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Part 4 Systems > BGT Final Final > Flashcards

BGT Final Final Flashcards

1
Q

What is another name for the Brayton cycle?
Why is it called this?
What are the stages of the cycle?

A

CONSTANT PRESSURE cycle;
PRESSURE is fairly CONSTANT across COMBUSTION section as VOLUME attempts to INCREASE, VELOCITY also INCREASES;
INTAKE, COMPRESSION, EXPANSION/POWER, EXHAUST

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2
Q

Describe the process of the Brayton cycle using diagrams?

A

A-B: AIR is COMPRESSED from ATMOSPHERIC
B-C: HEAT added by introducing and BURNING FUEL at CONSTANT PRESSURE, INCREASING VOLUME slight DECREASE in PRESSURE
C-D: Gas from COMBUSTION EXPAND through TURBINE and JET PIPE to ATMOSPHERE, energy turned to MECHANICAL POWER by TURBINE and remainder on discharge provides PROPULSIVE JET
D-A: PRESSURE is AMBIENT AIR, VOLUME DECREASES to equal AMBIENT AIR

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3
Q

What are the differences of Brayton cycle to Otto cycle?

A

Brayton: Combustion at constant PRESSURE, cycle CONTINUOUS, elimates 3 IDLE STROKES therefore MORE POWER for engine SIZE, HEATS at constant PRESSURE (VOLUME INCREASES) so NO PEAKS or FLUCTUATION only requires LIGHT FABRICATION of COMBUSTION chamber and LOW OCTANE FUEL;
Otto: Combustion at constant VOLUME, cycle INTERMITTENT, PEAKS of PRESSURE in excess of 1000PSI requiring HEAVY CYLINDER CONSTRUCTION and HIGH OCTANE FUEL

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4
Q

What are the comparative advantages of centrifugal and axial flow compressors?

A

CENTRIFUGAL: MORE ROBUST, EASIER to DEVELOP and MANUFACTURE, favoured in SMALLER ENGINES for SIMPLICITY and ROBUSTNESS
AXIAL: CONSUMES MORE AIR for SAME frontal AREA therefore MORE THRUST, HIGHER PRESSURE RATIOS, INCREASE PRESSURE by ADDITIONAL STAGES

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5
Q

What is the purpose of inlet guide vanes?

A

STATIONARY VANES that DIRECT airflow into ROTOR at MOST DESIRABLE

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6
Q

What are rotor blades and their purpose?

What is the construction of rotor blades?

A

The FIRST COMPONENT in the COMPRESSION stage that MOVES AIR REARWARD through each stage;
AERODYNAMICALLY DESIGNED with varying ANGLE of INCIDENCE

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7
Q

What are stator blades and their purpose?

What is the construction of rotor blades?

A

Receive AIR at HIGH VELOCITY from ROTOR blades and act DIFFUSERS changing KINETIC energy into PRESSURE energy;
AEROFOIL shaped BLADES

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8
Q

What are the advantages of multi spool compressors?

A

OPERATIONAL FLEXIBILITY:
HIGH COMPRESSION RATIOS;
QUICK ACCELERATION;
Better CONTROL of STALL CHARACTERISTICS

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9
Q

What is a compressor stall?

What are the symptoms of it?

A

ABRUPT LOSS of EFFICIENCY of the AXIAL FLOW COMPRESSOR in turbine engine when AoA of COMPRESSOR BLADES becomes excessive, IMBALANCE between RPM and INLET VELOCITY;
NO SYMPTOMS however if about to SURGE may experience VIBRATION, and FLUCTUATIONS in TEMP, FUEL FLOW and THRUST INDICATIONS

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10
Q

What are the causes of a compressor stall?

