Compressors, Gas Turbines, and Jet Engines MCQ

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

a) To ignite the fuel

b) To increase the pressure of the air

c) To convert kinetic energy to potential energy

d) To cool the air

Answer:

b) To increase the pressure of the air

Explanation:

In a gas turbine engine, the compressor's primary function is to increase the pressure of the incoming air before it enters the combustion chamber. This allows for more efficient combustion.

2. What is the Brayton cycle?

a) A thermodynamic cycle used in steam turbines

b) A thermodynamic cycle used in refrigeration systems

c) A thermodynamic cycle used in gas turbines

d) A thermodynamic cycle used in hydraulic turbines

Answer:

c) A thermodynamic cycle used in gas turbines

Explanation:

The Brayton cycle is a thermodynamic cycle that describes the workings of a constant pressure heat engine, commonly used in gas turbine engines.

3. What is an axial flow compressor?

a) A compressor where the flow of air is parallel to the axis of rotation

b) A compressor where the flow of air is perpendicular to the axis of rotation

c) A compressor that uses centrifugal force to compress air

d) A compressor that operates at subsonic speeds only

Answer:

a) A compressor where the flow of air is parallel to the axis of rotation

Explanation:

An axial flow compressor is a type of compressor in which the air or gas flows parallel to the axis of rotation. This design allows for high flow rates and high efficiency.

4. In a jet engine, what is the purpose of the turbine?

a) To compress the air

b) To produce thrust

c) To drive the compressor and other accessories

d) To ignite the fuel-air mixture

Answer:

c) To drive the compressor and other accessories

Explanation:

The turbine in a jet engine extracts energy from the high-temperature, high-pressure gases produced by the combustion process. This energy is used to drive the compressor and other accessories.

5. What is the function of the nozzle in a jet engine?

a) To slow down the exhaust gases

b) To direct the flow of air into the compressor

c) To increase the velocity of the exhaust gases

d) To mix fuel with air

Answer:

c) To increase the velocity of the exhaust gases

Explanation:

The nozzle in a jet engine accelerates the exhaust gases to high velocities, creating thrust according to Newton's third law of motion.

6. What is a centrifugal compressor?

a) A compressor that increases pressure using a rotating disk

b) A compressor that increases pressure using axial flow

c) A compressor used only in subsonic aircraft

d) A compressor that operates using the Venturi effect

Answer:

a) A compressor that increases pressure using a rotating disk

Explanation:

A centrifugal compressor increases the pressure of air or gas by using a rotating disk or impeller. The air enters the impeller axially and is accelerated radially outward.

7. What is the primary difference between a turbojet and a turbofan engine?

a) The type of fuel used

b) The presence of a fan at the front of the engine

c) The operating speed of the engine

d) The type of compressor used

Answer:

b) The presence of a fan at the front of the engine

Explanation:

The main difference between a turbojet and a turbofan engine is the presence of a fan at the front of a turbofan engine. The fan increases the volume of air accelerated by the engine, improving efficiency.

8. What is surge in the context of compressors?

a) A sudden decrease in pressure

b) A rapid increase in engine speed

c) A condition where the compressor cannot add enough energy to overcome the system resistance

d) A normal operating condition in all compressors

Answer:

c) A condition where the compressor cannot add enough energy to overcome the system resistance

Explanation:

Surge in compressors is a condition where the flow through the compressor is reduced to the point where it cannot add enough energy to overcome the system resistance, leading to a reversal of flow.

9. What is the function of stators in an axial compressor?

a) To compress the air

b) To direct the flow onto the rotor blades at the correct angle

c) To produce thrust

d) To reduce the temperature of the air

Answer:

b) To direct the flow onto the rotor blades at the correct angle

Explanation:

Stators in an axial compressor are stationary blades that direct the air onto the rotor blades at the correct angle. They also convert the velocity increase given by the rotor blades into a pressure increase.

10. In a gas turbine engine, what is the function of the combustor?

a) To compress the air

b) To cool the air

c) To mix air with fuel and ignite it

d) To extract energy from the exhaust gases

Answer:

c) To mix air with fuel and ignite it

Explanation:

The combustor in a gas turbine engine mixes the compressed air with fuel and ignites the mixture. This process releases energy in the form of high-temperature, high-pressure gases.

