【Advantages of turbofan engine】
The advantage of commercial large bypass over the turbofan hook of single-stage fans is that it has high fuel efficiency and three parts of heat engine efficiency: combustion chamber efficiency + internal efficiency + external efficiency.
The combustion chamber efficiency is how much heat these oils can burn, and how much is actually burned, and the core engine of Minfa (high-pressure compressed gas combustion chamber high-pressure turbine) even comes directly from the military hair, so it is the same as the military hair.
Internal efficiency is how much work can be brought in by the heat from the oil burning (driving turbines and jet propulsion). This value is positively correlated with the total pressure ratio of engines without afterburner (i.e. fighter engines), which is why commercial aviation pressures are getting higher and higher. In addition, it is positively correlated with the ratio of pre-turbine temperature to ambient temperature, which is why both commercial and military engines are pursuing higher pre-turbine temperature, which poses many challenges to turbine cooling systems and turbine materials and coatings.
External efficiency is how much forward energy is brought by the exhaust exhaust of these kinetic energy. And this thing is inversely related to the exhaust speed, that is, the closer the exhaust speed is to the intake speed, the closer the external efficiency is to 100%, and when the intake air and exhaust speed are the same, the external efficiency is 100%, of course, there will be no thrust at this time.
So the best solution to improve efficiency is to reduce the waste heat in the exhaust, use this part of the energy to push the more air, the better, and this part of the air is "pushed", the slower the better. In other words, raise the bypass ratio, push more air more gently.
Figure 2 The horizontal axis is the bypass ratio, and the Bs (red) of the vertical axis is the fuel consumption per unit of thrust per second.
Of course, the bypass ratio cannot be increased indefinitely, and the large bypass ratio also brings many disadvantages such as wing ground clearance, weight, windward area, engine shell wind resistance. Moreover, the fan of the dual-shaft architecture hangfa contradicts the low-pressure turbine revolution, and Leap should be the limit bypass ratio issued by the traditional dual-shaft architecture.
Military turbofan engines: more fuel efficient in subsonic flight, greater thrust boost potential when the afterburner is open.
As written above, military hair also has to pursue a little fuel efficiency, otherwise the combat radius is too limited. Turbofans are used, but the large turbofan has too large a windward area, too much weight, and so on. It is impossible to fly supersonically, it is impossible to maneuver with a fierce large overload.
Therefore, the engineer chose a small bypass ratio, but a multi-stage (generally 3-stage) low-pressure compressed gas with a larger pressure ratio, the core air flow pressed by the low-pressure compressor, part of which enters the inner duct, compresses and ignites to do work. Part goes into the outer duct, and at the rear of the low-pressure turbine, the inner and outer duct airflow mixes. (In fact, this is not particularly correct, nor did the engineer choose a small bypass than turbofan, originally the turbofan began to be a multi-stage fan, small bypass ratio, including commercial turbofans, such as Pratt & Whitney from JT3C JT3D, 3-stage low-pressure pressure gas replaced by 2-stage blades longer low-pressure pressure, opened an external duct).
This is also the internal hybrid turbofan, and then because the scorching core exhaust gas is cooled by the relatively cold outer duct air flow, the temperature of the mixture is reduced, and when it enters the afterburner, more oil can be sprayed, and the mixed gas stream can be burned until the oxygen is completely burned, and then ejected through the variable supersonic thrust nozzle. Because there are no metal parts in the core of the engine after adding the combustion chamber, it can be completely combusted, and if it is completely burned in the main combustion chamber, the high-pressure turbine will melt instantly.
The total engine pressure ratio of the fighter is much lower than that of the commercial development, not only due to structural weight considerations, but also when the fighter is flying at supersonic speeds, both the intake cone and the intake tract can provide certain (or even very high) compression. See: The most elegant and savage power - SR71 Blackbird's heart Pratt & Whitney J58.
If an aircraft only considers supersonic flight, it will not use turbofans, such as Concorde's power Olympus 593, twin-shaft turbojet with afterburner.
