Turbofan é um motor a reação utilizado em aeronaves projetadas especialmente para altas velocidades de cruzeiro, que possui um excelente desempenho em altitudes elevadas, entre 10.000 Metros e 15.000 Metros, apresentando velocidades na faixa de 700 Km/h até 1.000 Km/h.
O motor é constituido por um "fan" que complementa o fluxo de ar gerado pelos compressores de baixa pressão e alta pressão. É um tipo bem mais moderno de motorização, uma evolução natural do motor turbojato.
Cada tipo apresenta sutis diferenças no modo de operação, sendo que em todos os tipos a ventoinha ("fan") é uma extensão de um compressor de baixa pressão (LPC, ou Low Pressure Compressor) montado logo atrás dela.
Praticamente todos os motores que impulsionam os aviões comerciais a jato atualmente são turbofans. Eles são muito apreciados por sua eficiência e por serem relativamente pouco ruidosos.
Turbofans são menos utilizados em aeronaves militares, nas quais altas velocidades e baixo peso são necessários, ao passo que ruído e eficiência são menos importantes.
[editar] Introdução
Em um turbojato, o ar passa por uma via de admissão antes de ser comprimido a uma pressão maior por um compressor em formato de ventoinha. O ar comprimido passa por um combustor onde é misturado com o combustível (como querosene) e então detonado. Os gases da combustão passam então por uma turbina, onde sua energia é usada para mover o compressor. Apesar do processo de expansão na turbina reduzir a pressão (e a temperatura) dos gases, normalmente ainda há energia suficiente para gerar um jato de alta velocidade, já que os gases se expandem à pressão atmosférica através do bocal de saída. Esse processo normalmente produz um impulso na direção oposta à do jato de gases (o empuxo). Ao contrário de um motor cíclico, um turbojato utiliza um processo contínuo.
O processo descrito acima é, estritamente, para um turbojato de eixo simples. Após a Segunda Guerra Mundial, turbojatos de eixo duplo foram desenvolvidos para facilitar o manejo dos sistemas de compressão. A adoção de um sistema de dois eixos permite que o sistema de compressão permite que o arranjo seja divido em dois, com um compressor de baixa pressão (LPC) sobrecarregando um compressor de alta pressão (HPC, High Pressure Compressor). Cada compressor é montado em um eixo distinto (coaxial), que é movido por sua própria turbina (i.e turbina de alta pressão e turbina de baixa pressão). Fora isso, um turbojato de dois eixos é muito similar a um turbojato de eixo simples.
Os turbofans modernos evoluíram do turbojato de eixo duplo, basicamente aumentando o tamanho relativo do compressor de baixa pressão até o ponto no qual uma parte (se não a maior parte) do ar passa pelo motor contornando o fluxo principal, correndo ao redor da câmara de combustão. Esse ar pode tanto expandir-se através de um bocal independente quanto ser misturado aos gases quentes que saem da turbina de baixa pressão, antes de se expandir através de um bocal comum. Apesar de gerarem um jato mais lento, os turbofans civis modernos são mais silenciosos que seus turbojatos equivalentes. Turbofans têm ainda uma maior eficiência térmica, que será explicada mais abaixo. Em um turbofan, o compressor de baixa pressão é freqüentemente chamado de ventoinha. Turbofans civis geralmente têm uma única ventoinha, enquanto que a maioria dos turbofans militares têm várias ventoinhas.
Turbohélices são turbinas a gás que transmitem quase toda sua energia para uma engrenagem que move uma hélice. Turbohélices ainda são populares em aeronaves pequenas e/ou lentas, como por exemplo transportes militares como o C-130 Hercules e o P-3 Orion.
Se o turbohélice é recomendável a velocidades moderadas e o turbojato é melhor a velocidades altas, imaginou-se que a velocidades medianas uma junção dos dois sistemas seria melhor. Esse motor é o turbofan (originalmente chamado de turbojato de contorno de ar pelos inventores). Outro nome ocasionalmente usado é "ventoinha interna", sendo que esse nome também é usado para hélices e ventoinhas utilizadas em aplicações verticais.
