Principle of a Gas Turbine Engine
Gas Turbine, Systems I
In WW-II and shortly thereafter, piston powered aircraft peaked in power, performance and complexity wise. Power went up to over 4000 bhp for large multi-row radial engines. Only to be defeated by the jet, which was developed by (among others) Germany's Dr. Hans von Ohain and separately in the UK by Sir Frank Whittle. Its principles are based on the "Aeolipile" of the ancient Greek scientist Hero and other great thinkers like Leonardo da Vinci and the laws of Isaac Newton.
Compared to a piston, the gas turbine has less parts and the moving parts rotate in only one direction without stopping and accelerating as the pistons normally do in a engine. Thus, a running gas turbine is basically free of the vibrations normally found in piston models, which translates in much longer service life (TBO) and higher reliability.
After describing the basic operating principles we continue with examining the major sections of a gas turbine, in this part we start with the compressors and airflow of this engine.
As seen from the front to the end, the parts a GTE consists of are: air intake, compressor, combustion chambers, turbine and exhaust. Compared to a piston Aero engine the GTE is a relatively simple engine although the processes remain largely the same.
To guide the air into the engine most manufacturers use the pitot type intake. The engines under the wing or mounted to the side of the fuselage of a jet airliner are a very good example of this. This type of air intake is suitable for subsonic airspeeds where the effect of ram air is optimal. Attitude changes have little effect. This type has even been used on single engine fighter aircraft (F-86 Sabre) although it results in a long duct ahead of the engine. More common is the divided type intake each side of the fuselage or wing root.
Supersonic air intake
For higher speeds in the trans and supersonic range special intakes need to position the shock wave in such a way as to take advantage of the compression of air (MIG-21). Although the maximum airflow for burning JET fuel remains the same, measures need to be taken to spill the excess air. The Concorde had such arrangement, at cruise speeds of Mach 2.02 compressor intake air speed was only Mach 0.7 to 0.8, thus just below the speed of sound.
There are two types of compressors used in gas turbines: the radial and axial flow compressor. Axial flow means along the axis and radial is on a right angle from the axis. Sometimes a combination of both compressors is used in an engine (Pratt and Whitney PT-6). Radial centrifugal flow compressors may be double sided and consist of an impeller and diffuser. The impeller accelerates and compresses air and the diffuser reduces air velocity while increasing pressure even more.
Most early jet engines were radial flow compressors derived from big piston engine super- and turbochargers. Robust and simple to design with a built-in ruggedness to handle harsh field conditions. The air is sucked in at the front by rotation of the impeller, then given a radial flow to the tips. Air is compressed and passes into a diffuser ring which decelerates the air and increases its pressure. Air is then collected for passage to the can burners or combustion chambers.
Axial compressors are usually multistage and use alternate rows of rotating and static blades to compress the air, where a pair of rotating and static blades is called a stage. Having thirteen or more of these stages are quite common these days. Specific fuel consumption decreases as compression is higher, easily obtained with a multistage axial flow compressor. Where the radial flow model handles FOD more easily the axial flow is more sensitive to damage by foreign objects.
Air going through the engine undergoes changes in velocity and pressure for several reasons. During compression, its pressure rises but the velocity may not become so high that combustion can not be maintained. After heat energy is added (by combustion of fuel) the velocity is increased to drive the turbine and this adds to the thrust.
Divergent and convergent passages are used to bring about the changes in velocity and pressure where needed in the engine, for example the air scoop connecting the compressor to the combustion chamber is a divergent type. Air velocity decreases, pressure and temperature increases as air passes a divergent passage
With a convergent passage air velocity increases and pressure and temperature decreases. An example are nozzle guide vanes which are convergent and direct gas to the turbine thus increasing kinetic energy to drive the turbine.