Chapter 1 - Gas Turbines Flashcards
What is the definition of mass?
Force required to overcome the inertia of a body
What is the definition of momentum?
Measure of the quantity of motion
What is the definition of power?
Rate of changing energy
What is the equation for power?
Energy / Time
Why is heat regarded as ‘disorderly energy’?
Because of its random patterns and the difficulty of channelling it efficiently
What is important to know about pressure energy?
A fluid under compression will attempt to flow from a high pressure to a low pressure, and in so doing, will or can do work
How do you convert Fahrenheit from Centigrade?
(9/5 x °C) + 32
What is Newton’s First Law of Motion?
Every object is compelled to change it’s state of motion by an external force acting upon it
What is Newton’s Second Law of Motion?
Rate of change of momentum of a body is proportional to the total force acting upon it
What is Newton’s Third Law of Motion?
For every action there is an equal and opposite reaction
What does Bernoulli’s theory state?
Sum of the pressure energy, the potential energy and the kinetic energy is constant through a tube
What is the difference between the gas turbine engine and the piston engine?
Gas turbine is continuous
Why can the ignition device be switched off after ignition is achieved in a modern jet engine?
The flame will be continuous, providing fuel is supplied
Why can the starter be switched off after ignition is achieved in a modern jet engine?
The hot gases crossing the turbine produce torque to drive the compressor
Ideally, what should the compression and exhaust phases be?
Isothermal - temperature of the working fluid should remain constant
Ideally, what should the combustion process be?
Isobaric - constant pressure
Ideally, what should the adding of energy to the working fluid be?
Adiabatic - no heat is lost from the mass flow ensuring that all the heat added remains to do useful work
What must air into the intake be?
‘Clean’, free from debris and also free from turbulence
What are all intakes subject to during flight manoeuvres?
‘Blanking off’
Why are perfectly round intakes preferred?
Little chance of setting up low-pressure areas within the intake which can then induce vortices into the airflow
How can the ‘Ram effect’ be induced in an intake?
The gas first passes through a convergent duct, then a divergent duct into the compressor inlet
Initially accelerates the gas, increasing the mass flow of air, then slowing this increased mass in order to raise its temperature
Increase in pressure can be achieved due purely to the forward speed of the aircraft
Why are pitot intakes not suitable for transonic velocities?
Shock wave forms at the forward lip of the intake
Disrupts airflow into the intake
What kind of intake is used at supersonic speeds?
Variable geometry which slow the air down and spill off excess air
What is the purpose of the dump valve in a supersonic intake?
Pressure obtained at the inlet duct may be excessive, so dump valves rid the intake of unwanted air
What are the two types of compressor?
- Centrifugal vane
- Axial flow
What is stationed around the disc or impeller of the centrifugal type of compressor?
‘Diffuser’ ring
Ring of stationary vanes formed with divergent cross section between them
Why is the centrifugal compressor’s efficiency considered to be relatively low?
High speed of rotation and drastic changes in the air-flow direction leading to a high temperature increase
What is the limit to the pressure ratio and tip speed of a centrifugal compressor?
4.5:1
1600 ft/sec
What are the main advantages of the centrifugal compressor?
- Simple and comparatively cheap to manufacture
- Robust in construction and less vulnerable to damage
What are the main disadvantages of the centrifugal compressor?
- High speed of rotation required
- Large frontal area
- Limited pressure ratio
- High temperature increase
What forms a ‘stage’ of a compressor?
Row of stators and a row of rotors
What does the convergent passage through an axial flow compressor ensure?
Uniform axial velocity
What is the difference between the single-spool and multi-spool compressor?
Single-spool - consists of one rotor assembly
Multi-spool - consists of two or more rotor assemblies, each driven by their own turbine at an optimum speed
What is the advantage of a multi-spool compressor?
- Greater pressure ratios
- Greater operating flexibility
Where does the ‘cold’ stream pass in a large fan fitted to the front of a single or twin-spool compressor?
Bypasses the other compressors and is ducted to atmosphere through the cold stream nozzle
What is the cross sectional area between each adjacent stator and rotor blade in an axial compressor?
Divergent
Why is the area between each stator and rotor blade divergent?
Causes the remaining velocity energy to be converted into pressure and temperature
What is the purpose of the stators between each rotor?
