TURBO FAN AND JET ENGINES Flashcards
THE FORWARD FAN DESIGN ON A TURBO FAN ENGINE CAN PRODUCE BETWEEN _____ TO ____ PERCENT OF THE TOTAL THRUST (DEPENDING ON SIZE OF FAN)
THE GAS GENERATOR EXHAUST GAS WILL PROVID THE REMAINING THRUST WICH IS BETWEE ____ AND ____ PERCENT.
30 TO 60
40 AND 70
On a forward-fan design, the fan blades look similar to rotor blades of the compressor that have been enlarged. The cross section of these fans are larger than the front area of the compressor. This allows a relatively large mass of air to bypass the gas generator. On average. this large mass of bypassed air can produce between
30 to 60 percent of the total thrust of a turbofan engine. Some engines have been designed with an even greater amount of thrust from the fans. These percentages depend upon the size of the fan, the turbine arrangements, and atmospheric conditions. Since the fans provide 30- 60 percent of the total thrust. the gas generator exhaust gases will provide the remaining thrust. which is between 40 and 70 percent of that total thrust.
The gas generator turbine section will extract __ percent of the total energy that leaves the burner section just to turn the compressor and accessories. The remaining____ percent will be used for thrust. Part of the___ percent will be converted to mechanical energy through the turbines to drive the fans. The other part will exit the exhaust section as kinetic energy.
75
25
25
These fans push a large mass of ___ which increases _____(T=ma).
air
thrust
Combined with the exhaust gases of the gas generator, the overall thrust is greater than the thrust of a turbojet at the same fuel consumption rate. Thus, to produce the same amount of thrust, the turbofan engine requires less fuel than a turbojet engine and therefore, the turbofan has a lower TSFC.
TURBO FAN REQUIRES LESS FUEL FOR THE SAME AMOUNT OF THRUST THAN A TURBO JET ENGINE.
One of the primary design goals of today’s engines is to maximize the bypass ratio while meeting mission requirements. The most fuel-efficient engines are those with the______ bypass ratio.
higher
Compared to the turbojet, the turbofan is quieter because of the generated thrust from the fans and not just burner combustion. Although performance is high, all turbofan engines are limited at high-end airspeeds and altitudes when compared to the turbojet.
TURBO FANS ARE QUIETER BECAUS OF GENERATED THRUST FROM THE FANS AND NOT JUST BURNER COMBUSTION.
The reduction of the high-end airspeed is due to the large mass of airflow from the fans that is accelerated to only moderate speeds. On a high bypass turbofan engine, more air is bypassed and therefore less thrust comes from the high velocity gases of the gas generator. The altitude limitation is from the lower density air at the high altitudes and therefore, a smaller mass of air is accelerated by the fans.
TURBO FANS FLY SLOWER.
TURBO FAN
ADVANTAGES
DISADVANTAGES
Advantages 1. Higher thrust at low airspeeds 2. Lower TSFC (FUEL BURN) 3. Shorter takeoff distance 4. Considerable noise reduction, 10 to 20 percent over the turbojet Disadvantages 1. Higher specific weight 2. Larger frontal area 3. Inefficient at higher altitudes
- What does the term “gas generator” mean?
- An engine which is operated by converting the energy of expanding gases into propulsive force.
- How is a turbojet constructed?
- Inlet, compressor, burner, turbine, exhaust nozzle.
- How does a turbojet produce thrust?
- By greatly accelerating a small mass of air and expanding the gases leaving the exhaust nozzle.
- What are the advantages and disadvantages of the turbojet?
- Best high speed and high altitude performance. Highest TSFC. Longest takeoff rolls.
- How is a turbofan constructed?
- Fan, compressor, burner, turbine, exhaust nozzle.
- How does a turbofan produce thrust?
- Fan pushes a large mass of air and provides 30-60% of the total thrust.
- How much thrust is produced by the fan on a turbofan? Gas Generator?
- 30-60% from the fan.
40-70% from the gas generator exhaust gases.
- What is meant by “bypass air?”
- Airflow from the fan “bypassed around the gas generator.”
- What are the advantages and disadvantages of the turbofan?
- ADV
- Better TSFC than a turbojet,
- shorter take off distance,
- can lift larger weights.
DIS ADV
1 Large frontal area, - slower, and
- can not fly as high as turbojet.
