Turbines are devices that spin in the presence of a moving fluid. The difference between water wheels or windmills and turbines is largely one of emphasis and degree. During the 18th and 19th centuries, much progress was made toward extracting the kinetic energy of flowing water by devising water turbines. Leonhard Euler, applying fluid mechanics, developed a water turbine as early as 1750. During the 18th century several engineers, such as Benôit Fourneyron, succeeded in building water turbines that by far outstripped conventional water wheels by giving the blades special shapes. The term "turbine" was coined by Fourneyron's professor Claude Burdin; he derived the term from turbo, a spinning object.
The most useful turbines for many purposes are those that can be propelled with energy from heat. A typical turbine based on heat is the steam turbine. The idea of a steam turbine is much older than the steam engine itself. Around 60 bce the Alexandrian Greek Heron (a.k.a. Hero) used jets of steam to turn a kettle. In 1629 the Italian engineer Giovanni Branca depicted in his machine book Le Machine a steam turbine in which a jet of steam is directed at the vanes of the same sort of apparatus as a water wheel. No doubt others observed that escaping steam is like the rushing wind and could be used to push mills just as the wind powers windmills.
When practical steam engines were built at the start of the 18th century, however, they moved a cylinder back and forth (reciprocating motion) instead of pushing a wheel around, although they could be made to turn wheels with various ingenious mechanisms. Reciprocating steam engines were bulky, had slow rotation speeds, and wasted much energy in the machine itself to move the heavy pistons back and forth. When first used to drive electric generators, reciprocating steam engines proved difficult to maintain at a fixed rotation speed as the load on the generator changed.
Turbines are as simple as reciprocating engines are complex. Because they have essentially only one moving part, they are sometimes called the perfect engines, almost directly turning heat into rotary motion.
The first to build a steam turbine was the British engineer Charles Algernon Parsons. In 1884 he completed a small turbine that rotated at 18,000 revolutions per minute and that delivered 10 horsepower. The Swedish engineer Carl Gustav de Laval, experimenting with steam turbines, achieved greater power and higher rotation rates. In 1890 he built a turbine consisting of a 30-cm (12-in.) disk with 200 blades mounted on a flexible axis. The steam was admitted to the blades by special nozzles (Laval nozzles) that accelerated the steam to very high velocities, thus transferring the energy of the steam in the form of kinetic energy to the blades.
Theory of operation
A working fluid contains potential energy (pressure head) and kinetic energy (velocity head). The fluid may be compressible or incompressible. Several physical principles are employed by turbines to collect this energy:
Impulse turbines
These turbines change the direction of flow of a high velocity fluid jet. The resulting impulse spins the turbine and leaves the fluid flow with diminished kinetic energy. There is no pressure change of the fluid in the turbine rotor blades. Before reaching the turbine the fluid's pressure head is changed to velocity head by accelerating the fluid with a nozzle. Pelton wheels and de Laval turbines use this process exclusively. Impulse turbines do not require a pressure casement around the runner since the fluid jet is prepared by a nozzle prior to reaching turbine. Newton's second law describes the transfer of energy for impulse turbines.
Reaction turbines
These turbines develop torque by reacting to the fluid's pressure or weight. The pressure of the fluid changes as it passes through the turbine rotor blades. A pressure casement is needed to contain the working fluid as it acts on the turbine stage(s) or the turbine must be fully immersed in the fluid flow (wind turbines). The casing contains and directs the working fluid and, for water turbines, maintains the suction imparted by the draft tube. Francis turbines and most steam turbines use this concept. For compressible working fluids, multiple turbine stages may be used to harness the expanding gas efficiently. Newton's third law describes the transfer of energy for reaction turbines.
Turbine designs will use both these concepts to varying degrees whenever possible. Wind turbines use an airfoil to generate lift from the moving fluid and impart it to the rotor (this is a form of reaction). Wind turbines also gain some energy from the impulse of the wind, by deflecting it at an angle. Crossflow turbines are designed as an impulse machine, with a nozzle, but in low head applications maintain some efficiency through reaction, like a traditional water wheel. Turbines with multiple stages may utilize either reaction or impulse blading at high pressure. Steam Turbines are usually more impulse while Gas Turbines more reaction type designs. At low pressure the operating fluid medium expands in volume for small changes in pressure. Under these conditions (termed Low Pressure Turbines) blading becomes strictly a reaction type design with the base of the blade solely impulse. The reason is due to the effect of the rotation speed for each blade. As the volume increases, the blade height increases, and the base of the blade spins at a slower speed relative to the tip. This change in speed forces a designer to change from impulse at the base, to a high reaction style tip.
The design of steam turbines developed into a science near the end of the 19th century. Better materials allowed the construction of turbine blades that are resistant to corrosion. Charles Curtis developed the multistage turbine in which the blades and disks become progressively larger when the steam expands. Parsons developed in 1894 the ship turbine engine. The slow-revolving turbine consisted of several sections of increasing diameter. High-pressure steam is admitted to the turbine and pressure differences in each section drive the turbine blades. The first ship to be equipped with such a steam turbine, the Turbinia, immediately established a speed record with 31 knots (57.5 km or 35.7 mi per hour). During the early years of the 20th century, most reciprocating steam engines were replaced by steam turbines (or by diesels). Steam turbines can deliver much more power than reciprocating engines and need less maintenance. Steam turbines also supplanted marine steam engines on ships.
A similar evolution took place for large internal combustion engines, mainly driven by the need for lightweight and powerful airplane engines. Most large modern airplanes are now powered by either turboprop or turbojet engines. These turbines are spun by the expansion of jet fuel instead of by the expansion of water into steam.
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