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Nile Valley University – Atbara, Sudan
Deanship of Graduate Studies
Master Degree Program in Mechanical Engineering
Faculty of Engineering and Technology
Mechanical Engineering Department
Thermal Power Plants
Chapter 4 – Gas Turbine
Associate Professor Dr. Eng. Osama Mohammed Elmardi Suleiman Khayal, Department of
Mechanical Engineering, College of Engineering and Technology, Nile Valley University - Atbara –
Sudan
osamamm64@gmail.comEmail address:
k9zOxAAAAAJ&hl=ar-hps://scholar.google.com/citaons?user=Google scholar citaon link:
Khayal-hps://www.researchgate.net/prole/OsamaResearchGate link:
Mobile phone number: Sudan, Atbara | 00249919305570
AUGUST 2024
4.1 Introducon
A gas turbine, gas turbine engine, or also known by its old name internal combuson turbine, is a
type of connuous ow internal combuson engine. The main parts common to all gas turbine
engines form the power producing part (known as the gas generator or core) and are in the ow
direcon:
Rotary gas compressor.
burning.
Compressor driven turbines.
Addional components must be added to the gas generator to suit its use. Common to all there is
an air intake but in dierent conguraons to suit the requirements of marine use, land use or ight
at speeds varying from constant to supersonic. A propelling nozzle is added to produce thrust for
ight. An addional turbine is added to drive the propeller (turbofan) to reduce fuel consumpon
(by increasing propulsion eciency) at subsonic ight speeds. An addional turbine is also required
to drive a helicopter, a ground vehicle transmission (turbine sha), a marine propeller or an electric
generator (power turbine). A greater rao of thrust to weight in ight is achieved by adding an
aerburner.
The basic operaon of a gas turbine is the Brayton cycle where air is the working uid and
atmospheric air ows through the compressor creang a pressure rise; Energy is then added by
spraying the fuel into the air and igning it so that combuson generates a high-temperature ow;
This high temperature compressed gas enters the turbine, creang a sha work output in the
process, which is used to drive the compressor. Unused energy escapes into exhaust gases that can
be reused for external work, such as producing direct thrust in a jet engine, or turning a second,
independent turbine (known as a power turbine) that can be connected to a propeller or electric
generator. The purpose of a gas turbine determines the design such that the desired power division
between thrust and sha work is achieved. The fourth step of the Brayton cycle (cooling the working
uid) is omied, as gas turbines are open systems that do not reuse the same air.
Gas turbines are used to power airplanes, trains, ships, electric generators, pumps, gas compressors,
and tanks.
4.2 Theory of Operaon
In an ideal gas turbine, gases undergo four thermodynamic processes: reversible adiabac or
isentropic compression, constant pressure combuson, reversible adiabac or isentropic expansion,
and constant pressure heat rejecon. Together these form the Brayton cycle, also known as the
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'constant pressure cycle'. It is disnguished from the Oo cycle in that all processes (compression,
ignion, and exhaust) occur at the same me and connuously.
In a real gas turbine, mechanical energy is changed irreversibly (due to internal fricon and
turbulence) into pressure and thermal energy when the gas is compressed (either in a centrifugal or
axial compressor). Heat is added to the combuson chamber and the specic volume of the gas
increases, accompanied by a slight loss in pressure. During expansion through the stator and rotor
passages in the turbine, an irreversible transfer of energy occurs again. Fresh air is taken in instead
of rejecng heat.
The air is drawn in by a compressor, called a gas generator, of either an axial or centrifugal design,
or a combinaon of the two. This air is then introduced into the combuson secon which can be of
an annular or canister design. In the combuson secon, approximately 70% of the air from the
compressor is directed around the combustor itself for cooling purposes. The remaining 30% of air
is mixed with fuel and ignited by the already burning air-fuel mixture, which then expands the energy
produced through the turbine. This expansion of the mixture leaves the combuson secon and
accelerates through the turbine secon to strike the turbine blades, causing the disk aached to
them to rotate, thus generang useful energy. Of the energy produced, only 60-70% is used to
operate the gas generator. The remaining energy is used to power the engine for which it is used,
typically in aeronaucal applicaons, where it is propelled into a turbojet engine, driving the
turboprop fan, the rotor or turbosha aachment, and the gearbox and propeller of the turboprop.
