Turbomachinery

Concepts, Applications, and Design

V. Dakshina Murty

• Éditeur: CRC Press
• Année: 2018
• ISBN:
  9781351793070
• Édition: 1
• Sujet: Naturwissenschaften
• Sous-sujet: Energie

Turbomachinery: Concepts, Applications, and Design is an introductory turbomachinery textbook aimed at seniors and first year graduate students, giving balanced treatment of both the concepts and design aspects of turbomachinery, based on sound analysis and a strong theoretical foundation. The text has three sections, Basic Concepts, Incompressible Fluid Machines; and Compressible Fluid Machines. Emphasis is on straightforward presentation of key concepts and applications, with numerous examples and problems that clearly link theory and practice over a wide range of engineering areas. Problem solutions and figure slides are available for instructors adopting the text for their classes.

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Introduction and History

 

The word turbomachine is derived from the Latin word turbo, which means whirl, or something that spins. The term describes various types of machines involving a pressure head, such as compressors, pumps, blowers, turbines, and so on. The common feature of all these machines is that they have a rotating shaft on which vanes that come into contact with the working fluid are mounted. Such contact produces a change in momentum of the working fluid, and this results in motion of the vanes or increased pressure on the fluid. A turbomachine can be defined as a device in which energy transfer occurs between a fluid in motion and a rotating shaft due to dynamic action that results in changes in pressure and fluid momentum. These are different from positive displacement machines such as reciprocating piston cylinders, because in such machines the work input or output is primarily due to moving boundaries. The development of turbomachines can be classified separately into that of hydraulic machines and gas/steam turbines, including compressors.

From a historical perspective, turbomachines have been around since the time of the Romans, who used paddle-type water wheels for grinding grain around 70 bc. Around the same time, the Chinese used similar machines, also in grinding mills. About a century later, in 62 ad, the Greek engineer Hero built the first steam turbine. Known as the aeolipile, it was a spherical vessel containing water and worked on the principle of reaction. As the water was heated, jets of steam issued out of the nozzles on the sides producing torque and thereby motion. Because of the miniscule amount of power it produced, this remained just a toy and its usefulness has essentially remained confined to science projects. However, from a modern turbomachinery point of view, it was a pure reaction machine. An interesting account of the effects of Hero’s invention on modern turbomachinery is given by Lyman (2004). Another device that is very similar in principle but works on water jets through the reaction principle is Barker’s mill. In this device, water enters the center of a rotor under a static head and emerges tangentially through arms protruding radially, thus providing torque through the reaction of the jet. This is similar in principle to a lawn sprinkler. Although the following centuries saw the invention of several types of water wheels, they were mostly used in grinding mills, water supply, food production, and mining.

The development of modern turbomachinery, as it pertains to hydraulic machines, can be traced to the Swiss mathematician Leonhard Euler. While working at the Berlin Academy of Sciences with his son, Albert, Euler published the now-famous “Euler turbine equation” in 1754. It is based on Newton’s second law that torque is proportional to the rate of change of angular momentum of a fluid. This ushered in a more scientific approach to the design and analysis of compressors, turbines, pumps, and other such devices. The term turbine itself was not coined until 1822, when a Frenchman named Claude Burdin used the Latin word turbo, turbinis to describe “that which spins, as a spinning top.” Most of these machines designed during the eighteenth and nineteenth centuries were predominantly waterwheels, either hydraulic turbines or pumps. It was Burdin’s student Benoit Fourneyron who improved his teacher’s work and is credited with building the first high-efficiency hydraulic turbine in 1824. His turbine achieved an efficiency of close to 85%, which was significantly higher than that of similar devices operating at the time which had efficiencies well below 50%. It is also interesting to note that his turbine was of the radial outflow type. These types of turbines were first introduced into the United States in 1843, at almost the same time as an axial flow turbine called the Jonval turbine, another European design, was invented.

The first inward flow turbine can be attributed to Poncelot, who conceptualized it in 1826. The credit for building the first one in 1838 goes to Howd, who also obtained a patent on it. However, it was James B. Francis who extensively analyzed and tested it for several years from 1849. Although the initial designs were purely radial, they were subsequently modified to accommodate mixed flows. Thus, all modern inward mixed flow turbines can be traced back to James Francis, and they are generally called Francis turbines. They are highly efficient and account for a major part of the hydroelectric power produced in the world today.

