report

Table of content

Topics page no

Certificate

Declaration

Acknowledgement

Preface

Abstract

Chapter 1

Introducation 1

Turbofan 3

Chapter 2

Turbofan engine 4

turbofan configration 7

Chapter 3

Types of turbofan engines 8-11

Chapter 4

Bypass of turbofan engine 12-14

Chapter 5

Working of turbofan engine &

Conclusion 15-21

Chapter 6

Turbofan engine terminology & application & references 22-26

Turbofan engine

Preface

A turbofan engine has a large fan at the front, which sucks in air. Most of the air flows around the outside of the engine, making it quieter and giving more thrust at low speeds. Most of today's airliners are powered by turbofans. In a turbojet all the air entering the intake passes through the gas generator, which is composed of the compressor, combustion chamber, and turbine. In a turbofan engine only a portion of the incoming air goes into the combustion chamber. The remainder passes through a fan, or low-pressure compressor, and is ejected directly as a "cold" jet or mixed with the gas-generator exhaust to produce a "hot" jet. The objective of this sort of bypass system is to increase thrust without increasing fuel consumption. It achieves this by increasing the total air-mass flow and reducing the velocity within the same total energy supply.

Chapter 1…………………………introduction about turbofan .

Chapter 2…………………………tell about what is turbo fan engine.

Chapter 3………………………….different types of turbofan engine

Chapter 4………………………….bypass turbo fan engine.

Chapter 5………………………….working of turbofan.

Chapter 6………………………….application of turbofan engine & terminology.

Turbofan engine

Abstract

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.

Turbofan engine

Chapter 1

Introducation

The turbofan or fanjet is a type of airbreathing jet engine that finds wide use in aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan", the turbo portion refers to a gas turbine engine which takes mechanical energy from combustion, and the fan, a ducted fan that uses the mechanical energy from the gas turbine to accelerate air rearwards. The ratio of the mass-flow of air bypassing the engine core compared to the mass-flow of air passing through the core is referred to as the bypass ratio. The engine produces thrust through a combination of these two portions working in concert; engines that use more jet thrust relative to fan thrust are known as low bypass tubine, while those that have considerably more fan thrust than jet are known as high bypass. Most commercial aviation jet engines in use today are of the high-bypass type, and most modern military fighter engines are low-bypass. Afterburners are not used on high-bypass turbofan engines but may be used on either low-bypass turbofan or turbojet engines.

Turbofan engine

fig.1turbofan

The turbofan is a type of aircraft jet engine based around a gas turbine engine.[1] Turbofans providethrust 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 thespecific 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 speed also reduces thrust at high speeds.

Turbofan engine

fig. 2 turbofan

Turbofan Early turbojet engines were very fuel-inefficient, as their overall pressure ratio and turbine inlet temperature were severely limited by the technology available at the time. The very first running turbofan was the German  (designated as the 109-007 by the RLM) which was operated on its testbed on April 1, 1943. The engine was abandoned later while the war went on and problems could not be solved. The British wartime MERTOVICK F3 axial flow jet was given a fan, as the Metrovick F.3 in 1943, to create the first British turbofan.

Turbofan engine

Fig3. CFMF6 turbofan

Improved materials, and the introduction of twin compressors such as in the olympus and the later PRratt & whitney engine, increased the overall pressure ratio and thus the thermodynamics efficiency of engines, but they also led to a poor propulsive efficiency, as pure turbojets have a high specific thrust/high velocity exhaust better suited to supersonic flight.

The original low bypass turbofan engines were designed to improve propulsive efficiency by reducing the exhaust velocity to a value closer to that of the aircraft. The RRC, the world's first production turbofan, had a bypass ratio of 0.3, similar to the modern GENERAL ELECTRIC  fighter engine. Civilian turbofan engines of the 1960s, such as the PRATT & WHITNEY  had bypass ratios closer to 1, but were not dissimilar to their military equivalents.

Chapter 2

Turbofan engine

The turbofan is a derivative of the turbojet engine. In a turbojet, air undergoes four main phases, a process known as the Brayton cycle:

1. A compression phase in a compressor, approximately adiabatic, where its pressure and temperature increase;

2. A heating phase in a combuster, where its temperature and volume increase at approximately constant pressure;

Turbofan engine

3. An expansion phase in a turbine, approximately adiabatic, where mechanical work is extracted from the air to power the compressor;

4. A further expansion phase in a nozzel where its speed increases as it returns to inlet pressure.

Thrust is provided by the difference in speed between the outlet and inlet.

