Turbofan Engine Report

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TURBOFAN ENGINES ABSTRACT In today’s world it is possible to travel to any part of the world within short span of time using air transport. Earlier aviation industry was not as well developed as today as they were using Rotary Piston IC engines which limited the travel speed and distance; moreover the fuel consumption of these engines was high leading to increased cost of transport. A significant breakthrough in aviation industry took place with the advent of turbojet engines which were Rotary -Reaction Turbine Engines which were much efficient than Rotary piston engines and all other engines such as turbofan, turboprop, and turboshaft engines were developed as improvement over turbojet engines The Turbofan is a type of air breathing jet engine that is very typically employed for aircraft propulsion, that is based around a gas turbine engine. Turbofans provide thrust using a combination of a ducted fan and a jet exhaust nozzle. Part of the airstream from the ducted fan passes through the core, providing oxygen to burn fuel to create power. However, the rest of the air flow bypasses the engine core and mixes with the faster stream from the core, significantly reducing exhaust noise. The substantially slower bypass airflow produces thrust more efficiently than the high-speed air from

Transcript of Turbofan Engine Report

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TURBOFAN ENGINES

ABSTRACT

In today’s world it is possible to travel to any part of the world within short span of time using air transport. Earlier aviation industry was not as well developed as today as they were using Rotary Piston IC engines which limited the travel speed and distance; moreover the fuel consumption of these engines was high leading to increased cost of transport. A significant breakthrough in aviation industry took place with the advent of turbojet engines which were Rotary -Reaction Turbine Engines which were much efficient than Rotary piston engines and all other engines such as turbofan, turboprop, and turboshaft engines were developed as improvement over turbojet engines

The Turbofan is a type of air breathing jet engine that is very typically employed for aircraft propulsion, that is based around a gas turbine engine. Turbofans provide thrust using a combination of a ducted fan and a jet exhaust nozzle. Part of the airstream from the ducted fan passes through the core, providing oxygen to burn fuel to create power. However, the rest of the air flow bypasses the engine core and mixes with the faster stream from the core, significantly reducing exhaust noise. The substantially slower bypass airflow produces thrust more efficiently than the high-speed air from the core, and this reduces the specific fuel consumption.

A few designs work slightly differently, having the fan blades as a radial extension of an aft-mounted low-pressure turbine unit.

Turbofans have a net exhaust speed that is much lower than a turbojet. This makes them much more efficient at subsonic speeds than turbojets, and somewhat more efficient at supersonic speeds up to roughly Mach 1.6, but have also been found to be efficient when used with continuous afterburner at Mach 3 and above. However, the lower exhaust speed also reduces thrust at high vehicle speeds.

All of the jet engines used in currently manufactured commercial jet aircraft are turbofans. They are used commercially mainly because they are more efficient and

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quieter in operation than turbojets. Turbofans are also used in many military jet aircraft, such as the F-15 Eagle and in unmanned aerial vehicles such as the RQ-4 Global Hawk.

INTRODUCTION

Jet Propulsion is the thrust imparting forward motion to an object, as a reaction to the

rearward expulsion of a high-velocity liquid or gaseous stream.

A simple example of jet propulsion is the motion of an inflated balloon when the air is

suddenly discharged. While the opening is held closed, the air pressure within the balloon is

equal in all directions; when the stem is released, the internal pressure is less at the open end than

at the opposite end, causing the balloon to dart forward. Not the pressure of the escaping air

pushing against the outside atmosphere but the difference between high and low pressures inside

the balloon propels it.

An actual jet engine does not operate quite as simply as a balloon, although the basic

principle is the same. More important than pressure imbalance is the acceleration due to high

velocities of the jet leaving the engine. This is achieved by forces in the engine that enable the

gas to flow backward forming the jet. Newton's second law shows that these forces are

proportional to the rate at which the momentum of the gas is increased. For a jet engine, this is

related to the rate of mass flow multiplied by the rearward-leaving jet velocity. Newton's third

law, which states that every force must have an equal and opposite reaction, shows that the

rearward force is balanced by a forward reaction, known as thrust. This thrusting action is similar

to the recoil of a gun, which increases as both the mass of the projectile and its muzzle velocity

are increased. High-thrust engines, therefore, require both large rates of mass flow and high jet-

exit velocities, which can only be achieved by increasing internal engine pressures and by

increasing the volume of the gas by means of combustion.

