if the through-flow velocity is to be kept nearly constant—i.e., the blades have to become shorter at higher pressures. An optimum balance of blade-tip speeds and airflow velocities often requires that the rotational speed of the front, low-pressure end of the compressor be less than that of the high-pressure end. This is achieved in large aircraft gas turbines by “spooled” shafts where the shaft for the low-pressure end, driven by the low-pressure portion of the turbine, is running at a different speed within the hollow high-pressure compressor/turbine shaft, with each shaft having its own bearings. Both twin- and triple-spool engines have been developed.
Air leaving the compressor must first be slowed down and then split into two streams. The smaller stream is fed centrally into a region where atomized fuel is injected and burned with a flame held in place by a turbulence-generating obstruction. The larger, cooler stream is then fed into the chamber through holes along a “combustion liner” (a sort of shell) to reduce the overall temperature to a level suitable for the turbine inlet. Combustion can be carried out in a series of nearly cylindrical elements spaced around the circumference of the engine called cans, or in a single annular passage with fuel-injection nozzles at various circumferential positions. The difficulty of achieving nearly uniform exit-temperature distributions in a short aircraft combustion chamber can be alleviated in stationary applications by longer chambers with partial internal reversed flow.
The turbine is normally based on the reaction principle with the hot gases expanding through up to eight stages using one- or two-spooled turbines. In a turbine driving an external load, part of the expansion frequently takes place in a high-pressure turbine that drives only the compressor while the remaining expansion takes place in a separate, “free” turbine connected to the load.
High-performance aircraft engines usually employ multiple spools. A recent large aircraft-engine design operating with an overall pressure ratio of 30.5:1 uses two high-pressure turbine stages to drive 11 high-pressure compressor stages on the outer spool, rotating at 9,860 revolutions per minute, while four low-pressure turbine stages drive the fan for the bypass air as well as four additional low-pressure compressor stages through the inner spool turning at 3,600 revolutions per minute (see below). For stationary units, a total of three to five total turbine stages is more typical.
High temperatures at the turbine inlet and high centrifugal blade stresses necessitate the use of special metallic alloys for the turbine blades. (Such alloys are sometimes grown as single crystals.) Blades subject to very high temperatures also must be cooled by colder air drawn directly from the compressor and fed through internal passages. Two processes are currently used: (1) jet impingement on the inside of hollow blades, and (2) bleeding of air through tiny holes to form a cooling blanket over the outside of the blades.
In a gas-turbine engine driving an electric generator, the speed must be kept constant regardless of the electrical load. A decrease in load from the design maximum can be matched by burning less fuel while keeping the engine speed constant. Fuel flow reduction will lower the exit temperature of the combustion chamber and, with it, the enthalpy drop available to the turbine. Although this reduces the turbine efficiency slightly, it does not affect the compressor, which still handles the same amount of air. The foregoing method of control is substantially different from that of a steam turbine, where the mass flow rate has to be changed to match varying loads.
An aircraft gas-turbine engine is more difficult to control. The required thrust, and with it engine speed, may have to be changed as altitude and aircraft speed are altered. Higher altitudes lead to lower air-inlet temperatures and pressures and reduce the mass flow rate through the engine. Aircraft now use complex computer-driven controls to adjust engine speed and fuel flow while all critical conditions are monitored continuously.
For start-up, gas turbines require an external motor which may be either electric or, for stationary applications, a small diesel engine.
Many other aspects enter into the design of a modern gas-turbine engine, of which only a few examples can be given. Much attention must be paid, especially in a multispool unit, to the design of all bearings, including the thrust bearings that absorb axial forces, and to the lubrication system. As an engine is started up and becomes hot, components elongate or “grow,” thereby affecting passage clearances and seals. Other considerations include bleeding air from the compressor and ducting it for turbine-blade cooling or for driving accessories.
By far the most important use of gas turbines is in aviation, where they provide the motive power for jet propulsion. Because of the significance of this application and the diversity of modern jet engines, the subject will be dealt with at length in a separate section of the article. The present discussion will touch on the use of gas turbines in electric power generation and in certain industrial processes, as well as consider their role in marine, locomotive, and automotive propulsion.
In the field of electric power generation, gas turbines must compete with steam turbines in large central power stations and with diesel engines in smaller plants. Even though the initial cost of a gas turbine is less than either alternative for moderately sized units, its inherent efficiency is also lower. Yet, a gas-turbine unit requires less space, and it can be placed on-line within minutes, as opposed to a steam unit that requires many hours for start-up. As a consequence, gas-turbine engines have been widely used as medium-sized “peak load” plants to run intermittently during short durations of high power demand on an electric system. In this case, initial costs, rather than fuel charges, become the prime consideration.
