Compressors, gas turbines and Jet Propulsion - Mechanical Engineering test questions

Compressors, gas turbines and Jet Propulsion - Mechanical Engineering test questions

1) For minimum work input to the compressor, the compression law followed is

a) pV^1.35 = C
b) pV^γ = C
c) pV^1.2 = C
d) pV = C

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2) If the index of compression is ‘n’, the work input to the air compressor

a) Remains same whatever the value of ‘n’.
b) Decreases with increase in value of ‘n’.
c) Increases with increase in value of ‘n’.
d) First increases and then decreases with increase of value of ‘n’.

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3) Why is the clearance volume in reciprocating air compressors provided?

a) To create turbulence in the air to be delivered.
b) To accommodate valves in the head of the compressor.
c) To increase the volumetric efficiency of the compressor.
d) To reduce the work done per kg of air delivered.

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4) With suction pressure being atmospheric, increase in delivery pressure with fixed clearance volume

a) Does not change volumetric efficiency.
b) Decreases volumetric efficiency.
c) Increases volumetric efficiency.
d) First increases volumetric efficiency and then decreases it.

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5) On which of the following does the pressure in centrifugal air compressor depends upon?

a) Compression index.
b) Inlet-temperature.
c) Impeller tip velocity.
d) All of these.

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6) Intercooling in multistage compressors is done

a) To cool the air at delivery.
b) To cool the air during compression.
c) To minimise the work of compression.
d) None of these.

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7) Intake air filters are provided on compressors

a) To remove dirt and dust from suction air.
b) To reduce temperature of suction air.
c) To reduce pressure of suction air.
d) None of these.

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8) ……………………. cannot be the probable cause when a water cooled compressor knocks during operation.

a) Too much clearance in crank pin.
b) Air filter choked.
c) Excessive cooling in intercooler.
d) Loose valve.

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9) In multistage reciprocating air compressors the compression will be isothermal if

a) Compressor runs at slow speed.
b) Compressor cylinder is cooled properly.
c) Pressure ratio is same for each stage.
d) Perfect inter cooling is provided to the air being compressed.

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10) The volumetric efficiency of an air compressor is the ratio of

a) Displacement volume to clearance volume.
b) Volume of air before compression to the volume of air after compression.
c) Displacement volume to volume of air sucked inside the cylinder.
d) Actual free air delivered to the displacement volume.

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11) Isothermal compression efficiency can be approached by using ………………even when the compressor is running at high speed.

a) Cold water spray.
b) Multi-stage compression.
c) Both a and b.
d) None of these.

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12) ………………is the most efficient method of compressing air.

a) Isothermally.
b) Polytropically.
c) Isentropically.
d) Adiabatically.

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13) The ratio of indicated power to shaft-power is known as ……………..efficiency.

a) Isothermal.
b) Volumetric.
c) Mechanical.
d) Adiabatic.

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14) At high altitude a compressor will draw

a) Less power.
b) More power.
c) Same power.
d) None of these.

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15) The absolute pressure of air at the outlet of a compressor is called …………….pressure.

a) Suction.
b) Critical.
c) Back.
d) Discharge.

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16) If W_c is the work done due to compression and W_bf is the work done due to back flow then the efficiency of the vane blower is

a) W_c/(W_c+W_bf)
b) W_bf/(W_c+W_bf)
c) (W_c+W_bf)/W_c
d) (W_c+W_bf)/W_bf

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17) …………….compressors is mostly used for supercharging I.C. engines?

a) Radial flow compressor.
b) Roots blower.
c) Axial flow compressor.
d) Reciprocating compressor.

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18) What is the best method of controlling compressor during peak load periods?

a) Variable speed.
b) Constant speed unloader.
c) Start-stop motor.
d) Relief valve.

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19) Slip factor for a centrifugal impeller with radial vanes is the ratio of

a) Isentropic work to Euler work.
b) Whirl velocity to the blade velocity at the impeller exit.
c) Isentropic temperature rise to actual temperature rise.
d) Stagnation pressure to static pressure.

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20) In a compressor the phenomenon of surging refers to

a) Fixed mass flow rate regardless of pressure ratio.
b) Reduction in lift force at high angles of incidence.
c) No pressure rise, there is only churning of flow.
d) Unsteady, periodic and reversed flow.

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21) An axial flow compressor ……………….as compared to centrifugal compressor.

a) Has large air handling ability per unit.
b) Has high pressure ratio per stage.
c) Needs a low starting torque.
d) None of these.

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22) Which of the following statement is incorrect regarding multistage compression?

a) Best suited for compression.
b) High volumetric efficiency.
c) Low power consumption per unit of air delivered.
d) None of these.