11. What is the main advantage of a turbofan engine over a turbojet engine?

a) Higher thrust

b) Lower fuel consumption

c) Higher speed

d) Simpler design

Answer:

b) Lower fuel consumption

Explanation:

The main advantage of a turbofan engine over a turbojet is its lower fuel consumption, particularly at subsonic speeds. The fan increases the mass flow through the engine, improving its efficiency.

12. What causes thrust in a jet engine?

a) The high pressure in the combustion chamber

b) The rotation of the turbine blades

c) The acceleration of air and exhaust gases out of the engine

d) The compression of air in the compressor

Answer:

c) The acceleration of air and exhaust gases out of the engine

Explanation:

Thrust in a jet engine is produced by the acceleration of air and exhaust gases out of the engine, following Newton's third law of motion (action and reaction).

13. What is the purpose of the diffuser in a jet engine?

a) To increase the velocity of air entering the compressor

b) To decrease the velocity and increase the pressure of air entering the compressor

c) To mix the air with fuel

d) To cool the air before it enters the combustor

Answer:

b) To decrease the velocity and increase the pressure of air entering the compressor

Explanation:

The diffuser in a jet engine slows down the incoming air, which increases its pressure before it enters the compressor. This process improves the efficiency of the compressor.

14. What is the efficiency of a jet engine primarily dependent on?

a) The altitude at which it operates

b) The temperature of the combustion chamber

c) The size of the engine

d) The type of fuel used

Answer:

b) The temperature of the combustion chamber

Explanation:

The efficiency of a jet engine is primarily dependent on the temperature in the combustion chamber. Higher temperatures lead to higher efficiencies, as they allow more complete combustion of fuel and better conversion of heat to mechanical energy.

15. What is the purpose of a gas turbine in a combined cycle power plant?

a) To generate electrical power directly

b) To provide heat for industrial processes

c) To drive a steam turbine for additional power generation

d) To increase the pressure of the working fluid

Answer:

c) To drive a steam turbine for additional power generation

Explanation:

In a combined cycle power plant, the gas turbine's primary purpose is to drive a generator for power production and to provide exhaust heat to generate steam, which then drives a steam turbine for additional power generation.

16. What type of compressor is typically used in small gas turbine engines?

a) Axial flow compressor

b) Centrifugal compressor

c) Reciprocating compressor

d) Screw compressor

Answer:

b) Centrifugal compressor

Explanation:

Centrifugal compressors are commonly used in small gas turbine engines due to their simpler design, lower cost, and compact size, making them suitable for small aircraft and helicopters.

17. What is the bypass ratio in a turbofan engine?

a) The ratio of air passing through the core to the air bypassing the core

b) The ratio of fuel to air in the combustion chamber

c) The ratio of thrust produced by the fan to the total thrust

d) The ratio of air intake area to the exhaust nozzle area

Answer:

a) The ratio of air passing through the core to the air bypassing the core

Explanation:

In a turbofan engine, the bypass ratio is the ratio of the mass flow of air that bypasses the engine core (around the outside of the engine) to the mass flow of air passing through the core. Higher bypass ratios generally lead to increased fuel efficiency and reduced noise.

18. In a centrifugal compressor, what effect does the diffuser have on the air flow?

a) Increases its velocity

b) Decreases its velocity and increases its pressure

c) Decreases its temperature

d) Increases its temperature

Answer:

b) Decreases its velocity and increases its pressure

Explanation:

In a centrifugal compressor, the diffuser slows down the high-velocity air coming from the impeller, converting its kinetic energy into pressure energy, thereby increasing the air pressure.

19. How does a ramjet engine produce thrust?

a) Using a compressor and turbine

b) By igniting fuel in a combustion chamber with no moving parts

c) Using a fan to accelerate air

d) Through the action of pistons

Answer:

b) By igniting fuel in a combustion chamber with no moving parts

Explanation:

A ramjet engine produces thrust by taking in air at high speeds (due to aircraft motion), compressing it in a diffuser, mixing it with fuel, igniting the mixture in a combustion chamber, and expelling the hot exhaust gases to produce thrust. It operates efficiently at high speeds and has no moving parts like compressors or turbines.