Jet-propulsion devices are used primarily in high-speed, high-altitude aircraft, in missiles,

and in spacecraft. The source of power is a high-energy fuel that is burned at intense pressures to

produce the large gas volume needed for high jet-exit velocities. The oxidizer required for the

combustion may be the oxygen in the air that is drawn into the engine and compressed, or the

oxidizer may be carried in the vehicle, so that the engine is independent of a surrounding

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atmosphere. Engines that depend on the atmosphere for oxygen include turbojets, turbofans,

turboprops, ramjets, and pulse jets. Non atmospheric engines are usually called rocket engines.

HISTORY

Jet power as a form of propulsion has been known for hundreds of years, although its use

for propelling vehicles that carry loads is comparatively recent. The earliest known reaction

engine was an experimental, steam-operated device developed about the first century B.C. by the

Greek mathematician and scientist Hero of Alexandria. Known as the Aeolipile, Hero's device

did no practical work, although it demonstrated that a jet of steam escaping to the rear drives its

generator forward. The aeolipile consisted of a spherical chamber into which steam was fed

through hollow supports. The steam was allowed to escape from two bent tubes on opposite sides

of the sphere, and the reaction to the force of the escaping steam caused the sphere to rotate.

The development (1629) of the steam turbine is credited to the Italian engineer Giovanni

Branca, who directed a steam jet against a turbine wheel, which in turn powered a stamp mill.

The first recorded patent for a gas turbine was obtained in 1791 by the British inventor John

Barber.

In 1910, seven years after the first flights by the American inventors Orville and Wilbur

Wright, the French scientist Henri Marie Coanda designed and built a jet-propelled biplane,

which took off and flew under its own power with Coanda as pilot. Coanda used an engine that

he termed a reaction motor, but, discouraged by the lack of public acceptance of his aircraft, he

abandoned his experiments.

During the next 20 years the gas turbine was developed further in both the United States

and Europe. One result of the experimental work of that period was the perfection in 1918 of a

turbo supercharger driver by an exhaust gas turbine for conventional aircraft engines. In the early

1930s many patents covering gas turbines were awarded to a number of European engineers. The

patent granted the British aeronautical engineer Sir Frank Whittle in 1930 is generally conceded

to have outlined the first practical form of the modern gas turbine. In 1935 Whittle applied his

basic design to the development of the W-1 turbojet engine, which made its first flight in 1941.

Meanwhile, the French aeronautical engineer René Leduc had exhibited (1938) a model

of the ramjet in Paris, and a jet airplane that was powered by an axial-flow turbojet designed by

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the German engineer Hans Joachim Pabst von Ohain made its first flight in 1939. In the

following year, under the direction of the aeronautical engineer Secundo Campini, the Italians

developed an airplane powered by a turboprop engine with a reciprocating-engine-driven

compressor. The first American-built jet airplane, the Bell XP-59, was powered by the General

Electric 1-16 turbojet, adapted from Whittle's design in 1942. The first jet engine of exclusively

American design was produced by Westinghouse Electric Corp. for the U.S. Navy in 1944.

From a principle first described in 1906, the pulse jet was developed by the German

engineer Paul Schmidt, who received his first patent in 1931. The V-1, or buzz bomb, first flown

in 1942, was powered by pulse jet. Also in the mid-1940s the first commercial airline flights

using turboprop engines occurred. In 1947 the Bell X-1 experimental airplane, powered by a

four-chambered liquid-rocket engine and carried to the stratosphere in the belly of a bomber for

launching, was the first pilot-operated craft to break the sound barrier. Subsequently the Douglas

Skyrocket experimental airplane, powered by a jet engine in addition to a liquid-rocket engine,

broke the sound barrier at low altitude after taking off under its own power.

The first commercial jet airplane, the British Comet, was flown in 1952, but this service

was stopped after two serious accidents in 1954. In the U.S., the Boeing 707 jet was the first jet

airplane to be tested commercially, in 1954. Commercial flights began in 1958.

The continuous development of jet propulsion for air power has resulted in such advances

as piloted aircraft capable of attaining speeds several times greater than the speed of sound, and

intercontinental ballistic missiles and artificial satellites launched by powerful rockets.