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23) The indicated work per unit mass of air delivered is

a) Not affected by clearance volume.
b) Inversely proportional to clearance volume.
c) Directly proportional to clearance volume.
d) None of these.

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24) The function of a diffuser in a compressor is

a) It converts pressure energy into kinetic energy.
b) It converts kinetic energy into pressure energy.
c) It increases degree of reaction.
d) None of these.

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25) In an axial flow compressor, stalling of blades is the phenomenon of

a) Air stream not able to follow the blade control.
b) Production of no air pressure.
c) Either of the above.
d) None of these.

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26) For minimum work in compressor operating between limits p1 and p3 the best intercooler pressure p2 is given by

a) p2 = √P₁ + P₃
b) p2 = √P₁ - P₃
c) p2 = (p₁ + p₂)/ 2
d) p2 = √P₁ P₃

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27) The compresses air line should ………………………to check moisture troubles.

a) Rise gradually towards the point of use.
b) Drop gradually towards the point of use.
c) Be laid horizontally.
d) None of these.

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28) The work ratio of closed cycle gas turbine plant depends upon

a) Only on pressure ratio.
b) Temperature ratio of the cycle and specific heat ratio.
c) Pressure ratio of the cycle and specific heat ratio.
d) Pressure ratio, temperature ratio and specific heat ratio.

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29) Ideally the compressor work and turbine work are ………..for a jet propulsion unit.

a) Not related to each other.
b) Equal.
c) Unequal.
d) Unpredictable.

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30) Compared to steam turbine, a gas turbine

a) Has less efficiency.
b) Has less weight per horse power developed.
c) Has compressor and combustion chamber.
d) All of these.

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31) In gas turbines, high air-fuel ratio

a) Increases power output.
b) Reduces exhaust temperature.
c) Improves thermal efficiency.
d) None of these.

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32) On which factor does the work ratio of simple gas turbine cycle depends?

a) Pressure ratio.
b) Maximum cycle temperature.
c) Minimum cycle temperature.
d) All of these.

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33) Which of the following statement is correct regarding ‘gas turbine’?

a) It requires lot of cooling water.
b) It has flat efficiency at part loads.
c) It can be used to generate power only.
d) It is capable of rapid start up and loading.

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34) A regenerator in a gas turbine cycle is used to

a) Heat the turbine exhaust before it enters the low pressure stage.
b) Heat the gases leaving the combustion chamber.
c) Heat the compressed air on its way to combustion chamber.
d) Heat the fuel being supplied to combustion chamber.

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35) The effect of ‘intercooling’ in a gas turbine plant is it

a) Increases in net work output but decreases in thermal efficiency.
b) Decreases in both compressor and turbine work.
c) Fall in both work ratio and thermal efficiency.
d) None of these.

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36) ‘Reheating’ in a gas turbine plant is done

a) To increase the power output.
b) To increase the thermal efficiency of the plant.
c) To increase the temperature of gases at exhaust.
d) All of these.

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37) What is work ratio?

a) It is the ratio of actual heat drop and isentropic heat drop in a turbine.
b) It is the ratio of nee work output and compressor work.
c) It is the ratio of compressor work and turbine work.
d) It is the ratio of net work output and turbine work

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38) How does regeneration improve the thermal efficiency of a simple open cycle gas turbine plant?

a) By decreasing the quantity of heat supplied in combustion chamber.
b) By lowering the work input to compressor.
c) By increasing the turbine plant.
d) All of these.

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39) The efficiency of a jet engine is higher

a) At low speeds.
b) At high altitudes.
c) At high speeds.
d) At low altitudes.

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40) The disadvantage of jet propulsion is

a) The thermal efficiency is lower, particularly at low pressure.
b) The plant is very noisy, materials costly and life short.
c) The compression ratio is not constant.
d) All of these.

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41) The basic requirement of a solid propellant is

a) Mechanical strength should be adequate specially at low temperature.
b) Non-toxic in nature.
c) Specific impulse should be high enough.
d) All of these.

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42) A jet engine works on the principle of

a) Conservation of mass.
b) Conservation of energy.
c) Conservation of momentum.
d) Conservation of discharge.

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43) Propulsive efficiency is the ratio of

a) Engine input and propulsive work.
b) Propulsive work and heat released by combustion of fuel.
c) Thrust power and propulsive power.
d) Thrust power and heat released by combustion of fuel.

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44) ………………is used to increase the thermal efficiency of aircraft jet engines.

a) Reheating.
b) Intercooler.
c) Regeneration.
d) High temperature and pressure.

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45) In gas turbines the pressure ratio is the ratio of

a) Highest pressure to exhaust pressure.
b) Pressure across turbines.
c) Exhaust pressure to inlet pressure.
d) None of these.

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