A

TURBULENT or DISRUPTED AIRFLOW to engine INLET - DECREASED GAS VELOCITY;
EXCESSIVE FUEL FLOW by abrupt engine ACCELERATIONS - INCREASED combustion back PRESSURE and DECREASED GAS VELOCITY;
EXCESSIVE LEAN fuel mixture by abrupt engine DECELERATION - DECREASED combust or back PRESSURE and INCREASED GAS VELOCITY;
CONTAMINATED or DAMAGED COMPRESSORS - DECREASED COMPRESSION and INCREASED GAS VELOCITY;
DAMAGED TURBINE COMPONENTS causing LOSS of POWER to COMPRESSOR and LOW COMPRESSION - DECREASED COMPRESSION and INCREASED GAS VELOCITY;
Engine operation ABOVE or BELOW designed RPM - INCORRECT RPM SETTING for PHASE of flight

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11
Q

What happens to airflow generally in a compressor stall?
What is the name and characteristics of a mild compressor stall?
WHat about a severe compressor stall?

A

Air flow SLOWS/STAGNATES/REVERSES;
TRANSIENT stall: NOT usually HARMFUL, CORRECT themselves after 1 OR 2 PULSATIONS;
HUNG stall: Significantly DECAY ENGINE PERFORMANCE, cause LOSS of POWER, DAMGE or FAIL ENGINE

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12
Q

Using disrupted airflow describe the process that leads to a compressor stall?
Include situation, symptoms (normal and extreme)

A

JET with 2 REAR MOUNTED ENGINES TURNS;
SIDESLIP causes LESS AIRFLOW and LOW INLET AIR VELOCITY;
INCREASE in EFFECTIVE AoA leading to possible NOISE/RPM fluctuation/EGT INCREASE;
Fixed by REDUCING POWER to correct proportion of RPM to INLET VELOCITY;
EXTREME caused by FUEL MALFUNCTION or foreign OBJECT INGESTION;
Leads to REVERSAL that put BENDING STRESS on REAR of compressor BLADES causing them to CONTACT STATOR VANES;
Can cause disintegration of ROTOR SYSTEM and complete ENGINE FAILURE

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13
Q

What is the purpose of the turbines?

Simply, how does it achieve this?

A

DRIVE the COMPRESSOR;
DRIVE the ACCESSORIES and SHAFT of TURBO PROP/SHAFT;
CONVERTS KINETIC and HEAT energy from the EXHAUST GAS to MECHANICAL WORK

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14
Q

Specifically, how does the turbine produce mechanical work?

A

EXTRACTS ENRRGY from gas by REDUCING PRESSURE;
Achieved since the PRESSURE is CONVERTED to VELOCITY by CONVERGENT NOZZLES formed at TRAILING EDGE of STATOR and ROTOR BLADES;
Then TURBINES ROTATE

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15
Q

List the turbine blade designs?

A

IMPULSE BLADE;
REACTION BLADE;
IMPULSE REACTION

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16
Q

What are the characteristics of impulse blade design?

A

STATOR VANE and ROTOR BLADE;
VANES form CONVERGENT DUCTS;
BLADES form STRAIGHT DUCTS;
ROTOR TURNS by IMPULSE as GASES IMPINGE on blades

17
Q

What are the characteristics of reaction blade design?

A

STATOR VANE and ROTOR BLADE;
VANES form STRAIGHT DUCTS;
BLADES form CONVERGENT DUCTS;
CONVERGENING ROTOR TURNS by REACTION FORCE from EXPANSION/ACCELERATION of GAS

18
Q

What are the characteristics of impulse-reaction blade design?
What is the most efficient set up?

A

STATOR VANE and ROTOR BLADE with COMPOUND CURVATURE;
TWIST distributes WORKLOAD EVENLY along length by keeping EXIT PRESSURE and VELOCITY uniform tip to base;
Accomplished by EXTRACTING different KINETIC ENERGY at VARIOUS blade STATIONS;
50% REACTION greatest at TIP, 50% IMPULSE greatest at ROOT

19
Q

What is process for creation of pressure thrust?

