The full name of turbofan engine is a kind of aircraft engine, which is developed from turbojet engine. Compared with turbojet, the main feature is that the area of the first-stage compressor is much larger, and it also acts as an air propeller (fan) to push back a part of the sucked air through the periphery of the jet engine. The part of the engine core through which air passes is called the internal duct, and the part outside the engine core through which only fan air passes is called the external duct. The turbofan engine is most suitable for the flight speed of 400 to 1000 km, so most aircraft engines now use turbofans as power sources.
The bypass ratio (also called bypass ratio) of turbofan engine is the ratio of the air mass that does not pass through the combustion chamber to the air mass that passes through the combustion chamber. A turbofan engine with a bypass ratio of zero is a turbojet engine. The early turbofan engines and turbofan engines used in modern fighters have low ductwork. For example, the world's first turbofan engine, Rolls-Royce Conway, has a bypass ratio of only 0.3. The bypass ratio of most modern civil aircraft engines is usually above 5. The turbofan engine with high bypass ratio consumes less fuel, but the thrust is equivalent to that of turbojet engine, and it is much quieter when running.
.. the birth of turbofan engine.
After World War II, with the passage of time and technological innovation, turbojet engines have been insufficient to meet the power demand of new aircraft. Especially the subsonic civil aircraft and large transport aircraft developed rapidly after World War II, the flight speed should reach high subsonic speed and the fuel consumption should be small, so the engine efficiency should be high. The efficiency of turbojet engine can no longer meet this demand, which shortens the range of the above aircraft. Therefore, for some time, there have been more and more large aircraft using turboprop engines. In fact, as early as 1930s, there were some rough early designs of jet engines with external bypass. In the 1940s and 1950s, early turbofan engines began to be tested. However, due to the very high requirements for the design and manufacture of fan blades. Therefore, it was not until the 1960s that people were able to manufacture fan blades that met the requirements of turbofan engines, thus opening the practical stage of turbofan engines. In 1950s, NACA (the predecessor of NASA) carried out very important scientific research on turbofan engine. 1955-56, the research results were transferred to General Electric Company (GE) for further development. GE successfully launched CJ805-23 turbofan engine at 1957, and then broke a lot of records of supersonic jet engine. But the earliest practical turbofan engine is Pratt &; Whitney JT3D turbofan engine. In fact, Pratt & Whitney started the turbofan engine development project later than GE. After learning the secret of GE's research and development of CJ805, they quickly stepped up their work and first launched the practical JT3D. 1960, rolls royce's "Conway" turbofan engine began to be adopted by Boeing 707 large long-range jet, becoming the first turbofan engine used by civil airliners. In 1960s, the large turbofan engines RB2 1 1-22B of rolls royce were adopted in Lockheed's Samsung passenger plane and Boeing's 747 Zhenbao passenger plane, which marked the full maturity of turbofan engines. Since then, turbojet engines have been quickly abandoned by the western civil aviation industry. Principle of turbofan jet engine The thrust of turboprop engine is limited, which affects the flight speed of aircraft. Therefore, it is necessary to improve the efficiency of jet engines. The efficiency of engine includes thermal efficiency and propulsion efficiency. The thermal efficiency can be improved by increasing the temperature of the gas in front of the turbine and the pressure increasing ratio of the compressor. Because high temperature, high density gas contains more energy. However, under the condition of constant flight speed, increasing the temperature in front of the turbine will naturally increase the exhaust speed. The gas with fast flow rate loses a lot of kinetic energy when it is discharged. Therefore, unilaterally increasing thermal power, that is, increasing the temperature in front of the turbine, will lead to a decline in propulsion efficiency. To improve engine efficiency in an all-round way, it is necessary to solve the contradiction between thermal efficiency and propulsion efficiency. The beauty of turbofan engine lies in increasing the temperature in front of the turbine without increasing the exhaust speed. The structure of turbofan engine is actually to add several stages of turbines in front of turbojet engine, and these turbines drive a certain number of fans. Like a common jet engine, a part of the airflow inhaled by the fan is sent to the compressor (the term is called "inner duct"), and the other part is directly discharged from the periphery of the turbojet engine casing ("outer duct"). Therefore, the gas energy of turbofan engine is divided into two kinds of exhaust streams generated by fan and combustion chamber respectively. At this time, in order to improve the thermal efficiency and increase the temperature before the turbine, more gas energy can be transferred to the external pipeline through the fan by appropriate turbine structure and increasing the diameter of the fan, thus avoiding a significant increase in exhaust speed. In this way, thermal efficiency and propulsion efficiency are balanced, and the efficiency of the engine is greatly improved. High efficiency means low fuel consumption and longer range.
Edit the advantages and disadvantages of this turbofan engine.
As mentioned above, the turbofan engine has high efficiency, low fuel consumption and long flight range.