The gas turbine is an internal combustion engine that uses air as the working fluid. The engine extracts ________ energy from fuel and converts it to_______ energy using the gaseous energy of the working fluid (air) to drive the engine and propeller, which, in turn, propel the airplane.
chemical
mechanical
________ energy from fuel as it is burned is converted to ________ energy in the form of high temperatures and high velocity as the air passes through the combustor. The gaseous energy is converted back to_______ energy in the turbine, providing power to drive the compressor and the output shaft.
Chemical
gaseous
mechanical
The efficiency of the turbine is determined by how well it extracts ______ energy from the hot, high-velocity _______.
MECHANICAL
gasses
The factors that affect the thrust of a gas turbine engine include:
The factors that affect the thrust of a gas turbine engine include:
Air Density
Airspeed
Ram Effect
Engine RPM
Density– Is the mass of a a substance per unit of its volume.
According to the thrust equation:
If mass of airflow increases___________.
If density of air increases- mass will increase and therefore ____________.
- thrust increases
thrust will increase
Air Temperature
Temperature Increase-____________
Temperature decrease- ____________
Air Temperature
Temperature Increase- thrust will decrease
Temperature decrease- thrust will increase
Air Pressure
As air pressure increases- density increases and_____________.
As air pressure decreases- density decreases and ____________
Air Pressure
As air pressure increases- density increases and thrust increases
As air pressure decreases- density decreases and thrust decreases
Altitude
As an aircraft climbs, temperature and pressure decrease. With an increase in altitude, the rate of thrust________ because the pressure drop is greater than the thrust increase resulting from temperature decreasing. At approximately 36,000 feet, temperature stabilizes. As a result temperature will no longer offset the density decrease. Altitude cruise level is found at 36,000 feet.
DECREASES
Airspeed
In the thrust equation, the difference between the inlet and exhaust velocities plays a major role in determining thrust available. As the inlet velocity approaches the magnitude of exhaust velocity. thrust is reduced. Therefore, if the mass of air and fuel is held constant, thrust will________ as airspeed increases.
decrease
Ram Effect
The thrust equation consists of two variables: mass and acceleration. As airspeed increases the difference between mass and acceleration causes a decrease in thrust. However, as you increase airspeed more air is being rammed into the inlet, increasing mass and pressure of inlet air. This offsets the decrease in acceleration and results in a neutral affect or slight increase in thrust at subsonic airspeeds. This is due to the compressibility of airflow as velocity increases toward supersonic. As airflow becomes compressible, mass due to ram effect increases at an increasing rate.
WITH RAM EFFECT AS YOU INCREASE AIRSPEED MORE AIR BEING RAMMED INTO THE INCLET, INCREASING MASS AND PRESSURE INLET. SO YOU GET A NEUTRAL EFFECT OR SLIGHT INCREASE IN THRUST AT SUBSONIC AIRSPEEDS.
The turbine section
The turbine section
•Drives the compressor
•The compressor drives the engine accessories
•Is designed to increase airflow velocity
Turbine Section Acceleration
•Acceleration is produced through the __________ or a combination of both
stators, rotors
Approximately 75% of the total pressure energy from exhaust is converted. The amount of energy converted is dependent on turbine load.The remaining 25 percent of available energy is used as thrust.
ONLY 25% USED AS THRUST
ON A TURBOFAN ENGINE THE AMOUT OF AIR THAT BYPASSES THE GAS GENERATOR IN COMPARISON WITH THE AMOUNT OF AIR THAT PASSES THROUGH THE GAS GENERATOR IS CALLED?
BYPASS RATIO
A PRINCIPLE ADVANTAGE OF AN AXIAL FLOW COMPRESSOR COMPARED TO A CENTRIFUGAL FLOW COMPRESSOR ON A TURBINE ENGINE IS?
ABILITY TO HANDLE LARGE VOLUME OF AIRFLOW
A PRINCILPLE DISADVANTAGE OF A CONTIFUGAL FLOW COMPRESSOR COMPARED TO AN EXIAL FLOW COMPRESSOR ON A TURBINE ENGINE IS?
THE PROVISION OF MORE THAN ONE OR TWO STAGES IS RARELY FEASIBLE
THE MAIN SYSTEMS ON A 3-SHAFT TURBINE ENGINE ARE?
LOW PRESSURE SYSTEM, INTERMEDIATE PRESSURE SYSTEM, HIGH PRESSURE SYSTEM.
