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NOTES BY AAFTAB M 9619468502 SUB: PEN 1
Ch.4 GAS TURBINE AND JET PROPULSION
GAS TURBINE AND JET PROPULSION
INTRODUCTION
In a gas turbine unit, first of all, the air is obtained from the atmospheric, and
compressed in an air compressor. The compressed air is then passed into combustion
chamber, where it is heated considerably. The hot air is then made to flow over the
moving blade of turbine, which imparts rotational motion to runner during this
process, the air get expanded, and we get mechanical output and finally it is
exhausted into atmosphere. A major part of power developed by the turbine is
consumed for driving the compressor, (which supplied the compressed air to
combustion chamber). The remaining power is utilized for doing some external
works.
Network = Work produced by turbine - work consumed by
compressor
Q.1 CLASSIFICATION OF GAS TURBINE:-
1. According to process of heat absorption :-
(a) Constant pressure gas turbine / the working cycle is joule or bray to
cycle.
(b) Constant volume cycle / the working cycle are Atkinson cycle.
2. According to path of working substance :-
(a) Closed cycle gas turbine.
(b) Open cycle gas turbine.
(c) Semi-closed gas turbine.
3. On the basic of action of expanding gases:-
(a) Impulse turbine.
(b) Reaction turbine.
4. On the basic of direction of flow.
(a) Axial flow.
(b) Radial flow
NOTES BY AAFTAB M 9619468502 SUB: PEN 2
Ch.4 GAS TURBINE AND JET PROPULSION
Q.2 APPLICATION OF GAS TURBINE :-
a. Generation of electric power :- The gas turbine are extensively used in the
generation of electric power. It is considered as a clean source of power
generation.
b. Turbojet and turbo propeller engine: - The gas turbines are used to drive
air compressors in turbojets. They also used to drive air compressor and
propeller in turbo propeller engines. The gas turbines in turbojet and turbo
propeller engine are operated in the temperature in the range of 8000C to
10000C.
c. Supercharger:-The gas turbines are also used to drive superchargers fitted
in the aviation gasoline engines as well as for heavy duty diesel engine.
d. Marine engines: - The gas turbines are used in marine engine unlike steam
turbine or steam engine, the gas turbines do not requires water storage tanks
or distillation plants.
e. Railway engines: - The gas turbine is also entered in the field of railway
engines. The first gas turbine locomotive was put into services in 1941 in
Switzerland.
f. Industry: - Gas turbine is also employed for numerous industrial purposes.
Example: - Blast of air for blast furnace in steel industry, oil and other
chemical industries.
NOTES BY AAFTAB M 9619468502 SUB: PEN 3
Ch.4 GAS TURBINE AND JET PROPULSION
Q.3 Explain Brayton Ideal cycle for gas turbine.
Figure: - P-V & T-S diagram for joules or bray ton cycle.
Processes are represented as follows: -
Process1.2: - Air compressed isentropic ally in compressor from P1 to P2 and
T1 to T2
Process2.3: - Heat is added at constant pressure, rise in temperature from T2 to
T3 (In heating chamber).
Process3.4: - High pressure, high temperature air expands isentropic ally from P3,
T3 to low temperature and pressure T4 and P4 respectively.
Mechanical work is obtained here.
Process4.1: - During this process, air is passed through heat exchanger (cooling
tower).Where it is cooled to its initial condition.
The air standard efficiency is calculated as,
Heat supplied, QS = m CP (T3 – T2)
Heat rejected, QR = m CP (T4 – T1)
Now,
Network done = QS – QR
Efficiency = Network done
Heat supplied
NOTES BY AAFTAB M 9619468502 SUB: PEN 4
Ch.4 GAS TURBINE AND JET PROPULSION
Q.4 GENERAL LAYOUT OF GAS TURBINE POWER PLANT
Layout of a gas turbine plant is given below.
The main components of a simple gas turbine plant are compressor, combustion
chamber, turbine and generator.
