Gas turbines

Gas Turbines
 A gas turbine is a machine delivering mechanical power
or thrust. It does this using a gaseous working fluid.
The mechanical power generated can be used by, for
example, an industrial device.
 The outgoing gaseous fluid can be used to generate
thrust. In the gas turbine, there is a continuous flow of
the working fluid. Its efficiency is 20 to 30%
 Major Applications of Gas Turbine
1. Aviation(self contained, light weight, don’t require cooling)
2. Power Generation
3. Oil and Gas industry
4. Marine propulsion

Working
Air is compressed(squeezed) to high pressure by a
compressor.
Then fuel and compressed air are mixed in a
combustion chamber and ignited.
Hot gases are given off, which spin the turbine.
Gas turbines burn fuels such as oil, natural gas and
pulverized(powdered) coal.

 Gas turbines have three main parts:
1. Air compressor
2. Combustion chamber
3. Turbine

Air compressor:
The air compressor and turbine are mounted at
either end on a common shaft, with the
combustion chamber between them.
Gas turbines are not self starting. A starting
motor is used.
The air compressor sucks in air and compresses
it, thereby increasing its pressure.

Combustion chamber:
In the combustion chamber, the compressed air
combines with fuel and the resulting mixture is
burnt.
The greater the pressure of air, the better the
fuel air mixture burns.
Modern gas turbines usually use liquid fuel, but
they may also use gaseous fuel, natural gas or
gas produced artificially by gasification of a
solid fuel

Turbine:
Hot gases move through a multistage gas
turbine.
Like in steam turbine, the gas turbine also has
stationary and moving blades.
The stationary blades
guide the moving gases to the rotor blades
adjust its velocity.
shaft of the turbine is coupled to a generator

Working Cycle:
Brayton cycle is the ideal cycle for gas-turbine.
1-2 isentropic compression (in compressor)
2-3 Const. pressure heat-addition (in combustion chamber)
3-4 isentropic expansion (in turbine)
4-1 const. pressure heat rejection (exhaust)

Thermal Efficiency for Closed Brayton Cycle:
For unit mass of air,
Heat supplied during process 2 – 3,‫1ݍ‬ = 𝐶𝑃 (𝑇3 − 𝑇2)
Heat rejected during process 4 – 1, ‫2ݍ‬ = 𝐶𝑃 (𝑇4 − 𝑇1)
Work done, 𝑊 = ‫1ݍ‬ − ‫2ݍ‬
𝑊 = 𝐶𝑃 (𝑇3 − 𝑇2) − CP(T4 − T1)
Thermal efficiency, 𝜂 =
Work done
Heat Supplied
=
CP (T3 − T2) − CP(T4 − T1)
CP (T3 − T2)
=1 −
(T4 − T1)
(T3 − T2)
-------eqn.(1)
Pressure ratio, ‫ݎ‬‫݌‬ =
P2
P1
=
P3
P4
For isentropic compression process (1 – 2),
T2
T1
= (P2/P1)(𝛾−1)/𝛾
= (rp)(𝛾−1)/𝛾
∴ T2= T1 (rp)(𝛾−1)/𝛾 -----------(2)

For isentropic compression process (3 – 4),
T3
T4
= (P3/P4)(𝛾−1)/𝛾
= (rp)(𝛾−1)/𝛾
∴ T3= T4 (rp)(𝛾−1)/𝛾
------(3)
From eqn. (1)
∴ 𝜂 =1 −
(T4 − T1)
(T3 − T2)
=1 −
T1(T4/T1 − 1)
T2(T3/T2−1))
=1 −
T1((rp
)(𝛾−1)/𝛾
− 1)
T2((rp
)(𝛾−1)/𝛾
−1))
=1 −
T1
T2
=1 − (P1/P2)(𝛾−1)/𝛾
𝜂 = 1 −
𝟏
(𝐫 𝐩
)(𝜸−𝟏)/𝜸

Types of Gas Turbines:
1. Open Cycle Gas Turbine
2. Closed Cycle Gas Turbine

Open Cycle Gas Turbine
1-2 Isentropic compression in compressor;
2-3 Constant-pressure heat addition in combustion
chamber;
3-4 Isentropic expansion in the turbine;

Open Cycle Gas Turbine
It consists of a compressor, combustion chamber and a
turbine. The compressor takes in ambient air and raises
its pressure. Heat is added to the air in combustion
chamber by burning the fuel and raises its temperature.
The heated gases coming out of combustion chamber are
then passed to the turbine where it expands doing
mechanical work.

Efficiency of compressor,
𝜂ܿ =
Isentropic Compression work
Actual Compression work
=
CP (T2 − T1)
CP (T2′ − T1)
=
(T2 − T1)
(T2′ − T1)
Efficiency of turbine,
𝜂t =
Actual turbine work
Isentropic turbine work
=
CP (T3−T4′)
CP (T3−T4)
=
(T3−T4′)
(T3−T4)
Actual Compression work, Wc = CP (T2' − T1)
Actual turbine work, Wt = CP (T3 − T4')
Actual net work = Wc − Wt = CP[(T3 − T4') − (T2' − T1)]
Thermal efficiency, 𝜂 =
Net Work
Heat Supplied
=
CP[(T3 − T4') − (T2' − T1)]
CP (T3 – T2' )
𝜂 =
(T3 −T2′) − (T4′− T1)
(T3 – T2′ )
𝜂 = 1 −
T4′ − T1
T3 – T2′
Turbine work ratio =
Wt
Wc