IN A GAS RUBINE ENGINE THE INLET, COMPRESSOR, AND DIFFUSER SECTIONS ARE RESPONSIBLE FOR?
BUILDING POTENTIAL ENERGY OF THE AIRFLOW.
WHAT IS A PRIMARY DESIGN LIMITATION OF A TURBOJET ENGINES?
TURBINE INLET TEMP
ON A JET ENGINE, THE PROCESS OF TAKING HIGH VELOCITY, LOW PRESSURE AIR, AND TURNING IT INTO LOW VELOCITY, HIGH PRESSURE AIR IS THE PRIMARY FUNCTION OF THE?
DIFFUSER
DESCRIBE THE PURPOSE OF THE COMPRESSOR SECTION OF A TURBOJET ENGINE?
INCREASE THE PRESSURE OF THE AIR FLOWING THROUGH THE ENGINE
THE SECTION OF AN ENGINE WHICH INCREASES THE ENERGY OF THE AIR RECEIVED FROM THE ENTRANCE DUCT, THEN DISCHARGES INTO THE DIFFUSER SECTION AND THE COMBUSTION CHAMBER IS CALLED?
COMPRESSOR
THE RUBINE SECTION OF AN ENGINE IS RESPONSIBLE FOR?
DRIVING THE COMPRESSOR AND THE ACCESSORIES.
IN THE COMBUSTION/BURNER SECTION OF AN EGINE, PRIMARY AIR IS USED FOR WHAT PURPOSE?
MIXING WITH FUEL PRIOR TO COMBUSTION.
THE PRIMARY FUNCTION OF THE COMPRESSOR SECTION IS A GAS TURBINE ENGINE IS
PROGRESSIVELY SQUEEZE AIR IN PREPARATION FOR COMBUSTION.
Recall that increasing the pressure of airflow will increase its _____________, while increasing the velocity of airflow will increase its_____________. In a gas turbine engine, the inlet, compressor, and diffuser sections build the potential energy of the airflow. In the burner, turbine and exhaust sections, this potential energy, along with the energy from fuel ignition, is converted into kinetic energy. The kinetic energy is then used for thrust production. Therefore, in the first half of a gas turbine engine, high pressures are desired, while high velocities are desired in the second half.
potential energy
kinetic energy
BRAYTON CYCLE
A gas turbine engine follows a cycle of operation known as the Brayton Cycle. This operating cycle consists of four events which occur simultaneously: intake, compression, combustion and exhaust.
HIGHEST PRESSURE?
Highest pressure is after the diffuser before the burner
HIGHEST TEMPERATURE?
Highest temperature is after the burner, before the turbine
Highest velocity
Highest velocity is after the exhaust nozzle
If the density of air _________, mass will increase, and therefore thrust will__________
increases
increase
As air temperature _________, air molecules tend to move apart. This results in a density decrease, and a resultant _________ in thrust
increases
decrease
As air pressure increases, air molecules tend to move closer together. This results in an increase in density, and therefore, thrust_________
increases
With an increase in altitude, however, the rate of thrust decreases because a pressure drop is greater than the thrust increase resulting from a temperature drop. This means an engine will produce ______ thrust as it increases in altitude
less
At approximately 36,000 feet (beginning of the isothermal layer), temperature stabilizes. As a result, temperature will no longer offset the density decrease due to pressure. Therefore, thrust decreases more rapidly. This altitude is also known as the_____________. At this altitude, thrust available plus low fuel flow and diminished drag combine to provide optimum performance for many engines
optimum cruise level
One of the most obvious factors that affects the thrust output is the rotational speed of the engine. With an increase in RPM, there is an increase in thrust. However, at low RPM there is very_______ increase in thrust with an increase in throttle. At higher rates of revolution, a small increase in throttle setting will produce a_______ increase in thrust
At lower settings, fuel consumption is high for the amount of thrust produced. For this reason, gas turbine engines are normally operated at or near their maximum RPM
little
large
Engine Pressure Ratio (EPR): The EPR gauge indicates the pressure ratio between the______ and _______ airflow. The EPR gauge is more widely used because it automatically accounts for some of the airflow variations at the inlet
inlet
exhaust
EGT (Exhaust Gas Temperature): Measured aft of the __________ section
turbine
TIT (Turbine Inlet Temperature): Temperature of the gases from the _____________of the engine as they enter the first stage of the turbine
TIT is the highest temperature inside a turbine engine and is difficult to measure which is why EGT is often measured instead
combustion section