It is important to note that in a turbojet the compressor and turbine taken together form a net-zero mechanical energy system, i.e. all the mechanical shaft power produced by the turbine is consumed by the compressor. The net output of a turbojet is not shaft power, instead it is the kinetic energy of the jet exhaust itself. Although the expansion process in the turbine reduces the gas pressure (and temperature), there remains considerable thermal energy and pressure in the gases leaving the turbine. These energy forms are partly converted into kinetic energy by expansion to ambient pressure through a propelling nozzel, forming a high-velocity flow which provides reactive propulsion.

After World War II, two-spool (or two-shaft) turbojets were developed to make it easier to throttle back compression systems with a high design overall pressure ratio (i.e., combustor inlet pressure/intake delivery pressure). Adopting the two-spool arrangement enables the compression system to be split in two, with a low pressure (LP) compressor supercharging a high pressure (HP) compressor. Each compressor is mounted on a separate (co-axial) shaft, driven by its own turbine (i.e., the HP turbine and LP turbine). Otherwise, a two-spool turbojet is much like a single-spool engine.

Modern turbofans evolved from the two-spool axial flow turbofan engine, essentially by increasing the relative size of the low pressure (LP) compressor to the point where some (if not most) of the air exiting the unit actually bypasses the core (or gas-generator) stream passing through the main combustor. Civil-aviation high-bypass turbofans usually have a single large fan disk, whereas most military-aviation low-bypass turbofans (e.g. combat and trainer aircraft applications) have multi-disk compressors as a compromise between greater power-to-weight ratios, supersonic performance, and the capability of using afterburners, versus the higher fuel economy of a high-bypass design. Modern military transport turbofan engines are virtually identical to their civilian counterparts.

Turbofan engine

Fig4. Turbofan engine

Turbo prop engines are gas-turbine engines that deliver almost all of their power to a shaft to drive a propeller. Turboprops remain popular on very small or slow aircraft, such as small commuter airliners, for their fuel efficiency at lower speeds, as well as on medium military 

Turbofan configuration

Turbofan engines come in a variety of engine configurations. For a given engine cycle (i.e., same airflow, bypass ratio, fan pressure ratio, overall pressure ratio and HP turbine rotor inlet temperature), the choice of turbofan configuration has little impact upon the design point performance (e.g., net thrust, SFC), as long as overall component performance is maintained. Off-design performance and stability is, however, affected by engine configuration.

Turbofan engine

As the design overall pressure ratio of an engine cycle increases, it becomes more difficult to throttle the compression system, without encountering an instability known as compressor surge.

1. Single-shaft turbofan

Although far from common, the single-shaft turbofan is probably the simplest configuration, comprising a fan and high pressure compressor driven by a single turbine unit, all on the same shaft. The SNECMA M53, which powers Mirage fighter aircraft, is an example of a single-shaft turbofan. Despite the simplicity of the turbo machinery configuration, the M53 requires a variable area mixer to facilitate part-throttle operation.

2. Aft-fan turbofan

One of the earliest turbofans was a derivative of the GENERAL ELECTRIC turbojet, known as the CJ805, which featured an integrated aft fan/low pressure (LP) turbine unit located in the turbojet exhaust jetpipe. Hot gas from the turbojet turbine exhaust expanded through the LP turbine, the fan blades being a radial extension of the turbine blades. This aft-fan configuration was later exploited in the General electric GE 36 UDF (propfan) Demonstrator of the early 80s. One of the problems with the aft fan configuration is hot gas leakage from the LP turbine to the fan.

3. Basic two spool

Many turbofans have the basic two-spool configuration where both the fan and LP turbine (i.e., LP spool) are mounted on a second (LP) shaft, running concentrically with the HP spool (i.e., HP compressor driven by HP turbine). The BR710 is typical of this configuration. At the smaller thrust sizes, instead of all-axial blading, the HP compressor configuration may be axial-centrifugal (e.g., General electric CFE 738), double-centrifugal or even diagonal/centrifugal (e.g., Pratt & Whitney PW 600).

