Editing Turbofan (section)

==Principles==
[[File:tfan-schematic-kk-20090106.png|thumb|450px|Schematic diagram illustrating a modern 2-spool turbofan engine installation in a nacelle. The low-pressure spool is colored blue and the high-pressure one orange.]]
The turbofan was invented to improve the fuel consumption of the turbojet. It achieves this by pushing more air, thus increasing the mass and lowering the speed of the propelling jet compared to that of the turbojet. This is done mechanically by adding a [[ducted fan]] rather than using viscous forces.<ref>Thrust Augmentation with Mixer/Ejector systems, Presz, Reynolds, Hunter, AIAA 2002-0230, p.3</ref>  A [[vacuum ejector]] is used in conjunction with the fan as first envisaged by inventor [[Frank Whittle]].<ref>Gas Turbine Aerothermodynamics With Special Reference To Aircraft propulsion, Sir Frank Whittle 1981,{{ISBN|0 08 026719 X}}, p.217</ref>

Whittle envisioned flight speeds of 500&nbsp;mph in his March 1936 UK patent 471,368 "Improvements relating to the propulsion of aircraft", in which he describes the principles behind the turbofan,<ref>Gas Turbine Aerothermodynamics With Special Reference To Aircraft propulsion, Sir Frank Whittle 1981,{{ISBN|0 08 026719 X}}, p.218</ref> although not called as such at that time. While the turbojet uses the gas from its thermodynamic cycle as its propelling jet, for aircraft speeds below 500&nbsp;mph there are two penalties to this design which are addressed by the turbofan.

First, the energy required for a given thrust increases as the exhaust air is propelled at ever greater speeds, so the efficiency can be improved by diverting energy to propel larger quantities of air at lower speeds than the core. A turbofan achieves this by using an additional turbine to drive a ducted fan to blow air that bypasses the high speed core. With a lower core thrust, most of the thrust now comes from the large mass of low speed bypass air, providing the same thrust with a reduced fuel burn.

The other penalty is that combustion is less efficient at lower speeds. Any action to reduce the fuel consumption of the engine by increasing its pressure ratio or turbine temperature to achieve better combustion causes a corresponding increase in pressure and temperature in the exhaust duct which in turn cause a higher gas speed from the propelling nozzle (and higher KE and wasted fuel). Although the engine would use less fuel to produce a pound of thrust, more fuel is wasted in the faster propelling jet. In other words, the independence of thermal and propulsive efficiencies, as exists with the piston engine/propeller combination which preceded the turbojet, is lost.<ref>{{Cite journal |last=Rubert |first=Kennedy F. |date=1945-02-01 |title=An analysis of jet-propulsion systems making direct use of the working substance of a thermodynamic cycle |url=https://ntrs.nasa.gov/citations/19930093532 |journal=NASA|language=en |page=}}</ref> In contrast, Roth<ref>{{Cite thesis |title=A theoretical treatment of technical risk in modern propulsion system design |url=https://ui.adsabs.harvard.edu/abs/2000PhDT.......101R |date=2000-09-01 |first=Bryce Alexander |last=Roth|bibcode=2000PhDT.......101R }} p.76</ref> considers regaining this independence the single most important feature of the turbofan which allows specific thrust to be chosen independently of the gas generator cycle.

The working substance of the thermodynamic cycle is the only mass accelerated to produce thrust in a turbojet which is a serious limitation (high fuel consumption) for aircraft speeds below supersonic. For subsonic flight speeds the speed of the propelling jet has to be reduced because there is a price to be paid in producing the thrust. The energy required to accelerate the gas inside the engine (increase in kinetic energy) is expended in two ways, by producing a change in momentum ( i.e. a force), and a wake which is an unavoidable consequence of producing thrust by an airbreathing engine<ref>{{Cite book |url=http://archive.org/details/sim_journal-of-aircraft_september-october-1966_3_5 |title=Journal of Aircraft September-October 1966: Vol 3 Iss 5 |date=September 1966 |publisher=American Institute of Aeronautics and Astronautics |pages=386 |language=English}}</ref> (or propeller). The wake velocity, and fuel burned to produce it, can be reduced and the required thrust still maintained by increasing the mass accelerated. A turbofan does this by transferring energy available inside the engine, from the gas generator, to a [[ducted fan]] which produces a second, additional mass of accelerated air.

