Gas turbine and jet engines

How the thrust is developed: Thrust is the pulling or pushing force developed by an aircraft engine. Aircraft need thrust to propel them through air. The required thrust may be developed by rotating pulling or pushing propellers by means of piston or reciprocating engines, or by throwing back masses of air by means of gas turbine engines. Spacecraft need thrust to propel them through space. They develop thrust using the power of their rocket engines and do not need air for support. Spacecraft carry the required supply of fuel and oxidizer with them.

A simple piston engine works on a four-stroke cycle, consisting of induction, compression, combustion and exhaust. But air flows straight through a jet engine. Yet a jet engine has the same four stages. The simplest of all jet engines is a ramjet. It is a tapered tube, open at both ends, into which fuel can be injected. When the tube moves, air flows into the tube, and this corresponds to the induction stage in the piston engine. Once inside the tube, the air slows down because the tube widens. The kinetic energy released by this loss of velocity is converted into pressure energy and heat, and this corresponds to the compression stage in the piston engine. Then fuel is injected and ignited. As in a piston engine, this causes a big rise in temperature, but there is very little rise in pressure because the exhaust gas is free to escape through the rear of the tube. This corresponds to the combustion stage in the piston engine. Finally the exhaust gases escape into the atmosphere, driving the engine forwards, which corresponds to the exhaust stage of the piston engine.

To understand the mechanics of thrust development, we must remember Newton's third law of motion: "For every force acting upon a body, there is an equal and opposite reaction". In our case, the "body" is the volume of air that is passing through the engine. We must also understand that the forward thrust occurs inside the engine itself, it is not caused by the high-pressure exhaust gases acting on the outside atmosphere. This is well illustrated by rocket engines which propel spacecraft through empty space.

Gas turbine engine main parts: The gas turbine engine consists of a rotary air compressor with an air intake, one or more combustion chambers, a turbine, and an exhaust outlet.

There are two basic types of rotary air compressors, one giving a centrifugal flow and the other an axial flow. Both types are driven by the engine turbine and are coupled direct to the turbine shaft.

The centrifugal flow compressor is a single or two-stage unit employing an impeller to accelerate the air and a diffuser to produce the required pressure rise. The axial flow compressor

(Fig. 2, item 2) is a multi-stage unit employing alternate rows of rotating and stationary blades to accelerate and diffuse the air until the required pressure rise is obtained. The centrifugal compressor is usually more robust than the axial compressor and is also easier to manufacture. The axial compressor, however, consumes far more air than a centrifugal compressor of the same frontal area and can also be designed for high pressure ratios much more easily. Since the airflow is an important factor in determining the amount of thrust, this means that the axial compressor engine will also give more thrust for the same frontal area.

The combustion chamber (item 3 of Figs. 1 and 2) has the difficult task of burning large quantities of fuel, supplied through the fuel burners, with extensive volumes of air, supplied by the compressor, and releasing the heat in such a manner that the air is expanded and accelerated to give a smooth stream of uniformly heated gas at all conditions required by the turbine. Although all combustion chambers work on the same principles, they may be installed in the engine in a number of different ways. The multiple combustion chamber layout is often used with engines having centrifugal compressors. Here a number of flame tubes, each with its own outer casing, are disposed radially round the engine. Annular combustion chambers are used with engines having axial compressors. With annular combustion chambers, the flame tube itself is in the form of a double ring.

The turbine has the task of providing the power to drive the compressor and accessories and, in the case of engines which do not make use solely of a jet for propulsion, of providing shaft power for a propeller or rotor. It does this by extracting energy from the hot gases released from the combustion system and expanding them to a lower pressure and temperature. The turbine may consist of several stages, each employing one row of stationary guide vanes and one row of moving blades.

The exhaust system passes the turbine discharge gases to atmosphere at a velocity, and in the required direction, to provide the resultant thrust.

The jet engine fundamentals: The jet engine is a device for developing thrust. All jet engines operate on the principle of developing thrust by throwing back large masses of hot air. The amount of thrust can be calculated as the product of the mass of air, flowing through the engine, by its acceleration. The jet engine is throwing back large masses of hot exhaust air, the opposite reaction to which is thrust.

All modern aviation jet engines consist of the following main parts:

1. Air intake unit and the fan

2. Low pressure stages of the axial compressor

3. Intermediate pressure stages of the axial compressor

4. High pressure stages of the axial compressor

5. The combustion chamber

6. High pressure stages of the gas turbine

7. Intermediate pressure stages of the gas turbine

8. Low pressure stages of the gas turbine

9. Exhaust nozzle, afterburner or thrust augmenter, and thrust reverser.

Low pressure stages of the gas turbine rotate the fan and low pressure stages of the axial compressor, forming one mechanical unit, called low pressure spool. Intermediate pressure stages of the gas turbine rotate intermediate pressure stages of the axial compressor, forming another mechanical unit, called intermediate pressure spool. High pressure stages of the gas turbine rotate high pressure stages of the axial compressor, forming the third mechanical unit, called high pressure spool.

All three spools rotate at their optimal speeds. Modern triple-spool jet engines feature very large diameter fan, minimum number of stages and very low level of noise.

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Дата добавления: 2015-07-11