A diferença entre um turbofan e uma hélice, além da propulsão direta, é que a entrada de ar do primeiro desacelera o ar antes que este chegue às lâminas da ventoinha. Ao passo que tanto hélices quanto ventoinhas são eficientes apenas a velocidades subsônicas, ventoinhas internas permitem boa eficiência a velocidades maiores.
Dependendo da propulsão específica, ventoinhas internas alcançam máxima eficiência entre 400 e 2000 km/h (250 a 1300 mph), sendo portanto a opção de motor mais comum tanto das companhias comerciais de hoje quanto das aeronaves militares supersônicas ou subsônicas, sejam de treino sejam de combate. Entretanto, é importante notar que turbofans utilizam grandes entradas de ar para desacelerar o ar a velocidades subsônicas (conseqüentemente reduzindo as ondas de choque através do motor).
Taxa de contorno (bypass ratio, quantidade de ar que contorna a câmara de combustão) é um parâmetro freqüentemente utilizado para classificar turbofans, apesar de a propulsão específica ser um parâmetro mais adequado.
O ruído de qualquer tipo de turbojato está fortemente relacionado com a velocidade dos gases expelidos. Turbofans com alta taxa de contorno (i.e. baixa propulsão específica) são relativamente silenciosos se comparados a turbojatos e a turbofans com baixa taxa de contorno (i.e. alta propulsão específica). Um motor com baixa propulsão específica tem, por definição, um jato de menor velocidade, como mostra a equação abaixo (aproximada) para propulsão:
onde:
massa de ar que entra
velocidade do jato totalmente expandido (na pluma de exaustão)
velocidade do avião
Reorganizando a equação acima, propulsão específica é dada por:
Portanto, para uma determinada velocidade, a propulsão específica é diretamente proporcional à velocidade do jato. Relativamente falando, motores com baixa propulsão específica têm um maior diâmetro para acomodar a grande quantidade de ar necessária para uma dada propulsão.
Aviões a jato são considerados ruidosos, mas um motor convencional a pistão ou um motor turbohélice desenvolvendo a mesma potência seria muito mais barulhento.
[editar] Os primeiros turbofans
Os primeiros turbofans eram muito ineficientes no consumo de combustível, já que sua taxa de pressurização e a temperatura da entrada da turbina eram muito limitadas pela tecnologia da época. O primeiro turbofan em operação foi o Daimler-Benz DB 670 (popularmente conhecido como 109-007) que foi testado em 1º de abril de 1943. O projeto foi posteriormente abandonado por causa da guerra e principalmente por causa de problemas que não foram resolvidos. Materiais melhores, e a introdução de compressores duplos, como no motor Pratt & Whitney JT3C, melhoraram a taxa de pressurização e conseqüentemente a eficiência termodinâmica dos motores, mas levaram a uma fraca eficiência propulsiva, já que turbojatos genuínos têm alta propulsão específica e alta velocidade de exaustão.
Os turbofans originais de baixa taxa de contorno foram projetados para melhorar a eficiência de propulsão reduzindo a velocidade de exaustão a um valor próximo do da velocidade da aeronave. O Rolls-Royce Conway, o primeiro turbofan a entrar em produção, tinha uma taxa de contorno de 0.3, similar ao moderno motor de caça General Electric F404. Turbofans civis dos anos 1960, como o Pratt & Whitney JT8D e o Rolls-Royce Spey tinham taxas de controno próximas a 1, mas não eram similares aos seus equivalentes militares. O distinto motor General Electric CF700 foi desenvolvido como um motor com ventoinha frontal, com uma taxa de contorno de 2.0. Ele era derivado do turbojato General ElectricJ85/CJ610 do T-38 Talon e do Learjet (2,850 lbf ou 12,650 N) para impulsionar o grande Rockwell Sabreliner 75/80 , assim como o Dassault Falcon 20 com um aumento de cerca de 50% no impulso (4,200 lbf ou 18,700 N). O CF700 foi o primeiro turbofan de tamanho reduzido do mundo a ser certificado pela Administração Federal de Aviação (FAA, Federal Aviation Administration ). Atualmente, há mais de 400 aeronaves equipadas com o CF700 em operação ao redor do mundo, com uma base de experiência de mais de 10 milhões de horas de serviço. O turbofan CF700 foi ainda usado para treinar astronautas durante o Projeto Apollo como o motor do Veículo de Pouso e Pesquisa Lunar.