The transfer of energy from the core to bypass air results in lower pressure and temperature gas entering the core nozzle (lower exhaust velocity), and fan-produced higher pressure and temperature bypass-air entering the fan nozzle. The amount of energy transferred depends on how much pressure rise the fan is designed to produce (fan pressure ratio). The best energy exchange (lowest fuel consumption) between the two flows, and how the jet velocities compare, depends on how efficiently the transfer takes place which depends on the losses in the fan-turbine and fan.<ref>{{Cite book |url=http://archive.org/details/sim_journal-of-aircraft_september-october-1966_3_5 |title=Journal of Aircraft September-October 1966: Vol 3 Iss 5 |date=September 1966 |publisher=American Institute of Aeronautics and Astronautics |pages=387 |language=English}}</ref>

The fan flow has lower exhaust velocity, giving much more thrust per unit energy (lower [[specific thrust]]). Both airstreams contribute to the gross thrust of the engine. The additional air for the bypass stream increases the ram drag in the air intake stream-tube, but there is still a significant increase in net thrust. The overall effective exhaust velocity of the two exhaust jets can be made closer to a normal subsonic aircraft's flight speed and gets closer to the ideal [[Aquatic locomotion#Efficiency|Froude efficiency]]. A turbofan accelerates a larger mass of air more slowly, compared to a turbojet which accelerates a smaller amount more quickly, which is a less efficient way to generate the same thrust (see the [[#Efficiency|efficiency]] section below).

The ratio of the mass-flow of air bypassing the engine core compared to the mass-flow of air passing through the core is referred to as the [[bypass ratio]]. Engines with more [[Propelling nozzle|jet thrust]] relative to fan thrust are known as ''low-bypass turbofans'', those that have considerably more fan thrust than jet thrust are known as ''high-bypass''. Most commercial aviation jet engines in use are high-bypass,<ref name="Hall"/><ref name="HackerBurghardt2009"/> and most modern fighter engines are low-bypass.<ref name="Verma2013">{{cite book| first = Bharat | last = Verma |title= Indian Defence Review: Apr–Jun 2012 |url= https://books.google.com/books?id=IvAzNhvLK6AC&pg=PA18!|access-date=October 25, 2015 |date=January 1, 2013|publisher=Lancer Publishers|isbn= 978-81-7062-259-8 |page= 18|quote= Military power plants may be divided into some major categories – low bypass turbofans that generally power fighter jets...}}</ref><ref name="Magill" /> [[Afterburner]]s are used on low-bypass turbofans on combat aircraft.

=== Bypass ratio ===
{{main|Bypass ratio}}
The ''bypass ratio (BPR)'' of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core.<ref>{{Citation | encyclopedia= Britannica | title=Bypass ratio | url = https://www.britannica.com/technology/bypass-ratio}}</ref> A bypass ratio of 6, for example, means that 6 times more air passes through the bypass duct than the amount that passes through the combustion chamber.

Turbofan engines are usually described in terms of BPR, which together with overall pressure ratio, turbine inlet temperature and fan pressure ratio are important design parameters. In addition BPR is quoted for turboprop and unducted fan installations because their high propulsive efficiency gives them the overall efficiency characteristics of very high bypass turbofans. This allows them to be shown together with turbofans on plots which show trends of reducing [[Thrust-specific fuel consumption|specific fuel consumption]] (SFC) with increasing BPR.<ref>{{Citation | archive-date = 2013-05-28 | publisher = MIT | url = http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node84.html | title = Thermodynamics | archive-url = https://web.archive.org/web/20130528034153/http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node84.html}}</ref> BPR can also be quoted for [[lift fan]] installations where the fan airflow is remote from the engine and doesn't flow past the engine core.

