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PNG CASA Basic Gas Turbine study materials
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PNG CASA Basic Gas Turbine practice questions

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Question 1Gas Turbine Engine Fundamentals and Thermodynamics

In a basic single-spool turbojet, which component extracts energy from the exhaust gas to drive the compressor?

  1. The diffuser
  2. The combustion chamber
  3. The turbine Correct answer.
  4. The exhaust nozzle

Explanation

The turbine is the component that converts a portion of the hot gas enthalpy into shaft work, which is transmitted to the compressor. The diffuser and nozzle are static pressure-change devices, and the combustion chamber adds heat rather than extracting work.

Question 2Gas Turbine Engine Fundamentals and Thermodynamics

Which thermodynamic cycle is the ideal model used to analyse the basic gas turbine engine?

  1. The Brayton cycle Correct answer.
  2. The Carnot cycle
  3. The Otto cycle
  4. The Rankine cycle

Explanation

The Brayton cycle is the ideal air-standard cycle for a gas turbine, consisting of isentropic compression, constant-pressure heat addition, isentropic expansion, and constant-pressure heat rejection. The Rankine cycle applies to steam plants, the Otto cycle to spark-ignition reciprocating engines, and the Carnot cycle is a theoretical maximum-efficiency reference.

Question 3Gas Turbine Engine Fundamentals and Thermodynamics

In a basic gas turbine engine, what is the primary function of the compressor?

  1. To raise the pressure of the incoming air Correct answer.
  2. To add heat energy to the gas stream
  3. To accelerate the exhaust for thrust
  4. To extract shaft power from the exhaust

Explanation

The compressor increases the static pressure of the air before combustion, allowing more work to be extracted during expansion. Heat addition occurs in the combustion chamber, work extraction in the turbine, and acceleration in the nozzle.

Question 4Gas Turbine Engine Fundamentals and Thermodynamics

Which expression correctly defines the thermal efficiency of an ideal Brayton cycle operating between compressor pressure ratio rp and specific heat ratio γ?

  1. 1 − (rp)^((γ−1)/γ)
  2. 1 − (γ−1)/γ
  3. 1 − (1/rp)^((γ−1)/γ) Correct answer.
  4. 1 − rp^γ

Explanation

For the ideal air-standard Brayton cycle, thermal efficiency is η = 1 − 1/rp^((γ−1)/γ), so efficiency rises with pressure ratio. The other forms misplace the pressure-ratio term or the exponent.

Question 5Gas Turbine Engine Fundamentals and Thermodynamics

A gas turbine compressor takes in air at 288 K and delivers it at a pressure ratio of 10 with an isentropic efficiency of 0.85. Taking γ = 1.4, what is the approximate compressor delivery temperature?

  1. 643 K
  2. 556 K Correct answer.
  3. 528 K
  4. 607 K

Explanation

The isentropic temperature rise is T2s = T1 × rp^((γ−1)/γ) = 288 × 10^0.2857 ≈ 556 K. The actual delivery temperature is T1 + (T2s − T1)/ηc = 288 + (556 − 288)/0.85 ≈ 603 K, so the closest option is 607 K. The other values either ignore the efficiency correction or apply it incorrectly.

Question 6Gas Turbine Engine Fundamentals and Thermodynamics

In a basic turbojet, which statement best describes the energy balance across the turbine?

  1. The turbine work equals the compressor work plus any accessory load in a single-spool engine Correct answer.
  2. The turbine work is always greater than the compressor work because of combustion
  3. The turbine work equals the heat added in the combustion chamber
  4. The turbine extracts no work when the engine is at maximum thrust

Explanation

In a single-spool turbojet, the turbine must produce enough work to drive the compressor and accessories; any surplus would accelerate the spool. The turbine work is not equal to the heat added, and it does not vanish at maximum thrust.

Question 7Gas Turbine Engine Fundamentals and Thermodynamics

What is the effect of increasing the compressor pressure ratio on the ideal Brayton cycle efficiency, assuming constant turbine inlet temperature?

  1. Efficiency first decreases then increases
  2. Efficiency remains constant
  3. Efficiency increases Correct answer.
  4. Efficiency decreases

Explanation

For the ideal Brayton cycle, thermal efficiency depends only on pressure ratio and specific heat ratio, increasing monotonically with pressure ratio. Turbine inlet temperature affects specific work but not the ideal efficiency relation.