[editar] Turbofans de baixa taxa de contorno
Um turbofan de alta propulsão específica e baixa taxa de contorno normalmente tem uma ventoinha de vários estágios, criando uma taxa de pressurização relativamente alta e, conseqüentemente, conseguindo uma alta velocidade de exaustão. A passagem central de ar precisa ser suficientemente grande para conseguir a força necessária para mover a ventoinha. Um menor fluxo de ar na passagem central e e uma maior taxa de contorno podem ser conseguidas aumentando-se a temperatura de entrada do disco da turbina de alta pressão.
Imagine uma situação hipotética na qual um novo turbofan de baixa taxa de contorno e exaustão mista substitua um velho turbojato, preferencialmente em uma aplicação militar. Diga-se que o novo motor tem o mesmo fluxo de ar e a mesma propulsão específica que o motor que está substituindo. Um fluxo de contorno apenas pode ser iniciado se a temperatura de entrada do motor for aumentada, para compensar para uma correspondente diminuição do fluxo de ar na passagem principal. Melhoramentos no refrigeramento do motor e novas tecnologias de materiais poderiam facilitar de uma maior temperatura de entrada, desconsiderando diminuição da temperatura do ar, o que resultaria numa provável queda na taxa de pressurização.
Feito de forma eficiente, o turbofan final provavelmente operaria a pressões de bocal maiores que as do turbojato, mas com uma temperatura de exaustão menor para reter a propulsão específica. Uma vez que o aumento de temperatura através de todo o motor (da entrada ao bocal) seria menor, o fluxo de combustível também seria menor, resultando em um menor consumo específico de combustível (SFC, specific fuel consumption).
Alguns turbofans militares de baixa taxa de contorno (como o F404) possuem lâminas de entrada variáveis, com dobradiças ao estilo de piano, para direcionar o ar diretamente ao primeiro disco. Isso aumenta a margem de sucção da ventoinha. As asas anguladas do F-111 alcançaram um grande alcance e uma grande capacidade de carga por ser o pioneiro no uso de tal motor, sendo que este é também o coração do famoso caça de superioridade aérea F-14 Tomcat, o qual usa os mesmos motores em um uma estrutura menor e mais ágil para conseguir um cruzeiro eficiente e uma velocidade de Mach 2.
[editar] Turbofans com pós-combustão
Since the 1970s, most jet fighter engines have been low/medium bypass turbofans with a mixed exhaust, afterburner and variable area final nozzle – the first afterburning turbofan was the Pratt & Whitney TF30. An afterburner is a combustor located directly upstream of the nozzle. When lit, prodigious amounts of fuel are burnt in the afterburner, raising the temperature of exhaust gases by a significant amount, resulting in a higher exhaust velocity/engine specific thrust. The variable geometry nozzle must open to a larger throat area to accommodate the extra volume flow when the afterburner is lit. Afterburning gives a significant thrust boost for take off, transonic acceleration and combat maneuvers, but is very fuel intensive. Consequently afterburning can only be selected for relatively short proportion of the mission.
Unlike the main combustor, where the integrity of the downstream turbine blades must be preserved, an afterburner can operate at the ideal maximum (stoichiometric) temperature (i.e. about 2100K(3780R)). Now, at a fixed total applied fuel:air ratio, the total fuel flow for a given fan airflow will be the same, regardless of the dry specific thrust of the engine. However, a high specific thrust turbofan will, by definition, have a higher nozzle pressure ratio, resulting in a higher afterburning net thrust and, therefore, a lower afterburning specific fuel consumption. However, high specific thrust engines have a high dry SFC. The situation is reversed for a medium specific thrust afterburning turbofan: i.e. poor afterburning SFC/good dry SFC. The former engine is suitable for a combat aircraft which must remain in afterburning combat for a fairly long period, but only has to fight fairly close to the airfield (i.e cross border skirmishes) The latter engine is better for an aircraft that has to fly some distance, or loiter for a long time, before going into combat. However, the pilot can only afford to stay in afterburning for a short period, before his/her fuel reserves become dangerously low.