4. Boosted two spool

Higher overall pressure ratios can be achieved by either raising the HP compressor pressure ratio or adding an intermediate-pressure (IP) Compressor between the fan and HP compressor, to supercharge or boost the latter unit helping to raise the overall pressure of the engine cycle to the very high levels employed today

Turbofan engine

(i.e., greater than 40:1, typically). All of the large American turbofans feature an IP compressor mounted on the LP shaft and driven, like the fan, by the LP turbine, the mechanical speed of which is dictated by the tip speed and diameter of the fan. The Rolls-Royce BR715 is a non-American example of this. The high bypass ratios (i.e., fan duct flow/core flow) used in modern civil turbofans tends to reduce the relative diameter of the attached IP compressor, causing its mean tip speed to decrease. Consequently more IPC stages are required to develop the necessary IPC pressure rise.

5. Three spool

Rolls-Royce chose a three spool configuration for their large civil turbofans where the intermediate pressure (IP) compressor is mounted on a separate (IP) shaft, running concentrically with the LP and HP shafts, and is driven by a separate IP turbine. The first three spool engine was the earlier  of 1967.

Chapter 3

Types of turbofan engine

Turbofan engine

1.low bypass turbofan

A high specific thrust/low bypass ratio turbofan normally has a multi-stage fan, developing a relatively high pressure ratio and, thus, yielding a high (mixed or cold) exhaust velocity.

The core airflow needs to be large enough to give sufficient core power to drive the fan. A smaller core flow/higher bypass ratio cycle can be achieved by raising the (HP) turbine rotor inlet temperature.

Imagine a retrofit situation where a new low bypass ratio, mixed exhaust, turbofan is replacing an old turbojet, in a particular military application. Say the new engine is to have the same airflow and net thrust (i.e. same specific thrust) as the one it is replacing.

A bypass flow can only be introduced if the turbine inlet temperature is allowed to increase, to compensate for a correspondingly smaller core flow. Improvements in turbine cooling/material technology would facilitate the use of a higher turbine inlet temperature, despite increases in cooling air temperature, resulting from a probable increase in overall pressure ratio.

Efficiently done, the resulting turbofan would probably operate at a higher nozzle pressure ratio than the turbojet, but with a lower exhaust temperature to retain net thrust. Since the temperature rise across the whole engine (intake to nozzle) would be lower, the (dry power) fuel flow would also be reduced, resulting in a better specific fuel consumption (SFC).

2.High bypass turbofan

The low specific thrust/high bypass ratio turbofans used in today's civil jetliners (and some military transport aircraft) evolved from the high specific thrust/low bypass ratio turbofans used in such [production] aircraft back in the 1960s.

Low specific thrust is achieved by replacing the multi-stage fan with a single stage unit. Unlike some military engines, modern civil turbofans do not have any stationary inlet guide vanes in front of the fan rotor. The fan is scaled to achieve the desired net thrust.

The core (or gas generator) of the engine must generate sufficient core power to at least drive the fan at its design flow and pressure ratio.

Turbofan engine

Through improvements in turbine cooling/material technology, a higher (HP) turbine rotor inlet temperature can be used, thus facilitating a smaller (and lighter) core and (potentially) improving the core thermal efficiency.

Reducing the core mass flow tends to increase the load on the LP turbine, so this unit may require additional stages to reduce the average stage loading and to maintain LP turbine efficiency. Reducing core flow also increases bypass ratio (5:1, or more, is now common).

Further improvements in core thermal efficiency can be achieved by raising the overall pressure ratio of the core. Improved blade aerodynamics reduces the number of extra compressor stages required.

With multiple compressors (i.e., LPC, IPC, and HPC) dramatic increases in overall pressure ratio have become possible. Variable geometry . enable high pressure ratio compressors to work surge-free at all throttle settings.

the first high-bypass turbofan engine was the General electric , designed in mid 1960s to power the lockheed military transport aircraft. The civil General electric CF56 engine used a derived design. Other high-bypass turbofans are the Pratt & whitney the three-shaft Rolls royse and the  CFM; also the smaller TF34. More recent large high-bypass turbofans include the PW 400, the three-shaft ROLL & ROYSE, the General electric GE 34 , produced jointly by GE and P&W.

For reasons of fuel economy, and also of reduced noise, almost all of today's jet airliners are powered by high-bypass turbofans. Although modern combat aircraft tend to use low bypass ratio turbofans, military transport aircraft mainly use high bypass ratio turbofans for fuel efficiency.