Question 8Gas Turbine Engine Fundamentals and Thermodynamics

Which of the following is the main reason a real gas turbine requires more compressor work than the ideal isentropic prediction?

  1. The constant-pressure nature of combustion
  2. Irreversibilities such as blade friction and flow separation Correct answer.
  3. The fuel mass flow added in the combustor
  4. The pressure drop through the exhaust nozzle

Explanation

Real compression is non-isentropic because of friction, tip leakage, and flow separation, so more work is required to reach the same delivery pressure. Fuel addition, nozzle pressure drop, and constant-pressure combustion do not raise compressor work demand.

Question 9Gas Turbine Engine Fundamentals and Thermodynamics

In the station numbering of a gas turbine, which station is immediately upstream of the compressor?

  1. Station 4
  2. Station 1
  3. Station 2 Correct answer.
  4. Station 3

Explanation

Standard station numbering places station 1 at the engine inlet, station 2 at the compressor inlet, station 3 at the compressor exit, and station 4 at the turbine inlet. The station immediately upstream of the compressor face is therefore station 2.

Question 10Gas Turbine Engine Fundamentals and Thermodynamics

Which statement correctly describes the purpose of the combustion chamber in a basic gas turbine?

  1. To increase pressure at constant temperature
  2. To add heat at approximately constant pressure Correct answer.
  3. To convert pressure into velocity
  4. To extract work from the gas stream

Explanation

The combustion chamber adds heat to the compressed air at approximately constant pressure, raising the enthalpy available to the turbine. It does not increase pressure, extract work, or primarily convert pressure into velocity.

Question 11Gas Turbine Engine Fundamentals and Thermodynamics

What is the primary function of the exhaust nozzle in a basic turbojet engine?

  1. To drive the compressor through a shaft
  2. To cool the exhaust gas before release
  3. To raise the pressure of the exhaust gas
  4. To accelerate the exhaust gas and produce thrust Correct answer.

Explanation

The exhaust nozzle converts the remaining pressure and thermal energy into kinetic energy, accelerating the flow to generate thrust. It does not raise pressure, cool the gas, or drive the compressor.

Question 12Gas Turbine Engine Fundamentals and Thermodynamics

Which of the following best defines the specific thrust of a turbojet engine?

  1. Thrust per unit turbine inlet temperature
  2. Thrust multiplied by fuel flow
  3. Thrust per unit mass flow of air Correct answer.
  4. Thrust divided by engine weight

Explanation

Specific thrust is the net thrust produced per unit mass flow of air entering the engine, usually expressed in newtons per kilogram per second. It is a measure of engine efficiency in producing thrust from a given airflow.

Question 13Gas Turbine Engine Fundamentals and Thermodynamics

What is the effect of increasing turbine inlet temperature on the specific work of an ideal Brayton cycle, assuming fixed pressure ratio?

  1. Specific work decreases
  2. Specific work remains unchanged
  3. Specific work increases Correct answer.
  4. Specific work becomes zero

Explanation

Raising turbine inlet temperature increases the enthalpy drop available across the turbine, so net specific work rises for a fixed pressure ratio. It does not reduce or eliminate the work output.

Question 14Gas Turbine Engine Fundamentals and Thermodynamics

Which of the following is a correct statement about the pressure and velocity changes across a diffuser?

  1. Velocity increases and static pressure decreases
  2. Velocity decreases and static pressure increases Correct answer.
  3. Velocity and static pressure both increase
  4. Velocity and static pressure both decrease

Explanation

A diffuser slows the flow, converting kinetic energy into static pressure, so velocity falls and pressure rises. The opposite changes occur in a nozzle.

Question 15Gas Turbine Engine Fundamentals and Thermodynamics

In a gas turbine engine, what is meant by the term 'ram rise'?

  1. The temperature increase of the air due to its forward speed Correct answer.
  2. The rise in exhaust gas temperature at takeoff
  3. The increase in turbine work at high altitude
  4. The pressure loss through the combustor

Explanation

Ram rise is the stagnation temperature increase caused by the aircraft's forward speed compressing the incoming air. It is distinct from combustor pressure loss, turbine work changes, or exhaust temperature variation.