If the engine has a power turbine added to drive an industrial generator or helicopter rotor, the exit
pressure will be as close to the inlet pressure as possible with just enough power remaining to
overcome the pressure loss in the exhaust pipes and exhaust. For a turboprop engine, there will be
a special balance between propeller power and jet thrust providing the most economical operaon.
In a jet engine, enough pressure and energy are extracted from the ow to operate the compressor
and other components. The remaining high-pressure gases are accelerated through a nozzle to
provide jet propulsion for the aircra.
The smaller the motor, the higher the sha rotaon rate must be to reach the desired blade p
speed. The blade p speed determines the maximum pressure raos that can be obtained by the
turbine and compressor. This in turn limits the maximum power and eciency the engine can have.
For the p speed to remain constant, if the rotor diameter is halved, the rotaon speed must double.
For example, large jet engines operate at between 10,000 and 25,000 rpm, while small turbines
rotate at up to 500,000 rpm.
Mechanically, gas turbines can be much less complex than reciprocang engines. A simple turbine
may have one main moving part, the compressor/sha/turbine rotor assembly, with other moving
parts in the fuel system. This in turn can translate into a price. For example, at a cost of 10,000 ℛℳ
for materials, the Jumo 004 engine proved cheaper than the Junkers 213 piston engine, which cost
35,000 ℛℳ, and needed only 375 hours of low-skilled labor to complete (including fabricaon,
assembly and shipping), compared to 1,400 for a car. BMW 801. But this also translates into poor
eciency and reliability. More advanced gas turbines (such as those found in modern jet engines or
combined cycle power plants) may have two or three shas, hundreds of compressor and turbine
blades, movable stator blades, and extensive external piping for fuel, oil, and air systems. They use
heat-resistant alloys and are made to strict specicaons that require precise manufacturing. All this
oen makes building a simple gas turbine more complex than building a piston engine.
Furthermore, to achieve opmum performance in modern gas turbine power plants, the gas must
be prepared to precise fuel specicaons. Fuel gas condioning systems process natural gas to reach
precise fuel specicaons before it enters the turbine in terms of pressure, temperature, gas
composion and the relevant Wobbe index.
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The primary advantage of a gas turbine engine is its power-to-weight rao. Since signicant useful
work can be generated by a relavely lightweight engine, gas turbines are well suited for aircra
propulsion.
Thrust bearings and sleeve bearings are an important part of the design. They are hydrodynamic oil
bearings or oil cooled rotang element bearings. Sheet bearings are used in some small machines
such as small turbines and also have strong potenal for use in small gas turbines/auxiliary power
units.
4.2 Creep
The main challenge facing the design of turbines, especially turbine blades, is to reduce creep caused
by the high temperatures and pressures that occur during operaon. Higher operang temperatures
are constantly sought for increased eciency, but this comes at the cost of higher creep rates.
Therefore, several methods have been used in an aempt to achieve opmal performance while
reducing creep, the most successful being high-performance coangs and single crystal superalloys.
These techniques work by reducing deformaon that occurs through mechanisms that can be
broadly classied as sliding dislocaon, climbing dislocaon, and diusive ow.
Protecve coangs provide thermal insulaon to the blade and provide resistance to oxidaon and
corrosion. Thermal barrier coangs (TBCs) are oen zirconium dioxide-based ceramics, and
oxidaon/corrosion resistant coangs are typically aluminum alloys or MCrAlY (where M is typically
Fe and/or Cr). The use of TBCs limits the temperature exposure of the super alloyed substrate,
reducing the diusion of acve species within the alloy and reducing dissociaon and vacancy creep.