Because of the implied low mean jet velocity, a high bypass ratio/low specific thrust turbofan has a high thrust lapse rate (with rising flight speed).

Consequently the engine must be over-sized to give sufficient thrust during climb/cruise at high flight speeds (e.g., Mach 0.83). Because of the high thrust lapse rate, the static (i.e., Mach 0) thrust is relatively high.

This enables heavily laden, wide body aircraft to accelerate quickly during take-off and consequently lift-off within a reasonable runway length.

Turbofan engine

Fig .5 high & low bypass turbofan

The turbofans on twin engined airliners are further over-sized to cope with losing one engine during take-off, which reduces the aircraft's net thrust by 50%. Modern twin engined airliners normally climb very steeply immediately after take-off. If one engine is lost, the climb-out is much shallower, but sufficient to clear obstacles in the flightpath.

The Soviet Union's engine technology was less advanced than the West's and its first wide-body aircraft, was powered by low-bypass engines. The yy 42 a medium-range, rear-engine aircraft seating up to 120 passengers introduced in 1980 was the first Soviet aircraft to use high-bypass engines.

3.After burning turbofan

Turbofan engine

Since the 1970s, most jet fighter engines have been low/medium bypass turbofans with a mixed exhaust, after burner and variable area final nozzle. An afterburner is a combustor located downstream of the turbine blades and directly upstream of the nozzle, which burns fuel from afterburner-specific fuel injectors.

When lit, prodigious amounts of fuel are burnt in the afterburner, raising the temperature of exhaust gases by a significant degree, resulting in a higher exhaust velocity/engine specific thrust.

The variable geometry nozzle must open to a larger throat area to accommodate the extra volume flow when the afterburner is lit. Afterburning is often designed to give a significant thrust boost for take off, transonic acceleration and combat maneuvers, but is very fuel intensive. Consequently afterburning can only be used for short portions of a mission.

Unlike the main combustor, where the downstream turbine blades must not be damaged by high temperatures, an afterburner can operate at the ideal maximum temperature (i.e., about 2100K/3780Ra/3320F).

At a fixed total applied fuel:air ratio, the total fuel flow for a given fan airflow will be the same, regardless of the dry specific thrust of the engine. However, a high specific thrust turbofan will, by definition, have a higher nozzle pressure ratio, resulting in a higher afterburning net thrust and, therefore, a lower afterburning specific fuel consumption (SFC).

However, high specific thrust engines have a high dry SFC. The situation is reversed for a medium specific thrust afterburning turbofan: i.e., poor afterburning SFC/good dry SFC. The former engine is suitable for a combat aircraft which must remain in afterburning combat for a fairly long period, but only has to fight fairly close to the airfield (e.g. cross border skirmishes)

The latter engine is better for an aircraft that has to fly some distance, or loiter for a long time, before going into combat. However, the pilot can only afford to stay in afterburning for a short period, before aircraft fuel reserves become dangerously low. Modern low-bypass military turbofans include the Pratt & whitnye W2, the Eurojet EW 100, the GE 199, the , and all of which feature a mixed exhaust, afterburner and variable area propelling nozzle.

Chapter 4

Bypass of turbofan & bypass ratio

Turbofan engine

1) The bypass ratio (BPR) of a turbofan engine is the ratio between the mass flow rate of air drawn through the fan disk that bypasses the engine core (un-combusted air) to the mass flow rate passing through the engine core that is involved in combustion to produce mechanical energy.  For example, a 10:1 bypass ratio implies that 10 kg of air passes around the combustion chamber through the ducted fan for every 1 kg of air passing through the combustion chamber.

2) The ducted fan, rather than combustion gases expanding in a nozzle, produces the vast majority of the thrust in high-bypass designs. 

3) A high bypass ratio provides a lower thrust specific fuel consumption (grams/sec fuel per unit of thrust in kN using SI units) for reasons explained below, especially at zero velocity (at takeoff) and at the cruise speed of most commercial jet aircraft; however, the lower exhaust velocities of high-bypass designs also figure strongly in lower noise output, which is a decided advantage over earlier low or zero bypass designs.

4) High bypass designs are by far the dominant type for all commercial passenger aircraft and both civilian and military jet transports.

