A gas turbine, also called a combustion turbine, is a type of internal combustion engine.
It has an upstream rotating compressor coupled to a downstream turbine, and
a combustion chamber in-between.
The basic operation of the gas turbine is similar to that of the steam power plant except
that air is used instead of water. Fresh atmospheric air flows through a compressor that
brings it to higher pressure. Energy is then added by spraying fuel into the air and
igniting it so the combustion generates a high-temperature flow. This high-temperature
high-pressure gas enters a turbine, where it expands down to the exhaust
pressure, producing a shaft work output in the process.
The turbine shaft work is used to drive the compressor and other devices such as
an electric generator that may be coupled to the shaft. The energy that is not used for
shaft work comes out in the exhaust gases, so these have either a high temperature or a
The purpose of the gas turbine determines the design so that the most desirable energy
form is maximized.
Gas turbines are used to power aircraft, trains, ships, electrical generators, or even tanks.
THEORY OF OPERATION
Gases passing through an ideal gas turbine undergo
three thermodynamic processes. These
are isentropic compression, isobaric (constant pressure) combustion and
isentropic expansion. Together, these make up the Brayton cycle.
In a practical gas turbine, gases are first accelerated in either a centrifugal or
axial compressor. These gases are then slowed using a diverging nozzle known
as a diffuser; these processes increase the pressure and temperature of the flow.
In an ideal system, this is isentropic. However, in practice, energy is lost to
heat, due to friction and turbulence. Gases then pass from the diffuser to
a combustion chamber, or similar device, where heat is added. , this occurs at
constant pressure (isobaric heat addition).
As there is no change in pressure, the specific volume of the gases increases. In
practical situations this process is usually accompanied by a slight loss in
pressure, due to friction. Finally, this larger volume of gases is expanded and
accelerated by nozzle guide vanes before energy is extracted by a turbine.
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 below), not
counting the fuel system. However, the required precision manufacturing for
components and temperature resistant alloys necessary for high efficiency often
make the construction of a simple turbine more complicated than piston
TYPES OF GAS TURBINES
Air breathing jet engines are gas turbines optimized to produce thrust from the
exhaust gases, or from ducted fans connected to the gas turbines. Jet engines that
produce thrust from the direct impulse of exhaust gases are often
called turbojets, whereas those that generate thrust with the addition of a ducted fan
are often called turbofans or (rarely) fan-jets.
Gas turbines are also used in many liquid propellant rockets, the gas turbines are
used to power a turbo pump to permit the use of lightweight, low pressure
tanks, which saves considerable dry mass.
Air intake- Preceding the compressor is the air intake (or inlet). It is designed to be as
efficient as possible at recovering the ram pressure of the air stream tube approaching the
intake. The air leaving the intake then enters the compressor.
Compressor- The compressor is driven by the turbine. The compressor rotates at a very
high speed, adding energy to the airflow and at the same time squeezing (compressing) it
into a smaller space. Compressing the air increases its pressure and temperature.
Combustion chamber- The burning process in the combustor is significantly different
from that in a piston engine. In a piston engine the burning gases are confined to a small
volume and, as the fuel burns, the pressure increases dramatically. In a turbojet the air
and fuel mixture passes unconfined through the combustion chamber. As the mixture
burns its temperature increases dramatically, but the pressure actually decreases a few
Turbine- Hot gases leaving the combustor are allowed to expand through the turbine.
Turbines are usually made up of metals which can resist the high temperature, and
frequently have built-in cooling channels.
A turboprop engine is a type of turbine engine which drives an external
aircraft propeller using a reduction gear. Turboprop engines are
generally used on small subsonic aircraft, but some large military and
civil aircraft, such as the Airbus A400M, Lockheed L-188 Electra
and Tupolev Tu-95, have also used turboprop power.
Thrust in a turboprop is sacrificed in favor of shaft power, which is
obtained by extracting additional power (up to that necessary to drive the
compressor) from turbine expansion. While the power turbine may be
integral with the gas generator section, many turboprops today feature a
free power turbine on a separate coaxial shaft. This enables the propeller
to rotate freely, independent of compressor speed.
Owing to the additional expansion in the turbine system, the residual
energy in the exhaust jet is low. Consequently, the exhaust jet produces
(typically) less than 10% of the total thrust.
Propellers are not efficient when the tips reach or
exceed supersonic speeds. For this reason, a reduction gearbox is placed
in the drive line between the power turbine and the propeller to allow the
turbine to operate at its most efficient speed. The gearbox is part of the
engine and contains the parts necessary to operate a constant speed
The turbofan engine is a jet engine with a large fan at the front. The fan sucks in air and most
of the air flows around the outside of the engine, which make it operate quietly and provides
more thrust at low speeds. Nowadays, most airliners are powered by turbofan engines.