It has been found that a coang of 1 to 200 micrometers can reduce blade temperatures by up to
200 °C (392 °F). Al forms 𝐴𝑙2𝑂3 bond layers at the interface of the TBC bond layer which provides
oxidaon resistance, but also leads to the formaon of an unwanted cross-diusion zone between
it and the substrate. Oxidaon resistance outweighs the disadvantages associated with the
interdiusion zone because it increases blade life and limits eciency loss caused by build-up on the
outer surface of the blades.
Nickel-based superalloys have improved strength and creep resistance due to their composion and
the resulng microstructure. Gamma nickel (𝛾) FCC is alloyed with aluminum and tanium in order
to accelerate the uniform dispersion of cohesive primary gamma (𝛾′) 𝑁𝑖3(𝐴𝑙,𝑇𝑖) phases. Finely
dispersed 𝛾′ deposits impede dislocaon moon and cause stress, increasing the stress required for
creep onset. Moreover, 𝛾′ is an ordered L12 phase which makes it dicult for dislocaons to bypass
it. More refractory elements such as rhenium and ruthenium can be added into the solid soluon to
improve creep strength. The addion of these elements reduces the diusion of the inial gamma
phase, thus maintaining fague strength, strength and creep resistance. The development of single-
crystalline superalloys has led to signicant improvements in creep resistance as well. Due to the
lack of grain boundaries, single crystals eliminate Coppel creep. Although single crystals have less
creep at high temperatures, they have much lower yield stresses at room temperature where the
strength is determined by the Hall-Petch relaonship. Care must be taken to opmize design
variables to limit high temperature creep while not reducing low temperature yield strength.
4.3 Types of Gas Turbines
4.3.1 Jet Engines
Air-breathing jet engines are gas turbines opmized to produce thrust from exhaust gases, or from
ducted fans connected to the gas turbine. Jet engines that produce thrust from direct thrust of
exhaust gases are oen called ramjet engines. While sll in service with many militaries and civilian
operators, jet engines have been mostly phased out in favor of the turbofan due to the jet engine's
low fuel eciency and high noise.
Those that generate thrust with the addion of a ducted fan are called turbofans or (rarely)
turbojets. These engines produce approximately 80% of their thrust via the ducted propeller, which
can be seen from the front of the engine. They come in two types, low-bypass and high-bypass
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turbofans, the dierence being the amount of air moved by the fan, called "bypass air". These
engines provide the advantage of greater thrust without addional fuel consumpon.
Gas turbines are also used in many liquid-propellant rockets, where gas turbines are used to drive
the turbopump to allow the use of lightweight, low-pressure tanks, reducing the empty weight of
the rocket.
Figure 1 below shows a model of an axial ow gas turbine, J85, cut into view. Flow is le to right,
mulstage compressor on the le, central combuson chambers and two-stage turbine on the right.
Figure 1 A model of an axial ow gas turbine, J85, cut into view. Flow is le to right, mulstage
compressor on the le, central combuson chambers and two-stage turbine on the right.
4.3.2 Turboprop Engines
A turboprop is a turbine engine that drives an aircra's propeller using a reducon gear to translate
the high turbine secon operang speed (oen in the tens of thousands) into the low thousands
necessary for ecient propeller operaon. The benet of using a turboprop is that it takes advantage
of the high power-to-weight rao in turbofan engines to drive the propeller, allowing the use of a
more powerful but also smaller engine. Turboprop engines are used in a wide range of business
aircra such as the Pilatus PC-12, passenger aircra such as the Beechcra 1900, small cargo aircra
such as the Cessna 208 Caravan or De Havilland Canada Dash 8, and large (usually military) aircra.
Such as the Airbus A400M transport aircra, Lockheed AC-130 and the 60-year-old Tupolev Tu-95
strategic bomber. While military turbofans can vary, in the civilian market there are two main engines
that can be found: the Pra & Whitney Canada PT6, which is a freeform turbofan, and the Honeywell
TPE331, a xed turbofan (formerly known as Garre AiResearch 331).