20. What is the key characteristic of a scramjet engine?

a) It operates at subsonic speeds

b) It compresses air using mechanical compressors

c) It operates at hypersonic speeds with supersonic combustion

d) It uses a fan to produce thrust

Answer:

c) It operates at hypersonic speeds with supersonic combustion

Explanation:

A scramjet (supersonic combustion ramjet) engine is designed to operate at hypersonic speeds, where the airflow through the engine remains supersonic, and combustion occurs at these high speeds.

21. In gas turbine engines, what is the role of the intercooler?

a) To cool the exhaust gases

b) To reheat the air between compressor stages

c) To cool the air between compressor stages

d) To inject fuel into the combustion chamber

Answer:

c) To cool the air between compressor stages

Explanation:

An intercooler in gas turbine engines is used to cool the air between stages of compression. This increases the efficiency of the compression process by reducing the work required to compress the cooler air.

22. What principle does a turbocharger in an internal combustion engine use?

a) Electric motor-driven forced induction

b) Exhaust gas-driven forced induction

c) Mechanical linkage to the engine crankshaft

d) Independent fuel combustion

Answer:

b) Exhaust gas-driven forced induction

Explanation:

A turbocharger uses the principle of exhaust gas-driven forced induction. It utilizes the engine's exhaust gases to drive a turbine, which then drives a compressor that forces more air into the combustion chamber, enhancing engine power.

23. What is meant by 'isentropic efficiency' of a compressor or turbine?

a) Efficiency compared to an ideal cycle operating under constant pressure

b) Efficiency compared to an ideal cycle operating under constant temperature

c) Efficiency compared to an ideal cycle with no heat transfer

d) Efficiency compared to an ideal cycle with no change in entropy

Answer:

d) Efficiency compared to an ideal cycle with no change in entropy

Explanation:

Isentropic efficiency is a measure of the performance of a compressor or turbine compared to an idealized process that operates isentropically, i.e., with no change in entropy or ideally reversible adiabatic processes.

24. What is stall in the context of axial compressors?

a) A condition where the compressor operates at maximum efficiency

b) A condition where there is a sudden drop in pressure across the compressor

c) A condition of flow instability that leads to a drop in performance

d) The maximum rotational speed of the compressor

Answer:

c) A condition of flow instability that leads to a drop in performance

Explanation:

Stall in axial compressors is a condition of flow instability characterized by local flow reversal, leading to a reduction in compressor performance and potential surge conditions.

25. What is the function of afterburners in jet engines?

a) To cool the exhaust gases

b) To reduce engine noise

c) To increase thrust by burning additional fuel in the exhaust

d) To improve fuel efficiency

Answer:

c) To increase thrust by burning additional fuel in the exhaust

Explanation:

Afterburners in jet engines are used to increase thrust. They burn additional fuel in the engine's exhaust, significantly increasing the exhaust velocity and hence the thrust, especially useful for military aircraft during takeoff and combat maneuvers.

26. What is a Francis turbine?

a) A type of wind turbine

b) A type of steam turbine

c) A type of reaction turbine used in hydroelectric power plants

d) A type of gas turbine

Answer:

c) A type of reaction turbine used in hydroelectric power plants

Explanation:

A Francis turbine is a type of reaction turbine used in hydroelectric power plants. It operates under a wide range of flow conditions and has a runner with fixed blades, designed to convert hydraulic energy into mechanical energy.

27. In a compressor, what is the significance of the surge line on a performance map?

a) It represents the most efficient operating condition

b) It separates the stable operating conditions from the unstable ones

c) It indicates the maximum achievable pressure

d) It shows the relationship between temperature and pressure

Answer:

b) It separates the stable operating conditions from the unstable ones

Explanation:

The surge line on a compressor performance map marks the boundary between stable and unstable operating conditions. Operating to the left of the surge line can lead to compressor surge, a condition of flow reversal that can damage the compressor.