A

EXHAUST gas pass to ATMOSPHERE through a CONVERGENT PROPELLING NOZZLE INCREASING VELOCITY;
During MOST operations excluding low thrust (subsonic) EXIT VELOCITY is SPEED of SOUND to the EGT and NOZZLE is CHOKED;
When CHOKED, VELOCITY CANNOT INCREASE unless TEMP is INCREASED;
When UPSTREAM TOTAL PRESSURE is INCREASED ABOVE CHOKED value the STATIC pressure of GAS is ABOVE ATMOSPHERIC;
This PRESSURE DIFFERENCE is PRESSURE THRUST and is EFFECTIVE over NOZZLE EXIT area;
It is ADDITIONAL to thrust from MOMENTUM CHANGE of GAS stream

20
Q

What do convergent and divergent passage cause air to do individually?

A

Convergent: GAS ACCELERATES;
Divergent: GAS DECELERATES and PRESSURE INCREASES

21
Q

What happens to the energy of gas when a convergent exhaust nozzle is used?
What can be used instead?
How does is this method more advantagous?

A

Energy is WASTED as GASES leaving do NOT EXPAND RAPIDLY enough to immediately achieve outside air PRESSURE;
If HIGH PRESSURE RATIO ENGINE use CONVERGENT-DIVERGENT nozzle to RECOVER WASTE energy;
Uses PRESSURE energy to INCREASE gas VELOCITY more therefore INCREASING THRUST

22
Q

What are the requirements of oils?

A 
LOW VOLATILITY;
ANTI FOAMING;
LOW LACQUER and COKE DEPOSITS;
HIGH FLASH POINT;
LOW POUR POINT;
FILM STRENGTH;
WIDE TEMPERATURE RANGE;
HIGH VISCOSITY INDEX
23
Q

Why are low volatility, anti foaming, low lacquer and coke deposits, and high flash point desirable qualities of oil?

A

LV: To MINIMISE EVAPORATION at ALTITUDE;
AF: For POSITIVE LUBRICATION;
LL+CD: Keeps SOLID PARTICLES MINIMAL
HFP: INCREASES the TEMP that FLAMMABLE VAPOURS will IGNITE when NEAR FLAME source

24
Q

Why are low pour point, film strength, wide temperature range, and high viscosity index desirable qualities of oil?

A

LPP: LOWEST TEMP oil will GRAVITY FLOW
FS: Excellent COHESION and ADHESION allowing OIL molecules to stick together under COMPRESSION and stick to surfaces under CENTRIFUGAL forces;
WTR: -60°F to +400°F
HVI: RETAINS VISCOSITY when HEATED

25
Q

What is the purpose of magnetic plugs and chips in the oil system?
What is the principle of operation of each?

A

Fitted to RETURN SIDE of oil system to COLLECT FERROUS DEBRIS in RETURN OIL;
Magnetic plug: PERMANENT MAGNET inserted in oil, READILY REMOVED and INSPECTION good for EARLY WARNING of FAILURE;
Chip detector: MAGNETIC PLUGS that incorporate ELECTRONIC DEVICE that TRANSMITS signal to COCKPIT when there is SIGNIFICANT COLLECTIONS/SIZE of DEBRIS

26
Q

What is the requirement of fuel systems?

A

Supply PRECISE amount of FUEL in ALL GROUND and AIR CONDITIONS;
FREE of DANGEROUS OPERATIONAL CHARACTERISTICS;
INCREASE and DECREASE POWER on command to obtain THRUST in ANY CONDITION

27
Q

How is the fuel metering unit driven?
What combinations can the FMU operate under?
How is it controlled?

A

ENGINE DRIVEN;
MECHANICAL/HYDRAULIC/ELECTRICAL/PNEUMATIC;
HYDRO-MECHANICAL/HYDRO-PNEUMATIC/ELECTRO-HYDROMECHANICAL;
NOT DIRECTLY PILOT, from COMPUTER instead

28
Q

Excluding the FMU what other control devices are included in the turbine engine?
What is their purpose?

A 
PRESSURE CONTROL;
FLOW CONTROL;
PRESSURE RATIO CONTROL;
ACCELERATION and SPEED CONTROL;
Achieved by using a form of GOVERNOR;
LIMIT ENGINE from EXCEEDING set PARAMETERS
29
Q

What are fuel nozzles alternative name?