Present the air at the correct angle to the next stage of rotor blades
What is the reason for the small pressure increase through each stage?
To prevent air breakaway at the blades, and subsequent blade stall
What is the purpose of the ‘twist’ of each axial compressor blade?
Give the correct angle of incidence at each point of the blade, to maintain a fairly uniform axial velocity of the air
Why are stator blades usually shrouded in the front stages?
Minimise the vibrational effect of flow variations on the longer blades
How much engine thrust can the high by-pass ratio fan produce?
75%
What is the purpose of the containment ring?
Prevent a detached fan blade to penetrate the engine casing and hazard the aircraft
Shaped to ‘flip’ the blade so that it will be swept down the fan duct before the next fan blade can hit it
What prevents blade flutter in typical fan blades?
Mid-span shrouds (also called snubber or clappers)
What are the advantages of Wide Chord Fan Blades?
- Lighter
- Shorter
- Move more air for a given RPM
- Better able to centrifuge FOD into the fan duct
- More rigid, and thus do not require mid-span shrouds
What are the signs of compressor stall?
- Vibration
- Rumbling noise
- Inability of the engine to accelerate
- Rapid rise in exhaust gas temperature
What is surge?
Air at high pressure at the back of the compressor will travel to the low pressure zone at the front of the engine
Recognised by loud bang or banging in the intake
At what speed are fixed stator axial compressors designed to work most efficiently at?
At the engine’s maximum cruise RPM
What is the purpose of Inlet Guide Vanes (IGV)/Variable Inlet Guide Vanes (VIGV)?
Give ‘whirl’ to the air entering the front of the compressor
Air is presented to the first stage rotor at the most efficient angle of attack
What will likely control the Variable Angle Stator Vanes (VSV)?
Full Authority Digital Engine Control (FADEC)
What is the purpose of a bleed valve?
Bleed off excess pressure towards the rear of the compressor, preventing stall and surge
What are the open and closed conditions of the bleed valve?
Open at idle RPM
Closed at higher RPM
What are the two main types of fuel nozzles/burners?
- Vaporising
- Atomising
Describe a vaporising type of fuel nozzle/burner?
Vaporises fuel by pre-heating it prior to combustion, as the burner nozzle is forward facing so that the flame is heating the fuel delivery pipe and vaporising the fuel
Known as ‘hockey-stick’
Raise problems when engine starting
Describe the atomising type of fuel nozzle/burner?
Passes through a swirl chamber where tangentially disposed holes or slots impart a swirl to the fuel by converting its pressure to kinetic energy
Fuel is atomised to form a cone shaped spray
How much air flow is used for cooling and at what temperatures can burning the fuel reach?
80%
2,000°C
How much air is required for the complete combustion of fuel?
15 lbs of air to 1 lb of fuel
15:1 by weight
What are the three main flows of airflow into the flame tube?
- Primary air flow
- Secondary air flow
- Tertiary airflow
What is the purpose of primary air flow?
Mixing with the fuel and to support combustion
What is the purpose of secondary air flow?
Shape the flame and complete combustion
What is the purpose of tertiary air flow?
Cut off the flame and reduce gas temperature to a figure acceptable to the turbine
What is mnemonic steel?
Special heat resistant steel
What does the secondary and tertiary air flow also form?
A boundary flow on the inside and outside of the flame tube
What are the four main types of combustion system?
- Multiple chamber
- Tube-annular (can-annular)
- Annular
- Reverse flow
What is the purpose of the interconnectors in a multiple chamber of combustion chamber?
- Equalise the pressure generated throughout the system during combustion
- Propagates the flame around all the chambers
What is the advantage of a reverse flow combustion system?
Significantly reduced engine length
What are the stationary guide vanes called in the turbine section?
Nozzle guide vanes (NGV)
What are the two basic types of turbine used to produce torque?
- Impulse type
- Reaction type
Describe the cross sectional geometry of impulse/reaction turbine blades?
Aerofoil shape with a partial ‘cup’ shape at the forward part of the blade
How does the impulse/reaction turbine extract energy from the gas?
Partial cup shape absorbs some of the kinetic energy from the air
Adjacent blades form a convergent duct which accelerates the gas flow and causes a further reaction, producing even greater rotational force
What is the purpose of the NGVs?
- Accelerate the gas stream to as high a velocity as possible by utilising a convergent shape
- Direct the gas at the most efficient angle of attack onto the turbine assembly
What is the primary factor that limits the amount of power a gas turbine can produce?
