Quiz: Mechanical Engineering 1 April 2020

Exam: USSSC JE

Topic: Miscellaneous

Each question carries 1 mark

Negative marking: 1/4 mark

Time: 10 Minutes

Q1. The centre of pressure for an inclined surface area

(a) Lies below the centroid of the surface

(b) Coincides with the centroid

(c) Lies above the centroid of the surface

(d) None of the above

L1Difficulty 3

QTags Mechanical

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Q2. A bucket of water hangs with a spring balance. If an iron piece is suspended into water from another support without touching the sides of the bucket, the spring balance will show

(a) an increased reading

(b) a decreased reading

(c) no change in reding

(d) increased or decreased reading depending on the depth of immersion

L1Difficulty 3

QTags Mechanical

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Q3. Match List-I (Measuring device) with List-II (Parameters) regarding a body partly submerged in a liquid and select the correct answer using the codes given below the lists:

List-I List-II

A. Anemometer 1. Flow rate

B. Piezometer 2. Velocity

C. Pitot tube 3. Static pressure

D. Orifice 4. Difference between static and stagnation pressure

Codes :

A B C D

(a) 1 3 4 2

(b) 1 2 3 4

(c) 2 3 4 1

(d) 2 4 3 1

L1Difficulty 3

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Q4. Priming is necessary in

(a) Centrifugal pumps to lift water from a greater depth

(b) centrifugal pumps to remove air in the suction pipe and casing

(c) Hydraulic turbine to remove air in the turbine casing

(d) Hydraulic turbine to increase the speed of turbine and to generate more power

L1Difficulty 3

QTags Mechanical

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Q5. Match List-I with List-II and select the correct answer using the codes given below the lists:

List I

Work done in a polytropic process

Work done in steady flow process

Heat transfer in a reversible adiabatic process

Work done in an isentropic process

List- II

-∫▒Vdp

Zero

(p_1 V_1-p_2 V_2)/(γ-1)

4. (p_1 V_1-p_2 V_2)/(n-1)

Codes

A B C D

(a) 4 1 3 2

(b) 1 4 2 3

(c) 4 1 2 3

(d) 1 2 3 4

L1Difficulty 3

QTags Mechanical

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Q6. In a heat engine operating in a cycle between a source temperature of 606°C and a sink temperature of 20°C, what will be the least rate of heat rejection per kW net output of the engine?

(a) 0.50 kW

(b) 0.667 kW

(c) 1.5 kW

(d) 0.0341 kW

L1Difficulty 3

QTags Mechanical

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Q7. Match list I with list II and select the correct answer using the codes given below the lists:

List – I List – II

A. Bell Coleman refrigeration 1. Compressor

B. Vapour compression refrigeration 2. Generator

C. Absorption refrigeration 3. Flash chamber

D. Jet refrigeration 4. Expansion cylinder

codes

A B V D

(a) 1 4 3 2

(b) 4 1 3 2

(c) 1 4 2 3

(d) 4 1 2 3

L1Difficulty 3

QTags Mechanical

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Q8. The knocking tendency in compression ignition engines increases with

(a) increase of coolant water temperature

(b) increase of temperature of inlet air

(c) decrease of compression ratio

(d) increase of compression ratio

L1Difficulty 3

QTags Mechanical

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Q9. Dimensionless time is represented by

(a) Biot number

(b) Fourier number

(c) Euler number

(d) Graetz number

L1Difficulty 3

QTags Mechanical

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Q10. In a counter flow heat exchanger, the product of specific heat and mass flow rate is same for the hot and cold fluids. If NTU is equal to 0.5 then the effectiveness of the heat exchanger is

(a) 1.0

(b) 0.5

(c) 0.33

(d) 0.2

L1Difficulty 3

QTags Mechanical

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SOLUTIONS

S1. Ans.(a)

Sol.

h=x ̅+I_G/(Ax ̅ ) sin^2 θ

x ̅ = Centre of gravity

I_G = Area moment of inertia of surface about its centroidal axis and parallel to free surface.

S2. Ans.(c)

Sol.

Since the water of bucket will exert force of buoyancy equal to the volume displaced by the iron block. Hence equal and opposite force will be exerted by iron block on water hence the reading of the spring balance will increase.

S3. Ans.(c)

Sol.

Anemometer: It is device for measuring wind speed, and it a common weather station instrument.

Orifice plate: It is a device used for measuring the volumetric flow. It uses the Bernoulli’s principle.

S4. Ans.(b)

Sol. filling the pump casing and the suction pipe with the liquid before it is started is known as priming. Unless the centrifugal pump is primed, it will not start as there is no centrifugal head impressed on the liquid. Positive displacement type reciprocating or rotary pump do not need priming because the air, if any, in the cylinder or casing is driven out by the piston or vanes.

S5. Ans.(c)

Sol.

A. Work done in a polytropic process: (p_1 V_1-p_2 V_2)/(n-1)

B. Work done in steady flow process -∫▒Vdp

C. Heat transfer in a reversible adiabatic process: zero

D. Work done is an isentropic process: (p_1 V_1-p_2 V_2)/(γ-1)

S6. Ans.(a)

Sol.

T_1=606+273=879 K

T_2=20+273=293 K

η=(T_1-T_2)/T_1

=(879-293)/879=W/Q_1 (since W = 1 kW)

0.6666=〖1/Q〗_1

Q_1=1.5 kW

Amount of heat rejected

Q_2=Q_1-W=1.5-1.0=0.5 kW

S7. Ans.(d)

Sol.

1. Bell Coleman refrigeration: Expansion cylinder

2. Vapour compression refrigeration: compressor

3. Absorption refrigeration: Generator

4. Jet Refrigeration: Flash chamber

S8. Ans.(c)

Sol.

Decrease of compression ratio will reduce temperature and increase delay period. This increases the knocking tendency in CI engine.

S9. Ans.(b)

Sol. Dimensionless time is represented by Fourier number.

Mathematically, Fourier number = (αt )/(L_C^2 )

Where

α = Thermal diffusivity

=(k/ρC)

L_C = characteristics length

t = time

S10. Ans.(c)

Sol. C=1

∴ ε=NTU/(1+NTU)

ε=0.5/1.5=1/3=0.33