Editing Gas turbine (section)

==Theory of operation==
[[File:Brayton cycle.svg|thumb|The [[Brayton cycle]] ]]

In an ideal gas turbine, gases undergo four [[thermodynamics|thermodynamic]] processes: an [[isentropic]] compression, an [[isobaric process|isobaric]] (constant pressure) combustion, an isentropic expansion and isobaric heat rejection. Together, these make up the [[Brayton cycle]], also known as the [[Brayton cycle|"constant pressure cycle"]].<ref name=":0" /> It is distinguished from the [[Otto cycle]], in that all the processes (compression, ignition combustion, exhaust), occur at the same time, continuously.<ref name=":0" />

In a real gas turbine, mechanical energy is changed irreversibly (due to internal friction and turbulence) into pressure and thermal energy when the gas is compressed (in either a centrifugal or axial [[Gas compressor|compressor]]). Heat is added in the [[combustor|combustion chamber]] and the [[specific volume]] of the gas increases, accompanied by a slight loss in pressure. During expansion through the stator and rotor passages in the turbine, irreversible energy transformation once again occurs. Fresh air is taken in, in place of the heat rejection.

Air is taken in by a compressor, called a [[gas generator]], with either an [[axial compressor|axial]] or [[centrifugal compressor|centrifugal]] design, or a combination of the two.<ref name=":0" /> This air is then ducted into the [[combustor]] section which can be of a [[Annular combustor|annular]], [[Can combustor|can]], or [[can-annular]] design.<ref name=":0" /> In the combustor section, roughly 70% of the air from the compressor is ducted around the combustor itself for cooling purposes.<ref name=":0" /> The remaining roughly 30% the air is mixed with fuel and ignited by the already burning [[air-fuel mixture]], which then expands producing power across the [[turbine]].<ref name=":0" /> This expansion of the mixture then leaves the combustor section and has its velocity increased across the [[turbine]] section to strike the turbine blades, spinning the disc they are attached to, thus creating useful power. Of the power produced, 60-70% is solely used to power the gas generator.<ref name=":0" /> The remaining power is used to power what the engine is being used for, typically an aviation application, being thrust in a [[turbojet]], driving the fan of a [[turbofan]], rotor or accessory of a [[turboshaft]], and gear reduction and propeller of a [[turboprop]].<ref name=":1" /><ref name=":0" />

If the engine has a power turbine added to drive an industrial generator or a helicopter rotor, the exit pressure will be as close to the entry pressure as possible with only enough energy left to overcome the pressure losses in the exhaust ducting and expel the exhaust. For a [[turboprop]] engine there will be a particular balance between propeller power and jet thrust which gives the most economical operation. In a [[turbojet engine]] only enough pressure and energy is extracted from the flow to drive the compressor and other components. The remaining high-pressure gases are accelerated through a nozzle to provide a jet to propel an aircraft.

The smaller the engine, the higher the rotation rate of the shaft must be to attain the required blade tip speed. Blade-tip speed determines the maximum pressure ratios that can be obtained by the turbine and the compressor. This, in turn, limits the maximum power and efficiency that can be obtained by the engine. In order for tip speed to remain constant, if the diameter of a rotor is reduced by half, the [[rotational speed]] must double. For example, large jet engines operate around 10,000–25,000 rpm, while micro turbines spin as fast as 500,000&nbsp;rpm.<ref>{{cite conference |url=http://www.isma-isaac.be/publications/PMA_MOD_publications/ISMA2006/181-198.pdf |title=Rotordynamic behaviour of a micro-turbine rotor on air bearings: modelling techniques and experimental verification, p. 182 |first1=T. |last1=Waumans |first2=P. |last2=Vleugels |first3=J. |last3=Peirs |first4=F. |last4=Al-Bender |first5=D. |last5=Reynaerts |publisher=International Conference on Noise and Vibration Engineering |conference=ISMA |year=2006 |access-date=7 January 2013 |archive-url=https://web.archive.org/web/20130225144713/http://www.isma-isaac.be/publications/PMA_MOD_publications/ISMA2006/181-198.pdf |archive-date=2013-02-25 |url-status=dead}}</ref>