Early commercial stationary plants employed aircraft units operating at reduced turbine-inlet temperatures. The high rotational speed of aircraft turbines required special gearing to drive electric generators. More recently, special units have been designed for direct operation (in the United States) at 3,600 revolutions per minute. Units in sizes up to 200,000 kilowatts have been built, although the majority of installations are less than 100,000 kilowatts. These turbines have operated up to 6,000 hours per year on either liquid fuels or natural gas. Typical turbine-inlet temperatures for large units range from about 980° to 1,260° C with turbine blade cooling used at the higher temperatures.
Efficiency can be improved by adding a regenerator to exploit the high turbine exhaust temperatures (typically about 480° to 590° C). Alternatively, if the gas turbine serves as a peak-load unit for a continuously running steam power plant, the hot exhaust gases can be used to preheat by means of a heat exchanger the combustion air entering a steam boiler. A modern development involves feeding the gas turbine exhaust directly into a steam generator where additional fuel is burned, producing steam of moderate pressure for a steam turbine. An overall thermal efficiency of nearly 50 percent is claimed for these combined units, making them the most fuel-efficient power plants currently available.
With sizes typically ranging from 1,000 to 50,000 horsepower, industrial gas-turbine engines can be used for many applications. These include driving compressors for pumping natural gas through pipelines, where a small part of the pumped gas serves as the fuel. Such units can be automated so that only occasional on-site supervision is required. A gas turbine can also be incorporated in an oil refining process called the Houdry process, in which pressurized air is passed over a catalyst to burn off accumulated carbon. The hot gases then drive a turbine directly without a combustion chamber. The turbine, in turn, drives a compressor to pressurize the air for the process. Small portable gas turbines with centrifugal compressors also have been used to operate pumps.
In this area of application, the gas-turbine engine has two advantages over steam- and diesel-driven plants: it is lightweight and compact. During the early 1970s a ship powered by a gas turbine capable of 20,000 horsepower was successfully tested at sea by the U.S. Navy over a period of more than 5,000 hours. Gas turbines were subsequently selected to power various new U.S. naval vessels.
During the 1950s and ’60s, manufacturers of locomotives built a number of vehicles powered by gas-turbine engines that use heavy oil. Although gas-turbine locomotives have had moderate success for long sustained runs, they have not been able to make significant inroads against diesel locomotives under normal running conditions, especially after increases in the relative cost of heavy fuel oils. Moreover, the inherent low efficiency of a simple open-cycle gas turbine becomes even worse at part-load or during idling when considerable fuel is needed to drive the compressor while producing little or no useful power.
Gas-turbine engines were proposed for use in automobiles from the early 1960s. In spite of their small size and weight for a given power output and their low exhaust emissions compared to gasoline engines, the disadvantages of high manufacturing costs, low thermal efficiency, and poor part-load and idling performance have proven gas-turbine cars to be uneconomical and impractical.
Development Of Gas Turbines
The earliest device for extracting rotary mechanical energy from a flowing gas stream was the windmill (see above). It was followed by the smokejack, first sketched by Leonardo da Vinci and subsequently described in detail by John Wilkins, an English clergyman, in 1648. This device consisted of a number of horizontal sails that were mounted on a vertical shaft and driven by the hot air rising from a chimney. With the aid of a simple gearing system, the smokejack was used to turn a roasting spit.
Various impulse and reaction air-turbine drives were developed during the 19th century. These made use of air, compressed externally by a reciprocating compressor, to drive rotary drills, saws, and other devices. Many such units are still being used, but they have little in common with the modern gas-turbine engine, which includes a compressor, combustion chamber, and turbine to make up a self-contained prime mover. The first patent to approximate such a system was issued to John Barber of England in 1791. Barber’s design called for separate reciprocating compressors whose output air was directed through a fuel-fired combustion chamber. The hot jet was then played through nozzles onto an impulse wheel. The power produced was to be sufficient to drive both the compressor and an external load. No working model was ever built, but Barber’s sketches and the low efficiency of the components available at the time make it clear that the device could not have worked even though it incorporated the essential components of today’s gas-turbine engine.
Although many devices were subsequently proposed, the first significant advance was covered in an 1872 patent granted to F. Stolze of Germany. Dubbed the fire turbine, his machine consisted of a multistage, axial-flow air compressor that was mounted on the same shaft as a multistage, reaction turbine. Air from the compressor passed through a heat exchanger, where it was heated by the turbine exhaust gases before passing through a separately fired combustion chamber. The hot compressed air was then ducted to the turbine. Although Stolze’s device anticipated almost every feature of a modern gas-turbine engine, both compressor and turbine lacked the necessary efficiencies to sustain operation at the limited turbine-inlet temperature possible at the time.
The first successful gas turbine, built in Paris in 1903, consisted of a three-cylinder, multistage reciprocating compressor, a combustion chamber, and an impulse turbine. It operated in the following way: Air supplied by the compressor was burned in the combustion chamber with liquid fuel. The resulting gases were cooled somewhat by the injection of water and then fed to an impulse turbine. This system, which had a thermal efficiency of about 3 percent, demonstrated for the first time the feasibility of a practical gas-turbine engine.