4.3.3 Aeroderivave Gas Turbines
Aerodynamic gas turbines are generally based on aircra gas turbine engines, and are smaller and
lighter than industrial gas turbines.
Pneumac derivaves are used in electric power generaon due to their ability to shut down and
handle load changes more quickly than industrial machines. It is also used in the marine industry to
reduce weight. Popular types include the General Electric LM2500, General Electric LM6000, and
aircra derivave versions of the Pra & Whitney PW4000, Pra & Whitney FT4, and Rolls-Royce
RB211.
Figure 2 below shows the LM6000 in an electrical power staon applicaon.
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Figure 2 The LM6000 in an electrical power staon applicaon
4.3.4 Amateur Gas Turbines
Increasing numbers of gas turbines are being used or even built by hobbyists. In their simplest form,
these are commercial turbines acquired through military surplus or junkyard sales, then put into
operaon for display as part of the engine collecng hobby. In its most extreme form, amateurs
rebuilt engines beyond professional repair and then used them to compete for world speed records.
The simplest form of self-build gas turbine uses an automove turbocharger as the core component.
The combuson chamber is manufactured and connected between the compressor and turbine
secons.
More advanced jet engines are also being manufactured, the thrust and light weight of which are
sucient to power large model aircra. Schreckling Design builds the engine enrely from raw
materials, including fabricang the centrifugal compressor wheel from plywood, epoxy and
laminated carbon ber lament.
Many small companies now manufacture small turbines and hobbyist parts. Most typical jet-
powered aircra now use these commercial and semi-commercial microturbines.
4.3.5 Auxiliary Power Units (APU)
Small gas turbines are used as auxiliary power units (APUs) to supply auxiliary power to larger,
mobile machines such as aircra, and are a turbine sha design. It supplies:
Compressed air for air circulaon similar to air condioning and venlaon,
Compressed air starng power for larger jet engines,
Mechanical strength of the gearbox to drive sha aachments, and
Electrical, hydraulic, and other power transmission sources to consuming devices far from the APU.
4.3.6 Industrial Gas Turbines for Power Generaon
Industrial gas turbines dier from aerospace designs in that the frames, bearings and blades are of
heavier construcon. They are also more closely integrated with the devices that power them oen
an electrical generator and the secondary power equipment used to recover the remaining energy
(largely heat).
They range in size from portable mobile staons to large, complex systems weighing more than one
hundred tons housed in purpose-built buildings. When a gas turbine is used only for power
generaon, its thermal eciency is about 30%. However, purchasing electricity may be cheaper than
generang it. Therefore, many engines are used in CHP (combined heat and power) conguraons
that can be small enough to be integrated into portable container conguraons.
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Gas turbines can be parcularly ecient when the waste heat from the turbine is recovered by a
heat recovery steam generator (HRSG) to drive a convenonal steam turbine in a combined cycle
conguraon. The 605 MW General Electric 9HA engine achieved an eciency rate of 62.22% with
temperatures up to 1,540°C (2,800°F). For 2018, GE is delivering 826 MWha at combined cycle
eciencies of more than 64% due to advances in addive manufacturing and combuson
breakthroughs, up from 63.7% in 2017 orders and on track to achieve 65% by the early 2020s. In
March 2018, GE Power achieved an overall eciency of 63.08% for its 7HA turbines.
Aerodynamically derived gas turbines can also be used in combined cycles, resulng in higher
eciency, but they will not be as high as specially designed industrial gas turbines. They can also be
operated in a cogeneraon conguraon: the exhaust is used for space heang or water heang, or
an absorpon chiller is powered to cool the intake air and increase power output, a technology
known as turbine intake air cooling.
Another important feature is their ability to turn on and o within minutes, saving energy during
peak or unscheduled demand. Because single cycle power plants (gas turbines only) are less ecient
than combined cycle plants, they are typically used as peak power plants, which operate anywhere
from several hours per day to a few tens of hours per year depending on electricity demand, cost
and capacity generaons for the region.
