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APPLIED THERMODYNAMICS
Engr. Ans Ahmed Memon
Lecturer
Department of Mechanical Engineering,
MUET, Jamshoro
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LECTURE CONTENTS
Gas Turbine
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WHAT IS A GAS TURBINE ?
• In steam, gas, or hydroelectric power plants, the device that drives
the electric generator is the turbine. As the fluid passes through
the turbine, work is done against the blades, which are attached to
the shaft. As a result, the shaft rotates, and the turbine produces
work.
• In a gas turbine, first of all the air is obtained from the atmosphere
and compressed in an air compressor.
• The compressed air is then passed into the combustion chamber,
where it is heated or burned.
• The hot air is then made to flow over the moving blades of the gas
turbine (reaction turbine) and air gets expanded, which imparts
the rotational motion to the shaft. Finally, it is exhausted into the
atmosphere. (in case of open cycle turbine).
• A major part of the power developed by the turbine is consumed
for driving the compressor. The remaining power is utilized for
doing some external work (to generate electricity). 5
CLASSIFICATION OF GAS TURBINE
An Open Cycle and Closed Cycle Gas Turbine
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OPEN CYCLE GAS TURBINE
• The Brayton cycle is used for gas turbines only when both
the compression and expansion processes take place in
rotating machinery.
• Gas turbines usually operate on an open cycle in which
fresh air at ambient conditions is drawn into the
compressor, where its temperature and pressure are
raised.
• The high pressure air proceeds into the combustion
chamber, where the fuel is burned at constant pressure.
• The resulting high-temperature (flue) gases then enters
the turbine, where they expand to the atmospheric
pressure while producing power.
• The exhaust gases leaving the turbine are thrown out (not
recirculated), causing the cycle to be classified as an open
cycle. 7
CLOSED CYCLE GAS TURBINE
• The open gas-turbine cycle described above can
be modeled as a closed cycle, by utilizing the air-
standard assumptions.
• Here the compression and expansion processes
remain the same, but the combustion process is
replaced by a constant-pressure heat-addition
process from an external source, and the exhaust
process is replaced by a constant pressure heat-
rejection process to the ambient air.
• The ideal cycle that the working fluid undergoes in
this closed loop is the Brayton cycle, which is made
up of four internally reversible processes.
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GAS TURBINE (BRAYTON) CYCLE
P-v & T-s Diagram
1-2 Isentropic compression
(in a compressor)
2-3 Constant-pressure heat
addition (heat exchanger)
3-4 Isentropic expansion (in
a turbine)
4-1 Constant-pressure heat
rejection (heat exchanger)
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BASIC STRUCTURE OF A GAS TURBINE
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BASIC STRUCTURE TURBO-JET ENGINE
Combustion Bye pass duct
Chamber
Fan
Compressor stages Turbin
e 11
BASIC STRUCTURE OF A GAS TURBINE
Com Turbin
Com
pressor e
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Axial bust or
10 cans 3
stages
Type Reverse Flow stages
Type
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BACK WORK RATIO OF A GAS TURBINE
• In gas-turbine power plants, the ratio of the
compressor work to the turbine work, called as
the back work ratio.
• Usually more than one-half of the turbine
work output is used to drive the compressor.
• The situation is even worse when the
isentropic efficiencies of the compressor and the
turbine are low.
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BACK WORK RATIO OF A GAS TURBINE
• However, steam power plants have only a few
percent of back work ratio.
• This is not surprising, since a liquid is compressed in
steam power plants instead of a gas, and the steady-
flow work is proportional to the specific volume of
the working fluid.
• A power plant with a high back work ratio requires a
larger turbine to provide the additional power
requirements of the compressor.
• Therefore, the turbines used in gas-turbine power
plants are larger than those used in steam power
plants of the same net power output.
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APPLICATIONS / USES OF A GAS TURBINE
• Work is supplied to these devices from an
external source through a rotating shaft.