The air is compressed in a compressor and the fuel is burned in the combustion
chamber when the compressed air is supplied from the compressor.
The burned high temperature gases are passed through the turbine.
The part of the work developed by the gases passing through the turbine is used
to run the compressor and remaining (30-35%) is used to generate the electrical
energy.
NOTES BY AAFTAB M 9619468502 SUB: PEN 5
Ch.4 GAS TURBINE AND JET PROPULSION
Q.5 GAS TURBINE FUELS AND THEIR PROPERTIES
A gas turbine can use wide variety of liquid and gaseous fuel. Natural gas and
various grades of fuel oils ranging from light petroleum to residual fuels are
regularly used as fuels.
Other fuels less frequently used include crude oils, refinery gas, propane and
synthetic fuels which include liquid and gaseous form.
The desirable properties of gas turbine fuels are
a. High volatility for ease in start-up and higher combustion efficiency
b. High calorific value gives high combustion efficiency.
c. It should be self-lubricating.
d. It should have less suspended impurities.
e. It should have less sulphur level. .
Q.6 OPEN CYCLE GAS TURBINE:-
NOTES BY AAFTAB M 9619468502 SUB: PEN 6
Ch.4 GAS TURBINE AND JET PROPULSION
An open cycle gas turbine is also called a continuous combustion gas turbine
as the combustion of fuel takes place continuously.
An open cycle gas turbine consists of a compressor, combustion chamber
and a gas turbine which drives the generator and compressor.
In this turbine, the air is first sucked from the atmosphere and then
compressed isentropic ally and then passed into combustion chamber.
The compressed air is heated by the combustion of fuel and the products of
combustion also get mixed up with the compressed air.
The hot gas is then made to flow over the turbine blades thus expanding it
and finally exhausting it into the atmosphere.
Q.7ADVANTAGES OF OPEN CYCLE
a) Once the turbine is brought up to the rated speed by the starting motor and
the fuel is ignited, the gas turbine will be accelerated from cold start to full
load without warm-up time.
b) The weight in kg per kW developed is less
c) Almost any hydro-carbon fuel from high octane gasoline to heavy diesel oils
can be used in the combustion chamber.
d) Open cycle plants occupy comparatively little space.
e) Open cycle gas turbine power plant, except those having an intercooler, does
not require cooling water. Therefore, the plant is independent of cooling
medium and becomes self contained.
Q.8 DISADVANTAGE OF OPEN CYCLE
a) The part load efficiency of the open cycle plant decrease rapidly as the
considerable percentage of power developed by the turbine is used to drive
the compressor.
b) The open cycle plant is sensitive to changes in the atmospheric air
NOTES BY AAFTAB M 9619468502 SUB: PEN 7
Ch.4 GAS TURBINE AND JET PROPULSION
temperature, pressure and humidity.
c) The open cycle gas turbine plant has high air rate compared to the other
cycles, therefore it result in increased loss of heat in the exhaust gases and
large diameter duct work is necessary.
d) It is essential that the dust should be prevented from entering into the
compressor in order to minimize erosion and deposition of it on the blades
which in turn reduces compressor efficiency.
e) The deposition of the carbon and ash on the turbine blades is not desirable
as it also reduces the efficiency of the turbine.
Q.9 CLOSED CYCLE GAS TUBINE PLANTS
A closed cycle gas turbine consists of a compressor, heating chamber, gas
turbine and a cooling chamber.
In this turbine, the air is compressed isentropic ally and then passed into the
heating chamber.
The compressed air is heated with the help of some external source and
made to flow over the turbine blades.
The gas, while flowing over the blades, gets, expanded.
From the turbine, the gas is passed to the cooling chamber where it is cooled
at constant pressure with the help of circulating water to its original
temperature.
Now the air is made to flow into the compressor again.
