Turbojet

In 1791, the English inventor John Barber proposed the idea of ​​a rotary engine with a reciprocating compressor, a combustion chamber and a gas turbine. In 1909, the Russian inventor N.V. Gerasimov patented the scheme of a gas turbine engine to create jet thrust (turbojet engine) ^{[1] [2] [3]} . A patent for the use of a gas turbine for aircraft movement was obtained in 1921 by the French engineer .

The first sample of the turbojet engine was demonstrated by the English engineer Frank Whittle on April 12, 1937 and the small private firm created by him. It was based on the theoretical work of .

The first useful application of a turbojet engine occurred in Germany on a Heinkel He 178 aircraft with a turbojet engine. The turbojet engine was developed by Hans von Ohain almost simultaneously with Whittle - the first launch in September 1937, was manufactured by Heinkel-Hirth Motorenbau. Pilot Erich Varzits made his first flight on August 27, 1939 .

The compressor draws in air, compresses it and directs it into the combustion chamber. In it, compressed air mixes with fuel, ignites and expands. Expanded gas makes the turbine rotate, which is located on the same shaft with the compressor. The rest of the energy moves into the tapering nozzle. As a result of the directed outflow of gas from the nozzle, jet thrust acts on the engine ^[4] .

The key characteristics of the turbojet engine are as follows:

1. Engine thrust.
2. Specific fuel consumption (mass of fuel consumed per unit of time to create a unit of thrust / power)
3. Air consumption (mass of air passing through each of the engine sections per unit time)
4. The degree of increase in total pressure in the compressor
5. The gas temperature at the outlet of the combustion chamber.
6. Weight and dimensions.

The degree of increase in the total pressure in the compressor is one of the most important parameters of the turbojet engine, since the effective efficiency of the engine depends on it. If in the first samples of turbojet engines ( Jumo-004 ) this indicator was 3, then in modern ones it reaches 40 ( General Electric GE90 ).

To increase the gas-dynamic stability of compressors, they are performed in two stages ( NK-22 ) or three stages ( NK-25 ). Each of the cascades operates with its own rotation speed and is driven by its cascade of turbines. The shaft of the 1st compressor stage (low pressure) rotated by the last (lowest speed) cascade of the turbine passes inside the hollow shaft of the compressor of the second stage (high pressure stage for a two-stage engine, medium-pressure stage for a three-stage). Engine cascades are also referred to as low, medium and high pressure rotors.

The combustion chamber of most turbojet engines has an annular shape and the turbine-compressor shaft passes inside the chamber ring. When entering the combustion chamber, the air is divided into 3 flows.

Primary air - enters through the front openings in the combustion chamber, is inhibited in front of the nozzles and is directly involved in the formation of the fuel-air mixture. Directly involved in the combustion of fuel. The fuel-air mixture in the fuel combustion zone in the WFD is close in composition to stoichiometric .

Secondary air - enters through the side openings in the middle part of the walls of the combustion chamber and serves to cool them by creating an air stream with a much lower temperature than in the combustion zone.

Tertiary air - enters through special air channels in the outlet part of the walls of the combustion chamber and serves to equalize the temperature field of the working fluid in front of the turbine.

From the combustion chamber, the heated working fluid enters the turbine, expands, setting it in motion and giving it part of its energy, and after it expands in the nozzle and flows out of it, creating reactive thrust.

Thanks to the compressor, the turbojet engine (in contrast to the ramjet engine ) can “pull off” and work at low flight speeds, which is absolutely necessary for the aircraft engine, while the pressure in the engine path and air flow are provided only by the compressor.

With an increase in the flight speed, the pressure in the combustion chamber and the flow rate of the working fluid increase due to the increase in the pressure of the oncoming air flow, which is inhibited in the inlet device (like in the ramjet) and enters the inlet of the lower compressor stage under a pressure higher than atmospheric pressure this increases the engine thrust.

The speed range in which the turbojet engine is effective is biased toward lower values ​​compared to the ramjet . The turbine-compressor unit, which allows creating a high flow rate and a high degree of compression of the working fluid in the region of low and medium flight speeds, is an obstacle to improving engine efficiency in the high-speed zone:

• The temperature that the turbine can withstand is limited, which imposes a limit on the amount of thermal energy supplied to the working fluid in the combustion chamber, and this leads to a decrease in the work produced by it during expansion.

Increasing the permissible temperature of the working fluid at the turbine inlet is one of the main directions for improving the turbojet engine. If for the first turbojet engines this temperature barely reached 1000 K, then in modern engines it approaches 2000 K. This is achieved both through the use of particularly heat-resistant materials from which the blades and disks of turbines are made, and through the organization of their cooling: air from medium stages of the compressor (much colder than the products of fuel combustion) is fed to the turbine and passes through complex channels inside the turbine blades.