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turbojet
This is a turbine engine. It is characterized by completely relying on airflow to generate thrust. Usually used as power for high-speed aircraft. Fuel consumption is higher than that of turbofan engine. There are two types of turbojet engines: centrifugal and axial. Centrifugal type was patented by British Sir Frank Whittle in 1930, but it was not until 194 1 that an airplane equipped with this engine went to heaven for the first time. It did not participate in World War II, but was born in Germany, and participated in 1945 as the first practical jet fighter Me-262. Compared with centrifugal turbojet engine, axial flow has the advantages of small cross section and high compression ratio. Today's turbojet engines are all axial flow.
Principle and working mode of. ...
Turbojet engine adopts jet propulsion, which avoids the inherent weakness of rocket and ramjet engine. Because the turbine-driven compressor is used, the engine also has enough pressure to generate strong thrust at low speed. The turbojet engine works according to the "duty cycle". It sucks air from the atmosphere, and after compression and heating, the air with energy and momentum is discharged from the propulsion nozzle at a speed as high as 2000 ft/s (6 10 m/s) or about 1400 mph (2253 km/h). When the high-speed jet flows out of the engine, it drives the compressor and turbine to continue to rotate at the same time, maintaining the "working cycle". The mechanical layout of turbine engine is relatively simple, because it only contains two main rotating parts, namely compressor and turbine, and one or several combustion chambers. However, not all aspects of this engine have this simplicity, because thermal and aerodynamic problems are more complicated. These problems are caused by the high working temperature of the combustion chamber and turbine, the changing airflow through the compressor and turbine blades, and the design of the exhaust system that exhausts the gas and forms a propulsion jet.
When the aircraft speed is lower than about 450 mph (724 km/h), the efficiency of pure jet engine is lower than that of propeller engine, because its propulsion efficiency depends largely on its flight speed; Therefore, pure turbojet engine is most suitable for higher flight speed. However, due to the airflow disturbance caused by the high tip speed of the propeller, the efficiency of the propeller drops rapidly above 350 mph (563 km/h). These characteristics make some medium-speed aircraft use a combination of propeller and gas turbine engine-turboprop engine, rather than a simple turbojet device.
The advantages of propeller/turbine combination are replaced by the introduction of internal and external ducted engines, ducted fan engines and propeller fan engines to some extent. These engines have larger flow and lower jet speed than pure jet engines, so their propulsion efficiency is equivalent to that of turboprop engines and exceeds that of pure jet engines.
Turbojet/ramjet engine is a combination of turbojet engine (usually used at various speeds below Mach 3) and ramjet engine, which has good performance at high Mach number. The engine is surrounded by a duct with an adjustable air inlet at the front end and an afterburner with an adjustable nozzle at the rear end. When taking off and accelerating, and under the flight condition of Mach number 3, the engine adopts the working mode of conventional turbojet engine; When the aircraft accelerates above Mach number 3, its turbojet mechanism is closed, and the air in the air passage bypasses the compressor with the help of guide vanes and directly flows into the afterburner, which becomes the combustion chamber of the ramjet. This engine is suitable for aircraft that require high-speed flight and maintain high Mach number cruise. Under these conditions, the engine works as a ramjet engine.
Turbine/rocket engine is similar to turbine/ramjet engine in structure, and an important difference is that it has its own oxygen for combustion. This engine has a low-pressure compressor driven by a multi-stage turbine, and the power to drive the turbine is generated by burning fuel and liquid oxygen in the rocket combustion chamber. Because the gas temperature can be as high as 3500 degrees, it is necessary to inject additional fuel into the combustion chamber for cooling before the gas enters the turbine. Then, this oil-rich mixture (gas) is diluted by air from the compressor, and the remaining fuel is burned in the conventional afterburner system. Although this engine is smaller and lighter than the turbo/ramjet engine, it consumes more fuel. This trend makes it more suitable for interceptors or spacecraft launch vehicles. These aircraft require high-altitude and high-speed performance, usually require high acceleration performance, and do not need long battery life.
..... structure
admitting port
The main structure of axial-flow turbojet engine is shown in the figure. Air enters the inlet first, because the flight state of the aircraft is changing, and the inlet needs to ensure that the air can finally enter the next structure smoothly: the compressor. The main function of the air inlet is to adjust the air to the state that the engine can run normally before entering the compressor. When flying at supersonic speed, both the nose and the inlet will produce shock waves, and the air pressure will increase after passing through the shock waves, so the inlet can play a certain role in precompression, but the improper position of the shock waves will cause uneven local pressure and even damage the compressor. Therefore, there is a shock wave adjusting cone at the inlet of supersonic aircraft, and the position of shock wave is adjusted according to airspeed.