What is propulsion?

The word is derived from two Latin words: pro meaning before or forwards and pellere

means to drive. Propulsion means to push forward or drive an object forward. A propulsion

system is a machine that produces thrust to push an object forward. On airplanes, thrust is

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usually generated through some application of Newton's third law of action and reaction. The

engine accelerates a gas or working fluid, and the reaction to this acceleration produces a force

on the engine.

A general derivation of the thrust equation shows that the amount of thrust generated

depends on the mass flow through the engine and the exit velocity of the gas. Different

propulsion systems generate thrust in slightly different ways.

THEORY

What is a Turbofan Engine?

A turbofan engine is the most modern variation of the basic gas turbine engine. As with

other gas turbines, there is a core engine. In the turbofan engine, a fan in the front and an

additional turbine at the rear surrounds the core engine. The fan and fan turbine are composed of

many blades, like the core compressor and core turbine, and are connected to an additional shaft.

All of this additional turbo machinery is colored green on the schematic diagram as shown in Fig

1 below.

Fig 1:- Schematic diagram of turbofan engine

As with the core compressor and turbine, some of the fan blades turn with the shaft and

some blades remain stationary. The fan shaft passes through the core shaft for mechanical

reasons. This type of arrangement is called a two-spool engine (one "spool" for the fan, one

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"spool" for the core.) Some advanced engines have additional spools for sections of the

compressor, which provides for even higher compressor efficiency.

Jet Engine Thrust

The force produced by a jet engine is expressed in terms of kilograms of thrust. This is a

measure of the mass or weight of air moved by an engine times the acceleration of the air as it

goes through the engine. Technically, if the aircraft were to stand still and the pressure at the exit

plane of the jet engine was the same as the atmospheric pressure, the formula for the jet engine

thrust would be:

Weight of air in kilograms per second * velocity Thrust = ___________________________________________ 9.81 (normal acceleration due to gravity, in meter per second 2)

Imagine an aircraft standing still, capable of handling 97.522 kilograms of air per second.

Assume the velocity of the exhaust gases to be 1,500 feet per second. The thrust would then be:

Thrust = 97.522 kg of air per second * 457.2 m / s 9.81 m / s 2

= 9.941 * 457.2

Thrust = 4545.025 kg.

If the pressure at the exit plane is not the same as the atmospheric pressure and the

aircraft were not standing still, the formula would be somewhat different.

It is not very practical to try to compare jet engine output in terms of horsepower. As a

rule of thumb, however, it may be noted that that at 375 miles per hour (mph), one pound of

thrust equals one horsepower, at 750 mph one pound of thrust equals two horsepower.

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Thrust Equation for Turbojet-Type Engines

The thrust equation for a turbojet can be derived from the general form of Newton's second law

(i.e., force equals the time rate of change of momentum),

f = d (MV) / dt.

The nozzle of the turbojet is usually designed to take the exhaust pressure back to free

stream pressure. The thrust equation for a turbojet is then given by the general thrust equation

with the pressure-area term set to zero. If the free stream conditions are denoted by a "0"

subscript and the exit conditions by an "e" subscript, the thrust F is equal to the mass flow rate m

times the velocity V at the exit minus the free stream mass flow rate times the velocity.

F = [m * V]e - [m * V]0

This equation contains two terms. Aerodynamicists often refer to the first term m as the

Gross Thrust since this term is largely associated with conditions in the nozzle. The second term

m is called the ram drag and is usually associated with conditions in the inlet. For clarity, the

engine thrust is then called the net thrust. Our thrust equation indicates that net thrust equals

gross thrust minus ram drag. If we divide both sides of the equation by the mass flow rate, we

obtain and efficiency parameter called the specific thrust that greatly simplifies the performance

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PARTS OF A TURBOFAN ENGINE

The different parts of a Turbofan engine are as shown in Fig 10 below:-

Fig 10:- Parts of a Turbofan Engine

Fan - The fan is the first component in a turbofan. The fan pulls air into the engine. The large

spinning fan sucks in large quantities of air. It then, speeds the air up and splits it into two parts. One

part continues through the "core" or center of the engine, where it is acted upon by the other engine

components. The second part "bypasses" the core of the engine, instead traveling through a duct that

surrounds the core to the back of the engine where it produces much of the force that propels the

airplane forward.