5) Military combat aircraft usually use engines with low bypass ratios to compromise between fuel economy and the requirements of combat: high power-to-weight ratios, supersonic performance, and the ability to use afterburners, all of which are more compatible with low bypass engines.

6) A good example of the differences between a pure jet engine and a low-bypass turbofan may be seen in the Spey turbofan used in the f-5  

Turbojet engines are relatively inefficient as Brayton cycle engines because they directly convert thermal energy from combusting fuel into kinetic energy in the form of a high-velocity reaction jet directed through an expansion nozzle instead of producing mechanical power; therefore, pounds force or kilonewtons—not horsepower or kilowatts, as in propeller or turboprop engines—measure the power of a turbojet.

Turbofans, conversely, are very efficient Brayton cycle engines because their gas turbine convert thermal energy from combustion into mechanical shaft power: the essential difference between a turbojet and turbofan gas turbine is that the turbine stage in a turbojet is designed to extract only a fraction of the available thermal

Turbofan engine

energy in the high pressure and temperature exhaust, producing only enough mechanical energy to run the compressor stage as a net-zero mechanical energy system (ignoring very small mechanical outputs to run auxiliary equipment such as generators) and leaving a relatively high temperature and back pressure exhaust at the turbine exit for effective reaction propulsion.  

The gas turbine on a turbofan has additional turbine disks and stators, which are sufficient to convert most of the available thermal energy into mechanical work and leave an exhaust plume of greatly reduced temperature, pressure, and velocity. The back pressure at the turbine exit of a high bypass turbofan that maximally converts thermal energy into mechanical energy should be close to ambient pressure because the increased thrust derived from the ducted fan more than compensates the low direct jet propulsive efficiency of such an engine.In a high-bypass turbine engine, the gas turbine uses thermal energy from combustion to turn a ducted fan that slightly increases the velocity of a large amount of air.

Only the limitations of weight and materials (e.g., the strengths and melting points of materials in the turbine) reduce the efficiency at which a turbofan gas turbine converts this thermal energy into mechanical energy, for while the exhaust gases may still have available energy to be extracted, each additional stator and turbine disk retrieves progressively less mechanical energy per unit of weight, and increasing the compression ratio of the system by adding to the compressor stage to increase overall system efficiency increases temperatures at the turbine face.Nevertheless, high-bypass engines have a high propulsive efficiency because even slightly increasing the velocity of a very large volume and consequently mass of air produces a very large change in momentum and thrust: thrust is the engine's mass flow (the amount of air flowing through the engine) multiplied by the difference between the inlet and exhaust velocities in—a linear relationship—but the kinetic energy of the exhaust is the mass flow multiplied by one-half the square of the difference in velocities.

The Rolls–Royce Conway turbofan engine, developed in the early 1950s, better uses this energy.  In the Conway, an otherwise normal axial-flow turbojet was equipped with an oversized first compressor stage (the one closest to the front of the engine) and centered inside a tubular  nacelle(in effect, a ducted fan arrangement): while the inner portions of the compressor worked "as normal" and provided air into the core of the engine, the outer portion blew air around the engine to provide extra thrust. The Conway had a very small bypass ratio of only 0.3, but the improvement in fuel economy was notable; as a result, it and its derivatives like the Spey became some of the most popular jet engines in the world.

Turbofan engine

The growth of bypass ratios grew during the 1960s gave jetliners fuel efficiency that could compete with that of piston-powered planes. Pratt & Whitney and General Electricdeveloped most of the very large high-bypass engines in the United States, which for the first time was besting the United Kingdom in engine design. Rolls-Royce also started the development of the high-bypass turbofan, and although it caused considerable trouble at the time, the RB.211 would go on to become one of their most successful products.

Today, almost all jet engines have some bypass. Modern engines in slower aircraft, such as airliners, have bypass ratios up to 17:1; in higher-speed aircraft, such as fighters, bypass ratios are much lower, around 1.5; and craft designed for speeds up to Mach 2 and somewhat above have bypass ratios below 0.5 . Concorde and Tu-144 had no bypass to reduce inlet drag during extended supersonic cruise at Mach 2.