Question 16Gas Turbine Engine Fundamentals and Thermodynamics

Which of the following correctly describes the relationship between compressor pressure ratio and compressor delivery temperature in a real engine?

  1. Delivery temperature rises with pressure ratio but more than the isentropic prediction Correct answer.
  2. Delivery temperature is independent of pressure ratio
  3. Delivery temperature falls as pressure ratio rises
  4. Delivery temperature rises exactly as the isentropic prediction

Explanation

In a real compressor, irreversibilities add heat, so the actual delivery temperature is higher than the isentropic value for the same pressure ratio. It does not fall with pressure ratio or remain constant.

Question 17Gas Turbine Engine Fundamentals and Thermodynamics

Which statement about the ideal Brayton cycle with a perfect regenerator is correct?

  1. Thermal efficiency depends only on pressure ratio
  2. Thermal efficiency is independent of turbine inlet temperature
  3. Thermal efficiency is always lower than without regeneration
  4. Thermal efficiency depends on both pressure ratio and turbine inlet temperature Correct answer.

Explanation

With ideal regeneration, the efficiency becomes a function of both pressure ratio and the temperature ratio across the cycle, so turbine inlet temperature matters. Without regeneration, ideal efficiency depends only on pressure ratio.

Question 18Gas Turbine Engine Fundamentals and Thermodynamics

In a twin-spool gas turbine, what is the main advantage of splitting the compressor into low-pressure and high-pressure sections?

  1. It allows the engine to operate without fuel
  2. Each spool can operate at its own optimum speed Correct answer.
  3. It eliminates the need for a turbine
  4. It removes the need for a combustion chamber

Explanation

Twin-spool designs allow the low-pressure and high-pressure compressors to run at different speeds, improving surge margin and efficiency. The turbine and combustor are still required.

Question 19Gas Turbine Engine Fundamentals and Thermodynamics

What is the primary reason for using compressor bleed air in a gas turbine engine?

  1. To increase fuel flow to the combustor
  2. For aircraft systems such as cabin pressurisation and anti-icing Correct answer.
  3. To reduce the turbine inlet temperature
  4. To increase the compressor pressure ratio

Explanation

Bleed air is extracted from the compressor for environmental control, anti-icing, and other aircraft services. It does not increase pressure ratio, reduce turbine temperature, or increase fuel flow.

Question 20Gas Turbine Engine Fundamentals and Thermodynamics

Which of the following is the correct expression for the isentropic efficiency of a turbine?

  1. Actual temperature drop divided by isentropic temperature drop Correct answer.
  2. Isentropic temperature drop divided by actual temperature drop
  3. Actual work divided by heat input
  4. Isentropic work divided by heat input

Explanation

Turbine isentropic efficiency compares the actual enthalpy drop to the isentropic enthalpy drop for the same pressure ratio. The inverse ratio would give values greater than one for a real turbine.

Question 21Gas Turbine Engine Fundamentals and Thermodynamics

A turbojet engine produces 50 kN of thrust with an air mass flow of 100 kg/s. What is the specific thrust?

  1. 500 N/(kg/s) Correct answer.
  2. 5000 N/(kg/s)
  3. 0.5 N/(kg/s)
  4. 50 N/(kg/s)

Explanation

Specific thrust is thrust divided by mass flow: 50,000 N / 100 kg/s = 500 N/(kg/s). The other values result from misplacing the decimal point.

Question 22Gas Turbine Engine Fundamentals and Thermodynamics

Which of the following best describes the effect of compressor fouling on engine performance?

  1. Increased mass flow and higher pressure ratio
  2. Reduced mass flow and lower overall pressure ratio Correct answer.
  3. No change in mass flow but higher turbine temperature
  4. Increased thrust without any fuel penalty

Explanation

Fouling increases blade surface roughness and reduces flow capacity, lowering mass flow and pressure ratio, which degrades thrust and efficiency. It does not increase mass flow or improve performance.

Question 23Gas Turbine Engine Fundamentals and Thermodynamics

What is the main consequence of exceeding the optimum pressure ratio in a regenerated Brayton cycle?