Compressor - The compressor is the first component in the engine core. The compressor

squeezes the air that enters it into smaller areas, resulting in an increase in the air pressure. This results

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in an increase in the energy potential of the air. The squashed air is forced into the combustion

chamber.

Combustor - In the combustor the air is mixed with fuel and then ignited. This process results in

high temperature, high energy airflow. The fuel burns with the oxygen in the compressed air, producing

hot expanding gases.

Turbine - The high energy airflow coming out of the combustor goes into the turbine, causing

the turbine blades to rotate. This rotation extracts some energy from the high-energy flow that is used

to drive the fan and the compressor. The gases produced in the combustion chamber move through the

turbine and spin its blades. The task of a turbine is to convert gas energy into mechanical work to drive

the compressor.

Nozzle - The nozzle is the exhaust duct of the engine. The energy depleted airflow that passed

the turbine, in addition to the colder air that bypassed the engine core, produces a force when exiting

the nozzle that acts to propel the engine, and therefore the airplane, forward. The combination of the

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hot air and the cold air are expelled and produce an exhaust which causes a forward thrust. The nozzle

may be preceded by a mixer, which combines the high temperature air coming from the engine core

with the lower temperature air that was bypassed in the fan. This results in a quieter engine than if the

mixer was not present.

Afterburner - In addition to the basic components of a gas turbine engine, one other

process is occasionally employed to increase the thrust of a given engine. Afterburning (or

reheat) is a method of augmenting the basic thrust of an engine to improve the aircraft takeoff,

climb and (for military aircraft) combat performance. Afterburning consists of the introduction

and burning of raw fuel between the engine turbine and the jet pipe propelling nozzle, utilizing

the unburned oxygen in the exhaust gas to support combustion. The increase in the temperature

of the exhaust gas increases the velocity of the jet leaving the propelling nozzle and therefore

increases the engine thrust. This increased thrust could be obtained by the use of a larger engine,

but this would increase the weigh and overall fuel consumption. In other words Afterburner is a

device for increasing the thrust (forward-directed force) of a gas-turbine (jet) airplane engine.

Additional fuel is sprayed into the hot exhaust duct between the turbojet (engine) and the

tailpipe. The fuel ignites, providing a burst of speed. Afterburning is used for a short increase of

power during takeoff, or during combat in military aircraft.

WORKING PRINCIPLE

How does a turbofan engine work?

The engine inlet captures the incoming air. Some of the incoming air passes through the fan and

continues on into the core compressor and then the burner, where it is mixed with fuel and combustion

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occurs. The hot exhaust passes through the core and fan turbines and then out the nozzle, as in a basic

turbojet. This airflow is called the core airflow and is denoted by m . The rest of the incoming air,

colored blue on the figure, passes through the fan and bypasses, or goes around the engine, just like the

air through a propeller. The air that goes through the fan has a velocity that is slightly increased from

free stream. This airflow is called the fanflow, or bypass flow, and is denoted by m . The ratio of m to

m is called the bypass ratio. So a turbofan gets some of its thrust from the core and some of its thrust

from the fan. The ratio of the air that goes around the engine to the air that goes through the core is

called the bypass ratio.

Fig 4:- Thrust of a Turbofan engine

The total mass flow rate through the inlet is the sum of the core and fan flows

m = m + m

A turbofan gets some of its thrust from the core and some of its thrust from the fan. If we

denote the exit of the core as station "e", the exit of the fan as station "f", and the free stream as station

"0", we can use the basic thrust equation for each stream to obtain the total thrust:

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F = m - m * V0 + (m * V)e - m * V0

We can combine the terms multiplying V0 and use the definition of the bypass ratio bpr to

obtain the final thrust equation:

F = (m * V)e + bpr * m * Vf - (m * V)0

Because the fuel flow rate for the core is changed only a small amount by the addition of

the fan, a turbofan generates more thrust for nearly the same amount of fuel used by the core.