Two other devices with intermittent gas action, both developed at about the same time, deserve mention. A 10,000-revolutions-per-minute unit built in Paris in 1908 had four explosion chambers located on the periphery of a de Laval impulse turbine. Each chamber, containing air and fuel, was fired sequentially to provide a nearly continuous flow of high-temperature, high-pressure gases that were fed through nozzles to the turbine wheel. The momentary partial vacuum created by the hot gases rushing from the explosion chamber was used to draw in a new charge of air.
Of greater significance was the “explosion” turbine developed by Hans Holzwarth of Germany, whose initial experiments started in 1905. In this system, a compressor introduced a charge of air and fuel into a constant-volume combustion chamber. After ignition, the hot, high-pressure gas escaped through spring-loaded valves into nozzles directed against the blading of a turbine. The valves remained open until the gas was discharged, at which point a fresh charge was brought into the combustion chamber. Since the pressure increase in the compressor was only about one-fourth of the maximum pressure reached after combustion, the unit could operate even though the compressor efficiency was low. Holzwarth and various collaborators continued to develop the explosion turbine for more than 30 years until it was eventually superseded by the modern gas-turbine engine.
To be successful, a steady-flow engine based on the ideas first proposed by Stolze depends not only on high efficiencies (more than 80 percent) for both the rotating compressor and the turbine but also on moderately high turbine-inlet temperatures. The first successful experimental gas turbine using both rotary compressors and turbines was built in 1903 by Aegidus Elling of Norway. In this machine, part of the air leaving a centrifugal compressor was bled off for external power use. The remainder, which was required to drive the turbine, passed through a combustion chamber and then through a steam generator where the hot gas was partially cooled. This combustion gas was cooled further (by steam injected into it) to 400° C, the maximum temperature that Elling’s radial-inflow turbine could handle. The earliest operational turbine of this type delivered 11 horsepower. Many subsequent improvements led to another experimental Elling turbine, which by 1932 could produce 75 horsepower. It employed a compressor with 71-percent efficiency and a turbine with an efficiency of 82 percent operating at an inlet temperature of 550° C. Norway’s industry, however, was unable to capitalize on these developments, and no commercial units were built. The first industrial success did not come until 1936, when the Swiss firm of Brown Boveri independently developed a gas turbine for the Houdry process (see above).
Also during the mid-1930s a group headed by Frank Whittle at the British Royal Aircraft Establishment (RAE) undertook efforts to design an efficient gas turbine for jet propulsion of aircraft. The unit produced by Whittle’s group worked successfully during tests; it was determined that a pressure ratio of about 4 could be realized with a single centrifugal compressor running at roughly 17,000 revolutions per minute. Shortly after Whittle’s achievement, another RAE group, led by A.A. Griffith and H. Constant, began developmental work on an axial-flow compressor. Axial-flow compressors, though much more complex and costly, were better suited for detailed blade-design analysis and could reach higher pressures and flow rates and, eventually, higher efficiencies than their centrifugal counterparts.
Independent parallel developments in Germany, initiated by Hans P. von Ohain working with the manufacturing firm of Ernst Heinkel, resulted in a fully operational jet aircraft engine that featured a single centrifugal compressor and a radial-inflow turbine. This engine was successfully tested in the world’s first jet-powered airplane flight on Aug. 27, 1939. Subsequent German developments directed by Anselm Franz led to the Junkers Jumo 004 engine for the Messerschmitt Me-262 aircraft, which was first flown in 1942. In Germany as well as in Britain, the search for higher temperature materials and longer engine life was aided by experience gained in developing aircraft turbosuperchargers.
Before the end of World War II gas-turbine jet engines built by Britain, Germany, and the United States were flown in combat aircraft. Within the next few decades both propeller-driven gas-turbine engines (turboprops) and pure jet engines developed rapidly, with the latter assuming an ever larger role as airplane speeds increased.
Because of the significant advances in gas-turbine engine design in the years following World War II, it was expected that such systems would become an important prime mover in many areas of application. However, the high cost of efficient compressors and turbines, coupled with the continued need for moderate turbine-inlet temperatures, have limited the adoption of gas-turbine engines. Their preeminence remains assured only in the field of aircraft propulsion for medium and large planes that operate at either subsonic or supersonic speeds. As for electric power generation, large central power plants that use steam or hydraulic turbines are expected to continue to predominate. The prospects appear bright, nonetheless, for medium-sized plants employing gas-turbine engines in combination with steam turbines. Further use of gas-turbine engines for peak power production is likely as well. These turbine engines also remain attractive for small and medium-sized, high-speed marine vessels and for certain industrial applications.
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