Engine Major applications Bypass ratio

Rolls-Royce/Snecma Olympus

593 (turbojet)Concorde 0:1

SNECMA M88 Rafale 0.30:1

General Electric F404F/A-18, T-50, F-117, X-29, X-

310.34:1

Pratt & Whitney F100 F-16, F-15 0.36:1

Eurojet EJ200 Typhoon 0.4:1

Klimov RD-33 MiG-29, Il-102 0.49:1

Saturn AL-31 Su-27, Su-30, J-10 0.59:1

Turbofan engine

Pratt & Whitney JT8D DC-9, MD-80, 727, 737 0.96:1

Kuznetsov NK-321 Tu-160 1.4:1

Rolls-Royce Tay Gulfstream IV, F70, F100 3.1:1

PowerJet SaM146 SJ 100 4.43:1

Pratt & Whitney PW2000 757, C-17 5.9:1

Progress D-436 Yak-42, Be-200, An-148 6.2:1

General Electric GEnx 747, 787 8.5:1

Rolls-Royce Trent 900 A380 8.7:1

General Electric GE90 777 9:1

Rolls-Royce Trent 1000 787 10:1

Pratt & Whitney PW1000G Bombardier CSeries 12:1

Table1 .Engine different bypass ratios

Turbofan engine

Chapter 5

Working of turbofan engine

Figure 6 is a sketch of a turbofan cross section. In this side view, the components or stations have been numbered. The air travels through the engine from left to right, starting at the fan (number 1 on the figure) and progressing towards the exhaust nozzle (number 5 in the figure). The components and what occurs at each station are described in detail below.

fig.6 turbofan engine

1. Air Intake/Ingestion

The fan is responsible for producing the majority of the thrust generated by a turbofan engine and is easily visible when looking at the front of the engine, as seen in Figure 3. The fan is directly connected to the low pressure compressor (LPC) and the low pressure turbine (LPT) by way of a shaft known as the low pressure shaft.

Turbofan engine

Fig7. Turbofan engine viewed from front ,with the fan visible

The fan is station 1 in Figure 6 (above). Ambient air enters the engine by passing through the fan. Most of the air that passes through the fan travels around the core of the engine (the center of the engine where the compressor, combustor, turbine, and exhaust nozzle are located). This air that travels around the core is known as bypass air (it bypasses the core, as seen in Figure 6). Bypass air is accelerated out of the back of the engine by the fan thereby creating thrust. It never interacts with the compressor, combustor, turbine, or exhaust nozzle. The remaining air enters the core of the engine. This air has been somewhat accelerated by the fan, and immediately enters the low pressure compressor.

2. Compression

The purpose of compression is to prepare the air for combustion by adding energy in the form of pressure and heat. The compressor is divided into two portions: the low pressure compressor, mentioned above, and the high pressure compressor. The compressor is station 2 in Figure 6. Both compressors function in a similar manner; however, they interact with different parts of the turbofan engine.

Turbofan engine

The Low Pressure Compressor(LPC)

The LPC is directly connected to the fan and the low pressure turbine (LPT) by the low pressure shaft. The LPC has rows of spinning blades which push the air further back into the engine. As the air is being forced rearward, the LPC’s cross sectional area decreases, causing the volume of air to decrease. From the ideal gas law, this implies that the air is becoming pressurized and the temperature is increasing. Immediately after the air passes through the LPC, it enters the high pressure compressor. The High Pressure Compressor (HPC) The high pressure compressor, or HPC, is located directly downstream of the LPC and directly upstream of the combustor. The HPC is connected directly to the high pressure turbine by the high pressure shaft. Like the LPC, the HPC has rows of spinning blades which force the air flow rearward into a higher pressure and higher temperature state due to a decrease in volume. The HPC typically has more rows of blades when compared to the LPC. Air exiting the HPC has a high temperature and pressure and is now ready for combustion

The High Pressure Compressor (HPC)

The high pressure compressor, or HPC, is located directly downstream of the LPC and directly upstream of the combustor. The HPC is connected directly to the high pressure turbine by the high pressure shaft. Like the LPC, the HPC has rows of spinning blades which force the air flow rearward into a higher pressure and higher temperature state due to a decrease in volume. The HPC typically has more rows of blades when compared to the LPC. Air exiting the HPC has a high temperature and pressure and is now ready for combustion.

fig8.turbofan compressors

Turbofan engine

2. Combustion

Combustion occurs within the combustor, a stationary chamber within the core of the engine, which is station 3 in Figure 6. The combustor is directly downstream of the HPC and directly upstream of the high pressure turbine. The purpose of the combustor is to add even more energy to the air flow by way of heat addition. Within the combustor, fuel is injected and mixed with the air. This fuel-air mixture is then ignited, creating a dramatic increase in temperature and energizing the flow, propelling it rearward towards the high pressure turbine.