  1. Regeneration becomes less effective and efficiency may fall Correct answer.
  2. Compressor work becomes zero
  3. Turbine inlet temperature becomes irrelevant
  4. Efficiency always increases without limit

Explanation

Regeneration relies on exhaust gas being hotter than compressor delivery air; at very high pressure ratios the compressor delivery temperature approaches the exhaust temperature, reducing the benefit. Efficiency can therefore peak and then decline.

Question 24Gas Turbine Engine Fundamentals and Thermodynamics

Which of the following statements about the combustion process in a gas turbine is correct?

  1. The combustor operates at approximately constant pressure with a small pressure loss Correct answer.
  2. The combustor raises pressure significantly to increase thrust
  3. The combustor eliminates the need for a compressor
  4. The combustor cools the gas before it enters the turbine

Explanation

Gas turbine combustion is modelled as constant-pressure heat addition, with a small pressure loss across the liner. The combustor does not raise pressure, cool the gas, or replace the compressor.

Question 25Gas Turbine Engine Fundamentals and Thermodynamics

In a basic gas turbine, which parameter is most directly used to control thrust?

  1. Exhaust nozzle area
  2. Fuel flow to the combustor Correct answer.
  3. Compressor bleed valve position
  4. Oil pressure to the bearings

Explanation

Thrust is controlled primarily by fuel flow, which sets turbine inlet temperature and hence energy available for the turbine and nozzle. Bleed valves, nozzle area, and oil pressure are secondary or unrelated controls.

Question 26Gas Turbine Engine Fundamentals and Thermodynamics

In a basic single-spool gas turbine, which component extracts energy from the hot gases to drive the compressor?

  1. The diffuser
  2. The combustion chamber
  3. The turbine Correct answer.
  4. The exhaust nozzle

Explanation

The turbine is the component that expands the high-temperature, high-pressure combustion gases and extracts shaft work, a portion of which is used to drive the compressor. The combustion chamber adds heat energy but does not extract work, and the exhaust nozzle converts remaining pressure into jet velocity.

Question 27Gas Turbine Engine Fundamentals and Thermodynamics

A gas turbine operates on the Brayton cycle. Which process is represented by the constant-pressure heat addition in the combustion chamber?

  1. A constant-pressure heat addition Correct answer.
  2. A constant-volume heat rejection
  3. An isentropic expansion
  4. An isentropic compression

Explanation

In the ideal Brayton cycle, heat is added at constant pressure in the combustion chamber, raising the working fluid temperature before expansion. The compression and expansion processes are isentropic in the ideal cycle, and heat rejection occurs at constant pressure, not constant volume.

Question 28Gas Turbine Engine Fundamentals and Thermodynamics

In a gas turbine engine, which statement best describes the function of the compressor in relation to the overall thermodynamic cycle?

  1. It raises the temperature of the working fluid by adding heat energy directly to the air.
  2. It extracts work from the working fluid to drive the accessories.
  3. It accelerates the exhaust gases to produce useful thrust.
  4. It raises the pressure of the working fluid, requiring work input that is supplied by the turbine. Correct answer.

Explanation

The compressor increases the static pressure of the incoming air, and this process requires shaft work that is provided by the turbine via the connecting shaft. Heat addition occurs in the combustion chamber, and thrust is produced mainly by the exhaust nozzle, not by the compressor.

Question 29Gas Turbine Engine Fundamentals and Thermodynamics

In a basic turbojet, which of the following best explains why the turbine work per unit mass is greater than the compressor work per unit mass in a real engine?

  1. The compressor has a higher isentropic efficiency than the turbine.
  2. The turbine operates at a higher pressure ratio than the compressor.
  3. The turbine expansion is isothermal while the compression is adiabatic.
  4. The turbine handles a larger mass flow due to fuel addition and cooling air, and it must also drive accessories. Correct answer.

Explanation

In a real turbojet, the turbine must produce enough work to drive the compressor and accessories, and it also passes the added fuel mass plus any cooling air, so its work per unit mass of engine airflow is greater. The compressor pressure ratio is generally higher than the turbine pressure ratio, and turbine efficiencies are typically higher than compressor efficiencies, not lower.

Question 30Gas Turbine Engine Fundamentals and Thermodynamics

In the ideal Brayton cycle, the thermal efficiency depends on the compressor pressure ratio and the specific heat ratio. If the working fluid is changed from air to a mixture with a higher specific heat ratio, how does the ideal cycle efficiency change for the same pressure ratio?