Considering a constant core (i.e. fixed pressure ratio and turbine inlet temperature), core and bypass jet velocities equal and a particular flight condition (i.e. Mach number and altitude) the fuel consumption per lb of thrust (sfc) decreases with increase in BPR. At the same time gross and net thrusts increase, but by different amounts.<ref>Jet Propulsion, Nicholas Cumpsty 2003,{{ISBN|978 0 521 54144 2}}, Figure 7.3 Predicted variation in thrust and sfc with bypass ratio for a constant core</ref> There is considerable potential for reducing fuel consumption for the same core cycle by increasing BPR.This is achieved because of the reduction in pounds of thrust per lb/sec of airflow (specific thrust) and the resultant reduction in lost kinetic energy in the jets (increase in propulsive efficiency).<ref>"Practical considerations in designing the engine cycle", M G Philpot, AGARD LS 183, Steady and Transient Performance Prediction,{{ISBN|92 835 0674 X}}, p.2-12</ref>

If all the gas power from a gas turbine is converted to kinetic energy in a propelling nozzle, the aircraft is best suited to high supersonic speeds. If it is all transferred to a separate big mass of air with low kinetic energy, the aircraft is best suited to zero speed (hovering). For speeds in between, the gas power is shared between a separate airstream and the gas turbine's own nozzle flow in a proportion which gives the aircraft performance required. The trade off between mass flow and velocity is also seen with propellers and helicopter rotors by comparing disc loading and power loading.<ref>{{Cite web|title = Flight global | url = https://www.flightglobal.com/pdfarchive/view/1964/1964%20-%202596.html|format=PDF|website=Flightglobal.com}}</ref> For example, the same helicopter weight can be supported by a high power engine and small diameter rotor or, for less fuel, a lower power engine and bigger rotor with lower velocity through the rotor.

Bypass usually refers to transferring gas power from a gas turbine to a bypass stream of air to reduce fuel consumption and jet noise. Alternatively, there may be a requirement for an afterburning engine where the sole requirement for bypass is to provide cooling air. This sets the lower limit for BPR and these engines have been called "leaky" or continuous bleed turbojets<ref>{{Citation | publisher = Jane's | title = All The World's Aircraft 1975–1976 | editor-first = John W.R. | editor-last = Taylor | place = Paulton House, 8 Sheperdess Walk, London N1 7LW | page = 748}}</ref> (General Electric YJ-101 BPR 0.25) and low BPR turbojets<ref>{{Citation | title = Proceedings | date = 15 April 2015 | publisher = ASME | doi = 10.1115/84-GT-230 | url = http://proceedings.asmedigitalcollection.asme.org/proceeding.aspx?articleid=2275853| doi-access = free }}</ref> (Pratt & Whitney PW1120). Low BPR (0.2) has also been used to provide [[Compressor stall#Axi-symmetric stall or compressor surge|surge margin]] as well as afterburner cooling for the [[Pratt & Whitney J58]].<ref>{{Citation | title = PW tales | work = Road runners Internationale | url = http://roadrunnersinternationale.com/pw_tales.htm}}</ref>

===Efficiency===
[[File:Gas turbine efficiency.png|thumb|Propulsive efficiency comparison for various gas turbine engine configurations]]

[[Propeller (aeronautics)|Propeller]] engines are most efficient for low speeds, [[turbojet]] engines for high speeds, and turbofan engines between the two. Turbofans are the most efficient engines in the range of speeds from about {{convert|500|to|1000|km/h|kn mph|abbr= on}}, the speed at which most commercial aircraft operate.<ref name= grc_nasa>{{cite web|url= http://www.grc.nasa.gov/WWW/K-12/airplane/aturbf.html |title= Turbofan Engine |publisher= GRC NASA |access-date= 2010-11-24}}</ref><ref name="Neumann_2004_1984_pp228-230">{{cite book | last = Neumann | first = Gerhard | author-link = Gerhard Neumann | year = 2004 | orig-year = first published by Morrow 1984<!--removed excess additional detail--> | title = Herman the German: Just Lucky I Guess |publisher = Authorhouse | location = Bloomington, Indiana, US | isbn = 1-4184-7925-X | pages = 228–30}}</ref>