4. Expansion

Expansion occurs within the high pressure and low pressure turbines. The turbines are station 4 in Figure 2. Similar in appearance to the compressors, the turbines have rows of blades which spin (as seen in Figure 4). The purpose of the turbines is to extract energy from the flow which is then used to spin the compressors and the fan. The spinning fan draws more air through the core of the engine which continues the entire process, and it pulls more bypass air around the engine, generating continuous thrust.

The High Pressure Turbine(HPT)

The high pressure turbine, or HPT, is located directly downstream of the combustor and directly upstream of the low pressure turbine. The HPT is driven by the high pressure air that passes through it. The HPT’s cross sectional area is initially small and then increases downstream. This change in area allows the air to expand, increasing in volume thereby decreasing in pressure and temperature. This decrease in pressure andtemperature, along with the energy used to spin the turbine, correspond to a decrease in the overall energy in Figure 6: Rows of turbine blades the air flow. Air exiting the HPT is significantly cooler and less pressurized than the air entering; however, it still has viable

Turbofan engine

energy which will be extracted by the low pressure turbine. As mentioned earlier, the HPT is connected to the HPC by the high pressure shaft. The high pressure shaft spins the HPC when the HPT is spun by the air passing through it. This interaction ensures that the HPC will be pulling air into the combustor continuously, thus feeding the HPT highly energized, combusted air continuously.

The Low Pressure Turbine (LPT)

The low pressure turbine, or LPT, is located directly downstream of the HPT and directly upstream of the exhaust nozzle. The LPT functions exactly as the HPT does; however, it is connected to the LPC and the fan via the low pressure shaft. Therefore, when the LPT is driven by the air passing through it, is also drives the LPC and the fan. When the LPC is spinning, it provides the HPC with air to feed to the combustor. When the fan is spinning, it provides the LPC with air to feed to the HPC, and it produces thrust by accelerating bypass air out of the engine. Air exiting the LPT is significantly cooler than when entering, but it is still hotter than the ambient air. This hot air exits the LPT and immediately enters the exhaust nozzle.

Fig9.rows of turbine blades

Turbofan engine

Exhaust

The exhaust nozzle is located directly downstream of the LPT and it is the last component that the air flow touches before exiting the engine. An example of an exhaust nozzle can be seen in Figure 5. The exhaust nozzle is at station 5 in Figure 2 and it is stationary, like the combustion chamber. The purpose of the exhaust nozzle is to propel the core flow out of the engine, providing additional thrust. This is accomplished by way of its geometry or shape. The nozzle also helps regulate pressures within the engine to keep the other components functioning properly and efficiently.

Conclusion

This entire process can be broken down into steps as follows:

1. Air enters the engine through the fan, which is being driven by the low pressure turbine.

2. Most of the air is accelerated out of the back of the engine, creating thrust.

3. A portion of the air enters the core of the engine where it travels through the low pressure and high pressure compressors.

4. This compressed air is then mixed with fuel and ignited in the combustor where heat energy is added.

5. Heat energy and pressure energy cause the air to expand through the turbines, spinning the high pressure and low pressure turbines which in turn spin the fan and the compressors.

6. Air exits the turbines and exits the engine through the exhaust nozzle, which also generates thrust, propelling the engine and vehicle forward. This process is continuous, with each component doing a specific task to keep the engine as a whole running. Overall, the idea behind a turbofan engine is relatively simple: add

Turbofan engine

energy to the air flowing through the engine then extract this energy in order to drive the fan and create thrust. Turbofans are the main powerplant propelling jet liners across our country and around the world. With some general physics and chemistry concepts, one can gain an understanding of these machines that have revolutionized the way we travel and have a greater appreciation for the technology behind them.

Turbofan engine

Chapter 6

Overall Terminology used in turbofan engine

AfterburnerExtra combustor immediately upstream of final nozzle (also called reheat)

AugmentorAfterburner on low-bypass turbofan engines.

Average stage loadingConstant × (delta temperature)/[(blade speed) × (blade speed) × (number of stages)]

BypassAirstream that completely bypasses the core compression system, combustor and turbine system.

