Gas turbine (redirected from Gas turbines)

Gas turbine

One of a class of heat engines which use fuel energy to produce mechanical output power, either as torque through a rotating shaft (industrial gas turbines) or as jet power in the form of velocity through an exhaust nozzle (aircraft jet engines). The fuel energy is added to the working substance, which is gaseous in form and most often air, either by direct internal combustion or indirectly through a heat exchanger. The heated working substance, air co-mixed with combustion products in the usual case of internal combustion, acts on a continuously rotating turbine to produce power. The gas turbine is thus distinguished from heat engine types where the working substance produces mechanical power by acting intermittently on an enclosed piston, and from steam turbine engines where the working substance is water in liquid and vapor form. See Internal combustion engine, Steam turbine

Gas turbine engines depend on the principle of the air cycle, where, ambient air is first compressed to a maximum pressure level, at which point fuel heat energy is added to raise its temperature, also to a maximum level. The air is then expanded from high to low pressure through a turbine. The expansion process through the turbine extracts energy from the air, while the compression process requires energy input.

As the air moves through the engine, the turbine continuously provides energy sufficient to drive the compressor. In addition, because the turbine expansion process starts from a high temperature that comes from the fuel energy released by combustion, surplus energy beyond that required for compression can be extracted from the air by further expansion. At the point where the turbine has provided sufficient energy to power the compressor, the air pressure remains higher than the outside ambient level. This higher pressure represents available energy in the air that can be turned into useful output power by a final expansion process that returns the air pressure to ambient. The exhaust air leaves the engine with pressure equal to the outside, but at a higher temperature. As with any heat engine, the high exhaust temperature represents wasted energy that will dissipate into the outside atmosphere. See Compressor

From an energy accounting standpoint, the sequence of processes acting on the air from front to rear constitutes a full cycle. It starts with the outside air entering at its initial state, and is completed when the air returns again to both ambient pressure and temperature levels. The series of cycle processes includes the final outside dissipation of the wasted exhaust energy, inevitable for every heat engine according to Carnot's principle. The ideal version of the gas turbine cycle is known as the Brayton cycle. See Brayton cycle, Carnot cycle

For any completed cycle, the total energy added from the fuel sources will always be equal to the sum of the useful output energy and the wasted exhaust energy. The thermal efficiency, which is the ratio of net output energy to fuel input energy for the cycle, measures the engine's ability to minimize wasted energy. A thermal efficiency of 60% means that for every 100 units of added energy 60 units will be available as useful output while 40 units will leave the engine as high-temperature exhaust.

Another performance measure is the specific power, which is the ratio of output power to quantity of working substance mass flow rate. Gas turbine engines, in comparison with other types of heat engines, are characterized not only by high levels of efficiency but also by very high levels of specific power. They are especially useful for applications that need compact power.

By far the most common mechanical arrangement for the gas turbine is an in-line axial flow positioning of all components (see illustration). In the ground-based engine, the inlet at the front guides the incoming air into the compressor, which in turn delivers high-pressure air into the combustor section. The combustor burns the injected fuel at a high reaction temperature, using some of the air itself as an oxygen source. The combustion products in the combustor mix with the remaining unused air to reach a uniform equilibrium temperature, still high but diluted down from the reaction temperature. The hot, high-pressure combustor exit air enters the compressor drive turbine, where it expands down in pressure toward, but stays higher than, ambient level. This expansion process results in output shaft power that can be delivered directly to the compressor through a connecting rotating shaft. Starting from the exit of the compressor drive turbine, net output power remains available. This power can be realized through the process of further pressure expansion completely down to the ambient level. For ground-based applications, the final expansion takes place through a power turbine whose output shaft is connected to the external load. In the single-spool arrangement the power turbine and compressor drive turbine are indistinguishably combined into one unit which, together with the compressor and the output load, is connected to a common shaft. For aircraft applications, either a power turbine extracts useful power to drive a propeller through a separate shaft (turboprop), or the expansion process takes place through a nozzle which acts to convert some of the thermal energy into velocity energy to be used for jet propulsion. See Aircraft engine, Jet propulsion

Simple gas turbine component arrangements

Gas turbines characteristically produce smooth and linear throttle response over their entire operating range. Rotor speeds normally vary continuously over this range without the need for the gear shifting and clutch mechanisms found in piston engines. The governing fuel control senses rotor speeds, pressures, and temperatures to maintain stable, steady power or thrust output and, when needed, ensure rapid accelerations and decelerations. The control is programmed, normally by electronic input, to guard against harming the engine during throttle changes by governing the appropriate fuel input rate. Most important, during throttle transients the control functions to prevent turbine overheating, burner blowout, and compressor surge.