In a turbojet (zero-bypass) engine, the high temperature and high pressure exhaust gas is accelerated when it undergoes expansion through a [[propelling nozzle]] and produces all the thrust. The compressor absorbs the mechanical power produced by the turbine. In a bypass design, extra turbines drive a [[ducted fan]] that accelerates air rearward from the front of the engine. In a high-bypass design, the ducted fan and nozzle produce most of the thrust. Turbofans are closely related to [[turboprop]]s in principle because both transfer some of the gas turbine's gas power, using extra machinery, to a bypass stream leaving less for the hot nozzle to convert to kinetic energy. Turbofans represent an intermediate stage between [[turbojet]]s, which derive all their thrust from exhaust gases, and turbo-props which derive minimal thrust from exhaust gases (typically 10% or less).<ref name= srm>[http://www.srmuniv.ac.in/downloads/turbofan-2012.pdf "The turbofan engine"] {{Webarchive|url=https://web.archive.org/web/20150418181832/http://www.srmuniv.ac.in/downloads/turbofan-2012.pdf |date= 2015-04-18}}, p. 7. [[SRM Institute of Science and Technology]], Department of Aerospace Engineering.</ref> Extracting shaft power and transferring it to a bypass stream introduces extra losses which are more than made up by the improved propulsive efficiency. The turboprop at its best flight speed gives significant fuel savings over a turbojet even though an extra turbine, a gearbox and a propeller are added to the turbojet's low-loss propelling nozzle.<ref>{{cite book | title = Gas Turbine Theory | edition = 2nd | last1 = Cohen | last2 = Rogers | last3 = Saravanamuttoo | publisher = Longmans | year = 1972 |isbn = 0-582-44927-8 |page = 85}}</ref> The turbofan has additional losses from its greater number of compressor stages/blades, fan and bypass duct.{{clarify|date=May 2021}}

Froude, or propulsive, efficiency can be defined as:

<math display="block">\eta_f = \frac {2}{1 + \frac {V_j}{V_a}}</math>

where:
* {{math|{{var|V}}{{sub|{{var|j}}}}}} = thrust equivalent jet velocity
* {{math|{{var|V}}{{sub|{{var|a}}}}}} = aircraft velocity

===Thrust===
While a turbojet engine uses all of the engine's output to produce thrust in the form of a hot high-velocity exhaust gas jet, a turbofan's cool low-velocity bypass air yields between 30% and 70% of the total thrust produced by a turbofan system.<ref>{{cite book |url= http://www.faa.gov/library/manuals/aircraft/airplane_handbook/media/FAA-H-8083-3B.pdf |title= FAA-H-8083-3B Airplane Flying Handbook Handbook|publisher=Federal Aviation Administration |year= 2004 |url-status=dead|archive-url= https://web.archive.org/web/20120921094453/http://www.faa.gov/library/manuals/aircraft/airplane_handbook/media/FAA-H-8083-3B.pdf |archive-date=2012-09-21}}</ref>

The thrust ('''''F<sub>N</sub>''''') generated by a turbofan depends on the [[effective exhaust velocity]] of the total exhaust, as with any jet engine, but because two exhaust jets are present the thrust equation can be expanded as:<ref>{{Cite web|url=https://www.grc.nasa.gov/WWW/K-12/airplane/turbfan.html|title=Turbofan Thrust|website=Grc.nasa.gov|access-date=1 March 2022}}</ref>

<math display="block">F_N = \dot{m}_e v_{he} - \dot{m}_o v_o + BPR\, (\dot{m}_c) v_f</math>

where:
* {{math|{{var|{{overset|•|m}}}}{{sub|{{var|e}}}}}} = the mass rate of hot combustion exhaust flow from the core engine
* {{math|{{var|{{overset|•|m}}}}{{sub|{{var|o}}}}}} = the mass rate of total air flow entering the turbofan = {{math|{{var|{{overset|•|m}}}}{{sub|{{var|c}}}} + {{var|{{overset|•|m}}}}{{sub|{{var|f}}}}}}
* {{math|{{var|{{overset|•|m}}}}{{sub|{{var|c}}}}}} = the mass rate of intake air that flows to the core engine
* {{math|{{var|{{overset|•|m}}}}{{sub|{{var|f}}}}}} = the mass rate of intake air that bypasses the core engine
* {{math|{{var|v}}{{sub|{{var|f}}}}}} = the velocity of the air flow bypassed around the core engine
* {{math|{{var|v}}{{sub|{{var|he}}}}}} = the velocity of the hot exhaust gas from the core engine
* {{math|{{var|v}}{{sub|{{var|o}}}}}} = the velocity of the total air intake = the true airspeed of the aircraft
* {{mvar|BPR}} = bypass ratio