NOTES BY AAFTAB M 9619468502 SUB: PEN 8
Ch.4 GAS TURBINE AND JET PROPULSION
Q.10 ADVANTAGES OF CLOSED CYCLE
a) The machine can be smaller and cheaper than the machine used to develop
the same power using open cycle plant due to increase in back pressure at
the turbine exit.
b) The closed cycle avoids erosion of the turbine blades due to the
contaminated gases and fouling of compressor blades due to dust.
c) The need for filtration of the incoming air which is a severe problem in open
cycle plant is completely eliminated.
d) As indirect heating is used in closed cycle plant, the inferior oil or even solid
fuel can be used in the furnace.
e) The maintenance cost is low and reliability is high due to longer useful life.
f) The thermal efficiency is more for same maximum and minimum
temperature limits as with the open cycle plant due to decreased pressure
NOTES BY AAFTAB M 9619468502 SUB: PEN 9
Ch.4 GAS TURBINE AND JET PROPULSION
ratios.
Q.11 DISADVANTAGES OF CLOSED CYCLE: -
a) The system is dependent on external means as considerable quantity of
cooling water is required in the pre-cooler.
b) Higher internal pressures involve complicated design of all components and
high quality material is required which increases the cost of the plant.
c) The response to the load variations is poor compared to the open cycle plant.
d) It requires very big heat exchangers as the heating of working fluid is done
indirectly which results in larger space for heat exchangers.
NOTES BY AAFTAB M 9619468502 SUB: PEN 10
Ch.4 GAS TURBINE AND JET PROPULSION
Q.12 MODIFIED GAS TURBINE POWER PLANTS: - (IMP)
To improve the efficiency of the gas turbine power plant following features can
be implemented.
i. Gas turbine with inter cooling.
ii. Gas turbine with Reheating.
iii. Gas turbine with Regeneration.
(1) GAS TURBINE WITH INTERCOOLING: -
NOTES BY AAFTAB M 9619468502 SUB: PEN 11
Ch.4 GAS TURBINE AND JET PROPULSION
The major portion of the power developed by the gas turbine is utilized by the
compressor which can be reduced by compressing the air in two stages with an
inter cooler between the two.
This helps in improving the efficiency of the gas turbine.
In this arrangement, the air is first compressed in the low pressure compressor
which increases its pressure and temperature.
Then this air is passed to an intercooler which reduces the temperature of the
compressors to its original temperature at constant pressure.
The compressed air is once again compressed in the high pressure compressor
and then passed through the heating chamber and turbine.
Finally, the air is cooled in the cooling chamber and again passed into low
pressure compressor.
NOTES BY AAFTAB M 9619468502 SUB: PEN 12
Ch.4 GAS TURBINE AND JET PROPULSION
(2) GAS TURBINE WITH REHEATING: -
The output of a gas turbine can be considerably improved by expanding the
hot air in two stages with a reheated between the compressor and turbines.
In this arrangement, the air is first compressed in the compressor, passed
into heating chamber and then to the first turbine.
The air is once again passed on to another heating chamber and then to the
second turbine.
Finally, the air is cooled in the cooling chamber and again passed into the
compressor.
(3) GAS TURBINE WITH REGENERATION: -
NOTES BY AAFTAB M 9619468502 SUB: PEN 13
Ch.4 GAS TURBINE AND JET PROPULSION
In gas-turbine engines, the temperature of the exhaust gas leaving the turbine
is often considerably higher than the temperature of the air leaving the
compressor.
Therefore, the high pressure air leaving the compressor can be heated by
transferring heat to it from the hot exhaust gases in a counter-flow heat
exchanger, which is also known as a regenerator or a recuperate.
Q.13 Jet propulsionWorking principle: -
1. The principle of jet propulsion involves impacting momentum to a
Mass of fluid in such a way that the reaction of imparted momentum provides
A propulsive force.
