turbines

Credit for invention of the steam turbine is given both to the British engineer Sir Charles Parsons , for invention of the reaction turbine and to Swedish engineer Gustaf de Laval , for invention 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 periphery.
The word "turbine" was coined in 1822 by the French mining engineer Claude Burdin from the Latin turbo, or vortex , in a memoir, "Des turbines hydrauliques ou machines rotatoires à grande vitesse", which he submitted to the Académie royale des sciences in Paris.
Several physical principles are employed by turbines to collect this energy: Impulse turbines change the direction of flow of a high velocity fluid or gas jet.
There is no pressure change of the fluid or gas in the turbine blades , as in the case of a steam or gas turbine, all the pressure drop takes place in the stationary blades .
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.

A pressure casement is needed to contain the working fluid as it acts on the turbine stage or the turbine must be fully immersed in the fluid flow .

In the case of steam turbines, such as would be used for marine applications or for land-based electricity generation, a Parsons type reaction turbine would require approximately double the number of blade rows as a de Laval type impulse turbine, for the same degree of thermal energy conversion.

Whilst this makes the Parsons turbine much longer and heavier, the overall efficiency of a reaction turbine is slightly higher than the equivalent impulse turbine for the same thermal energy conversion.

Steam turbines were traditionally more impulse but continue to move towards reaction designs similar to those used in gas turbines.
The turbine pressure ratio is a function of and the turbine efficiency.
The primary numerical classification of a turbine is its specific speed. This number describes the speed of the turbine at its maximum efficiency with respect to the power and flow rate.

The gas flow in most turbines employed in gas turbine engines remains subsonic throughout the expansion process.
With axial turbines , some efficiency advantage 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 invented by Swedish Engineer Fredrik Ljungström in Stockholm, and in partnership with his brother Birger Ljungström he obtained a patent in 1894.

The design is essentially a multi-stage radial turbine offering great efficiency, four times as large heat drop per stage as in the reaction turbine, extremely compact design and the type met particular success in backpressure power plants. However, contrary to other designs, large steam volumes are handled with difficulty and only a combination with axial flow turbines admits the turbine to be built for power greater than ca 50 MW.
Ceramic blades are more brittle 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 blades.
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.
Bladeless turbine uses the boundary layer effect and not a fluid impinging upon the blades as in a conventional turbine.
Cross-flow turbine , also known as Banki-Michell turbine, or Ossberger turbine.
Curtis combined the de Laval and Parsons turbine by using a set of fixed nozzles on the first stage or stator and then a rank of fixed and rotating blade rows, as in the Parsons or de Laval, typically up to ten compared with up to a hundred stages of a Parsons design.

Use of a small section of a Curtis arrangement, typically one nozzle section and two or three rows of moving blades, is usually termed a Curtis 'Wheel' and in this form, the Curtis found widespread use at sea as a 'governing stage' on many reaction and impulse turbines and turbine sets. This practice is still commonplace today in marine steam plant.
A gas turbine , for example, may refer to an internal combustion machine that contains a turbine, ducts, compressor, combustor, heat-exchanger, fan and an alternator.
Combustion turbines and steam turbines may be connected to machinery such as pumps and compressors, or may be used for propulsion of ships, usually through an intermediate gearbox to reduce rotary speed.