In areas with a shortage of baseload and aerload power plant capacity or low fuel costs, a gas
turbine power plant may operate regularly for most hours of the day. Large single cycle gas turbines
typically produce 100-400 MW of electrical power and have a thermodynamic eciency of 35-40%.
4.3.7 Industrial Gas Turbines for Mechanical Drive
Industrial gas turbines used solely for mechanical drive or used in conjuncon with a recovery steam
generator dier from power generang sets in that they are oen smaller and feature a double sha
design rather than a single sha. The power range is from 1MW to 50MW. These motors are
connected directly or via a gearbox to either a pump or compressor set. The majority of the facilies
are used in the oil and gas industries. Mechanical drive applicaons increase eciency by about 2%.
Oil and gas rigs require these engines to drive compressors to inject gas into wells to push the oil up
through another bore, or to compress the gas for transportaon. They are also oen used to provide
power to the plaorm. These plaorms do not need to use an engine in conjuncon with a CHP
system because the gas is obtained at a very low cost (oen free of gas aring). The same companies
use pump sets to push uids into the ground and through pipelines at dierent intervals.
4.3.8 Compressed Air Energy Storage
A recent development seeks to improve eciency in another way, by separang the compressor and
turbine with a compressed air reservoir. In convenonal turbines, up to half of the energy generated
is used to drive the compressor. In a compressed air energy storage conguraon, energy is used to
drive the compressor, and compressed air is released to drive the turbine when needed.
4.4 External Combuson
Most gas turbines are internal combuson engines, but it is also possible to manufacture external
combuson gas turbines that are, eecvely, a turbine version of a hot air engine. These systems
are commonly referred to as EFGT (externally red gas turbines) or IFGT (indirect red gas turbines).
External combuson has been used for the purpose of using pulverized coal or nely ground biomass
(such as sawdust) as fuel. In the indirect system, a heat exchanger is used and only clean air without
combuson products is transmied through the power turbine. Thermal eciency is lower in
indirect type of external combuson; However, the turbine blades are not exposed to the products
of combuson and lower quality (and therefore cheaper) fuels can be used.
When using external combuson, it is possible to use the exhaust air from the turbine as the primary
combuson air. This eecvely reduces global heat loss, although heat loss associated with
combuson exhaust remains unavoidable.
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Closed-cycle gas turbines based on supercrical helium or carbon dioxide also hold promise for use
in high-temperature solar and nuclear power generaon in the future.
4.5 Passenger Road Vehicles (Cars, Bikes, and Buses)
A number of experiments were conducted on gas turbine cars, the largest of which was Chrysler.
Recently, there has been some interest in using turbo engines for hybrid electric vehicles. For
example, a consorum led by small gas turbine company Bladon Gates received investment from
the Technology Strategy Board to develop an ultra-lightweight range extender (ULRE) for the next
generaon of electric vehicles. The goal of the consorum, which includes luxury car maker Jaguar
Land Rover and leading electric machinery company SR Drives, is to produce the rst commercially
viable and environmentally friendly gas turbine generator designed specically for automove
applicaons.
The common turbocharger for gasoline or diesel engines is also a derivave of the turbine.
4.6 Advances in Technology
Gas turbine technology has evolved steadily since its incepon and connues to evolve.
Development is acvely working on the producon of smaller gas turbines and more powerful and
ecient engines. What is aiding these developments is computer-based design (specically
computaonal uid dynamics and nite element analysis) and also the development of advanced
materials: basic materials with high strength at high temperatures (for example, single-crystal
superalloys that exhibit anomalous yield strength) or thermal barrier coangs that protect structural
materials from constantly high temperatures. These advances allow higher compression raos and
turbine inlet temperatures, more ecient combuson and beer cooling of engine parts.
Computaonal uid dynamics (CFD) has contributed to signicant improvements in the performance
and eciency of gas turbine engine components by enhancing understanding of the complex viscous
ow and heat transfer phenomena involved. For this reason, CFD is one of the main computaonal
tools used in the design and development of gas turbine engines.