The other type of reaction turbine was conceived by a Czech professor named Victor Kaplan and was not developed until much later. It is named after him, and is an axial flow type. Viktor Kaplan obtained his first patent for an adjustable blade propeller turbine in 1912. Although he was not very successful in the initial designs due to cavitation problems, subsequent design modifications made the turbines suitable for low head applications such as rivers. Today, all axial flow adjustable pitch propeller turbines bear his name. They are highly efficient at both full and partial loads and especially suitable for large flows and low heads.

The third type of hydraulic turbine, which is quite different from the reaction turbines described earlier, is the Pelton turbine. This works on the principle of high-velocity jets impinging on a set of blades. Thus, the turbine extracts energy from the impulse of moving water. The high velocity is obtained by transporting the water from very large heads through pipes called penstock and converting it to jets that are then directed, either singly or multiply, onto blades. The entire energy conversion takes place before the water enters the runner and, as such, there is no pressure drop in the Pelton turbine.

Similar to hydraulic turbines, pumps also date back several hundreds of years. The earliest pump concept can be traced to around 2000 bc, when the Egyptians used “shadoofs” to lift water. These devices consisted of long poles mounted on a seesaw with a bucket mounted at one end and a rope attachment on the other. Around 200 bc, the Archimedean screw pump was devised to lift both liquids and mixtures of liquids and solids. Many inventions of the past five to six hundred years, including gear pumps, piston pumps, and plunger pumps belong to the class of positive displacement pumps. In the eighteenth and nineteenth centuries, increased interest and research in moving fluids led to the discovery of various types of pumps and their subsequent design modifications. Although the first patent for a centrifugal pump can be traced to the British engineer John Gwynne in 1851, the first mention of a vaned diffuser, by Sir Osborne Reynolds, did not occur for another quarter of a century, in 1875, in connection with the so-called turbine pump. The following two decades saw several pump manufacturers enter the pump industry. They included Sulzer, Rateau, Byron Jackson, Parsons, Allis Chalmers, and Worthington.

Compared with the invention of hydraulic turbines and pumps, the arrival of compressible flow machines, such as compressors and gas/steam turbines, is more recent. Except for Hero’s turbine, which was more of a toy or experiment with no power output, the earliest modern steam turbine can be traced to medieval times, when Taqi-al-Din produced a prime mover for rotating a spit in 1551. Mechanical details of this device are given by Hassan (1976). Almost a century later, John Wilkins and Giovanni Branca produced similar devices, in 1629 and 1648. Although James Watt built the steam engine in 1776, it was based on a reciprocating piston and cylinder, a mechanism that is not a turbine. In 1831, William Avery built the first useful steam turbine and obtained a patent for it. It was an extension of Hero’s turbine from almost two millennia earlier! From an engineering perspective, the first steam turbine that had a major impact was built by Sir Charles Parsons in 1884. It was a multistage axial flow reaction turbine that produced 10 hp when spinning at 18,000 rpm. Around the same time, in France, Auguste Rateau developed a pressure-compounded impulse turbine in 1900. In the United States, Charles G. Curtis constructed a velocity-compounded two-stage steam turbine in 1901. As demand for power grew in the twentieth century, interest in steam turbines also continued to grow. Some of the important developments included the harnessing of regenerative feed heating in 1920 and of the reheat cycle in 1925. There was also a steady increase in the inlet pressures over the years. Entry pressure into the turbine, which was about 15 bar at the end of World War I, increased to 30 bar by 1930, and further increased to 100 bar during the 1960s.

Unlike the steam turbine, which forms a part of the Rankine cycle, the development of compressors, which are major part of the Brayton cycle, took place over a much longer period. It needs to be emphasized that the term steam turbine here signifies the complete steam plant or cycle, including the boiler, feed pump, and the turbine itself. Compared with the gas turbine cycle, the steam turbine is much easier to design, construct, and operate. This is mainly due to the ease with which water can be made to flow through a boiler and the ease with which the resulting high-pressure steam is directed to the turbine to produce power. Even in the most inefficient scenarios, the turbine produces enough work to drive the feed pump, and thus net power output is always positive from the cycle. This is true for a reciprocating-type expander also; that is, the power produced by the expander is greater than the power consumed by the compressor.

