Brief Introduction to Different Types of Turbine rotors

Published: 10th March 2010
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A turbine is a rotary engine that extracts energy from a fluid or air flow and converts it into useful work.


The simplest turbines have one moving part, a rotor congregation, which is a shaft or drum, with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and inform rotational energy to the rotor. Near the beginning turbine examples are windmills, water wheels. Gas, steam, and water turbines normally have a casing around the blades that contains and controls the working fluid. Credit for development of the steam turbine is given both to the British Engineer Sir Charles Parsons (1854-1931), for discovery of the reaction turbine and to Swedish Engineer Gustaf de Laval (1845-1913), for discovery of the impulse turbine. Modern steam turbines frequently employ both reaction and impulse in the same unit, typically varying the degree of reaction and impulse from the blade root to its fringe.





Theory of operation Turbines impulse





A working fluid contains potential energy (force head) and kinetic energy (speed head). The fluid may be compressible or incompressible. Several physical principles are working by turbines to collect this energy.





Impulse turbines





These turbines change the direction of flow of a high velocity fluid or gas jet. The resulting impulse spins the turbine and grass the fluid flow with diminished kinetic energy. There is no pressure change of the fluid or gas in the turbine rotor blades as in the case of a steam or gas turbine, all the pressure drop takes place in the motionless blades.





Before reaching the turbine, the fluid's pressure head is changed to velocity head by accelerating the fluid with a nozzle. Pelt-on wheels and de Laval turbines use this process entirely. Impulse turbines do not require a pressure casement around the rotor since the fluid jet is created by the nozzle prior to reaching the blading on the rotor. Newton's second law describes the transfer of energy for impulse turbines.








Types of turbines





* Steam turbines. are used for the production of electricity in thermal power plants, such as plants using coal or fuel oil or nuclear power. They were once used to directly drive mechanical devices such as ships' propellers but mainly such applications now use decrease gears or a transitional electrical step, where the turbine is used to generate electricity, which then powers an electric motor connected to the mechanical load. Turbo electric ship machinery was particularly popular in the period immediately before and during WWII, primarily due to a lack of sufficient gear-cutting facilities in shipyards.





* Gas turbines. are sometimes referred to as turbine engines. Such engines usually quality an inlet, fan, compressor, combustor and nozzle (possibly other assemblies) in addition to one or more turbines.





* Transonic turbine. The gas flow in most turbines employed in gas turbine engines remains subsonic all through the development process. In a transonic turbine the gas-flow becomes supersonic as it exits the nozzle guide vanes, while the downstream velocities normally become subsonic. Transonic turbines operate at a higher pressure ratio than normal but are usually less efficient and singular.








* Contra-rotating turbines. With axial turbines, some efficiency improvement can be obtained if a downstream turbine rotates in the opposite direction to an upstream unit. However, the complication can be counter-productive. A contra-rotating steam turbine, usually known as the Ljungström turbine, was originally made-up by Swedish Engineer Fredrik Ljungström (1875-1964), in Stockholm and in partnership with his brother Birger Ljungström he obtained a copyright in 1894. The design is essentially a multi-stage radial turbine (or pair of 'nested' turbine rotors) and met with some success, particularly in marine applications, where its compact size and low weight lend itself well to turbo-electric applications. In this radial arrangement, the overall efficiency is typically less than that of Parsons or de Laval turbines.





* Stator less turbine. Multi-stage turbines have a set of static (meaning stationary) inlet guide vanes that direct the gas-flow onto the rotating rotor blades. In a stator-less turbine the gas-flow exiting an upstream rotor impinge onto a downstream rotor without an intermediate set of stator vanes individual encountered.





* Ceramic turbine. Conventional high-pressure turbine blades (and vanes) are made from nickel based alloys and often utilize complicated internal air-cooling passages to prevent the metal from overheating. In recent years, experimental ceramic blades have been manufactured and tested in gas turbines, with a view to increasing Rotor Inlet Temperatures along with possibly, eliminating air-cooling. Ceramic blades are more delicate than their metallic counterparts, and carry a greater risk of catastrophic blade failure. This has tended to limit their use in jet engines and gas turbines, to the stator (stationary) blades.





* Shrouded turbine. Many turbine rotor blades have shrouding at the top, which interlocks with that of adjacent blades, to increase damping and thereby reduce blade flutter. In large land-based electricity generation steam turbines, the shrouding is often complemented, especially in the long blades of a low-pressure turbine, with lacing wires. These are wires which pass through holes drilled in the blades at suitable distances from the blade root and the wires are usually brazed to the blades at the point where they pass through. The lacing wires are designed to reduce blade flutter in the central part of the blades. The introduction of lacing wires significantly reduces the instances of blade failure in large or low-pressure turbines.





* Shroud less turbine. Currently observe is, wherever possible, to eliminate the rotor shrouding, thus reducing the centrifugal load on the blade and the cooling requirements.





* Bladeless turbine. Uses the boundary layer effect and not a fluid impinging upon the blades as in a conventional turbine.








* Water turbines


• Pelt-on turbine, a type of impulse water turbine.


• Francis turbine, a type of widely used water turbine.


• Kaplan turbine, a variation of the Francis Turbine.


* Wind turbine. These normally operate as a single stage without nozzle and inter stage guide vanes. An exception is the Elaine Belle, which has a stator and a rotor, thus being a true turbine.








Uses of turbines





• Nearly all electrical power on Earth is produced with a turbine of some type. Very high efficiency steam turbines control about 40% of the thermal energy, with the rest exhausted as waste heat.





• Most jet engines rely on turbines to supply mechanical work from their working fluid and fuel as do all nuclear ships and power plants.





• Turbines are regularly part of a larger machine. A gas turbine, for example, may refer to an internal combustion machine that contains a turbine, ducts, compressor, combustor, heat-exchanger, fan and (in the case of one designed to produce electricity) an alternator. However, it must be noted that the collective machine referred to as the turbine in these cases is designed to transfer energy from a fuel to the fluid passing through such an internal combustion device as a means of propulsion, and not to transfer energy from the fluid passing through the turbine to the turbine as is the case in turbines used for electricity provision etc.





• Reciprocating piston engines such as aircraft engines can use a turbine powered by their exhaust to drive an intake-air compressor, a configuration known as a turbocharger (turbine supercharger) or colloquially, a "turbo".





• Turbines can have very high power density (ie the ratio of power to weight, or power to volume). This is because of their ability to operate at very high speeds. The Space Shuttle's main engines use turbo pumps (machines consisting of a pump driven by a turbine engine) to feed the propellants (liquid oxygen and liquid hydrogen) into the engine's combustion chamber. The liquid hydrogen turbo pump is slightly larger than an automobile engine (weighing approximately 700 lb) and produces nearly 70,000 hp (52.2 MW).





• Turbo expanders are widely used as sources of refrigeration in industrial processes.





• Turbines could also be used as powering system for a remote controlled plane that creates thrust and lifts the plane of the ground.





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