Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
7.4.1. Fundamental Thermodynamics
The core of the basic gas turbine consists of three essential elements: The compressor, which
mechanically increases the energy of the air (raising the pressure and temperature); the
combustor, in which fuel is burned (further raising the temperature of the pressurized air); and
the turbine, which mechanically extracts enough energy from the hot compressed gas to drive
the compressor (thereby reducing the pressure and temperature of the gas). A fraction of the net
energy remaining in the gas after it leaves the turbine is then available to be used in different
ways, as shown in Figure 7-7. Case (a) uses an additional turbine stage to mechanically convert
the energy to shaft work (such as might be required to drive a propeller or electric generator);
case (b) is the turbojet, in which a nozzle is used to accelerate the gas (converting some of the
energy into kinetic energy), producing a high-speed jet that can be used to propel the vehicle;
and case (c) is the turbofan, in which a further turbine converts most of the energy of the gas into
shaft work to drive the bypass air compressor, thereby producing the bypass jet that propels the
vehicle. For aircraft applications, the weight of the engine must be low in relation to the power
output. This constraint has kept the aircraft type of gas turbine simple-much simpler than those
now being built for land-based power generation. (The implications of going to more complicated
configurations are considered briefly in Section 7.4.4.)
Figure 7-8: Gas turbine thermal efficiency.
The overall efficiency of an aircraft engine, h0 , is the mechanical power created by the thrust
divided by the energy input rate of the fuel flow. It is convenient to express overall efficiencies by h0 = htherm x hp , where htherm is the thermal efficiency and hp is the
propulsion efficiency; these terms are considered below.
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