Compared to the turbojet, the turbofan engine has many advantages. In a turbojet all the air
passes through the compressor, combustion chamber, and turbine. In a turbofan engine only a
proportion of the incoming air goes into the gas generator. The rest of the air is directly
ejected out of the engine, or mixed with the gas generator exhaust to produce a "hot" jet. The
aim of this system is to increase the thrust without increasing fuel consumption. It achieves
this by increasing the total mass of air that passes through the engine and reduces the velocity
within the same total energy supply.
TYPES OF TURBOFANS-
Low Bypass Turbofan
High Bypass Turbofan
The Turbo-shaft engine is another form of gas-turbine engine, which is
widely used in helicopters. It operates like a turboprop system.
However, it does not have a propeller but drives the helicopter rotor
instead. The turbo-shaft engine is designed to keep the speed of the
helicopter rotor independent from the rotating speed of the gas generator.
It allows the rotating speed of the rotor to remain constant even when the
rotating speed of the generator is varied to adjust the amount of power it
The Ramjet engine is the simplest jet engine. It has no moving parts. It is
essentially a turbojet engine without the rotating machinery inside the engine.
So its compression ratio depends wholly on its forward speed. Because of this
fact, it can not produce static thrust and it produces very little thrust, when the
speed is below the speed of the sound. Consequently, a ramjet vehicle cannot
take off by itself. So, other means, such as another aircraft may be needed to
help it to take off. This engine is used in guided-missile system, and space
A ramjet is designed around its inlet. An object moving at high speed
through air generates a high pressure region upstream. A ramjet uses this
high pressure in front of the engine to force air through the tube, where it
is heated by combusting some of it with fuel. It is then passed through a
nozzle to accelerate it to supersonic speeds. This acceleration gives the
ramjet forward thrust.
Inlet- Ramjets try to exploit the very high dynamic pressure within the air
approaching the intake lip. An efficient intake will recover much of the
free stream stagnation pressure, which is used to support the combustion
and expansion process in the nozzle.
Combustor- As with other jet engines, the combustor's job is to create hot
air, by burning a fuel with the air at essentially constant pressure.
Nozzles- The propelling nozzle is a critical part of a ramjet design, since it
accelerates exhaust flow to produce thrust. acceleration is typically
achieved via a convergent-divergent nozzle.
Gas turbine are mainly divided into two group-
I .Constant pressure combustion gas turbine :
i) Open cycle, ii) Closed cycle
II. Constant volume combustion gas turbine :
In almost all the field open cycle gas turbine
plants are used. Closed cycle plants were
introduced at one stage because of their ability to
burn cheap fuel.
DISADVANTAGES OF CLOSED
CYCLE OVER OPEN CYCLE
Advantages of closed cycle:
i) Higher thermal efficiency
ii) Reduced size
iii) No contamination
iv) Improved heat transmission
v) Improved part load h
vi) Lesser fluid friction
vii) No loss of working medium
viii) Greater output and
ix) Inexpensive fuel.
Disadvantages of closed cycle:
ii) Large amount of cooling water is required. This limits its use of stationary installation
or marine use
iii) Dependent system
iv) The wt of the system pre kW developed is high comparatively, not economical for
v) Requires the use of a very large air heater.
Jet propulsion is thrust produced by passing a jet of matter (typically air or water) in
the opposite direction to the direction of motion. By conservation of momentum, the
moving body is propelled in the opposite direction to the jet.
A number of animals, including cephalopods, sea
hares, arthropods, and fish have convergently evolved jet propulsion mechanisms.
This is most commonly used in the jet engine, but is also the means of propulsion
utilized by NASA to power various space craft.
JET PROPULSION IN ANIMALS
Jet propulsion in cephalopods is produced by water being exhaled through
a siphon, which typically narrows to a small opening to produce the maximum
exhalent velocity. The water passes through the gills prior to exhalation, fulfilling
the dual purpose of respiration and locomotion. Sea hares (gastropod mollusks)
employ a similar means of jet propulsion, but without the sophisticated
neurological machinery of cephalopods they navigate somewhat more clumsily.
Some teleost fish have also developed jet propulsion, passing water through the
gills to supplement fin-driven motion.
In some dragonfly larvae, jet propulsion is achieved by the expulsion of water from
a specialized cavity through the anus. Given the small size of the organism, a great
speed is achieved.
Scallops and cardiids, siphonophores, tunicates (such as salps), and some
jellyfish also employ jet propulsion. The most efficient jet-propelled organisms are
the salps,which use an order of magnitude less energy (per kilogram per meter)