Bypass ratioBypass airflow /core compression inlet airflow.

CoreTurbomachinery handling the airstream that passes through the combustor.

Core powerResidual shaft power from ideal turbine expansion to ambient pressure after deducting core compression power.

Core thermal efficiencyCore power/power equivalent of fuel flow.

DryAfterburner (if fitted) not lit.

Turbofan engine

EGTExhaust Gas Temperature.

EPREngine Pressure Ratio.

FanTurbofan LP compressor.

Fan pressure ratioFan outlet total pressure/intake delivery total pressure.

Flex tempUse of artificially high apparent air temperature to reduce engine wear.

Gas generatorAt engine core.

HP compressorHigh pressure compressor (also HPC).

HP turbineHigh pressure turbine.

Intake ram dragPenalty associated with jet engines picking up air from the atmosphere (conventional rocket motors do not have this drag term, because the oxidiser travels with the vehicle).

IEPRIntegrated engine pressure ratio.

IP compressorIntermediate pressure compressor (also IPC).

Turbofan engine

IP turbine

Intermediate pressure turbine (also IPT).LP compressorLow pressure compressor (also LPC).

LP turbineLow pressure turbine (also LPT).

Net thrustNozzle total gross thrust - intake ram drag (excluding nacelle drag, etc., this is the basic thrust acting on the airframe).

Overall pressure ratioCombustor inlet total pressure/intake delivery total pressure.

Overall thermal efficiencyThermal efficiency * propulsive efficiency.

Propulsive efficiencyPropulsive power/rate of production of propulsive kinetic energy (maximum propulsive efficiency occurs when jet velocity equals flight velocity, which implies zero net thrust!).

Specific fuel consumption   (SFC) Total fuel flow/net thrust (proportional to flight velocity/overall thermal efficiency).

Spooling upaccelerating, marked by a delay.

Turbofan engine

Application

Most modern jet engines are actually turbofans, where the low pressure compressor acts as a fan, supplying supercharged air to not only the engine core, but to a bypass duct.

The bypass airflow either passes to a separate 'cold nozzle' or mixes with low pressure turbine exhaust gases, before expanding through a 'mixed flow nozzle'.Forty years ago there was little difference between civil and military jet engines, apart from the use of afterburning in some (supersonic) applications.

Civil turbofans today have a low specific thrust (net thrust divided by airflow) to keep jet noise to a minimum and to improve fuel efficiency.

Consequently the bypass ratio (bypass flow divided by core flow) is relatively high (ratios from 4:1 up to 8:1 are common). Only a single fan stage is required, because a low specific thrust implies a low fan pressure ratio.Today's military turbofans, however, have a relatively high specific thrust, to maximize the thrust for a given frontal area, jet noise being of little consequence.

Multi-stage fans are normally required to achieve the relatively high fan pressure ratio needed for a high specific thrust. Although high turbine inlet temperatures are frequently employed, the bypass ratio tends to be low (usually significantly less than 2.0).

Turbofan engine

REFERENCES

1. Marshall Brain. "How Gas Turbine Engines Work". howstuffworks.com. Retrieved 2010-11-24.

2. "Turbofan Engine". www.grc.nasa.gov. Retrieved 2010-11-24. 3. Neumann, Gerhard(2004) [1984], Herman the German: Just Lucky I Guess,

Bloomington, IN, USA: Authorhouse, ISBN1-4184-7925-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.

4. Spittle, Peter. "Gas turbine technology"Rolls-Royce plc, 2003. Retrieved: 21 July 2012.

5. "Metrovick F3 Cutaway - Pictures & Photos on FlightGlobal Airspace". Flightglobal.com. 2007-11-07. Retrieved 2013-04-29. "1954 | 0985 | Flight Archive". Flightglobal.com. 1954-04-09. Retrieved 2013-04-29.

6. PDF C. Riegler, C. Bichlmaier:, 1st CEAS European Air and Space Conference, 10–13 September 2007, Berlin, Germany

7. PS-90A turbofan, Aviadvigatel, 2011-01-17 8. Turbofan Engine Family for Regional Jet, Aviadvigatel, 2011-01-17

Stati

c pressurenormal

meaning of

pressure. Excludes

any kinetic energy

effects

Specific thrust

Thermal efficiency

Total fuel flow

Total

Turbine rotor inlet temperature

Turbofan engine

Turbofan engine