Maximum temperature that can be tolerated at the turbine inlet (Turbine Inlet Temperature TIT)
What other factor must be considered where there is more than one turbine stage?
Interstage (or Intermediate) Turbine Temperature (ITT)
In a twin spool engine, what will the forward most turbine drive?
The HP compressor
The first turbine the air will encounter
What is a free power turbine (Power Turbine, Free Turbine or Gas Coupled Turbine)?
No direct link to any of the engine compressors
How is blade cooling of turbine blades achieved?
Percentage of mass flow of air is passed through holes formed in the blades
Air comes from compressor stage, which is relatively cool (400°C)
Why must blade cooling air come from the rear of the HP compressor?
Colder air could cause thermal shock leading to stress cracks forming
What is the most common type of blade attachment?
‘Fir tree’ fixing
Describe ‘fir tree’ fixing?
Very accurate machining is required to ensure that loading is shared by all serrations
Free in the serrations when the turbine is stationary, and is stiffened in the root by centrifugal loading when the turbine is rotating
At what speed does gas enter the exhaust system from the turbine section?
750 - 1200 ft/sec
Why is there a divergent area between an exhaust cone and the outer wall?
To decrease the speed of the airflow from the turbine section, to reduce the effect of high friction losses
What is the secondary purpose of the exhaust cone?
Prevents hot gases from flowing across the rear face of the turbine disc
What are the three methods of reducing the noise from a jet engine?
- Use a smaller throttle setting (thrust/power) for take-off
- Wrap the engine in a noise insulating material
- Reduce the velocity of the jet efflux or increase the velocity of the surrounding (ambient) air at the exhaust
Where is the noisiest part of a high by-pass engine?
Fan
What are the requirements of a thrust reverse system?
- Not affect engine operation, either when in use or not
- Be able to withstand high temperatures
- Be mechanically strong but light in weight
- Be ‘fail safe’
What is the difference between the propeller thrust reverse system and the typical turbo engine?
Propeller - reversal of thrust is 180° forward
Turbo - 45° to the horizontal in the forward direction
What is the actual amount of useful reverse thrust?
Approximately 50% of normal forward thrust
What are the main types of thrust reverser?
- Clamshell door
- Retractable ejector
- Cold stream cascade reverser
How is the retractable ejector actuated?
Hydraulically
What is another name for the retractable ejector?
Bucket target type
Describe cold stream reversers?
Hydraulically or pneumatically operated doors, called translation cowls, that uncover a set of cascade vanes
Blocker doors also simultaneously block off the cold stream and divert it through those vanes
How is it ensured that high velocity engine exhaust gases will not damage the thrust reverse system as it deploys?
Throttle lever must be in the minimum power position (fully to the rear) prior to selecting reverse thrust
What is the main disadvantage of a single spool engine?
Takes too long to accelerate (spool up)
What is the tip speed usually in a high by-pass turbofan engine?
Around Mach 1.3
What is the weight reduction of a typical low by-pass engine over a pure jet of similar mass flow?
20%
How can a by-pass engine obtain the same power at the turbine to drive the compressors and the accessories than a pure jet?
Turbine inlet temperature (TIT) is elevated and the pressure ratio is also increased
How much energy is available for jet thrust in a turbo-prop engine?
10%
What is the main difference between a turbo-prop and turbo-jet engine?
Turbo-prop engine usually has more stages
How does Boyles’ and Charles’ Law relate to how a gas turbine engine works?
If a mass of air which is not confined in volume is heated, there will also be a subsequent increase in the volume of the gas while the pressure remains approximately constant
What is the Brayton Cycle?
The working cycle of the gas turbine engine which goes through the stages of compression, combustion and expansion plotted on a graph of volume against pressure
What are the two types of centrifugal compressor?
Single and double-entry
Approximately what percentage of compressor delivery air is used for initial combustion?
20%
What safety features are there to protect against asymmetry of the thrust reverser system?
If the operating system of one thrust reverser were to fail, a mechanical lock would prevent it from deploying until it is restored, and in any case not until WOW switch is made
The operational reverser will be automatically isolated
What are the types of by-pass engines?
Low by-pass - ratio of 1:1
High by-pass - ratio of over 1:1 to more than 6.5:1