The simple cycle eciency of early gas turbines was praccally doubled by incorporang
intercooling, regeneraon (or recovery), and reheang. These improvements, of course, come at the
cost of increased inial and operang costs, and can only be jused if the reducon in fuel costs
osets the increase in other costs. Relavely low fuel prices, the general desire of the industry to
reduce installaon costs, and the huge increase in simple cycle eciency to about 40 percent, did
not leave much desire to choose these modicaons.
On the emissions side, the challenge is to increase turbine inlet temperatures while at the same me
reducing the maximum ame temperature in order to reduce NOx emissions and meet the latest
emissions regulaons. In May 2011, Mitsubishi Heavy Industries achieved a turbine inlet
temperature of 1,600 °C (2,900 °F) on a 320 MW gas turbine, and 460 MW in gas turbine combined
cycle power generaon applicaons where the total thermal eciency exceeds 60%.
Commercially compable shim bearings were introduced to gas turbines in the 1990s. It can
withstand over one hundred thousand on/o cycles, eliminang the need for an oil system. The
applicaon of microelectronics and power switching technology has made it possible to develop
commercially viable electricity generaon by microturbines for distribuon and vehicle propulsion.
In 2013, GE began developing the GE9X with a compression rao of 61:1.
4.6 Advantages and Disadvantages
The advantages and disadvantages of gas turbine engines are as follows:
Advantages include:
1. Very high power-to-weight ratio compared to reciprocating engines.
2. Smaller than most reciprocating motors with the same power rating.
3. The smooth rotation of the main shaft produces much less vibration than a reciprocating
engine.
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4. Having fewer moving parts than reciprocating motors results in lower maintenance costs
and increased reliability/availability over the service life.
5. Greater reliability, especially in applications requiring high, sustained power output.
6. The waste heat is almost entirely dissipated in the exhaust. This results in a high temperature
exhaust stream that can be used to boil water in a combined cycle, or for cogeneration.
7. Lower combustion pressures than reciprocating engines in general.
8. High shaft speeds in smaller "free turbine units", although larger gas turbines used in power
generation operate at synchronous speeds.
9. Low cost and consumption of lubricating oil.
10. It can operate on a wide range of fuel types.
11. Very low toxic emissions of carbon dioxide and hydrocarbon due to excess air, complete
combustion and no “suppression” of flame on cold surfaces.
Disadvantages include:
1. Core engine costs can be high due to the use of exotic materials, especially in applications
requiring high reliability (such as aircraft propulsion).
2. Less efficient than reciprocating engines at idle speed.
3. Longer start-up than reciprocating motors.
4. Less responsive to changes in power demand compared to reciprocating engines.
5. The characteristic whine can be difficult to suppress. Exhausts (especially on jet engines) can
also make a distinctive sound.
References
1. Staonary Combuson Gas Turbines including Oil & Over-Speed Control System descripon
"Aircra Gas Turbine Technology" by Irwin E. Treager, McGraw-Hill, Glencoe Division, 1979, ISBN 0-
07-065158-2.
2. Gas Turbine Theory by H.I.H. Saravanamuoo, G.F.C. Rogers and H. Cohen, Pearson Educaon,
2001, 5th ed., ISBN 0-13-015847-X .
3. Leyes II, Richard A.; Fleming, William A. (1999). The History of North American Small Gas Turbine
Aircra Engines. Washington, DC: Smithsonian Instuon. ISBN 978-1-56347-332-6.
4. R. M. "Fred" Klaass and Christopher DellaCorte, "The Quest for Oil-Free Gas Turbine Engines," SAE
Technical Papers, No. 2006-01-3055, available at sae.org.
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6. Aircra Engines and Gas Turbines, Second Edion by Jack L. Kerrebrock, The MIT Press, 1992, ISBN
0-262-11162-4 .
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8. Gas Turbine Performance, 2nd Edion by Philip Walsh and Paul Fletcher, Wiley-Blackwell, 2004
ISBN 978-0-632-06434-2
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