Closed Cycle Gas Turbine
1-2 Isentropic compression in compressor;
2-3 Constant-pressure heat addition in combustion
chamber;
3-4 Isentropic expansion in the turbine;
4-1 Constant pressure heat rejection in heat exchanger

Closed Cycle Gas Turbine
It uses air as working medium. In closed cycle gas turbine plant, the
working fluid (air or any other suitable gas) coming out from
compressor is heated in a heater by an external source at constant
pressure.
The high temperature and high-pressure air coming out from the
external heater is passed through the gas turbine.
The fluid coming out from the turbine is cooled to its original
temperature in the cooler using external cooling source before
passing to the compressor.
The working fluid is continuously used in the system without its
change of phase and the required heat is given to the working fluid
in the heat exchanger.

Compare Open cycle and Closed cycle Gas turbines
S no. Open cycle gas turbine Closed cycle gas turbine
1 Low thermal efficiency High thermal efficiency
2 Working fluid is replaced
continuously circulated
Working fluid is circulated
continuously
3 High cost fuels are used Low cost fuels can be used
4 Turbine blade wear is high
due to contamination of air in
combustion chamber
Turbine blade wear is low as gas does
not get contaminated while flowing
through combustion chamber
5 Suitable for moving
applications
Suitable for stationary installations
6 Maintenance cost is low Maintenance cost is high
7 Low power to weight ratio High power to weight ratio
8 System is simple System is complex

Methods of improving thermal
efficiency of open cycle gas turbine
1. Intercooling
2. 2. Reheating
3. 3. Regeneration

Intercooling
A compressor in a gas turbine cycle utilizes the major
percentage of power developed by the gas turbine. The
work required by the compressor can be reduced by
compressing the air in two stages and incorporating an
intercooler between the two as shown in Fig.
1-2’ Low Pressure
Compression
2’-3 Intercooling
3-4’ High Pressure
Compression
4’-5 Heat addition in
combustion chamber
5-6’ Expansion in turbine

Work input (with intercooling),
= 𝐶‫′2𝑇(݌‬ − 𝑇1) + 𝐶‫′4𝑇(݌‬ − 𝑇3) -------(1)
Work input (without intercooling),
= 𝐶‫𝐿𝑇(݌‬ ′ − 𝑇1) = 𝐶‫2𝑇(݌‬ ′ − 𝑇1) + 𝐶‫′𝐿𝑇(݌‬ − 𝑇2′) ------(2)
By comparing the equations (1) and (2) it can be observed that the work
input with intercooling is less than the work input without intercooling,
as 𝐶‫4𝑇(݌‬ ′ − 𝑇3) < 𝐶‫𝐿𝑇(݌‬ ′ − 𝑇2 ′)

Reheating
 The output of gas turbine can be improved by
expanding the gasses in two stages with a reheater
between the two turbines.
 The H.P. turbine drives the compressor and the LP
turbine provides useful power output.
1-2’ Compression
in compressor
2’-3 Heat addition in CC
3-4’ Expansion in HP turbine
4’-5 Heat addition in
reheater
5-6’ Expansion in LP turbine

Work output (with reheating) = 𝐶‫3𝑇(݌‬ − 𝑇4′) + 𝐶‫5𝑇(݌‬ − 𝑇6′)------(a)
Work output (without reheating) = 𝐶‫3𝑇(݌‬ − 𝑇𝐿′)
=𝐶‫3𝑇(݌‬ − 𝑇4′) + 𝐶‫′4𝑇(݌‬ − 𝑇𝐿′) ------(b)
Since the pressure lines diverse to the right on T-s diagram, it can be
seen that the temperature difference (𝑇5 − 𝑇6′) is always greater
than (𝑇4′ − 𝑇𝐿′). So the reheating increases the net work.

Regeneration
 The temperature of exhaust gases leaving the turbine of a gas
turbine engine is considerably higher than the temperature of air
delivered by the compressor.
 Therefore, high pressure air leaving the compressor can be heated
by hot exhaust gases, thereby reducing the mass of fuel supplied in
the combustion chamber. Hence the thermal efficiency can be
increased.
 The heat exchanger used to transfer the heat from exhaust gases to
compressed air is known as regenerator.

1-2’ ---- Compression in compressor
2’-3 ---- Heat addition into the compressed air during its passage
through the heat exchanger
3-4 ---- Heat addition in the combustion chamber
4-5’ ---- Expansion in turbine
5’-6 ---- Heat rejection in heat exchanger to the compressed air

The effectiveness of the heat exchanger is given by,
ε = ‫݁ݏܽ݁ݎܿ݊ܫ‬ ݅݊ ݁݊‫ݐ‬ℎ݈ܽ‫ݕ݌‬ ‫ݎ݁݌‬ ݇݃ ‫݂݋‬ ܽ݅‫ݎ‬
‫݈ܾ݈݁ܽ݅ܽݒܣ‬ ݅݊ܿ‫݁ݏܽ݁ݎ‬ ݅݊ ݁݊‫ݐ‬ℎ݈ܽ‫ݕ݌‬ ‫ݎ݁݌‬ ݇݃ ‫݂݋‬ ܽ݅‫ݎ‬
=
T3−T2′
T5′− T2′

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