2. It may be achieved by expanding the gas, which is at high temperature
And pressure, through a nozzle due to which a high velocity jet of hot gases
Is produced that gives a propulsive force (in opposite direction due to it
Reaction) for jet propulsion open cycle gas turbine is most suitable.
Q.14Propulsion system may classified as: -
i. Turbo jet
ii. Turbo prop
iii. Ram jet
Intermittent combustion system: -
Pulse jet or flying bomb.
Self contained rocket engine (non air breathing engine)
i. Liquid propellant
ii. Solid propellant
NOTES BY AAFTAB M 9619468502 SUB: PEN 14
Ch.4 GAS TURBINE AND JET PROPULSION
Q.15 Explain Turbo jet: -
It consist of a diffuser at entrance which slows down the air (entering at
velocity equal to plane speed) and parts of kinetic energy of the air
stream is converted into pressure; this types of compression is called as
ram compression.
The air is further compressed to a pressure of 3 to 4 bar in a rotary
compressor (usually axial flow type)
The compressed air then enters the combustion chambers (c.c) where
fuel is added. The combustion of fuel takes place at sensibly at constant
pressure and subsequently temperature rises rapidly.
The hot gases then enter the gas turbine where partial expansion takes
place. The power produced is just sufficient to drive the compressor, fuel
pump and other auxiliaries.
The exhaust gases from the gas turbine which are at a higher pressure
than atmosphere are expanded in a nozzle and a very high velocity jet is
produced which provides a forward motion to the air craft by jet
reaction, (Newton’s third law of motion)
At higher speed turbo-jet gives higher propulsion efficiency.
The turbo jets are most suited to air-craft travelling above 800km/hr.
NOTES BY AAFTAB M 9619468502 SUB: PEN 15
Ch.4 GAS TURBINE AND JET PROPULSION
Advantages of turbo-jet: -
1. Construction much simpler.
2. Engine vibrations are absent.
3. Much higher speeds possible (more than 3000km/hr).
4. Power supply is uninterrupted and smooth.
5. Weight to power ratios superior.
6. Rate of climber higher.
7. Frontal area smaller.
8. Fuel can be burnt over a larger range of mixture strength.
Disadvantages of turbo-jet: -
1. Less efficient.
2. Life of the unit comparatively shorter.
3. The turbojet becomes rapidly inefficient below 550km/hr.
4. More noisy (than reciprocating engine)
5. Requires longer strip since length of take off is too much.
6. At take off the thrust is low, this effect is overcome by boosting.
__________________________________________________________________
_
NOTES BY AAFTAB M 9619468502 SUB: PEN 16
Ch.4 GAS TURBINE AND JET PROPULSION
Q.15 Explain Turbo-prop: -
Working: -
In turbo prop, the expansion of gases takes place partly in turbine (80%) and
partly in nozzle (20%). The power developed by the turbine is consumed in
running the compressor and the propeller. The propeller and jet produced by the
nozzle give forward motion to the aircraft.
The turbo prop entails the advantages of turbo jet (i.e. low specific weight
and simplicity in design) and propeller (i.e. high power for takeoff and high
propulsion efficiency at speed below 600km/hr)
The overall efficiency of turbo prop is improved by providing the rise takes
place in the diffuser. This pressure rise takes place in the diffuser. This
pressure rise takes place in the diffuser. This pressure rise takes place due
to conversion of kinetic energy of incoming air (equal to air craft velocity)
into pressure energy by the diffuser. This of compression is known as
“Ram effect”.
Application: -
Turbo prop engines are currently used both to power helicopters
and for various land and marine applications.
NOTES BY AAFTAB M 9619468502 SUB: PEN 17
Ch.4 GAS TURBINE AND JET PROPULSION
Q.16 Explain Ram Jet Engine: -
Ram jet is also called athodyd, lorin tube or flying stovepipe. Ram jet
engines have the capability to fly at supersonic speeds.
Ram jet engines consists of a diffuser (used for compression) combustion
chamber and nozzle.
