ch.4 gas turbine and jet propulsionnotes by aaftab m 9619468502 sub: pen 3 ch.4 gas turbine and jet...

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



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.



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



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.



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:-



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



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.



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



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.



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: -



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.



(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: -



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



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.



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.

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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.



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.



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.



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.



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.



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.

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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.



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



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.



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.

ch.4 gas turbine and jet propulsionnotes by aaftab m 9619468502 sub: pen 3 ch.4 gas turbine and jet...

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