This means that a turbofan is very fuel efficient. In fact, high bypass ratio turbofans are nearly as

fuel efficient as turboprops. Because the fan is enclosed by the inlet and is composed of many

blades, it can operate efficiently at higher speeds than a simple propeller. That is why turbofans

are found on high speed transports and propellers are used on low speed transports. Low bypass

ratio turbofans are still more fuel efficient than basic turbojets. Many modern fighter planes

actually use low bypass ratio turbofans equipped with afterburners. They can then cruise

efficiently but still have high thrust when dog fighting. Even though the fighter plane can fly

much faster than the speed of sound, the air going into the engine must travel less than the speed

of sound for high efficiency. Therefore, the airplane inlet slows the air down from supersonic

speeds.

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Fig 5:- ROLLS-ROYCE TAY TURBOFAN ENGINE

As an example for the turbofan engine consider the Rolls-Royce Tay turbofan engine as

shown in the Fig 5.This Rolls-Royce Tay turbofan engine pushes nearly three times as much air

through the bypass ducts as it pushes through the central core of the engine, where the air is

compressed, mixed with fuel, and ignited. Turbofan engines like the Rolls-Royce Tay are not as

powerful as turbojets, but they are quieter and more efficient.

The turbofan engine is an improvement on the basic turbojet. Part of the incoming air is

only partially compressed and then bypassed in an outer shell beyond the turbine. This air is then

mixed with the hot turbine-exhaust gases before they reach the nozzle. A bypass engine has

greater thrust for takeoff and climb, and increased efficiency; the bypass cools the engine and

reduces noise level.

In some fan engines the bypass air is not remixed in the engine but exhausted directly. In

this type of bypass engine, only about one-sixth of the incoming air goes through the whole

engine; the remaining five-sixths is compressed only in the first compressor or fan stage and then

exhausted. Different rotational speeds are required for the high- and low-pressure portions of the

engine. This difference is achieved by having two separate turbine-compressor combinations

running on two concentric shafts or twin spools. Two high-pressure turbine stages drive the 11

high-pressure compressor stages mounted on the outer shaft, and 4 turbine stages provide power

for the fan and 4 low-pressure compressor stages on the inner shaft. To move an airplane through

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the air, thrust is generated by some kind of propulsion system. Most modern airliners use

turbofan engines because of their high thrust and good fuel efficiency.

An example of an engine of this type is the JT9D-3 jet engine, which weighs about 3850

kg (about 8470 lb) and can develop a takeoff thrust of about 20,000 kg (about 44,000 lb). This is

more than double the thrust available for the largest commercial planes before the Boeing 747.

WORKING STAGES OF THE TURBOFAN ENGINE

1.

2.

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3.

4.

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5.

6.

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7.

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8.

9.

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TYPES OF JET ENGINES

Fig 7:- JET ENGINES

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The three most common types of jet engines are the turbojet, turboprop, and turbofan. Air

entering a turbojet engine is compressed and passed into a combustion chamber to be oxidized.

Energy produced by the burning fuel spins the turbine that drives the compressor, creating an

effective power cycle. Turboprop engines are driven almost entirely by a propeller mounted in

front of the engine, deriving only 10 percent of their thrust from the exhaust jet. Turbofans

combine the hot air jet with bypassed air from a fan, also driven by the turbine. The use of

bypass air creates a quieter engine with greater boost at low speeds, making it a popular choice

for commercial airplanes.

REFERENCES

1. http://www.freepatentsonline.com/3508403 2. Marshall Brain. "How Gas Turbine Engines Work". howstuffworks.com.

Retrieved 2010-11-24.3. "Turbofan Engine". www. grc.nasa.gov. Retrieved 2010-11-24.4. Neumann, Gerhard (2004) [1984], Herman the German: Just Lucky I Guess,

Bloomington, IN, USA: Authorhouse, ISBN 1-4184-7925-X. First published by Morrow in 1984 as Herman the German: Enemy Alien U.S. Army Master Sergeant. Republished with a new title in 2004 by Authorhouse, with minor or no changes., pp. 228–230.

5. C. Riegler, C. Bichlmaier: "The geared turbofan technology - Opportunities, challenges and readiness status", 1st CEAS European Air and Space Conference, 10–13 September 2007, Berlin, Germany

6. Actually a Bristol engine design taken on by Rolls-Royce when they took over Bristol

7. PS-90A turbofan, Aviadvigatel, 2011-01-178. Turbofan Engine Family for Regional Jet, Aviadvigatel,2011-01-17

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9. http://science.howstuffworks.com/transport/flight/modern/turbine.htm