Turbojet engine is a kind of 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.
The main structure of the intake 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 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. The combustion chamber and turbine air enter the combustion chamber after being compressed by the compressor and mix with kerosene for combustion, and expand 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. The shape and structure of nozzle and afterburner nozzle (or nozzle) determine the state in which the airflow 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. Turbojet engines are suitable for a wide range of navigation, from low-altitude subsonic to high-altitude supersonic aircraft. The legendary fighter MiG -25 (Flying Fox) of the former Soviet Union used the turbojet engine of Rurika 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.