Therefore, Compressors, Pumps, Fans involve
work inputs, while Turbines, Engines involve
work output.
Uses of Gas Turbine:
• The main use for the gas turbine at the present
time is in the aircraft field.
• Also, gas turbine units are increasingly used for
electric power generation.
• Gas turbines are also used in marine propulsion.
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APPLICATIONS / USES OF A GAS TURBINE
Therefore, gas turbines are
classified in two groups:
Shaft Power Gas Turbines:
• It is a gas turbine whose main
purpose is to deliver shaft
power.
• It is mostly used in Power
generation.
Jet Engine Gas Turbines:
• It is a turbine whose main
purpose is to deliver thrust.
• It is mostly used in Aircrafts
(Space Application). 16
APPLICATIONS OF GAS TURBINES
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APPLICATIONS OF GAS TURBINES
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THERMAL EFFICIENCY OF BRAYTON CYCLE
The thermal efficiency of an ideal Brayton cycle
depends on the pressure ratio of the gas turbine and the
specific heat ratio of the working fluid.
The thermal efficiency increases with both of these
parameters, which is also the case for actual gas
turbines.
The air in gas turbines performs two important functions:
i. It supplies the necessary oxidant for the combustion of
the fuel, and it serves as a coolant to keep the
temperature of various components within safe limits.
ii.The second function is accomplished by drawing in more
air than is needed for the complete combustion of the
fuel.
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THERMAL EFFICIENCY OF BRAYTON CYCLE
From T-S and P-V diagram it has been noticed that all four
processes of the Brayton cycle are executed in steady flow
devices;
Therefore, they should be analyzed as steady-flow
processes. When the changes in kinetic and potential
energies are neglected, the energy balance for a steady-
flow process can be expressed, on a unit–mass basis as:
(q in q out ) (W in W out ) h exit
hinlet
Therefore, heat transfer to and from the working fluid
are:
q in h3 h 2 c p (T3 T 2 )
q out h 4 h1 c p (T 4 T1
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)
THERMAL EFFICIENCY OF BRAYTON CYCLE
Then the thermal efficiency of the ideal Brayton cycle
under the cold- air standard assumptions becomes:
th W net q in q out 1 q out
q in
q in
c p (T 4
th 1
cT1p )(T3 qTin2 )
T1 ( T 4
th 1 1) T1
T3 (1)
T (
2
T2
1)
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THERMAL EFFICIENCY OF BRAYTON CYCLE
Process 1-2 and 3-4 are isentropic and pressure values are:
P2=P3 , and P 4=P1 Thus ,
T2 P ( k 1) / P ( k 1) /
k k
T3
T1 P21 T4
P43
Substituting these equations into the thermal efficiency relation and
simplified as,
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th 1 ( k 1) / (2) r p P2
r pk
P1
Where, rp is pressure ratio and k is specific heat ratio.
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DEVIATION OF ACTUAL FROM IDEAL BRAYTON CYCLE
i-e. Solved Ex. Numerical 23
COMPARISON OF
GAS TURBINE & STEAM TURBINE
Gas Turbine Steam Turbine
• The important components are • The important components are
compressor and combustion steam boiler & its accessories.
chamber. • The mass of steam turbine per
• The mass of gas turbine per kw kw developed is more.
developed is less. • It requires more space for
• It requires less space for
installation. installation.
• The installation and running • The installation and running
cost is less. cost is more.
• The starting of gas turbine is • The starting of steam turbine is
very easy and quick. difficult and taking long time.
• Its control is easy, by changing • Its control is difficult, with
load conditions. changing load conditions.
• It does not depend on water • A steam turbine depends on
supply.
• Its efficiency is less. water supply.
• Its efficiency is more. 24
Working principle of a gas turbine
https://www.youtube.com/watch?
v=BUn5-0VG3Hw
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Any Question….???
THANK YOU…!
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