Mechanically, gas turbines ''can'' be considerably less complex than [[Reciprocating engine]]s. Simple turbines might have one main moving part, the compressor/shaft/turbine rotor assembly, with other moving parts in the fuel system. This, in turn, can translate into price. For instance, costing {{currency|10,000|RM}} for materials, the Jumo 004 proved cheaper than the [[Junkers Jumo 213|Junkers 213]] piston engine, which was {{currency|35,000|RM}},<ref>Christopher, John. ''The Race for Hitler's X-Planes'' (The Mill, Gloucestershire: History Press, 2013), p.74.</ref> and needed only 375 hours of lower-skill labor to complete (including manufacture, assembly, and shipping), compared to 1,400 for the [[BMW 801]].<ref>Christopher, p.75.</ref> <!--Christopher isn't clear if this only applied to the 004s, so confirmation is wanted here....-->This, however, also translated into poor efficiency and reliability. More advanced gas turbines (such as those found in modern [[turbofan|jet engines]] or combined cycle power plants) may have 2 or 3 shafts (spools), hundreds of compressor and turbine blades, movable stator blades, and extensive external tubing for fuel, oil and air systems; they use temperature resistant alloys, and are made with tight specifications requiring precision manufacture. All this often makes the construction of a simple gas turbine more complicated than a piston engine.

Moreover, to reach optimum performance in modern gas turbine power plants the gas needs to be prepared to exact fuel specifications. Fuel gas conditioning systems treat the natural gas to reach the exact fuel specification prior to entering the turbine in terms of pressure, temperature, gas composition, and the related [[Wobbe index]].

The primary advantage of a gas turbine engine is its power to weight ratio.{{citation needed|date=January 2019}} 
Since significant useful work can be generated by a relatively lightweight engine, gas turbines are perfectly suited for aircraft propulsion.

[[Thrust bearing]]s and [[journal bearings]] are a critical part of a design. They are [[Fluid bearing|hydrodynamic oil bearings]] or oil-cooled [[rolling-element bearing]]s. [[Foil bearing]]s are used in some small machines such as micro turbines<ref>{{cite journal |url=http://www.uwm.edu.pl/wnt/technicalsc/tech_12/B19.pdf |title=Development of the foil bearing technology |first1=Krzysztof|last1=Nalepa | first2=Paweł|last2=Pietkiewicz |first3=Grzegorz|last3=Żywica |s2cid-access=free |via=Uniwersytet Warmińsko-Mazurski w Olsztynie | journal=Technical Sciences|volume=12|pages=229–240|s2cid= 44838086|doi=10.2478/v10022-009-0019-2 |date=November 2009 |doi-broken-date= 1 November 2024|access-date=2022-03-01}}</ref> and also have strong potential for use in small gas turbines/[[auxiliary power unit]]s<ref>{{cite conference |last=Agrawal |first=Giri L. |title=Foil Air/Gas Bearing Technology – An Overview |conference=ASME 1997 International Gas Turbine and Aeroengine Congress and Exhibition |pages=V001T04A006 |date=2 June 1997 |doi=10.1115/97-GT-347 |url= http://proceedings.asmedigitalcollection.asme.org/proceeding.aspx?articleid=2088143 |access-date=23 July 2018|isbn=978-0-7918-7868-2 }}</ref>