Modern low-bypass military turbofans include the Pratt & Whitney F119, the Eurojet EJ200 and the General Electric F110, all of which feature a mixed exhaust, afterburner and variable area propelling nozzle. Non-afterburning engines include the Rolls-Royce/Turbomeca Adour (afterburning in the SEPECAT Jaguar) and the unmixed, vectored thrust, Rolls-Royce Pegasus.
[editar] Turbofans de alta taxa de contorno
The low specific thrust/high bypass ratio turbofans used in today's civil jetliners (and some military transport aircraft) evolved from the high specific thrust/low bypass ratio turbofans used in such aircraft back in the 1960s.
Low specific thrust is achieved by replacing the multi-stage fan with a single stage unit. Unlike some military engines, modern civil turbofans do not have any stationary inlet guide vanes in front of the fan rotor. The fan is scaled to achieve the desired net thrust.
The core (or gas generator) of the engine must generate sufficient Core Power to at least drive the fan at its design flow and pressure ratio. Through improvements in turbine cooling/material technology, a higher (HP) turbine rotor inlet temperature can be used, thus facilitating a smaller (and lighter) core and (potentially) improving the core thermal efficiency. Reducing the core mass flow tends to increase the load on the LP turbine, so this unit may require additional stages to reduce the average stage loading and to maintain LP turbine efficiency. Reducing core flow also increases bypass ratio (5:1, or more, is now common).
Further improvements in core thermal efficiency can be achieved by raising the overall pressure ratio of the core. Improved blade aerodynamics reduces the number of extra compressor stages required. With multiple compressors (i.e. LPC, IPC, HPC) dramatic increases in overall pressure ratio have became possible. Variable geometry (i.e. stators) enable high pressure ratio compressors to work surge-free at all throttle settings.
The first high-bypass turbofan engine was the General Electric TF39, built to power the Lockheed C-5 Galaxy military transport aircraft. The civil General Electric CF6 engine used a derived design. Other high-bypass turbofans are the Pratt & Whitney JT9D, the three-shaft Rolls-Royce RB211 and the CFM International CFM56. More recent large high-bypass turbofans include the Pratt & Whitney PW4000, the three-shaft Rolls-Royce Trent, the General Electric GE90, and the General Electric GEnx.
The significantly higher thrust provided by high-bypass turbofan engines also made civil wide-body aircraft practical and economical. In addition to the vastly increased thrust, these engines are also generally quieter. This is not so much due to the higher bypass ratio, as to the use of low pressure ratio, single stage, fans, which significantly reduce specific thrust and, thereby, jet velocity. The combination of a higher overall pressure ratio and turbine inlet temperature improves thermal efficiency. This, together with a lower specific thrust (better propulsive efficiency), leads to a lower specific fuel consumption.
For reasons of fuel economy, and also of reduced noise, almost all of today's jet airliners are powered by high-bypass turbofans. Although modern military aircraft tend to use low bypass ratio turbofans, military transport aircraft (e.g. C-17 ) mainly use high bypass ratio turbofans (or turboprops) for fuel efficiency.
The Soviet Union's engine technology was less advanced than the West's and its first wide-body aircraft, the Ilyushin Il-86, was powered by low-bypass engines. The Yakovlev Yak-42, a medium-range, rear-engined aircraft seating up to 120 passengers was the first Soviet aircraft to use high-bypass engines.
[editar] Configurações de turbofans
Turbofan engines come in a variety of engine configurations. For a given engine cycle (i.e. same airflow, bypass ratio, fan pressure ratio, overall pressure ratio and HP turbine rotor inlet temperature), the choice of turbofan configuration has little impact upon the design point performance (e.g. net thrust, SFC), as long as overall component performance is maintained. Off-design performance and stability is, however, affected by engine configuration.
