Jet Engine Definition

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JET ENGINES

Thermodynamics at its Finest

Dan Aglione

March 17, 2011

English 202C

Section 032


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AUDIENCE AND SCOPE

This document addresses high school students and beginner science undergraduates who have an

interest in the aviation industry. More specifically, jet engines are an enormous engineering field

in both military and commercial research. An overview of an engines components and

functions should help students decide if aerospace is a favorable major selection. Because these

students do not have in depth engineering experience, this description will provide basic

background information with enough detail to instill engine principles and understanding. This

should be sufficient for students to get an overview of engine study and properly decide if they

wish to study it further.

INTRODUCTION

Jet engines were an incredible invention of the 20 th century. They truly revolutionized the skies

in numerous ways. Jet engines made air travel one of the fastest and safest forms of

transportation known today. The military advancements enabled jet fighters to reach altitudes

and speeds never before possible by preexisting propeller technology.

Although these engines are exceedingly intricate and have gone through thousands of

improvements over the years, their basic thermodynamic principles are rather straightforward. Ajet engine is a device that converts airflow into forward thrust. It is this thrust that allows the

aircraft to reach high enough speeds needed for flight. This is done via several processes inside

the engine that change the incoming airs temperature, pressure, and velocity into useful kinetic

(or motion) energy.

Before the process of a jet engine can be examined, it is first essential to mention some common

variations of engines. The earliest engine produced was tagged the turbojet engine. This design

was defined for combusting all of the intake air, however, was later found to be fuel inefficient

and noisy. Turboprop engines then came about where a signature propeller was placed in front

of the air intake. Finally, a newer approach was developed called the turbofan engine. Theseengines have an inlet fan that pulls some air into combustion, while the rest of the air bypasses

the engine core entirely. This design is the most common today and will be the type discussed in

detail in the following document.


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THRUST

A jet engines primary purpose is to generate thrust. Forward thrust is essentially the force

exhibited by an engine due to a high velocity jet of air being pushed backwards. In accordance

with Newtons Third Law of Motion that states every action has an equal and opposite reaction,

the amount of thrust an engine creates is equal to the force of the exiting air stream.

The relationship between thrust and exit air speed is given by this equation:

T - thrust

dm/dt - mass flow rate of the air (mass of air entering the engine per time)v - velocity of exiting jet of air relative to the atmospheric air speed

This relationship implies that in order to achieve a lot of thrust, both the mass of the air as wellas its exiting velocity are the key factors. With that being said, jet engine design revolves around

these two items. Engine components work together to capture large amounts of air and

accelerate it as fast as possible. How this happens is described in detail in the next section.

INSIDE A JET ENGINE

As mentioned before, the processes included in a jet engine all function to increase the exit speed

of a large mass of air. Turbofans contain huge intakes to capture as much air mass as possible at

the entrance. The interior of the engine focuses on inducing the vigorous exit speeds by varying

the airs pressure, temperature, and volume along its path.

Although not all turbofan engines are exactly identical, they all work on the same principles and

share the same major parts. There are six (6) primary components inside of a turbofan engine,

all displayed in figure 1.

They are as follows:


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1. Intake2. Inlet Fan3.

Compressor

4. Combustion Chamber5. Turbine6. Exhaust Nozzle

1. Intake

The air intake is the first component that contacts the atmospheric air

during flight. The intake (illustrated in figure 2) is the front of the

engine housing that acts as a diffuser. A diffuser is simply a device

that has a gradually increasing cross-sectional area. The jet of air

experiences a higher pressure and a lower velocity as an effect of

this geometry due to basic conservation of energy principles. Oncethe air passes through the intake, it is traveling slower than when it

entered and at a pressure higher than atmospheric.

2. Inlet Fan

Next, the air contacts an inlet fan placed directly behind the intake duct which rotates about a

central shaft connected to the turbine (see fig. 3). The spinning fan pulls in the incoming airand accelerates it backwards. Some of this air continues on to the compressor (primary air

stream). The rest bypasses the compressor and escapes to the back of the engine (secondary

air stream). Figure 4 displays the difference between the two stream lines. Air captured in

red denotes the primary air going to the compressor whereas air captured in blue represents

the bypassing secondary stream line.

Figure 1: Turbofan Schematic

Figure 2: Air Intake


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The amount of air going to each location specifies a bypass ratio. The bypass ratio relates

how much of the total inlet air is sent around the compressor as opposed to going through it.

Military aircraft have smaller ratios (about 2:1) which yield tremendous power. On the other

hand, commercial aircraft have a typical bypass ratio of 5:1 which is more fuel efficient and

reduces noise levels. For subsonic aircraft (flying below the speed of sound), it is both

uneconomical and impractical to compress 100% of the air.

3. Compressor

The compressor is responsible for greatly raising the pressure and temperature of the air

jet. The compressor contains many blades, called airfoils, which are connected to the

turbine shaft. Each airfoil is comprised of a pair of rotating blades (rotors) and non-

rotating blades (stators). The rotors accelerate the air into the stators, where the collisions

squeeze the air into gradually smaller and smaller spaces.

A compressor has many pairs of rotors and stators that, when compiled one after the

other, are effective at increasing the airs pressure. Air can exit the compressor forty

times denser than when outside of the engine. At this stage, the velocity of the air isalmost negligible.

Figure 5 (shown on next page) features a compressor with labeled rotors and stators.

Notice the directions of the blades.