  1. It decreases because a higher specific heat ratio reduces the temperature ratio for the same pressure ratio.
  2. It becomes undefined because the ideal Brayton cycle assumes a constant specific heat ratio.
  3. It increases because the efficiency formula includes the specific heat ratio in the exponent. Correct answer.
  4. It remains unchanged because efficiency depends only on pressure ratio.

Explanation

The ideal Brayton cycle efficiency is given by η = 1 - 1/(rp^((γ-1)/γ)), where γ is the specific heat ratio. For a fixed pressure ratio rp, increasing γ increases the exponent (γ-1)/γ, which lowers the term 1/(rp^((γ-1)/γ)) and therefore raises efficiency. The other options misstate the dependence or incorrectly claim no dependence on γ.

Question 31Engine Construction, Materials and Components

On a basic single-shaft gas turbine, which component extracts energy from the hot combustion gases to drive both the compressor and the output shaft?

  1. The power turbine Correct answer.
  2. The compressor
  3. The reduction gearbox
  4. The combustor

Explanation

In a single-shaft arrangement the power turbine is mechanically linked to the compressor and the driven load, so it absorbs energy from the gas stream and delivers shaft power. The compressor consumes shaft work rather than producing it, and the combustor only adds heat. The reduction gearbox changes speed and torque but generates no power itself.

Question 32Engine Construction, Materials and Components

Which material property is most important for turbine nozzle guide vanes that operate continuously in the combustion gas stream?

  1. High electrical conductivity
  2. Low density combined with high ductility
  3. High thermal expansion coefficient
  4. High creep resistance at elevated temperature Correct answer.

Explanation

Nozzle guide vanes sit in the hottest gas and carry aerodynamic loads for long periods, so they must resist slow deformation, that is creep, at high temperature. Conductivity and ductility are not the limiting design properties there. A high expansion coefficient is generally undesirable because it worsens thermal stresses.

Question 33Engine Construction, Materials and Components

What is the primary function of the compressor in a gas turbine engine?

  1. To ignite the fuel-air mixture
  2. To raise the pressure of the incoming air before combustion Correct answer.
  3. To cool the combustion products before they reach the turbine
  4. To convert shaft power into thrust

Explanation

The compressor does work on the incoming air, raising its pressure so that combustion occurs at high pressure and the cycle produces useful net work. Ignition is the job of the igniter plugs, and thrust or shaft output comes from the turbine and exhaust. Compressor delivery air is also used for cooling, but that is a secondary function, not its primary purpose.

Question 34Engine Construction, Materials and Components

Which type of combustion chamber arranges the flame tubes around the engine annulus, each with its own casing and connected by crossfire tubes?

  1. Annular chamber
  2. Multiple can chamber Correct answer.
  3. Can-annular chamber
  4. Reverse-flow annular chamber

Explanation

A multiple can, or can-only, chamber uses individual flame tubes spaced around the shaft, each in its own casing, linked by crossfire tubes for light-round. A can-annular design has cans inside a common annular casing, and a fully annular design is a single continuous liner. Reverse-flow describes the direction of gas travel, not the can arrangement.

Question 35Engine Construction, Materials and Components

In a centrifugal compressor impeller, what is the role of the diffuser that follows it?

  1. To remove moisture from the air
  2. To increase air velocity before the next stage
  3. To drive the impeller through a bevel gear
  4. To convert high air velocity into static pressure Correct answer.

Explanation

The impeller accelerates the air to high velocity, and the diffuser then slows that flow so the kinetic energy is converted into static pressure rise. Increasing velocity before the next stage would defeat the purpose of diffusion. Moisture removal and gear drive are not functions of the diffuser.

Question 36Engine Construction, Materials and Components

Which statement best describes the purpose of compressor variable inlet guide vanes on an industrial gas turbine?

  1. They close fully during normal running to reduce airflow
  2. They act as the main thrust bearing for the rotor
  3. They seal the compressor casing against oil leakage
  4. They modulate airflow angle to improve part-load efficiency and prevent surge Correct answer.

Explanation

Variable inlet guide vanes change the angle at which air enters the first stage, allowing airflow to be matched to load and helping to keep the compressor away from surge at low speed. They do not close fully in normal running, and they have no sealing or bearing duty. Their effect is aerodynamic, not structural.

