371
11.1 General Concept and Components
means a heat exchanger with heat transfer through metal walls. The term regenerator refers to a device with a rotating tube matrix of ceramic material, with the hot
and cold gas alternately flowing through it.
Figure 11.1 suggests a machine with a single shaft. Most power gas turbines for
electricity generation are built this way, but multi-shaft machines exist as well. A
typical gas turbine for traction (vehicle, ship, propeller aircraft) has two turbine parts.
Then, the high pressure part drives the compressor and the low pressure part drives
the external load. The machine is divided into a gas generator, encompassing the
compressor, the combustion chamber and the HP turbine and a power turbine, with
different rotational speeds of both shafts. The gas generator turns at a rather high rotational speed, whereas the power turbine turns at a lower speed, adapted to the load. For
traction, the torque as a function of speed is advantageous. At a fixed fuel flow rate,
the power generated by the gas generator and supplied to the power turbine is constant.
So, the product of the torque and the rotational speed on the outgoing shaft is then approximately constant. Some machines have a gas generator with two parts (we discuss
the reason in the next section): compressor split into two parts and turbine split into
two parts. The machine then has three shafts. A compressor part connected to a turbine
part is called a spool. With multi-shaft machines, the shafts are mostly concentric, with
the outgoing shaft, as on Fig. 11.1, at the compressor side. This is no general rule, but
most applications benefit from an outgoing shaft at the cold side of the machine.
11.1.2 Comparison with Other Thermal Engines
Gas turbines show similarities, but differences as well, with reciprocating internal combustion engines and steam turbines. Within a reciprocating engine, the gas
completes also a cycle of compression, heating by combustion and expansion, but
the difference is that the stages of the cycle occur within the same space, but at
different times. By alternating cold and hot stages, the thermal load on the walls is
lower at a given combustion temperature. Reciprocating engines thus allow higher
combustion temperatures. Further, the combustion happens for a large part under
a constant volume. A high combustion temperature is then very advantageous for
efficiency. The combustion in a gas turbine is at constant pressure. The consequence
is that the simple-cycle efficiency does not depend much on the combustion temperature (see Sect. 11.3.3). Both machine types reach comparable efficiencies at
present. With a simple cycle, the efficiency of a gas turbine amounts to about 40 %.
This is lower than the efficiency of a diesel engine, about 45 %, but some extensions, analysed later, allow efficiency improvement with gas turbines. With both
machine types a high combustion temperature is important for a high power density,
or power per volume occupied. For this aspect, gas turbines have a very significant
advantage over reciprocating engines, as flow through a gas turbine is continuous and occurs at high speed, with a through-flow Mach number of the order of
0.5. Large state-of-the-art land-based gas turbines feature about 1500 °C as inlet
temperature of the turbine part and develop about 400 MW power. The dimensions
11.1 General Concept and Components
means a heat exchanger with heat transfer through metal walls. The term regenerator refers to a device with a rotating tube matrix of ceramic material, with the hot
and cold gas alternately flowing through it.
Figure 11.1 suggests a machine with a single shaft. Most power gas turbines for
electricity generation are built this way, but multi-shaft machines exist as well. A
typical gas turbine for traction (vehicle, ship, propeller aircraft) has two turbine parts.
Then, the high pressure part drives the compressor and the low pressure part drives
the external load. The machine is divided into a gas generator, encompassing the
compressor, the combustion chamber and the HP turbine and a power turbine, with
different rotational speeds of both shafts. The gas generator turns at a rather high rotational speed, whereas the power turbine turns at a lower speed, adapted to the load. For
traction, the torque as a function of speed is advantageous. At a fixed fuel flow rate,
the power generated by the gas generator and supplied to the power turbine is constant.
So, the product of the torque and the rotational speed on the outgoing shaft is then approximately constant. Some machines have a gas generator with two parts (we discuss
the reason in the next section): compressor split into two parts and turbine split into
two parts. The machine then has three shafts. A compressor part connected to a turbine
part is called a spool. With multi-shaft machines, the shafts are mostly concentric, with
the outgoing shaft, as on Fig. 11.1, at the compressor side. This is no general rule, but
most applications benefit from an outgoing shaft at the cold side of the machine.
11.1.2 Comparison with Other Thermal Engines
Gas turbines show similarities, but differences as well, with reciprocating internal combustion engines and steam turbines. Within a reciprocating engine, the gas
completes also a cycle of compression, heating by combustion and expansion, but
the difference is that the stages of the cycle occur within the same space, but at
different times. By alternating cold and hot stages, the thermal load on the walls is
lower at a given combustion temperature. Reciprocating engines thus allow higher
combustion temperatures. Further, the combustion happens for a large part under
a constant volume. A high combustion temperature is then very advantageous for
efficiency. The combustion in a gas turbine is at constant pressure. The consequence
is that the simple-cycle efficiency does not depend much on the combustion temperature (see Sect. 11.3.3). Both machine types reach comparable efficiencies at
present. With a simple cycle, the efficiency of a gas turbine amounts to about 40 %.
This is lower than the efficiency of a diesel engine, about 45 %, but some extensions, analysed later, allow efficiency improvement with gas turbines. With both
machine types a high combustion temperature is important for a high power density,
or power per volume occupied. For this aspect, gas turbines have a very significant
advantage over reciprocating engines, as flow through a gas turbine is continuous and occurs at high speed, with a through-flow Mach number of the order of
0.5. Large state-of-the-art land-based gas turbines feature about 1500 °C as inlet
temperature of the turbine part and develop about 400 MW power. The dimensions