As the design overall pressure ratio of an engine cycle increases, it becomes more difficult to throttle the compression system, without encountering an instability known as compressor surge. This occurs when some of the compressor aerofoils stall (like the wings of an aircraft) causing a violent change in the direction of the airflow. However, compressor stall can be avoided, at throttled conditions, by progressively:
1) opening interstage/intercompressor blow-off valves (inefficient)
and/or
2) closing variable stators within the compressor
Most modern American civil turbofans employ a relatively high pressure ratio High Pressure (HP) Compressor with several rows of variable stators to control surge margin. However, on the three-spool RB211/Trent the HP Compressor has a modest pressure ratio and can be throttled-back surge-free, without employing HP Compressor variable geometry.
[editar] Turbofan de eixo simples
Although far from common, the Single Shaft Turbofan is probably the simplest configuration, comprising a fan and high pressure compressor driven by a single turbine unit, all on the same shaft. The SNECMA M53, which powers Mirage fighter aircraft, is an example of a Single Shaft Turbofan. Despite the simplicity of the turbomachinery configuration, the M53 requires a variable area mixer to facilitate part-throttle operation.
[editar] Turbofans com ventoinha traseira
One of the earliest turbofans was a derivative of the General Electric J79 turbojet, known as the CJ805, which featured an integrated aft fan/low pressure (LP) turbine unit located in the turbojet exhaust jetpipe. Hot gas from the turbojet turbine exhaust expanded through the LP turbine, the fan blades being a radial extension of the turbine blades. This Aft Fan configuration was later exploited in the General Electric GE-36 UDF (propfan) Demonstrator of the early 80's. One of the problems with the Aft Fan configuration is hot gas leakage from the LP turbine to the fan.
[editar] Turbofan básico de dois eixos
Many turbofans have the Basic Two Spool configuration where both the fan and LP turbine (i.e. LP spool) are mounted on a second (LP) shaft, running concentrically with the HP spool (i.e. HP compressor driven by HP turbine). The Rolls-Royce BR710 is typical of this configuration. At the smaller thrust sizes, instead of all-axial blading, the HP compressor configuration may be axial-centrifugal (e.g. General Electric CFE738), double-centrifugal or even diagonal/centrifugal (e.g. Pratt & Whitney Canada PW600).
[editar] Turbofan melhorado de dois eixos
Higher overall pressure ratios can be achieved by either raising the HP compressor pressure ratio or adding an Intermediate Pressure (IP) Compressor between the fan and HP compressor, to supercharge or boost the latter unit helping to raise the overall pressure ratio of the engine cycle to the very high levels employed today (i.e. greater than 40:1, typically). All of the large American turbofans (e.g. General Electric CF6, GE90 and GEnx plus Pratt & Whitney JT9D and PW4000) feature an IP compressor mounted on the LP shaft and driven, like the fan, by the LP turbine, the mechanical speed of which is dictated by the tip speed and diameter of the fan. The high bypass ratios (i.e. fan duct flow/core flow) used in modern civil turbofans tends to reduce the relative diameter of the attached IP compressor, causing its mean tip speed to decrease. Consequently more IPC stages are required to develop the necessary IPC pressure rise.
[editar] Turbofan de três eixos
Rolls-Royce chose a Three Spool configuration for their large civil turbofans (i.e. the RB211 and Trent families), where the Intermediate Pressure IP compressor is mounted on a separate (IP) shaft, running concentrically with the LP and HP shafts, and is driven by a separate IP Turbine. Consequently, the IP compressor can rotate faster than the fan, increasing its mean tip speed, thereby reducing the number of IP stages required for a given IPC pressure rise. However, because the RB211/Trent designs have a higher IPC pressure rise than the American engines, the HPC pressure rise is less resulting in a shorter, lighter, more rigid engine. However, three spool engines are harder to both build and maintain. The greater rigidity means that there is less distortion of the engine casing under 'g' loads during flight, resulting in less blade tip rubbing and, therefore, a slower in-service deterioration of component performance and specific fuel consumption.