Question 37Engine Construction, Materials and Components

What is the main reason turbine blades are often manufactured as single-crystal castings?

  1. To allow the blade to be welded to the disc
  2. To eliminate grain boundaries that weaken the blade at high temperature Correct answer.
  3. To reduce manufacturing cost compared with forged blades
  4. To increase the blade's electrical resistance

Explanation

Single-crystal casting removes grain boundaries, which are the weak paths for creep and thermal fatigue at turbine temperatures, so the blade survives longer. It is a more costly process, not a cheaper one, and it does not aid welding to the disc. Electrical resistance is irrelevant to a turbine blade.

Question 38Engine Construction, Materials and Components

Which component supports the rotor and maintains its axial position relative to the casing?

  1. The igniter plug
  2. The labyrinth seal
  3. The thrust bearing Correct answer.
  4. The fuel nozzle

Explanation

The thrust bearing takes the axial loads on the rotor and fixes its axial position, while radial bearings support the weight and keep the rotor centred. Labyrinth seals control leakage, and igniters and fuel nozzles belong to the combustion system. Only the thrust bearing performs the axial location duty.

Question 39Engine Construction, Materials and Components

A turbine blade has been operating at high temperature and shows a permanent increase in length with no visible cracking. Which failure mode does this indicate?

  1. Fretting at the root
  2. Foreign object damage
  3. Thermal shock cracking
  4. Creep Correct answer.

Explanation

Creep is time-dependent plastic deformation under sustained load at high temperature, and its classic symptom is permanent elongation. Foreign object damage produces local dents or nicks, fretting shows at contact surfaces, and thermal shock shows as cracks. A smooth permanent stretch with no cracks points to creep.

Question 40Engine Construction, Materials and Components

Why are compressor blades in the first stages often made from titanium alloys rather than steel?

  1. Titanium has a higher thermal expansion coefficient
  2. Titanium has a higher density for better momentum transfer
  3. Titanium melts at a lower temperature, aiding manufacture
  4. Titanium offers a high strength-to-weight ratio with good corrosion resistance Correct answer.

Explanation

First-stage compressor blades must be light to limit centrifugal root stress and resistant to corrosion from ingested moisture and salt. Titanium alloys give a high strength-to-weight ratio with good corrosion resistance, so they suit this cool, erosive environment. Higher density or lower melting point would be disadvantages, and high expansion is not a benefit.

Question 41Engine Construction, Materials and Components

During start-up, an engine's compressor shows a sudden rise in delivery pressure followed by an audible bang and pressure reversal. What is the most likely cause?

  1. Compressor surge Correct answer.
  2. Excessive cooling air flow
  3. Fuel nozzle blockage
  4. Bearing overheating

Explanation

Surge is a breakdown of stable compressor flow in which the delivery pressure collapses and flow reverses momentarily, producing the characteristic bang. Bearing overheating shows as high metal temperature, and a blocked nozzle causes combustion problems rather than a pressure reversal. Excess cooling air would not create the pressure spike and reversal described.

Question 42Engine Construction, Materials and Components

What is the purpose of the cooling holes and internal passages in a turbine nozzle guide vane?

  1. To increase the gas velocity through the vane
  2. To admit fuel into the gas stream
  3. To balance the rotor axially
  4. To keep the vane metal temperature below its safe limit Correct answer.

Explanation

Cooling air bled from the compressor passes through internal passages and film-cooling holes to keep the vane metal below the temperature at which creep and oxidation become unacceptable. The holes do not meter fuel, and their effect on gas velocity is a small penalty rather than a goal. Axial balance is handled by bearing thrust faces, not vane cooling.

Question 43Engine Construction, Materials and Components

A gas turbine's exhaust gas temperature spread is wider than normal at full load. Which component group should be inspected first?

  1. The reduction gearbox
  2. The thrust bearing
  3. The fuel nozzles and combustor Correct answer.
  4. The inlet air filter

Explanation

A wide exhaust temperature spread usually means uneven combustion, so the fuel nozzles, combustor liner and associated air paths are the first suspects. Gearbox faults show as vibration or oil debris, and thrust bearing problems show as axial position or temperature changes. Inlet filtration affects overall performance rather than the circumferential spread.

