| |
Gas
turbines are described thermodynamically by the
Brayton cycle, in which air is compressed
isentropically, combustion occurs at constant
pressure, and expansion over the turbine occurs
isentropically back to the starting pressure.
In practice, friction, and turbulence cause:
1. non-isentropic compression: for a given overall
pressure ratio, the compressor delivery temperature
is higher than ideal.
2. non-isentropic expansion: although the turbine
temperature drop necessary to drive the compressor
is unaffected, the associated pressure ratio is
greater, which decreases the expansion available to
provide useful work.
3. pressure losses in the air intake, combustor and
exhaust: reduces the expansion available to provide
useful work.
Brayton cycle
Brayton cycle
As with all cyclic heat engines, higher combustion
temperature means greater efficiency. The limiting
factor is the ability of the steel, nickel, ceramic,
or other materials that make up the engine to
withstand heat and pressure. Considerable
engineering goes into keeping the turbine parts
cool. Most turbines also try to recover exhaust
heat, which otherwise is wasted energy. Recuperators
are heat exchangers that pass exhaust heat to the
compressed air, prior to combustion. Combined cycle
designs pass waste heat to steam turbine systems.
And combined heat and power (co-generation) uses
waste heat for hot water production.
Mechanically, gas turbines can be considerably less
complex than internal combustion piston engines.
Simple turbines might have one moving part: the
shaft/compressor/turbine/alternative-rotor assembly
(see image above), not counting the fuel system.
More sophisticated turbines (such as those found in
modern jet engines) may have multiple shafts
(spools), hundreds of turbine blades, movable stator
blades, and a vast system of complex piping,
combustors and heat exchangers.
As a general rule, the smaller the engine the higher
the rotation rate of the shaft(s) needs to be to
maintain tip speed. Turbine blade tip speed
determines the maximum pressure that can be gained,
independent of the size of the engine. Jet engines
operate around 10,000 rpm and micro turbines around
100,000 rpm.
Thrust bearings and journal bearings are a critical
part of design. Traditionally, they have been
hydrodynamic oil bearings, or oil-cooled ball
bearings. This is giving way to foil bearings, which
have been successfully used in micro turbines and
auxiliary power units. |