===Nozzles===
The cold duct and core duct's nozzle systems are relatively complex due to the use of two separate exhaust flows. In high bypass engines, the fan is situated in a short duct near the front of the engine and typically has a convergent cold nozzle, with the tail of the duct forming a low pressure ratio nozzle that under normal conditions will choke creating supersonic flow patterns around the core{{Citation needed|date=August 2020}}. The core nozzle is more conventional, but generates less of the thrust, and depending on design choices, such as noise considerations, may conceivably not choke.<ref>{{cite journal|url=https://dspace.lib.cranfield.ac.uk/bitstream/handle/1826/12476/Civil_turbofan_engine_exhaust_aerodynamics-2017.pdf|title=Civil Turbofan Engine Exhaust Aerodynamics: Impact of Bypass Nozzle After-body Design|journal=Aerospace Science and Technology|volume=73|date=February 2018|pages=85–95|doi=10.1016/j.ast.2017.09.002 |access-date=1 March 2022|last1=Goulos |first1=Ioannis |last2=Stankowski |first2=Tomasz |last3=MacManus |first3=David |last4=Woodrow |first4=Philip |last5=Sheaf |first5=Christopher |bibcode=2018AeST...73...85G |hdl=1826/12476 }}</ref> In low bypass engines the two flows may combine within the ducts, and share a common nozzle, which can be fitted with afterburner.

=== Noise ===
[[File:Boeing 747-8 VQ-BSK at LSZH (17754867239).jpg|thumb|Chevrons on a 747-8's [[General Electric GEnx|GEnx-2B]]]]
Most of the air flow through a high-bypass turbofan is lower-velocity bypass flow: even when combined with the much-higher-velocity engine exhaust, the average exhaust velocity is considerably lower than in a pure turbojet. Turbojet engine noise is predominately [[jet noise]] from the high exhaust velocity. Therefore, turbofan engines are significantly quieter than a pure-jet of the same thrust, and jet noise is no longer the predominant source.<ref name="Kempton2011">Kempton, A., [http://www.win.tue.nl/ceas-asc/Workshop15/CEAS-ASC_XNoise-EV_K1_Kempton.pdf "Acoustic liners for modern aero-engines"], 15th CEAS-ASC Workshop and 1st Scientific Workshop of X-Noise EV, 2011. Win.tue.nl.</ref> Turbofan engine noise propagates both upstream via the inlet and downstream via the primary nozzle and the by-pass duct. Other noise sources are the fan, compressor and turbine.<ref>{{Cite journal| title = Softly, softly towards the quiet jet | first = Michael J. T. | last = Smith | journal=[[New Scientist]] | date = 19 February 1970 | at = fig. 5}}</ref>

Modern commercial aircraft employ high-bypass-ratio (HBPR) engines with separate flow, non-mixing, short-duct exhaust systems. Their noise at takeoff is primarily from the fan and jet.<ref>{{cite book |last1=Huff |first1=Dennis |last2=Envia |first2=Edmane |date=October 2007 |editor-last=Crocker |editor-first=Malcolm |title=Handbook of Noise and Vibration Control |publisher=Wiley |pages=1096–1108 |chapter=Chapter 89: Jet Engine Noise Generation, Prediction, and Control |url= https://www.wiley.com/en-us/Handbook+of+Noise+and+Vibration+Control-p-9780471395997 |isbn= 978-0-471-39599-7}}</ref> The primary source of jet noise is the turbulent mixing of shear layers in the engine's exhaust. These shear layers contain instabilities that lead to highly turbulent vortices that generate the pressure fluctuations responsible for sound. To reduce the noise associated with jet flow, the aerospace industry has sought to disrupt shear layer turbulence and reduce the overall noise produced.<ref name="chevron technology">{{cite journal |url= https://www.researchgate.net/publication/273550214 | journal= Proceedings of the 13th Asian Congress of Fluid Mechanics 17–21 December 2010, Dhaka, Bangladesh | title=Evolution from 'Tabs' to 'Chevron Technology'–a Review | format= PDF | last1 =Zaman | first1 = K. B. M. Q. | last2 =Bridges | first2 = J. E. | last3 =Huff | first3 = D. L. | date=17–21 December 2010 | publisher=b[[NASA Glenn Research Center]] | place = Cleveland, [[Ohio|OH]] | access-date= January 8, 2025}}</ref>