The Turbo-Union RB199 military turbofan also has a three spool configuration.
[editar] Ventoinha ligada a caixa de engrenagens
As bypass ratio increases, the mean radius ratio of the fan and LP turbine increases. Consequently, if the fan is to rotate at its optimum blade speed the LP turbine blading will run slow, so additional LPT stages will be required, to extract sufficient energy to drive the fan. Introducing a reduction gearbox, with a suitable gear ratio, between the LP shaft and the fan, enables both the fan and LP turbine to operate at their optimum speeds. This is not a popular solution, since high power gearboxes tend to add considerably to the maintenance required for the engine. Typical of this configuration are the long established Honeywell TFE731 and the recent Pratt & Whitney Advanced Technology Fan Integrator (ATFI) demonstrator engine.
[editar] Aperfeiçoamento do funcionamento
Consider a mixed turbofan with a fixed bypass ratio and airflow. Increasing the overall pressure ratio of the compression system raises the combustor entry temperature. Therefore, at a fixed fuel flow there is an increase in (HP) turbine rotor inlet temperature. Although the higher temperature rise across the compression system implies a larger temperature drop over the turbine system, the mixed nozzle temperature is unaffected, because the same amount of heat is being added to the system. There is, however, a rise in nozzle pressure, because overall pressure ratio increases faster than the turbine expansion ratio, causing an increase in the hot mixer entry pressure. Consequently, net thrust increases, whilst specific fuel consumption (fuel flow/net thrust) decreases. A similar trend occurs with unmixed turbofans.
So turbofans can be made more fuel efficient by raising overall pressure ratio and turbine rotor inlet temperature in unison. However, better turbine materials and/or improved vane/blade cooling are required to cope with increases in both turbine rotor inlet temperature and compressor delivery temperature. Increasing the latter may require better compressor materials.
[editar] Aumento da propulsão
Thrust growth is obtained by increasing core power. There are two basic routes available:
a) hot route: increase HP turbine rotor inlet temperature
b) cold route: increase core mass flow
Both routes require an increase in the combustor fuel flow and, therefore, the heat energy added to the core stream.
The hot route may require changes in turbine blade/vane materials and/or better blade/vane cooling. The cold route can be obtained by one of the following:
- adding T-stages to the LP/IP compression
- adding a zero-stage to the HP compression
- improving the compression process, without adding stages (e.g. higher fan hub pressure ratio)
all of which increase both overall pressure ratio and core airflow.
Alternatively, the core size can be increased, to raise core airflow, without changing overall pressure ratio. This route is expensive, since a new (upflowed) turbine system (and possibly a larger IP compressor) is also required.
Changes must also be made to the fan to absorb the extra core power. On a civil engine, jet noise considerations mean that any significant increase in Take-off thrust must be accompanied by a corresponding increase in fan mass flow (to maintain a T/O specific thrust of about 30lbf/lb/s), usually by increasing fan diameter. On military engines, the fan pressure ratio would probably be increased to improve specific thrust, jet noise not normally being an important factor.
[editar] Discussão técnica
- Specific Thrust (net thrust/intake airflow) is an important parameter for turbofans and jet engines in general. Imagine a fan (driven by an appropriately sized electric motor) operating within a pipe, which is connected to a propelling nozzle. Fairly obviously, the higher the Fan Pressure Ratio (fan discharge pressure/fan inlet pressure), the higher the jet velocity and the corresponding specific thrust. Now imagine we replace this set-up with an equivalent turbofan - same airflow and same fan pressure ratio. Obviously, the core of the turbofan must produce sufficient power to drive the fan via the Low Pressure (LP) Turbine. If we choose a low (HP) Turbine Inlet Temperature for the gas generator, the core airflow needs to be relatively high to compensate. The corresponding bypass ratio is therefore relatively low. If we raise the Turbine Inlet Temperature, the core airflow can be smaller, thus increasing bypass ratio. Raising turbine inlet temperature tends to increase thermal efficiency and, therefore, improve fuel efficiency.