The air enters the ram jet plant with supersonic speed and is slowed down to
sonic velocity in the supersonic diffuser, consequently the pressure suddenly
in the supersonic diffuser to the formation of shock wave. The pressure of
the air is further increased in subsonic diffuser increasing the temperature of
air above the ignition temperature.
In the combustion chamber the fuel is injected through the injection nozzle.
The fuel air mixture is then ignited by means of spark plug and combustion
temperatures of order of 2000k are attained. The expansion of gases towards
the diffuser entrance is restricted by pressure barrier at the end of the
diffuser and as a result the hot gases are constrained to move towards the
nozzle and undergo expansion; the pressure energy is converted into kinetic
energy. This high velocity gases leaving the nozzle provides forward thrust
to the unit.
The performance of ramjet engine is obtained at fight speed of
1700km/hr to 2000km/hr.
NOTES BY AAFTAB M 9619468502 SUB: PEN 18
Ch.4 GAS TURBINE AND JET PROPULSION
Advantages of Ramjet engine: -
1. No moving parts.
2. Light in weight.
3. Wide variety of fuel can be used.
Limitations: -
1. It cannot be started at its own. A ramjet is a ways equipped with a
small turbo jet which starts the ramjet.
2. The fuel consumption is too large at low and moderate speeds.
3. To obtain steady combustion, certain elaborate devices in the form of
flame holders or pilot flame are required.
NOTES BY AAFTAB M 9619468502 SUB: PEN 19
Ch.4 GAS TURBINE AND JET PROPULSION
Q.17 ExplainROCKETS OR ROCKETS ENGINE: -
A rocket engine has its own oxidizer and does not depend on
surrounding air. Hence it can operate at any places even in vacuum. The fuel and
oxidizer are carried into the body of the unit which is to be propelled. The
combination of these two is termed as propellants i.e. (fuel+ oxidizers).
The propellants are of two types.
1. Solids propellants.
2. Liquid propellants.
1) Solid propellants:-
In solid propellant rockets, the fuel and oxidizer are intimately mixed and
formed into a solid grain which is placed in the combustor. Thus there is no
need of separate fuel supply system.
In restricted burning rocket engine, the burning of this propellant is restricted
so that it burns in one direction only. The restricted burning is also called as
cigarette burning, can be accomplished by pouring the charge. The restricted
burning rockets are preferred when the unit is required to deliver a small thrust
for a relative longer duration.
in unrestricted burning, the engine charge is free to burn on all surfaces. At
the same time, such rockets are used where it is desirable to develop large thrust
for a short period. I.e. in launching rockets called booster.
NOTES BY AAFTAB M 9619468502 SUB: PEN 20
Ch.4 GAS TURBINE AND JET PROPULSION
The combustor of solid rocket has to be large enough to store the entire Propellant
and also strong enough to withstand the sufficiently high combustion pressure
again the propellant being solid cannot be used to cool the combustor and nozzle
and there is danger of overheating. As such these rockets are suitable for producing
thrust only for shorter duration. Therefore, they are used extensively for missiles
and air craft booster, guided missiles and rockets projectiles.
LIQUID PROPELLENTS:-
Liquid propellant rockets are liquid propellants which are fed under
pressure from tanks into a thrust chamber. The liquid propellants usually
consist of a liquid oxidizer (oxygen) and liquid fuel (gasoline and alcohol).
Function of the feed system is to force the liquid propellants into the
combustion chamber.
NOTES BY AAFTAB M 9619468502 SUB: PEN 21
Ch.4 GAS TURBINE AND JET PROPULSION
The typical pump-pressurization rotor-motor system is shown in fig.
Construction:- it consist of fuel tank, an oxidizer tank two pumps and steam
turbine, combustion chamber and nozzle.
Working:- the fuel tank contains alcohol and oxidizer are supplied by pumps to
the combustion chamber where the fuel is ignited by means of electrical means.
