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Development History of Turbojet Engine

AddTime:2017-04-08 16:29:48   Views:     【 Big Mid Small 】   Print   Close

War need

Before the Second World War, all aircraft used piston engines as the power of the aircraft. The engine itself could not generate forward power, but needed to drive a pair of propellers to rotate in the air, so as to drive the aircraft forward. This combination of piston engine and propeller has always been the fixed propulsion mode of aircraft, and few people have questioned it.

By the end of the 1930s, especially in World War II, due to the need of war, the performance of aircraft had developed rapidly, flying at speeds of 700-800 kilometers per hour and at altitudes of over 10,000 meters, but it was suddenly found that the propeller aircraft seemed to have reached its limit, although engineers increased the power of the engine from 1,000 kilowatts to 2,000 kilowatts or even 3,000 kilowatts. KW, but the speed of the aircraft is still not significantly improved, the engine clearly felt "not strong enough".

Key problem

The problem lies in the propeller. When the speed of the airplane reaches 800 km/h, the tip of the propeller is actually close to the sound speed because the propeller is always rotating at high speed. The direct consequence of this transonic flow field is that the efficiency of the propeller drops sharply and the thrust decreases. At the same time, because of the large windward area of the propeller, the resistance of the propeller is also greater, and with the flight height. As the atmosphere becomes thinner, the power of the piston engine will drop sharply. These factors together determine that the piston engine + propeller propulsion mode has come to an end. In order to further improve flight performance, a new propulsion mode must be adopted. Jet engine emerged as the times require.

The principle of jet propulsion is not unfamiliar to us. According to Newton's third law, the forces acting on objects are reversed turbojet engines with equal magnitude and opposite direction.

Exertion. When the jet engine works, it sucks a lot of air from the front end and sprays out at high speed after combustion. In this process, the engine exerts force on the gas to accelerate it backwards, and the gas also gives the engine a reaction force to push the aircraft forward. In fact, this principle has long been applied in practice. The firecrackers we used to play fly up to the sky by the reaction force of the gunpowder gas emitted from the tail.

Breach

As early as 1913, French Engineer Ryan Loren won a patent for a jet engine, but it was a ramjet engine that could not work at low speeds at that time and lacked the high-temperature heat-resistant materials needed. In 1930, Frank Whittle obtained his first patent for using a gas turbine engine, but it wasn't until 11 years later that his engine was completing its first flight. Whittle's engine formed the basis of a modern turbojet engine.

progress

The structure of modern turbojet engine consists of intake port, compressor, combustion chamber, turbine and tail nozzle. There is an afterburner between the turbine and tail nozzle of fighter. Turbojet engine is still a kind of heat engine. It must follow the working principle of heat engine: input energy at high pressure and release energy at low pressure. Therefore, in terms of the principle of generating output energy, jet engine and piston engine are the same. They all need four stages: intake, pressurization, combustion and exhaust. The difference is that in piston engine, these four stages are carried out in turn in time, but in jet engine, they are carried out continuously, and the gas flows through each part of the jet engine in turn. The four working positions of the piston engine should be located.

Air first enters the intake port of the engine. When an aircraft is flying, it can be seen that the airflow flows to the engine at the flying speed. Because the flight speed of the aircraft varies, and the inflow speed adapted by the compressor has a certain range, the function of the intake port is to adjust the future flow to the appropriate speed through an adjustable pipe. In supersonic flight, the airflow velocity in front of the intake port and in the intake port decreases to subsonic speed. At this time, the stagnation of the airflow can increase the pressure by more than ten times or even tens of times, which greatly exceeds the pressure increase multiple in the compressor. Thus, a ramjet engine with only velocity ramming and no compressor is produced.

The compressor behind the intake port is designed to increase the pressure of the air flow. When the air flows through the compressor, the compressor working blade works on the air flow, which increases the pressure and temperature of the air flow. At subsonic speed, compressor is the main component of air turbocharging.

The high temperature and high pressure gas from the combustion chamber flows through the turbine installed on the same axis as the compressor. In a turbojet engine, the work done by the expansion of the gas flow in the turbine is exactly equal to the work consumed by the compressed air of the compressor and the work required by the transmission accessories to overcome friction. After combustion, the gas energy in front of the turbine increases greatly, so the expansion ratio in the turbine is much smaller than the compression ratio in the compressor. The pressure and temperature at the outlet of the turbine are much higher than that at the inlet of the compressor. The thrust of the engine comes from this part of the gas energy.

The high temperature and high pressure gas flowing out of the turbine continues to expand in the tail nozzle, and discharges from the nozzle to the rear along the engine axis at high speed. This speed is much greater than the speed at which the airflow enters the engine, which gives the engine a counteracting thrust.

Generally speaking, the higher the temperature of the air flow coming out of the combustion chamber, the greater the energy input, the engine.


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