About Gas Turbines

Gas turbines are a type of internal combustion engine that extract energy from a stream of compressed air and use it to do work. The work is usually used to drive an electric generator or a compressor. The air is compressed, combined with fuel, and then ignited. This causes the gas turbine to turn, which in turn powers an electric generator.

Types of gas turbines

  • The open cycle gas turbine (OCGT) is the most common type of gas turbine. It is used in power generation and oil & gas applications.
  • The closed cycle gas turbine (CCGT) is a newer type of gas turbine that is more efficient than the OCGT. It is used in power generation and industrial applications.
  • The aircraft gas turbine (AGT) is the type of gas turbine used in jet engines.
  • Gas turbines are classified based on their operating principle:
  • The impulse type Gas Turbine uses the momentum of a high velocity jet of fluid to turn the rotor.
  • The reaction type Gas Turbine uses the propelling action of the fluid to turn the rotor.

Wrapping Up

Gas turbines are used in a variety of applications, including power generation, jet engines, and marine propulsion.

Following are some of the multiple choice questions on the Gas Turbines with answers that will help the students in developing their knowledge.

Gas Turbines MCQ

1. Gas turbine works on

  • Brayton or Atkinson cycle
  • Carnot cycle
  • Rankine cycle
  • Ericsson cycle

2. An aftercooler is used to

  • remove impurities from air
  • reduce volume of air
  • cause moisture and oil vapour to drop out
  • cool the air

3. The overall efficiency of the compressed air system is the

  • Ratio of shaft output of the air motor to the shaft input to the compressor
  • Ratio of shaft input to the compressor to the shaft output of air motor
  • Product of shaft output of air motor and shaft input to the compressor
  • None of the above

4. The air power of the compressor is also known as

  • Indicated power
  • Brake power
  • Frictional power
  • None of these

5. Which of the following plants is smallest and lightest for generating a given amount of power?

  • Gas turbine plant
  • Petrol engine
  • Diesel engine
  • Solar plant

6. The degree of reaction in an axial flow compressor is defined as the ratio of static enthalpy rise in the

  • rotor to static enthalpy rise in the stator
  • stator to static enthalpy rise in the rotor
  • rotor to static enthalpy rise in the stage
  • stator to static enthalpy rise in the stage

7. In a reciprocating air compressor, the compression work per kg of air

  • increases as clearance volume increases
  • decreases as clearance volume increases
  • is independent of clearance volume
  • increases as clearance volume decreases

8. An axial compressor gives optimum performance at high speeds and large volume flows.

  • Agree
  • Disagree

9. The increase in pressure in a roots blower is entirely due to back flow of air.

  • Agree
  • Disagree

10. If p1, is the pressure of air entering the L.P. cylinder and p2 is the pressure of air leaving the L.P. cylinder (or intercooler pressure), then the ratio of cylinder diameters for a single acting, two stage reciprocating air compressor with complete intercooling is given by (where D1 = Dia. of L.P. cylinder, and D2 = Dia. of H.P. cylinder)

  • D1/D2 = p1p2
  • D1/D2 = p1/p2
  • D1/D2 = p2/p1
  • none of these

11. The ratio of the volume of free air delivery per stroke to the swept volume of the piston, is known as

  • compressor efficiency
  • volumetric efficiency
  • isothermal efficiency
  • mechanical efficiency

12. High air-fuel ratio in gas turbines

  • increases power output
  • improves thermal efficiency
  • reduces exhaust temperature
  • do not damage turbine blades

13. The maximum combustion pressure in gas turbine is __________ as compared to I.C. engine.

  • more
  • less

14. The compression ratio in a gas turbine is

  • 4
  • 1
  • 9
  • 12

15. The actual volume of air delivered by a compressor, When reduced to the normal temperature and pressure conditions is called compressor capacity.

  • Yes
  • No

16. When the temperature of air leaving the intercooler, in a two stage compression with intercooler, is __________ the original atmospheric air temperature, then the intercooling is known as perfect or complete intercooling.

  • equal to
  • less than
  • more than
  • none of the above

17. Intercooling in gas turbines

  • decreases net output but increases thermal efficiency
  • increases net output but decreases thermal efficiency
  • decreases net output and thermal efficiency both
  • increases net output and thermal efficiency both

18. A closed cycle gas turbine gives __________ efficiency as compared to an open cycle gas turbine.

  • same
  • lower
  • higher
  • none of the above

19. A large clearance Volume in a reciprocating compressor results in

  • reduced volume flow rate
  • increased volume flow rate
  • lower suction pressure
  • lower delivery pressure

20. In a centrifugal compressor, an increase in speed at a given pressure ratio causes

  • increase in flow
  • decrease in flow
  • increase in efficiency
  • increase in flow and decrease in efficiency

21. A rotary compressor is driven by an

  • electric motor
  • engine
  • either (a) or (b)
  • none of these

22. Workdone by a two-stage reciprocating air compressor per cycle is equal to the workdone in LP. cylinder and H.P. cylinder.