28. What are bleed valves in a gas turbine engine used for?

a) Releasing excess fuel from the engine

b) Reducing the air pressure in the combustion chamber

c) Extracting air from the compressor to prevent surge

d) Cooling the turbine blades

Answer:

c) Extracting air from the compressor to prevent surge

Explanation:

Bleed valves in a gas turbine engine are used to extract or 'bleed' air from the compressor stages. This helps in stabilizing the compressor operation and preventing conditions like surge and stall.

29. What is a propelling nozzle in a jet engine?

a) A device to inject fuel into the combustion chamber

b) A nozzle used to direct the airflow into the compressor

c) A nozzle that accelerates the exhaust gases to produce thrust

d) A component used to cool the engine

Answer:

c) A nozzle that accelerates the exhaust gases to produce thrust

Explanation:

A propelling nozzle in a jet engine is a critical component that accelerates the exhaust gases leaving the engine to high speeds, creating thrust according to Newton's third law of motion.

30. How does a radial turbine differ from an axial turbine?

a) In a radial turbine, the fluid flows parallel to the axis, whereas in an axial turbine, it flows perpendicular to the axis

b) In a radial turbine, the fluid flows perpendicular to the axis, whereas in an axial turbine, it flows parallel to the axis

c) Radial turbines are used only in jet engines, while axial turbines are used in power plants

d) Radial turbines are less efficient than axial turbines

Answer:

b) In a radial turbine, the fluid flows perpendicular to the axis, whereas in an axial turbine, it flows parallel to the axis

Explanation:

In a radial turbine, the working fluid changes direction and flows radially outward or inward relative to the axis of rotation, unlike an axial turbine where the flow is parallel to the axis.

31. What role does the diffuser play in a centrifugal compressor?

a) It increases the velocity of the air

b) It decreases the velocity and increases the pressure of the air

c) It cools the air before it enters the compressor

d) It injects fuel into the air stream

Answer:

b) It decreases the velocity and increases the pressure of the air

Explanation:

In a centrifugal compressor, the diffuser slows down the high-velocity air exiting the impeller. This reduction in kinetic energy is converted into an increase in pressure.

32. In jet engines, what is the bypass air used for in high bypass turbofan engines?

a) To cool the engine

b) To increase fuel efficiency and reduce noise

c) To provide additional thrust

d) Both b and c

Answer:

d) Both b and c

Explanation:

In high bypass turbofan engines, the bypass air (air that does not go through the core) is used to increase fuel efficiency and reduce noise. It also contributes to thrust, making these engines suitable for commercial airliners.

33. What is the primary advantage of using a gas turbine over a reciprocating engine for power generation?

a) Higher power-to-weight ratio

b) Lower operating temperatures

c) Simpler fuel requirements

d) More consistent power output

Answer:

a) Higher power-to-weight ratio

Explanation:

Gas turbines have a higher power-to-weight ratio compared to reciprocating engines, making them more efficient for power generation, especially in applications where weight and space are critical.

34. What is choking in the context of compressors and nozzles?

a) A condition where the flow rate decreases suddenly

b) The point at which the flow reaches the speed of sound and cannot increase further

c) A blockage in the compressor or nozzle

d) A decrease in pressure leading to flow reversal

Answer:

b) The point at which the flow reaches the speed of sound and cannot increase further

Explanation:

Choking is a phenomenon in compressors and nozzles where the flow reaches the speed of sound (Mach 1) at the throat of the nozzle or compressor passage. Beyond this point, the flow rate does not increase with a further decrease in downstream pressure.

35. What is a free turbine in a gas turbine engine?

a) A turbine that is not mechanically connected to the compressor

b) A turbine that operates without any load

c) A turbine that does not use any fuel

d) A turbine that can rotate in either direction

Answer:

a) A turbine that is not mechanically connected to the compressor

Explanation:

In a free turbine engine, the power turbine (which drives the load, such as a generator or propeller) is not mechanically connected to the gas generator turbine (which drives the compressor). This allows for more flexibility in operation and is commonly used in turboprop and turboshaft engines.