- Naturally, as altitude increases there is a decrease in air density and, therefore, the net thrust of an engine. There is also a flight speed effect, termed Thrust Lapse Rate. Consider the approximate equation for net thrust again:
With a high specific thrust (e.g. fighter) engine, the jet velocity is relatively high, so intuitively one can see that increases in flight velocity have less of an impact upon net thrust than a medium specific thrust (e.g. trainer) engine, where the jet velocity is lower. The impact of thrust lapse rate upon a low specfic thrust (e.g. civil) engine is even more severe. At high flight speeds, high specific thrust engines can pick-up net thrust through the ram rise in the intake, but this effect tends to diminish at supersonic speeds because of shock wave losses. - Thrust growth on civil turbofans is usually obtained by increasing fan airflow, thus preventing the jet noise becoming too high. However, the larger fan airflow requires more power from the core. This can be achieved by raising the Overall Pressure Ratio (combustor inlet pressure/intake delivery pressure) to induce more airflow into the core and by increasing turbine inlet temperature. Together, these parameters tend to increase core thermal efficiency and improve fuel efficiency.
- Some high bypass ratio civil turbofans use an extremely low area ratio (less than 1.01), convergent-divergent, nozzle on the bypass (or mixed exhaust) stream, to control the fan working line. The nozzle acts as if it has variable geometry. At low flight speeds the nozzle is unchoked (less than a Mach Number of unity), so the exhaust gas speeds up as it approaches the throat and then slows down slightly as it reaches the divergent section. Consequently, the nozzle exit area controls the fan match and, being larger than the throat, pulls the fan working line slightly away from surge. At higher flight speeds, the ram rise in the intake increases nozzle pressure ratio to the point where the throat becomes choked (M=1.0). Under these circumstances, the throat area dictates the fan match and, being smaller than the exit, pushes the fan working line slightly towards surge. This is not a problem, since fan surge margin is much better at high flight speeds.
- The off-design behaviour of turbofans is illustrated under compressor map and turbine map.
- Because modern civil turbofans operate at low specific thrust, they only require a single fan stage to develop the required fan pressure ratio. The desired overall pressure ratio for the engine cycle is usually achieved by multiple axial stages on the core compression. Rolls-Royce tend to split the core compression into two with an intermediate pressure (IP) supercharging the HP compressor, both units being driven by turbines with a single stage, mounted on separate shafts. Consequently, the HP compressor need only develop a modest pressure ratio (e.g.~4.5:1). US civil engines use much higher HP compressor pressure ratios (e.g. ~23:1 on the General Electric GE90) and tend to be driven by a two stage HP turbine. Even so, there are usually a few IP axial stages mounted on the LP shaft, behind the fan, to further supercharge the core compression system. Civil engines have multi-stage LP turbines, the number of stages being determined by the bypass ratio, the amount of IP compression on the LP shaft and the LP turbine blade speed.
- Because military engines usually have to be able to fly very fast at Sea Level, the limit on HP compressor delivery temperature is reached at a fairly modest design overall pressure ratio, compared with that of a civil engine. Also the fan pressure ratio is relatively high, to achieve a medium to high specific thrust. Consequently, modern military turbofans usually only have 5 or 6 HP compressor stages and only require a single stage HP turbine. Low bypass ratio military turbofans usually have one LP turbine stage, but higher bypass ratio engines need two stages. In theory, by adding IP compressor stages, a modern military turbofan HP compressor could be used in a civil turbofan derivative, but the core would tend to be too small for high thrust applications.
[editar] Feitos recentes na tecnologia das lâminas
The turbine blades in a turbofan engine are subject to high heat and stress, and require special fabrication. New material construction methods and material science have allowed blades, which were originally polycrystalline (regular metal), to be made from lined up metallic crystals and more recently mono-crystalline (i.e. single crystal) blades, which can operate at higher temperatures with less distortion.