The pump is driven with the helps steam turbine. Here steam is produced by
mixing a very concentrated hydrogen-peroxide and potassium permanganate. The
products of combustion are discharged from combustion chamber through the
nozzle. The hot combustion product then undergoes adiabatic expansion down to
the atmospheric pressure in nozzle. The reaction to the high velocity jet gives
propulsive force. Quite often the liquid propellant is circulated around the walls of
the combustor and nozzle and it provides the necessary cooling effect.
__________________________________________________________________
_
Q.18 What are Desired characteristics for rocket propellants: -
1. High calorific value.
2. Reliable smooth ignitions.
3. Stability and ease of handling and storing.
4. Low toxicity and corrosiveness.
5. Highest possible density so that it occupies less space.
6. High boiling point at low pressure otherwise it evaporate during
storage.
Q.19 Write Applications of Rockets: -
1. Long range artillery.
2. Lethal weapon.
NOTES BY AAFTAB M 9619468502 SUB: PEN 22
Ch.4 GAS TURBINE AND JET PROPULSION
3. Signaling and firework display.
4. Jet assisted take-off.
5. For satellite.
6. For space ships.
7. Research.
Q.20 Explain Constant Volume Combustion Turbine: -
In a constant volume combustion turbine the compressed air from an
air compressor C is admitted into the combustion chamber D through the value A.
When the value A is closed, the fuel is admitted into the chamber by means of a
spark plug S. The combustion takes place at constant volume with increase of
pressure. The value B opens and the hot gases flow to the turbine T, and finally,
they are discharged, into atmosphere. The energy of the gases is thereby converted
into mechanical energy. For continuous running of the turbine these operations are
repeated.
A,B = Values
C = Compressor
D = Combustion chamber
P = Fuel pump
S = Spark plug
T = Turbine
NOTES BY AAFTAB M 9619468502 SUB: PEN 23
Ch.4 GAS TURBINE AND JET PROPULSION
The main demerit associated with this type of turbine is that the pressure difference
and velocities of hot gases are not constant, so speed of turbine fluctuates.
Q.21 What are various Rocket Fuels: -
(A) Bipropellant fuel: -
various oxidizers and fuel of bipropellants are:
Oxidizers Fuel
1. Liquid oxygen Liquid hydrogen
2. Liquid oxygen Ethyl alcohol
3. Liquid oxygen Gasoline
4. Nitric acid Aniline
(B) Monopropellant fuel: -
A monopropellant as the name indicates is one which is capable of releasing
its chemical energy without the addition of an oxidizer. The monopropellants have
been widely used in solid propellant rockets.
NOTES BY AAFTAB M 9619468502 SUB: PEN 24
Ch.4 GAS TURBINE AND JET PROPULSION
Q.22 Differentiate between
Closed cycle Gas turbine Open cycle Gas turbine
1) The compressed air is heated in a
heating chamber. Since the gas is
heated by an external sources, so the
amount of gas remains the same.
The compressed air is heated in
combustion chamber. The products of
combustion get mixed up in the heated
air.
2) The gas from the turbine is passed
into the cooling chamber.
The gas from the turbine is exhausted
into the atmosphere.
3) The working fluid is circulated
continuously.
The working fluid is replaced
continuously.
4) The fluid with better thermodynamic
Properties can be used.
Only air can be used as the working
fluid.
5) The turbine blades do not wear away
Earlier, as the enclosed gas does not
Contaminated while flowing through
The heating chamber.
The turbine blades wear away earlier,
as the air from the atmosphere gets
contaminated while flowing through
the combustion chamber.
6) Since the air, from the turbine, is
Cooled by circulating water, it is best
Suited for stationary installation or
Marine uses.
Since the air, from the turbine, is
discharged into the atmosphere, it is
best suited for moving vehicle.
7) Its maintenance cost is high. Its maintenance cost is low.
8) The mass of installation per KW is
More.
The mass of installation per KW is less.