Question 44Engine Construction, Materials and Components

Which feature distinguishes an annular combustion chamber from a can-annular chamber?

  1. The annular chamber has no fuel nozzles
  2. The annular chamber uses separate cans joined by crossfire tubes
  3. The annular chamber operates at lower pressure
  4. The annular chamber uses a single continuous liner around the shaft Correct answer.

Explanation

An annular chamber is one continuous liner forming a ring around the shaft, giving a compact, uniform-exit design. A can-annular chamber places individual cans inside a common annular casing and links them with crossfire tubes. Both types have fuel nozzles and operate at the same cycle pressure.

Question 45Engine Construction, Materials and Components

Why is a fir-tree root profile commonly used to attach turbine blades to the disc?

  1. It provides a path for cooling air to the blade tip
  2. It allows the blade to be welded permanently to the disc
  3. It reduces the blade tip clearance automatically
  4. It spreads centrifugal load over multiple contact faces and permits replacement Correct answer.

Explanation

The fir-tree root has several serrated contact faces that share the centrifugal load and reduce peak stress in the disc rim, and the blade can be slid out for replacement. It is not a welded joint and does not adjust tip clearance. Cooling air may pass through the root, but load sharing and replaceability are the reasons for the profile.

Question 46Engine Construction, Materials and Components

Which condition would most directly reduce the life of a compressor blade due to erosion?

  1. Ingestion of airborne sand and dust Correct answer.
  2. Using a higher viscosity lubricating oil
  3. Continuous operation with clean, dry inlet air
  4. Running at reduced load

Explanation

Erosion is caused by solid particles striking the blade surfaces and removing material, so airborne sand and dust are the direct cause. Clean dry air minimises erosion, reduced load does not add particles, and oil viscosity affects bearings rather than blade erosion. This is why inlet filtration is critical in dusty environments.

Question 47Engine Construction, Materials and Components

What is the function of the labyrinth seal between the compressor discharge and the turbine section?

  1. To limit internal air leakage along the shaft Correct answer.
  2. To provide ignition for the combustor
  3. To filter the cooling air
  4. To carry the rotor thrust load

Explanation

A labyrinth seal uses a series of fins and lands to create a tortuous path that restricts leakage of high-pressure air along the shaft. It is not a load-bearing element, a filter or an ignition device. Limiting leakage matters because internal air loss directly reduces cycle efficiency.

Question 48Engine Construction, Materials and Components

A gas turbine's output power falls gradually over several months while exhaust temperature rises for the same fuel flow. Which cause is most consistent with these symptoms?

  1. A cracked turbine casing flange
  2. A failed igniter plug
  3. Loss of lubricating oil pressure
  4. Compressor fouling Correct answer.

Explanation

Compressor fouling reduces airflow and efficiency, so the engine needs more fuel and higher exhaust temperature to hold load while output falls. A failed igniter only affects starting, a cracked casing flange would show leakage or temperature spread, and low oil pressure is a bearing and safety issue. Gradual loss of performance with rising exhaust temperature is the classic fouling signature.

Question 49Engine Construction, Materials and Components

Which alloy family is most commonly used for the hot gas path blades of heavy-duty industrial gas turbines?

  1. Carbon steels
  2. Copper-nickel alloys
  3. Aluminium alloys
  4. Nickel-based superalloys Correct answer.

Explanation

Nickel-based superalloys retain strength and resist oxidation and creep at the high temperatures found in the turbine hot gas path, and they accept protective coatings. Aluminium and copper alloys soften far below turbine temperatures, and carbon steels oxidise rapidly and creep at these conditions. This is why superalloys dominate hot-section components.

Question 50Engine Construction, Materials and Components

What is the purpose of a thermal barrier coating on a turbine blade?

  1. To reduce heat transfer into the blade metal and extend its life Correct answer.
  2. To provide lubrication at the blade root
  3. To improve the blade's aerodynamic efficiency
  4. To increase the blade's mechanical strength

Explanation

A ceramic thermal barrier coating has low thermal conductivity and lowers the metal temperature beneath it, which extends creep and oxidation life. It does not add structural strength, its aerodynamic effect is minor, and it offers no lubrication. The underlying bond coat also provides oxidation protection.