• The turbine absorbs part of the energy of the working fluid before it enters the nozzle.

As a result, the maximum velocity of the jet flow out of the turbojet engine is lower than that of the ramjet, which, in accordance with the formula for the jet propulsion thrust in the design mode, when the pressure at the nozzle exit is equal to the ambient pressure, ^[5]

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${\displaystyle \mathrm{<span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">G</span></span>}}${\ displaystyle G}   - second consumption of the mass of the working fluid through the engine,
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limits from above the range of speeds at which the turbojet engine is effective to M = 2.5 - 3 (M is the Mach number ). At these and higher flight speeds, the braking of the oncoming air flow creates a degree of pressure increase measured in tens of units, the same or even higher than that of high-pressure compressors, and even greater compression becomes undesirable, since the air heats up, and this limits the amount of heat that can be communicated to him in the combustion chamber. Thus, at high flight speeds (for M> 3), the turbine-compressor unit becomes useless, and even counterproductive, since it only creates additional resistance in the engine path, and under these conditions, ramjet engines become more efficient.

Afterburner

Although there is an excess of oxygen in the combustion chamber in the turbojet engine, this power reserve cannot be realized directly - by increasing the fuel consumption in the chamber - due to the limitation of the temperature of the working fluid entering the turbine. This reserve is used in engines equipped with an afterburner located between the turbine and nozzle. In afterburner mode, additional fuel is burned in this chamber, the internal energy of the working fluid increases before the nozzle expands, as a result of which its outflow speed increases, and the engine thrust increases, in some cases, by more than 1.5 times, which is used by combat planes when flying at high speeds. In the afterburner, a stabilizer is used, the function of which is to reduce the speed behind it to near-zero values, which ensures stable combustion of the fuel mixture. With afterburner fuel consumption is significantly increased, turbojet engines with afterburner practically did not find application in commercial aviation, with the exception of Tu-144 and Concord aircraft, whose flights have already stopped.

Hybrid turbofan engine / ramjet

In the 1960s, the United States created the Pratt & Whitney J58 hybrid turbojet / ramjet engine , which was used on the SR-71 Blackbird strategic reconnaissance aircraft . Until Mach number M = 2.4, he worked as a turbojet engine with afterburner, and at higher speeds channels were opened, through which air from the inlet device entered the afterburner, bypassing the compressor, combustion chamber and turbine, the fuel supply to the afterburner increased, and she began to work as ramjet. Such a work scheme made it possible to expand the speed range of effective engine operation to M = 3.2. At the same time, the engine was inferior in weight characteristics to both turbojet engines and ramjet engines, and this experience was not widely used.

Hybrid Turbojet / RD

When flying in the atmosphere, engines of this type use oxygen from atmospheric air, and when flying outside the atmosphere, liquid oxygen from fuel tanks is used as an oxidizing agent. Engines of this type were planned to be used in the HOTOL project and outlined in the Skylon project ^[6] .

Adjustable nozzles

The turbojet engines, the jet velocity of which can be both subsonic and supersonic at various modes of engine operation, are equipped with adjustable nozzles. These nozzles consist of longitudinal elements called flaps , movable relative to each other and set in motion by a special drive, which allows changing the nozzle geometry upon the command of a pilot or automatic engine control system. At the same time, the dimensions of the critical (narrowest) and output sections of the nozzle are changed, which allows optimizing the operation of the engine during flights at different speeds and operating modes of the engine. [one]

Scope

Turbojet engines most actively developed as engines for all kinds of military and commercial aircraft until the 70-80s of the XX century. Currently, turbojet engines have lost a significant part of their niche in the aircraft industry, being supplanted by more economical dual-circuit turbojet engines (turbojet engines).

• Samples of aircraft equipped with turbojet engines
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  Attack aircraft Su-25 UB with two turbojet engines R-95Sh.

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  MiG-21 bis fighter with turbojet engine R-25-300 .

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  The Concorde supersonic airliner with four Rolls-Royce Olympus 593 turbofan engines.

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  Su-24 of Sukhoi firm with afterburner single-circuit turbofans AL-21F .

Dual turbojet engine

For the first time, a dual-circuit turbojet engine was proposed by the creator of the first efficient turbojet engine, Frank Whittle, in the early 1930s. Since 1937, the Soviet scientist and designer A.M. Lyulka investigated this principle and submitted an application for the invention of a dual-circuit turbojet engine (copyright certificate on April 22, 1941). The first examples of turbojet engines with afterburners were created at Rolls-Royce in the second half of the 1940s, and Conway became the first production.

The basis of dual-circuit turbojet engines (hereinafter referred to as the turbojet engine ) is the principle of involving an additional mass of air in the creation of thrust in order, first of all, to increase the efficiency of a jet engine in a dense atmosphere. This part of the air is forced through the external circuit of the engine.

