TURBINE

any of various devices that convert the energy in a stream of fluid into mechanical energy. The conversion is generally accomplished by passing the fluid through a system of stationary passages or vanes that alternate with passages consisting of finlike blades attached to a rotor. By arranging the flow so that a tangential force, or torque, is exerted on the rotor blades, the rotor turns, and work is extracted. Turbines can be classified into four general types according to the fluids used: water, steam, gas, and wind. Although the same principles apply to all turbines, their specific designs differ sufficiently to merit separate descriptions. A water turbine uses the potential energy resulting from the difference in elevation between an upstream water reservoir and the turbine-exit water level (the tailrace) to convert this so-called head into work. Water turbines are the modern successors of simple waterwheels, which date back about 2,000 years. Today, the primary use of water turbines is for electric power generation. The greatest amount of electrical energy comes, however, from steam turbines coupled to electric generators. The turbines are driven by steam produced in either a fossil-fuel-fired or a nuclear-powered generator. The energy that can be extracted from the steam is conveniently expressed in terms of the enthalpy change across the turbine. Enthalpy reflects both thermal and mechanical energy forms in a flow process and is given by the sum of the internal thermal energy and the product of pressure times volume. The available enthalpy change through a steam turbine increases with the temperature and pressure of the steam generator and with reduced turbine-exit pressure. For gas turbines, the energy extracted from the fluid also can be expressed in terms of the enthalpy change, which for a gas is nearly proportional to the temperature drop across the turbine. In gas turbines the working fluid is air mixed with the gaseous products of combustion. Most gas-turbine engines include at least a compressor, a combustion chamber, and a turbine. These are usually mounted as an integral unit and operate as a complete prime mover on a so-called open cycle where air is drawn in from the atmosphere and the products of combustion are finally discharged again to the atmosphere. Since successful operation depends on the integration of all components, it is important to consider the whole device, which is actually an internal-combustion engine, rather than the turbine alone. For this reason, gas turbines are treated in the article internal-combustion engine. The energy available in wind can be extracted by a wind turbine to produce electric power or to pump water from wells. Wind turbines are the successors of windmills, which were important sources of power from the late Middle Ages through the 19th century. Fred Landis any of various devices that convert the energy in a stream of fluid into mechanical energy by passing the stream through a system of fixed and moving fanlike blades and causing the latter to rotate. There are four broad classes of turbines: water (hydraulic), steam, wind, and gas. The most important application of the first three is the generation of electricity, while gas turbines are most frequently employed in aircraft to provide the motive power for jet propulsion. The principles of turbine operation were applied in ancient times. The waterwheel, the ancestor of the water turbine, was used for grinding grain by the Romans about 70 BC. Early devices of this sort were simple paddle wheels immersed in streams where the flow of water was available to turn the wheels. A precursor of the steam-driven turbine was supposedly constructed by Hero of Alexandria about the 1st century AD. This device operated on the principle of reaction; rotation was achieved by steam issuing from curved tubes in a manner similar to that of water issuing from a rotating lawn sprinkler. The windmill, from which the modern wind turbine developed, was already in use by the mid-7th century AD in Persia. Windmills with vertical sails on horizontal shafts appear to have reached Europe through contact with the Arabs several hundred years later. Additional reading General principles are considered in Aubrey F. Burstall, A History of Mechanical Engineering (1963); G.T. Csanady, Theory of Turbomachines (1964); and Calvin Victor Davis and Kenneth E. Sorensen (eds.), Handbook of Applied Hydraulics, 3rd ed. (1969, reprinted 1984). Discussions of steam and wind turbines are provided by W.G. Steltz and A.M. Donaldson (eds.), Aero-thermodynamics of steam turbines (1981); and Gary L. Johnson, Wind Energy Systems (1985). Fred Landis