Nickel-based superalloys are used for HP turbine blades in almost all of the modern jet engines. The temperature capabilities of turbine blades have increased mainly through four approaches: the manufacturing (casting) process, cooling path design, thermal barrier coating (TBC), and alloy development.
Although turbine blade (and vane) materials have improved over the years, much of the increase in (HP) turbine inlet temperatures is due to improvements in blade/vane cooling technology. Relatively cool air is bled from the compression system, bypassing the combustion process, and enters the hollow blade or vane. After picking up heat from the blade/vane, the cooling air is dumped into the main gas stream. If the local gas temperatures are low enough, downstream blades/vanes are uncooled and solid.
Strictly speaking, cycle-wise the HP Turbine Rotor Inlet Temperature (after the temperature drop across the HPT stator) is more important than the (HP) turbine inlet temperature. Although some modern military and civil engines have peak RITs of the order of 3300 °R (2840 °F) or 1833 K (1560 °C), such temperatures are only experienced for a short time (during take-off) on civil engines.
[editar] Fabricantes de turbofans
The turbofan engine market is dominated by General Electric, Rolls-Royce plc and Pratt & Whitney, in order of market share. GE and SNECMA of France have a joint venture, CFM International which, as the 3rd largest manufacturer in terms of market share, fits between Rolls Royce and Pratt & Whitney. Rolls Royce and Pratt & Whitney also have a joint venture, International Aero Engines, specializing in engines for the Airbus A320 family, whilst finally, Pratt & Whitney and General Electric have a joint venture, Engine Alliance marketing a range of engines for aircraft such as the Airbus A380.
[editar] General Electric
GE Aircraft Engines, part of the General Electric Conglomerate, currently has the largest share of the turbofan engine market. Some of their engine models include the CF6 (available on the Boeing 767, Boeing 747, Airbus A330 and more), GE90 (only the Boeing 777) and GEnx (developed for the Airbus A350 & Boeing 787 currently in development) engines. On the military side, GE engines power many U.S. military aircraft, including the F110, powering 80% of the US Air Force's F-16 Vipers and the F404 and F414 engines, which power the Navy's F/A-18 Hornet and Super Hornets. Rolls Royce and General Electric are jointly developing the F136 engine to power the Joint Strike Fighter.
[editar] CFM International
CFM International is a joint venture between GE Aircraft Engines and SNECMA of France.
They have created the very successful CFM56 series, used on Boeing 737 and Airbus aircraft.
[editar] Rolls-Royce
Rolls-Royce plc is the second largest manufacturer of turbofans and is most noted for their RB211 and Trent series, as well as their joint venture engines for the Airbus A320 and Boeing MD-90 families (IAE V2500 with Pratt & Whitney and others), the Panavia Tornado (Turbo-Union RB199) and the Boeing 717 (BR700). Rolls Royce, as owners of the Allison Engine Company, have their engines powering the C-130 Hercules and several Embraer regional jets. Rolls-Royce Trent 970s were the first engines to power the new Airbus A380. It was also Rolls-Royce Olympus[1]/SNECMA jets that powered the now retired Concorde although they were turbojets rather than turbofans. The famous thrust vectoring Pegasus[1] engine is the primary powerplant of the Harrier "Jump Jet" and its derivatives.
[editar] Pratt & Whitney
Pratt & Whitney is third behind GE and Rolls-Royce in market share. The JT9D has the distinction of being chosen by Boeing to power the original Boeing 747 "Jumbo jet". The PW4000 series is the successor to the JT9D, and powers some Airbus A310, Airbus A300, Boeing 747, Boeing 767, Boeing 777, and MD-11 aircraft. The PW4000 is certified for 180-minute ETOPS when used in twinjets. The second family is the 100 inch (2.5 m) fan engine developed specifically for the Airbus A330 twinjet, and the third family has a diameter of 112 inch designed to power Boeing 777. The Pratt & Whitney F119 and its derivative, the F135, power the United States Air Force's F-22 Raptor and the international F-35 Lightning II, respectively. Rolls Roy
留言列表