After passing through the inlet, the air enters the low-pressure compressor, sometimes called a fan. After that, the flow is divided into two parts: into the external circuit and, bypassing the combustion chamber, further into the nozzle, and the other part into the internal circuit of the turbojet engine, where usually the last stages of the turbine drive the fan.

The most efficient and powerful turbofan engines make three-stage, two- and three-shaft. Another rotor is added to the two rotors of the internal circuit, also called a gas generator, in which the fan and the last cascade of the turbine are connected by a shaft located inside the gas generator shafts.

The turbofan engine parameter is the bypass ratio - the ratio of the mass flow of air through the external circuit to the flow rate through the internal circuit. The increase in efficiency is achieved by reducing the difference between the speed of the outflow of gases from the nozzle and the speed of the aircraft by increasing the air flow in the engine, that is, increasing the area of ​​entry into the engine. This leads to an increase in drag and mass.

Turbojet engines are performed with mixing the flow of circuits behind the turbine and without mixing, with a short external circuit. When mixing, the flows are mixed in a special chamber and leave the engine through a single nozzle with an equalized temperature. The presence of a mixing chamber leads to an increase in the size and mass of the engine, but increases the efficiency and reduces the noise created by the jet.

Turbofan engines, like turbofan engines, can be equipped with adjustable nozzles and afterburners for supersonic military aircraft.

Thrust Vector Control (UHT) / Thrust Vector Deviation (OWT)

Special rotary nozzles on some turbojet engines allow deflecting the flow of the working fluid flowing from the nozzle relative to the axis of the engine. OBT leads to additional loss of engine thrust due to the performance of additional work on the rotation of the flow and complicates the control of the aircraft. But these shortcomings are fully compensated by a significant increase in maneuverability and a decrease in the take-off run of the aircraft during take-off and run during landing, up to vertical take-off and landing. ATS is used exclusively in military aviation.

High Bypass Turbofan Engine / Turbofan Engine

Sometimes in the popular literature turbofan engines with a high degree of bypass (above 2) are called turbofan. In the English language literature, this engine is called turbofan with the addition of a refinement of high bypass (short bypass), in short - hbp. Turbofan engines with a high bypass ratio are performed, as a rule, without a mixing chamber. Due to the large input diameter of such engines, their outer nozzle is often made shortened in order to reduce engine weight.

Scope

We can say that from the 1960s to the present day in the aircraft engine industry - the era of turbofan engines. Various types of turbofan engines are the most common class of jet propellers used on airplanes, from high-speed fighter interceptors with small turbofan engines to small commercial and military transport aircraft with high turbofan engines.

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  TRDDFsm AL-31F .

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  Su-27 aircraft with two turbofan AL-31F turbofan

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  Turbofan engine with a high degree of bypass TF-39 (rear view)

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  Lockheed C-5 Galaxy aircraft with four turbofan engines TF-39

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  Turbofan F-107

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  Tomahawk cruise missile with turbofan F-107

In a propeller-driven engine, the flow of cold air is created by two coaxial, rotating in opposite directions, multi - blade saber - shaped screws driven from the turbine through the gearbox. The bypass ratio of such engines reaches 90.

Today, only one production model of this type of engine is known - D-27 ( ZMKB Progress named after Academician A. G. Ivchenko, Zaporozhye, Ukraine. ), Used on a Yak-44 aircraft with a cruising flight speed of 670 km / h , and on the An-70 with a cruising speed of 750 km / h.

Turboprop engines (TVD) or turboshaft engines (TVLD) refer to the WFD of an indirect reaction .

Structurally, a turbine engine is similar to a turbojet engine, in which the power developed by the last cascade of the turbine is transmitted to the propeller shaft (usually through a gearbox). This engine is not, strictly speaking, jet (the turbine exhaust reaction is not more than 10% of its total thrust), but traditionally they are referred to as the WFD. Turboprop engines are used in transport and civil aviation when flying at cruising speeds of 400-800 km / h.

In a fuel injection engine, the gas emanating from their combustion chamber is directed, firstly, to a turbine driving the compressor, and secondly, to a turbine connected to the drive shaft. The drive shaft is mechanically connected to the gearbox, which drives the rotor. Thus, in a fuel injection engine, the connection between the rotor and the output shaft is purely gas-dynamic. Such a technical solution is mainly used for power plants of helicopters due to the large moment of inertia of the rotor. In the case of mechanical connection of the rotor with the gas generator, starting the engine requires a high power starter.

Uses a nuclear reactor to heat air instead of burning kerosene. The main disadvantage is the strong radiation contamination of the used air. The advantage is the possibility of a long flight ^[7] .