In contrast, the major problem facing gas turbine cycles is the difficulty in obtaining appreciable pressure rises in compressors, especially those of the axial flow type. Since the flow in turbines is from high to low pressure, they are always successful, and, with some care, can also be designed to perform quite efficiently. However, compressors in general, and axial flow compressors in particular, are inherently inefficient due to the adverse pressure gradients. Early designs had such low efficiencies that the power produced by the turbine was not enough to drive the compressor, and, as such, the machines produced by the inventors never ran without external power input. The first US patent for a complete gas turbine was obtained by Charles Curtis in 1895. Following the unsuccessful efforts of Stolze in the early 1900s, Rene Armangand, along with Charles Lemale, produced a gas turbine in 1906 that produced positive net power, albeit at an efficiency of 3%. Other pioneers in the early stages of gas turbine research were Hans Holzwarth, Brown Boveri, and Sanford Moss. The first successful industrial gas turbine engine was created by Aurel Stodola in 1936. He used a twenty-stage axial compressor driven by a five-stage axial turbine. The isentropic efficiency of the compressor was around 85%.

Research on the application of gas turbines to aviation started around the same time as that on commercial applications. Efforts were underway almost simultaneously in the United Kingdom by Frank Whittle and in Germany by Hans von Ohain, Herbert Wagner, and Helmut Schelp. Although Whittle was the earliest of the four to complete the design in 1929, he had to overcome combustion problems with the liquid fuel and also material failure due to high temperature. It was not until 1942, when blades made of nickel–cadmium–cobalt were available, that he could make a successful flight, and his engine was taken to the General Electric laboratories in the United States. Whittle’s counterpart in Germany, Hans von Ohain, was more successful, since he avoided the liquid combustion problem by using hydrogen. His first successful flight was in 1939, three years ahead of Whittle’s. Herbert Wagner and Helmut Schlep had varying degrees of success in the late 1930s in terms of building a working engine.

Since the end of World War II, gas turbines have been the prime movers of civilian and military aircraft. Several of the smaller companies that existed during the period immediately after the war (1945–1950) either disappeared or consolidated into bigger companies. Today, the three major companies producing the largest engines are General Electric, Pratt and Whitney, and Rolls Royce.

A few closing comments are in order. As opposed to gas and steam turbines, hydraulic turbines exhibit much higher efficiencies. It is not uncommon for hydraulic turbines to have efficiencies as high as 93%–94%. Such high efficiencies coupled with no fuel cost (water power is free!) have resulted in almost no interest in research into hydraulic turbines. A similar argument applies to pumps. Consequently, the major research interest in these two types of machines has been confined to making runner materials more cavitation resistant. In terms of environmental considerations, there is also interest in making hydraulic turbines more “fish friendly.” Besides these two aspects, there is little incentive to improve the performance of hydro turbines or pumps. However, for gas turbines, due to their extensive use in aviation, there has been a constant need to maximize the thrust-to-weight ratio, especially for military applications. Hence, there has been consistent support from the military for gas turbine research. This scenario is unlikely to change in the near future.

 

PROBLEMS

The following problems are of the study and discussion type.

1.1Perform a literature search and make a timeline for hydraulic machines. Include both hydraulic turbines and pumps.

1.2Repeat Problem 1 for gas turbines (including compressors) and steam turbines.

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Dimensional Analysis

The technique of dimensional analysis is most useful in any field of engineering that involves some form of convective transport. These include fluid mechanics, heat transfer, soil mechanics, hydraulics and hydrology, and environmental transport. This technique has been successfully applied in the study of turbomachines, especially hydraulic turbines and pumps. Model testing and accompanying dimensional analysis are imperative in these areas, because the large si...

Table des matières

• Cover
• Half-Title
• Title
• Copyright
• Dedication
• Contents
• Preface
• List of Symbols
• 1. Introduction and History
• 2. Dimensional Analysis
• 3. Theory of Turbomachines
• 4. Hydraulic Turbines
• 5. Pumps
• 6. Fans and Blowers
• 7. Radial Gas Turbines
• 8. Axial Gas Turbines
• 9. Radial Compressors
• 10. Axial Compressors
• 11. Steam Turbines
• 12. Wind Turbines
• References
• Appendix I
• Appendix II
• Appendix III
• Index