Main Parts of the Gas Turbine Engine

Introductory

The gas turbine engine is essentially a heat engine using air as the working fluid to provide thrust. To achieve this, the air passing through the engine has to be accelerated; this means that the velocity or kinetic energy of the air is increased. To obtain this increase, the pressure energy is first of all increased, followed by the addition of heat energy, before final conversion back to kinetic energy in the form of a high velocity jet efflux.

Working cycle

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: centrifugal flow and axial flow compressors. Both types are driven by the engine turbine and are coupled direct to the turbine shaft. The function of the compressor is to accelerate the air, to diffuse it and to produce the required pressure rise.

The axial compressor consumes far more air than a centrifugal com­pressor of the same frontal area and can be designed to attain much higher pres­sure ratios. 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 chamberhas the diffi­cult task of burning large quantities of fuel supplied through the fuel burners with extensive volumes of air supplied by the compressor. The heat is released 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. This task must be accomplished with the minimum loss in pressure and with the maximum heat release.

(The amount of fuel added to the air will depend upon the temperature rise required. However, the maximum temperature is limited to within the range of 850 to 1700 deg. С by the materials from which the turbine blades and nozzles are made. The air has already been heated to between 200 and 550 deg. С by the work done during compression, giving a temperature rise requirement of 650 to 1150 deg. С from the combustion process.)

The turbine has the task of providing the power to drive the compressor and accessories. It does this by extracting energy from the hot gases released from the combustion system and expanding them to a lower pressure and temperature. High stresses are involved in this process, and for efficient operation, the turbine blade tips may rotate at speeds over 1, 500 feet per second. The continuous flow of gas to which the turbine is exposed may have an entry temperature between 850 and 1, 700 deg. С and may reach a velocity of over 2, 500 feet per second in parts of the turbine.

The turbine may consist of several stages. Each stage employs one row of stationary nozzle guide vanes and one row of moving blades.

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

Because the turbojet engine is a heat engine, the higher the temperature of combustion the greater is the expansion of the gases. The combustion temperature, however, must not exceed a value that gives a turbine gas entry temperature suitable for the design and materials of the turbine assembly.

The use of air-cooled blades in the turbine assembly permits a higher gas temperature and a consequently higher thermal efficiency.