Figure 3: Inlet FanFigure 4: Primary/Secondary

Air Streams

Secondary Air Stream

(Bypassed Air)Primary Air Stream

(To Compressor)


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4. Combustion Chamber

The air enters a combustion chamber once it has been

sufficiently compressed. At this point, the air is very dense

and traveling at slow enough speeds to allow for

combustion. Inside the chamber, jet fuel pumped from thewings is continuously sprayed to mix with the air. The

air/fuel mixture ignites via a flame (see fig. 6), thus

escalating the airs temperature and pressure significantly.

The slow moving air prevents from blowing out the flame.

Some of the bypass air enters the chamber through tiny

holes to both insulate and cool the reaction. Jet fuel burns

at very high temperatures, so the secondary air stream is

important to keep combustion in safe conditions. If the air

is allowed to get too hot, it can severely damage the

chamber and subsequent turbine blades.

Figure 5: Axial Compressor

Figure 6: Combustion

Chamber

Stators

Rotors


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5. Turbine

The high-energy combusted air rushes out of the chamber and into the turbine. The

turbine is a vital component in a jet engine. Its purpose is to supply the shaft power thatruns the inlet fan and compressor. A turbine is basically an axial compressor in reverse.

It is a series of blades that spin

around a central shaft as shown to

the right in figure 7. The difference

is that compressor blades move the

air, whereas turbine blades are

moved by the air.

The turbine uses the energy from

the combustion process to apply

work to the shaft. The hot,

pressurized gas from the

combustion chamber enters the turbine and expands intensely. The air pushes itself

through the turbine blades in order to cool down and reduce its pressure. This forcing

action turns the turbine blades at an extremely high rate. The turbine then spins its

central axis shaft which simultaneously spins the compressor and fan blades.

Powering the compressor and fan is the only reason to have a turbine in the jet engine.

The air of course loses energy by contacting all of the blades, reducing its velocity in theprocess, but currently this is the best known technology.

6. Exhaust Nozzle

Before exiting the engine and providing thrust, the air passes

through a nozzle. The exhaust nozzle acts similar to a

diffuser in that it does not provide any energy. Its

functionality rests only in the geometry. The nozzle has a

continuously decreasing cross-sectional area that accelerates

the fluid. This shape can be seen on the nozzles of an F-18

displayed in figure 8. Again, by conservation of energy

Figure 7: Turbine Blades

Figure 8: F-18 Hornet Exhaust Nozzles


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principles, decreasing the area will automatically lower the pressure and increase the

velocity.

Nozzles are an easy, inexpensive way of propelling the air without providing additional

energy. This concept holds true for any fluid and can be demonstrated using a garden

hose by plugging up some of the spout with your thumb. The water flow rate remains the

same but its spraying speed goes up.

CONCLUSION

Jet engines are undoubtedly an inspiring engineering marvel. Their fundamentals are simple yet

their applications are endless. It is clear that advanced feats can be accomplished with

elementary understanding of thermodynamics.

In a jet engine, immense thrust is generated by compressing, burning, and expanding airflow.

Incoming air is initially pressurized by the intake. Some of this air is accelerated to the

compressor via the inlet fan while the other air is blown around. The compressor squeezes the

air together, increasing its pressure and reducing its speed. Continuous combustion occurs in the

combustion chamber by mixing the pressurized airflow with jet fuel. This superheated air bursts

through the turbine blades and out the exhaust nozzle, thus producing a large forward thrust.

Figure 9 depicts a summarizing diagram below.

Like most processes, jet engines have interdependently linked components. The turbine, for

example, powers the inlet fan and compressor. The compressor slows the airflow to allow

successful combustion. As in all great design, the true ingenuity does not lie in mixing together

a whole bunch of parts, but in effectively relating each element in a synchronized, cohesive

manner.

Figure 9: Airflow Diagram


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Works Cited

Brain, Marshall. "How Gas Turbine Engines Work."How Stuff Works. N.p., 1 Apr. 2000. Web.

12 Mar. 2011. .

Heppenheimer, T.A. "Jet Engines." US Centennial of Flight Commission. N.p., n.d. Web. 12

Mar. 2011.

.

"Jet Engine." Solar Navigator. N.p., 2006. Web. 11 Mar. 2011.

.

Journey Through a Jet Engine. Rolls Royce. Web. 12 Mar. 2011..

"Turbofan Engine." Ed. Tom Benson. NASA, n.d. Web. 11 Mar. 2011.

.
http://science.howstuffworks.com/transport/flight/modern/turbine.htmhttp://www.centennialofflight.gov/essay/Evolution_of_Technology/jet_engines/Tech24.htmhttp://www.centennialofflight.gov/essay/Evolution_of_Technology/jet_engines/Tech24.htmhttp://www.solarnavigator.net/aviation_and_space_travel/jet_engine.htmhttp://www.rolls-royce.com/interactive_games/journey03/index.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/aturbf.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/aturbf.htmlhttp://www.rolls-royce.com/interactive_games/journey03/index.htmlhttp://www.solarnavigator.net/aviation_and_space_travel/jet_engine.htmhttp://www.centennialofflight.gov/essay/Evolution_of_Technology/jet_engines/Tech24.htmhttp://www.centennialofflight.gov/essay/Evolution_of_Technology/jet_engines/Tech24.htmhttp://science.howstuffworks.com/transport/flight/modern/turbine.htm

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Jet Engine Definition