  • True
  • False

23. Rotary compressors are used for delivering

  • small quantities of air at high pressures
  • large quantities of air at high pressures
  • small quantities of air at low pressures
  • large quantities of air at low pressures

24. The efficiency of a jet engine is higher at

  • low speeds
  • high speeds
  • low altitudes
  • high altitudes

25. The type of rotary compressor used in gas turbines, is of

  • centrifugal type
  • axial flow type
  • radial flow type
  • none of these

26. In the aircraft propellers

  • the propulsive matter is ejected from within the propelled body
  • the propulsive matter is caused to flow around the propelled body
  • its functioning does not depend upon the presence of air
  • none of the above

27. For a two stage reciprocating compressor, compression from p1 to p3 is with perfect intercooling and no pressure losses. If compression in both the cylinders follows the same polytropic process and the atmospheric pressure is pa, then the intermediate pressure p2 is given by

  • p2 = (p1 + p3)/2
  • p2 = p1.p3
  • P2 = Pa x p3/p1
  • p2 = pa p3/p1

28. The inlet pressure is always __________ the discharge pressure.

  • equal to
  • less than
  • more than
  • none of the above

29. The clearance volume in the compressor is kept minimum because it effects on volumetric efficiency.

  • Correct
  • Incorrect

30. The ratio of workdone per cycle to the stroke volume of the compressor is known as

  • compressor capacity
  • compression ratio
  • compressor efficiency
  • mean effective pressure

31. Intercooling in multi-stage compressors is done

  • to cool the air during compression
  • to cool the air at delivery
  • to enable compression in two stages
  • to minimise the work of compression

32. The capacity of a compressor is expressed in

  • kg/m2
  • kg/m3
  • m3/min
  • m3/kg

33. The increase in pressure in a vane blower takes place first due to compression and then due to back flow of air.

  • Correct
  • Incorrect

34. An open cycle gas turbine works on the same cycle as that of a closed cycle gas turbine.

  • Yes
  • No

35. The total heat rejected in a reciprocating air compressor is equal to the sum of the heat rejected during polytropic compression per kg of air and heat rejected in the intercooler per kg of air.

  • True
  • False

36. The rotary compressors are suitable for large discharge of air at low pressure.

  • True
  • False

37. The volumetric efficiency for reciprocating air compressors is about

  • 10 to 40%
  • 40 to 60%
  • 60 to 70%
  • 70 to 90%

38. A rocket works with maximum overall efficiency when air-craft velocity is __________ the jet velocity.

  • equal to
  • one-half
  • double

39. The compressed air may be used

  • in gas turbine plants
  • for operating pneumatic drills
  • in starting and supercharging of I. engines
  • all of the above

40. In a four stage compressor, if the pressure at the first and third stage are 1 bar and 16 bar, then the delivery pressure at the fourth stage will be

  • 1 bar
  • 16 bar
  • 64 bar
  • 256 bar

41. The reason for volumetric efficiency of reciprocating compressor being less than 100 percent is

  • Pressure drop across the valves
  • Superheating in compressor
  • Clearance volume and leakages
  • All of these

42. In a jet engine, the air-fuel ratio is

  • 30:1
  • 40:1
  • 50:1
  • 60:1

43. Free air is the air at

  • Atmospheric conditions at any specific location
  • 20°C and 1 kg/cm² and relative humidity of 36%
  • 0°C and standard atmospheric conditions
  • 15°C and 1 kg/cm²

44. Standard air is the air at

  • Atmospheric conditions at any specific location
  • 20°C and 1 kg/cm² and relative humidity 36%
  • 0°C and standard atmospheric conditions
  • 15°C and 1 kg/cm²

45. The work-done on a compressor will be minimum if air is taken from

  • The atmosphere
  • A source at 0° C
  • A source of low temperature air
  • A source of high temperature air

46. In a double acting compressor, the air is compressed

  • In one cylinder
  • In two cylinders
  • In a single cylinder on both sides of the piston
  • In two cylinders on both sides of the piston

47. The maximum delivery pressure in a rotary air compressor is

  • 10 bar
  • 20 bar
  • 30 bar
  • 50 bar

48. Propulsion efficiency of the following order is obtained in practice

  • 34%
  • 50%
  • 60%
  • 72%

49. The type of rotary compressor used in aeroplanes, is of

  • Centrifugal type
  • Axial flow type
  • Radial flow type
  • None of these

50. Isothermal compression though most efficient, but is not practicable because

  • It requires very big cylinder
  • It does not increase pressure much
  • It is impossible in practice
  • Compressor has to run at very slow speed to achieve it

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