26-04-2011, 04:28 PM
Submitted by
SHARATH. P
seminar.ppt (Size: 3.41 MB / Downloads: 639)
INTRODUCTION TO JET ENGINES
TURBOFAN 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
‘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).
‘TURBOFAN ENGINES’
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.
‘TURBOFAN ENGINES’
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.
‘TURBOFAN ENGINES’
From Newton’s second law
By conservation of momentum, the moving body is propelled in the opposite direction to the jet.
How?
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,
‘TURBOFAN ENGINES’
AIR INTAKE
FAN
COMPRESSOR
COMBUSTION CHAMBER
TURBINE
NOOZLE
AFTER BURNER
THRUST REVERSER
‘TURBOFAN ENGINES’
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.
‘TURBOFAN ENGINES’
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’
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.
‘TURBOFAN ENGINES’
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.
‘TURBOFAN ENGINES’
Process 1-2:
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.
Process 2-3:
Isentropic compression of air.
Process 2’-3’ shows the actual compression of air.
Process 3-4:
Ideal addition of heat at constant pressure p3=p4.
Process 3’-4 shows the actual addition of heat at constant process p3=p4.
Process 4-5:
Isentropic expansion of gas in the turbine.
Process 4-5’ shows the actual expansion in the nozzle.
Process 5-6:
Isentropic expansion of gas in the nozzle.
Process 5’-6’ shows the actual expansion of gas in the nozzle.
‘TURBOFAN ENGINES’
Thermal efficiency
Compressor isentropic efficiency:
Turbine isentropic efficiency:
Propulsive efficiency
Overall efficiency:
‘TURBOFAN ENGINES’
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.
‘TURBOFAN ENGINES’
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
SHARATH. P
seminar.ppt (Size: 3.41 MB / Downloads: 639)
INTRODUCTION TO JET ENGINES
TURBOFAN 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
‘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).
‘TURBOFAN ENGINES’
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.
‘TURBOFAN ENGINES’
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.
‘TURBOFAN ENGINES’
From Newton’s second law
By conservation of momentum, the moving body is propelled in the opposite direction to the jet.
How?
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,
‘TURBOFAN ENGINES’
AIR INTAKE
FAN
COMPRESSOR
COMBUSTION CHAMBER
TURBINE
NOOZLE
AFTER BURNER
THRUST REVERSER
‘TURBOFAN ENGINES’
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.
‘TURBOFAN ENGINES’
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’
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.
‘TURBOFAN ENGINES’
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.
‘TURBOFAN ENGINES’
Process 1-2:
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.
Process 2-3:
Isentropic compression of air.
Process 2’-3’ shows the actual compression of air.
Process 3-4:
Ideal addition of heat at constant pressure p3=p4.
Process 3’-4 shows the actual addition of heat at constant process p3=p4.
Process 4-5:
Isentropic expansion of gas in the turbine.
Process 4-5’ shows the actual expansion in the nozzle.
Process 5-6:
Isentropic expansion of gas in the nozzle.
Process 5’-6’ shows the actual expansion of gas in the nozzle.
‘TURBOFAN ENGINES’
Thermal efficiency
Compressor isentropic efficiency:
Turbine isentropic efficiency:
Propulsive efficiency
Overall efficiency:
‘TURBOFAN ENGINES’
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.
‘TURBOFAN ENGINES’
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