Aircraft with air intake on both sides or belly will be affected by the boundary layer (or boundary layer) of the fuselage because the inlet is close to the fuselage, and a boundary layer adjustment device will be installed. The so-called boundary layer refers to a layer of air flowing close to the surface of the fuselage. Its velocity is much lower than the surrounding air, but its static pressure is higher than the surrounding air, forming a pressure gradient. Because of its low energy, it is not suitable for entering the engine and needs to be eliminated. When the aircraft has a certain angle of attack (AOA), due to the change of pressure gradient, boundary layer separation will occur at the part where the pressure gradient increases (such as leeward side), that is, the boundary layer that was originally close to the fuselage will suddenly separate at a certain point, forming turbulence. Turbulence is relative to laminar flow, which is simply an irregular moving fluid. Strictly speaking, all flows are turbulent. At present, the mechanism and simulation of turbulence are not clear. But that doesn't mean turbulence is not good. In many parts of the engine, such as the combustion process, we should make full use of turbulence.
compressor
The compressor consists of stator blades and rotor blades. A pair of stator blades and rotor blades is called the first stage. The stator is fixed on the engine frame, and the rotor is connected with the turbine through the rotor shaft. The active turbojet engine is generally an 8- 12 compressor. The more stages, the greater the pressure. When the fighter suddenly performs a high-g maneuver, the air pressure flowing into the front stage of the compressor will drop sharply, and the pressure in the rear stage will be very high. At this time, the high-pressure air in the rear stage will expand reversely, and the engine is extremely unstable, which is called "surge" in engineering. This is the most fatal accident of the engine, which is likely to cause shutdown or even structural damage. There are several ways to prevent "surge" Experience shows that surge mostly occurs between the 5th and 6th stages of the compressor, and a bleed ring is set in the second interval to release the pressure in time when the pressure is abnormal, so as to avoid surge. Or the rotor shaft is made into two concentric hollow cylinders, which are respectively connected with the front-stage low-pressure compressor and the turbine, and the rear-stage high-pressure compressor and the other turbine. The two rotor groups are independent of each other, so the speed can be automatically adjusted when the pressure is abnormal, and surge can also be avoided.
Combustion chamber and turbine
Air enters the combustion chamber after being compressed by the compressor, and is mixed with kerosene for combustion, and expands to do work; And then flows through the turbine to drive the turbine to rotate at high speed. Because the turbine and the compressor rotor are connected on the same shaft, the rotation speed of the compressor and the turbine is the same. Finally, the high-temperature and high-speed gas is ejected through the nozzle, and the power is provided through the reaction. At first, the combustion chamber was several small cylindrical combustion chambers, which were juxtaposed in a ring around the rotor shaft. Each cylinder is not sealed, but a hole is opened in a proper place, so that the whole combustion chamber is connected. Later, it developed into a compact annular combustion chamber, but the overall fluid environment was not as good as that of a cylindrical combustion chamber, and a combined combustion chamber combining the advantages of the two appeared.
Steam turbines always work under extreme conditions, and there are extremely strict requirements for their materials and manufacturing technology. At present, powder metallurgy hollow blades are mostly cast in one piece, that is, all blades and disks are cast at one time. Compared with the early days, each blade and disc were cast separately and then connected by tenon, which saved a lot of connection quality. The manufacturing materials are mostly high temperature resistant alloy materials, and the hollow blades can be cooled by cold air. The new engine developed for the fourth generation fighter will be equipped with ceramic powder metallurgy blades with more outstanding high temperature performance. These measures are aimed at improving one of the most important parameters of turbojet engine: the pre-turbine temperature. High pre-swirl temperature means high efficiency and high power.
Nozzle and afterburner
The shape and structure of the nozzle (or nozzle) determine the airflow state that is finally eliminated. Early low-speed engines used simple convergent nozzles to improve speed. According to Newton's third law, the greater the ejection velocity of gas, the greater the reaction force the aircraft will get. However, the growth rate of this method is limited, because the final airflow speed will reach the speed of sound, and then there will be a shock wave to stop the increase of gas speed. Supersonic jet can be obtained by using convergent-divergent nozzle (also known as Laval nozzle). The maneuverability of the aircraft mainly comes from the aerodynamic force provided by the wing surface. When the maneuverability is high, the thrust of the jet can be directly used. There are two schemes in history, that is, installing gas control surface at the nozzle or directly using deflectable nozzle (also known as thrust vector nozzle or vector thrust nozzle), and the latter has entered the practical application stage. The superb maneuverability of the famous Russian Su -30 and Su -37 fighters benefited from the AL-3 1 thrust vector engine of Rurika Design Bureau. The representative of the rudder is the American X-3 1 technical verification machine.
After the high-temperature gas passes through the turbine, it still contains some oxygen that is not consumed in time. If kerosene is continuously injected into this gas, it can still burn and generate additional thrust. Therefore, some high-performance fighter engines add afterburner (or afterburner) behind the turbine to achieve the purpose of greatly improving the engine thrust in a short time. Generally speaking, afterburner can increase the maximum thrust by 50% in a short time, but the fuel consumption is amazing. Generally, afterburner is only used for take-off or fierce air combat, and cannot be used for long-term supersonic cruise.
..... use
Turbojet engines are suitable for a wide range of navigation, from low-altitude subsonic to high-altitude supersonic aircraft. MiG -25 is a legendary fighter in the former Soviet Union. It used the turbojet engine of Liurika Design Bureau as the power, and once set a fighter speed record of Mach 3.3 and a ceiling record of 37,250 meters. This record is unlikely to be broken for some time.
Compared with turbofan engine, turbojet engine has poor fuel economy, but its high-speed performance is better than turbofan engine, especially at high altitude and high speed.