===Creep===
A major challenge facing turbine design, especially [[turbine blades]], is reducing the [[creep (deformation)|creep]] that is induced by the high temperatures and stresses that are experienced during operation. Higher operating temperatures are continuously sought in order to increase efficiency, but come at the cost of higher creep rates. Several methods have therefore been employed in an attempt to achieve optimal performance while limiting creep, with the most successful ones being high performance coatings and single crystal [[superalloy]]s.<ref>{{cite conference |first1=Brian |last1=Hazel |first2=Joe |last2=Rigney |first3=Mark |last3=Gorman |first4=Brett |last4=Boutwell |first5=Ram |last5=Darolia |title=Superalloys 2008 (Eleventh International Symposium) |chapter=Development of Improved Bond Coat for Enhanced Turbine Durability |doi=10.7449/2008/Superalloys_2008_753_760 |conference =Superalloys |publisher=The Minerals, Metals & Materials Society |location=US |year=2008|pages=753–760 |isbn=978-0-87339-728-5 |doi-access=free }}</ref> These technologies work by limiting deformation that occurs by mechanisms that can be broadly classified as dislocation glide, dislocation climb and diffusional flow.

Protective coatings provide [[thermal insulation]] of the blade and offer [[oxidation]] and [[corrosion]] resistance. Thermal barrier coatings (TBCs) are often stabilized [[zirconium dioxide]]-based ceramics and oxidation/corrosion resistant coatings (bond coats) typically consist of [[aluminide]]s or MCrAlY (where M is typically Fe and/or Cr) alloys. Using TBCs limits the temperature exposure of the superalloy substrate, thereby decreasing the diffusivity of the active species (typically vacancies) within the alloy and reducing dislocation and vacancy creep. It has been found that a coating of 1–200 μm can decrease blade temperatures by up to {{convert|200|C}}.<ref>{{Cite web|url=https://www.phase-trans.msm.cam.ac.uk/2003/Superalloys/coatings/index.html|title=Coatings for Turbine Blades|website=www.phase-trans.msm.cam.ac.uk}}</ref> 
Bond coats are directly applied onto the surface of the substrate using pack carburization and serve the dual purpose of providing improved adherence for the TBC and oxidation resistance for the substrate. The Al from the bond coats forms Al<sub>2</sub>O<sub>3</sub> on the TBC-bond coat interface which provides the oxidation resistance, but also results in the formation of an undesirable interdiffusion (ID) zone between itself and the substrate.<ref>A. W. James et al. "Gas turbines: operating conditions, components and material requirements"</ref> The oxidation resistance outweighs the drawbacks associated with the ID zone as it increases the lifetime of the blade and limits the efficiency losses caused by a buildup on the outside of the blades.<ref>Tamarin, Y. Protective Coatings for Turbine Blades. 2002. ASM International. pp 3–5</ref>

Nickel-based superalloys boast improved strength and creep resistance due to their composition and resultant [[microstructure]]. The gamma (γ) FCC nickel is alloyed with aluminum and titanium in order to precipitate a uniform dispersion of the coherent {{chem2|Ni3(Al,Ti)}} gamma-prime (γ') phases. The finely dispersed γ' precipitates impede dislocation motion and introduce a threshold stress, increasing the stress required for the onset of creep. Furthermore, γ' is an ordered L1<sub>2</sub> phase that makes it harder for dislocations to shear past it.<ref>A. Nowotnik "Nickel-Based Superalloys"</ref> Further [[Refractory]] elements such as [[rhenium]] and [[ruthenium]] can be added in solid solution to improve creep strength. The addition of these elements reduces the diffusion of the gamma prime phase, thus preserving the [[Fatigue (material)|fatigue]] resistance, strength, and creep resistance.<ref>Latief, F. H.; Kakehi, K. (2013) "Effects of Re content and crystallographic orientation on creep behavior of aluminized Ni-based single crystal superalloys". Materials & Design 49 : 485–492</ref> The development of single crystal superalloys has led to significant improvements in creep resistance as well. Due to the lack of grain boundaries, single crystals eliminate [[Coble creep]] and consequently deform by fewer modes – decreasing the creep rate.<ref>Caron P., Khan T. "Evolution of Ni-based superalloys for single crystal gas turbine blade applications"</ref> Although single crystals have lower creep at high temperatures, they have significantly lower yield stresses at room temperature where strength is determined by the Hall-Petch relationship. Care needs to be taken in order to optimize the design parameters to limit high temperature creep while not decreasing low temperature yield strength.