Fan noise may come from the interaction of the fan-blade wakes with the pressure field of the downstream fan-exit stator vanes. It may be minimized by adequate axial spacing between blade trailing edge and stator entrance.<ref>{{cite journal |title=Designing the JT-9D Engine to meet Low Noise Requirements for Future Transports |last1=Kester |first1=JD |last2=Slaiby |first2=TG |journal=SAE Transactions |date=1968 |volume=76 |issue=2 |id=paper 670331 |page=1332 |jstor=44565020 |doi=10.4271/670331}}</ref>
At high engine speeds, as at takeoff, shock waves from the supersonic fan tips, because of their unequal nature, produce noise of a discordant nature known as "buzz saw" noise.<ref>{{cite magazine |title=Quiet Propulsion |first=M.J.T. |last=Smith |magazine=Flight International |date=17 August 1972 |page=241}}</ref><ref name= buzz_saw>{{Citation | first = A. | last = McAlpine | url = http://www.southampton.ac.uk/engineering/research/projects/buzz_saw_noise_and_non_linear_acoustics.page | title = Research project: Buzz-saw noise and nonlinear acoustics | publisher = University of Southampton}}</ref>

All modern turbofan engines have [[acoustic liner]]s in the [[nacelle]] to damp their noise. They extend as much as possible to cover the largest surface area. The acoustic performance of the engine can be experimentally evaluated by means of ground tests<ref name="Schuster2010">{{Citation | last1 = Schuster | first1 = B. | last2 = Lieber | first2 = L. | last3 = Vavalle | first3 = A. | title = Optimization of a seamless inlet liner using an empirically validated prediction method | work = 16th AIAA/CEAS Aeroacoustics Conference | year = 2010 | place = Stockholm, [[Sweden|SE]] |doi=10.2514/6.2010-3824| isbn = 978-1-60086-955-6 | s2cid = 113015300 }}</ref> or in dedicated experimental test rigs.<ref name="Ferrante2011">{{Citation | last1 = Ferrante | first1 = P. G. | last2 = Copiello | first2 = D. | last3 = Beutke | first3 = M. | title = Design and experimental verification of 'true zero-splice' acoustic liners in the universal fan facility adaptation (UFFA) modular rig | work = 17h AIAA/CEAS Aeroacoustics Conference | year = 2011 | id = AIAA-2011-2728 | place = Portland, OR |doi=10.2514/6.2011-2728| isbn = 978-1-60086-943-3 }}</ref>

In the [[aerospace]] industry, <!--link target-->{{anchor|Chevrons}}'''chevrons''' are the "saw-tooth" patterns on the trailing edges of some [[jet engine]] nozzles<ref name=NASA>{{cite web |url= http://www.nasa.gov/topics/aeronautics/features/bridges_chevron_events.html |title= NASA Helps Create a More Silent Night |last1=Banke |first1=Jim |date= 2012-12-13 |publisher=[[NASA]] |access-date= January 12, 2013}}</ref> that are used for [[noise control|noise reduction]]. The shaped edges smooth the mixing of hot air from the engine core and cooler air flowing through the engine fan, which reduces noise-creating turbulence.<ref name=NASA/> Chevrons were developed by GE under a [[NASA]] contract.<ref name="chevron technology" /> Some notable examples of such designs are [[Boeing 787]], [[Boeing 737 Max]] and [[Boeing 747-8]]{{snd}} on the [[Rolls-Royce Trent 1000]] (787), [[General Electric GEnx]] (787, 748-8) and [[CFM LEAP]] (1B variant only; 737 Max) engines.<ref>{{Citation | archive-date = 2014-03-25 | place = [[China|CN]] | work = 13th ACFM | title = Invited | url = http://www.afmc.org.cn/13thacfm/invited/201.pdf | publisher = AFMC | archive-url = https://web.archive.org/web/20140325205124/http://www.afmc.org.cn/13thacfm/invited/201.pdf}}</ref>