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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 project and implimentationile 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 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.
Turbofan engine report.doc (Size: 827.5 KB / Downloads: 568)
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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 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 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.
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 "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.
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
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 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 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.
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 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:
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.
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 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
TYPES OF JET ENGINES
Fig 7:- JET ENGINES
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.
Why are there different types of engines?
If we think about Newton's first law of motion, we realize that an airplane propulsion system must serve two purposes. First, the thrust from the propulsion system must balance the drag of the airplane when the airplane is cruising. And second, the thrust from the propulsion system must exceed the drag of the airplane for the airplane to accelerate. In fact, the greater the difference between the thrust and the drag, called the excess thrust, the faster the airplane will accelerate.
Some aircraft, like airliners and cargo planes, spend most of their life in a cruise condition. For these airplanes, excess thrust is not as important as high engine efficiency and low fuel usage. Since thrust depends on both the amount of gas moved and the velocity, we can generate high thrust by accelerating a large mass of gas by a small amount, or by accelerating a small mass of gas by a large amount. Because of the aerodynamic efficiency of propellers and fans, it is more fuel efficient to accelerate a large mass by a small amount. That is why we find high bypass fans and turboprops on cargo planes and airliners.
Some aircraft, like fighter planes or experimental high speed aircraft require very high excess thrust to accelerate quickly and to overcome the high drag associated with high speeds. For these airplanes, engine efficiency is not as important as very high thrust. Military aircraft typically employ afterburning turbojets. Future hypersonic aircraft will employ some type of ramjet or rocket propulsion.
Why are Turbofan Engines so popular?
The turbofan engine has gained popularity for a variety of reasons. As shown in Fig 6 below, one or more rows of compressor blades extend beyond the normal compressor blades. The result is that four times as much air is pulled into the turbofan engine as in the simple turbojet. However, most of this excess air is ducted through bypasses around the power section and out the rear with the exhaust gases. Also, a fan burner permits the burning of additional fuel in the fan air stream. With the burner off, this engine can operate economically and efficiently at low altitudes and low speeds. With the burner on, the thrust is doubled by the burning fuel, and it can operate on high speeds and high altitudes fairly efficiently. The turbofan has greater thrust for takeoff, climbing, and cruising on the same amount of fuel than the conventional turbojet engine.
With better all-around performance at a lower rate of fuel consumption, plus less noise resulting from its operation, it is easy to understand why most new jet-powered airplanes are fitted with turbofan engines. This includes military and civilian types.
Fig 6: - Turbofan Engine
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seminar.ppt (Size: 3.41 MB / Downloads: 517)
INTRODUCTION TO JET ENGINES
WORKING PRINCIPLE AND COMPONENTS OF TURBOFANS
TYPES OF TURBOFAN ENGINES
EQUATIONS OF NET THRUST
CYCLE FOR TURBOFAN ENGINES
MERITS AND DEMERITS OF TURBOFAN ENGINES
What is the difference between a turbo jet and turbo fan engine?
A turbojet is one of the oldest kinds of Jet engine designs. The air flow enters the jet engine at one end and is compressed while it travels through rows of rotating blades (or stages).
A turbofan is a air breathing ,turbine powered engine is the most widely used engine on modern aircraft today. It uses a fan or a series of fans to compress the air.
There are a large number of different types of Turbofan Engines, all of which achieve forward thrust from the principle of jet propulsion.
“Jet propulsion is motion produced by passing a jet of fluid (e.g. air or water) in the opposite direction to the direction of motion”.
Jet Propulsion is the thrust imparting forward motion to an object.
From Newton’s second law
By conservation of momentum, the moving body is propelled in the opposite direction to the jet.
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. F= m dv/dt. For a Turbofan 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 which is a forward motion to an object .
This thrusting action is similar to the recoil of a gun,
The air with high pressure enter into the inlet of engine.
The fan or the propeller is the first component in a turbofan. 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.
The compressor is the main component in the engine core, driven by turbine .The compressor squeezes the air that enters it into smaller areas, resulting in an increase in the air pressure in 13 stages. This results in an increase in the energy potential of the air. The squashed air is forced into the combustion.
The compressor rotates at very high speed, Compressing the air increases its pressure and temperature.
In the combustor the air reaches is mixed with fuel injected and ignited. The fuel burns with the oxygen in the compressed air, producing hot expanding gases.This process results in reaching a high temperature of , high energy airflow.
The high energy airflow coming out of the combustor goes into the turbine and gas expands, 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.
The nozzle is the exhaust duct of the engine. The energy 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.
Turbofan engines come in two varieties: high bypass and low bypass. With high bypass turbofans, majority of the total engine thrust (as much as 80%) is produced by the bypass air.
With low bypass turbofans, majority of the thrust is produced by the exhaust gases.
The low specific thrust/high bypass ratio turbofans used in today's civil jetliners (and some military transport aircraft) which is evolved from the high specific thrust/low bypass ratio turbofans used in such [production] aircraft back in the 1960s.
Advantages of high bypass than low bypass turbofan engines
Low specific thrust is achieved by replacing the multi-stage fan with a single stage unit.
High-bypass turbofan engines are generally quieter than the earlier low bypass ratio civil engines. This is not so much due to the higher bypass ratio as to the use of a low pressure ratio, single stage fan which significantly reduces specific thrust and, thereby, jet velocity.
The combination of a higher overall pressure ratio and turbine inlet temperature improves thermal efficiency. This, together with a lower specific thrust (better propulsive efficiency), leads to a lower specific fuel consumption.
Thermodynamics of a jet engine are modeled approximately by a Brayton Cycle. The Brayton cycle is a thermodynamic cycle that describes the workings of the gas turbine engine, basis of the Turbofan engine and others.
The air entering from atmosphere is diffused isentropically from velocity C1 down to zero (i.e., C2 = 0). This indicates that the diffuser has an efficiency of 100%, this is termed as ram compression.
Process 1-2’ is the actual process.
Isentropic compression of air.
Process 2’-3’ shows the actual compression of air.
Ideal addition of heat at constant pressure p3=p4.
Process 3’-4 shows the actual addition of heat at constant process p3=p4.
Isentropic expansion of gas in the turbine.
Process 4-5’ shows the actual expansion in the nozzle.
Isentropic expansion of gas in the nozzle.
Process 5’-6’ shows the actual expansion of gas in the nozzle.
Compressor isentropic efficiency:
Turbine isentropic efficiency:
Very high power-to-weight ratio, compared to reciprocating engines;
Moves in one direction only, with far less vibration than a reciprocating engine.
Since fans are less noisy then blades.
High operation speeds (more than 3000km/h achieved) than turbojet at (550 km/s).
Low lubricating oil cost and consumption of fuel.
Rate of climb is higher.
With the advent of turbofan Engines Which were Rotary -Reaction Turbine Engines which were much efficient than Rotary Piston Engines.
Since fans are less noisy then blades.
High operation speeds (more than 3000km/h achieved) than turbojet at (550 km/s).
Low consumption of fuel.
Rate of climb is higher.
A high thrust can be produced compared to turbojet
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