What is their purpose?

A

INJECTORS/DISTRIBUTORS;

ATOMISING/VAPOURISING FUEL to ensure RAPID BURNING

30
Q

What is the construction of the simplex type and duplex type fuel nozzles?

A

Simplex: EARLY JETS, CHAMBER induces SWIRL into FUEL and FIXED AREA ATOMISING ORIFICE;
Duplex: PRIMARY and MAIN FLOW TUBE with 2 INDEPENDENT ORIFICES (1 much smaller), SMALL handles LOWER FLOW and LARGER handles HIGH FLOW as fuel PRESSURE INCREASE

31
Q

What is bypass ratio?
What are the functions of the magnitude of bypass ratios?
What is a typical bypass ratio value? What are they reaching now?
What type of aircraft is it used on?

A

MASS AIRFLOW through BYPASS DUCT to the MASS AIRFLOW through PRIMARY GAS PATH in same TIMEFRAME;
SIZE of BYPASS DUCT, PRIMARY FLOW INTAKE and VELOCITY of air;
2:1 - 5:1 up to 30:1
SOME TURBOJET and ALL TURBOFAN

32
Q

What is the principle of after burning?

What is it primarily used for?

A

Introducing and burning FUEL in EXHAUST GASES between TURBINE and PROPELLING NOZZLE of engine;
INCREASED TEMP results in INCREASED JET VELOCITY and INCREASED THRUST;
Improving TAKEOFF and CLIMB PERFORMANCE

33
Q

Why can after burning realistically be achieved?

A

Approximately 1/3 of TOTAL AIR is COMPLETELY BURNT, so much of it is still AVAILABLE for further EXPANSION when MIXED and BURNT with more FUEL

34
Q

How does the arrangement of the spray bars in the after burner affect the airflow?

A

Since TEMP is usually 1700°C or HIGHER, BARS are placed so FLAME is CONCENTRATED around AXIS of PIPE;
Only a PROPORTION of UNBURNT AIR is USED in COMBUSTION, the rest flows along the WALLS of JET PIPE to PROVIDE COOLING BARRIER

35
Q

What is the purpose of water cooling?

A

POWER output is proportional to MASS AIRFLOW and therefore AIR DENSITY;
WATER is used to COOL to INCREASE AIR DENSITY to RESTORE/INCREASE POWER produced/available

36
Q

When an aircraft utilises injection cooling, when is water/methanol used?

A

Pure water: If COMPLETELY CONSUMED during TAKE OFF ROLL (B707);
Methanol added: NOT COMPLETELY CONSUMED during TAKEOFF ROLL so ANIT FREEZE qualities of methanol are used (TURBO PROP/HELICOPTER)

37
Q

What are the methods of injecting coolant into the airflow?

Which is more suitable for axial flow compressor engines? Why?

A

COOLANT sprayed DIRECTLY into COMPRESSOR INLET;
COOLANT INJECTED into COMBUSTION CHAMBER INLET;
Combustion chamber inlet since MORE EVEN DISTRIBUTION can be obtained and GREATER QUANTITY can be satisfactorily INJECTED

38
Q

What happens when spraying into compressor inlet?

What is the purpose of added methanol when spraying into the compressor inlet?

A

TEMP of COMPRESSOR INLET AIR and AIR DENSITY and THRUST are INCREASED;
If only WATER was injected TURBINE INLET TEMP would DECREASE;
With METHANOL, TURBINE INLET TEMP is RESTORED by it BURNING in COMBUSTION CHAMBER;
POWER is RESTORED WITHOUT adjusting FUEL FLOW

39
Q

What happens when spraying into combustion chamber?

A

INCREASES MASS AIRFLOW through TURBINE, relative to COMPRESSOR;
PRESSURE and TEMP drop across TURBINE is REDUCED, INCREASING JET PIPE PRESSURE, INCREASING THRUST;
REDUCED TURBINE INLET TEMP due to injection allows scheduled INCREASE of FUEL FLOW

Part 4 Systems (4 decks)

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