# Multiple choice question for engineering

## Set 1

1. Reflectivity is defined as

a) Fraction of total energy transmitted by the body

b) Fraction of energy reflected by the body

c) Fraction of total energy absorbed by the body

d) Fraction of total energy absorbed and radiated by the body

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2. Of the radiant energy 350W/m^{2} incident upon a surface 250W/m^{2} is absorbed, 60W/m^{2} is reflected and the remainder is transmitted through the surface. Workout the value for reflectivity for the surface material

a) 0.181

b) 0.171

c) 0.161

d) 0.151

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3. A thin metal plate of 4 cm diameter is suspended in atmospheric air whose temperature is 290 K. This plate attains a temperature of 295 K when one of its face receives radiant energy from a heat source at the rate of 2 W. If heat transfer coefficient on both surfaces of the plate is stated to be 87.5 W/m ^{2} K, workout the energy lost by reflection

a) 0.8 W

b) 0.3 W

c) 0.9 W

d) 0.10 W

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4. The value of reflectivity depends upon

(i) Body temperature

(ii) Wavelength of reflected radiation

(iii) Nature of surface of body

Identify the correct statement

a) 1 and 2

b) 2 and 3

c) 2 only

d) 1 and 3

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5. When a surface absorbs a certain fixed percentage of striking radiation the surface is called

a) Grey body

b) Black body

c) White body

d) Opaque body

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6. Which one is true regarding gases?

a) Reflectivity is unity

b) Absorptivity is zero

c) Reflectivity is zero

d) Transmissivity is zero

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7. Thin glass plate is an example of

a) Transparent body

b) Opaque body

c) Black body

d) White body

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8. Radiant energy with an intensity of 800 W/m^{2} strikes a flat plate normally. The absorptivity is thrice the reflectivity and twice the transmissivity. Determine the rate of reflection

a) 518.20 W/m^{2}

b) 418.20 W/m^{2}

c) 318.20 W/m^{2}

d) 218.20 W/m^{2}

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_{0 }= 218.20 W/m

^{2}.

9. The total radiant energy A impinging upon a body would be partially or totally absorbed, reflected from its surface or transmitted through it. Identify the correct statement

a) B is the energy that is absorbed by the body

b) B is the energy that is reflected by the body

c) C is the energy that is absorbed by the body

d) D is the energy that is transmitted by the body

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10. Which one is true for opaque body?

a) Transmissivity is zero

b) Reflectivity is zero

c) Absorptivity is zero

d) Reflectivity is unity

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## Set 2

1. “The response of a thermocouple is defined as the time required for the thermocouple to reach the surrounding temperature when it is exposed to it”. Choose the correct answer

a) True

b) False

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2. The sensitivity of thermocouple is defined as the time required by thermocouple to reach how much percentage of its steady state values?

a) 43.3

b) 53.2

c) 63.3

d) 73.3

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3. The response time for different sizes and materials of thermocouple wires usually lie between

a) 0.04 to 2.5 seconds

b) 0.06 to 1.2 seconds

c) 0.02 to 0.04 seconds

d) 2.4 to 9.4 seconds

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4. A thermocouple junction of spherical form is to be used to measure the temperature of the gas stream. The junction is at 20 degree Celsius and is placed in a gas stream which is at 200 degree Celsius. Make calculations for time required by the thermocouple to reach 197 degree Celsius temperature. Assume the thermos-physical properties as given below

k = 20 W/ m K

h = 350 W/m^{2} K

c = 0.4 k J/kg K

p = 8000 kg/m^{3}

a) 1.094 seconds

b) 2.094 seconds

c) 3.094 seconds

d) 4.094 seconds

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_{a}/t

_{I – }t

_{a}= exponential (- p V c T/h A).

5. An egg with mean diameter of 4 cm and initially at 25 degree Celsius is placed in an open boiling water container for 4 minutes and found to be boiled at a particular level. For how long should a similar egg boil at the same level, when refrigerator temperature is 5 degree Celsius? Use lumped parameter theory and assume following properties of egg

k = 12 W/m K

h = 125 W/m^{2 }K

c = 2 k J/kg K

p = 1250 kg/m^{3}

a) 251.49 seconds

b) 261.49 seconds

c) 271.49 seconds

d) 281.49 seconds

### View Answer

_{t – }T

_{INFINITY}/T

_{i – }T

_{INFINITY}= e

^{ – b T}.

6. A person is found dead at 5 pm in a room whose temperature is 20 degree Celsius. The temperature of body is measured to be 25 degree Celsius, when found and heat transfer coefficient is estimated to be 8 W/m^{2} K. Modeling the body as a 30 cm diameter, 1.7 cm long cylinder. Estimate the time of death of person.

a) 13.55 seconds

b) 12.55 seconds

c) 11.55 seconds

d) 10.55 seconds

### View Answer

_{t – }T

_{INFINITY}/T

_{i – }T

_{INFINITY}= e

^{ – b T}.

7. The following data pertains to the junction of a thermocouple wire used to measure the temperature of the gas stream.

Density = 8500 kg/m^{3}, specific heat = 325 J/kg K, thermal conductivity = 40 W/m K and the heat transfer coefficient between the junction and gas = 215 W/m^{2 }K.

If thermocouple junction can be approximated as 1 mm diameter sphere, determine how long it will take for the thermocouple to read 99% of the initial temperature difference

a) 9.86 seconds

b) 8.86 seconds

c) 7.86 seconds

d) 6.86 seconds

### View Answer

_{ a}/t

_{i – }t

_{a}= exponential (- h A T/p V c).

8. A thermocouple junction in the form of 4 mm radius sphere is to be used to measure the temperature of a gas stream. The junction is initially at 35 degree Celsius and is placed in a gas stream which is at 300 degree Celsius. The thermocouple is removed from the hot gas stream after 10 seconds and kept in still air at 25 degree Celsius with convective coefficient 10 W/m^{2 }K. Find out the time constant of the thermocouple. Assume the thermos-physical properties as given below

h = 37.5 W/m^{2 }K

p = 7500 kg/m^{3}

c = 400 J/kg K

a) 6.67 seconds

b) 106.67 seconds

c) 206.67 seconds

d) 306.67 seconds

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9. What percentage of water an average human body can have?

a) 52%

b) 62%

c) 72%

d) 82%

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10. Heisler charts are valid if

a) Fourier number is equal to 0.2

b) Fourier number is less than 0.2

c) Fourier number is greater than 0.2

d) Fourier number is equal to 0.4

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## Set 3

1. Temperature and velocity profiles are identical when the dimensionless Prandtl number is

a) 1

b) 2

c) 3

d) 4

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2. Reynolds analogy is given by

a) Nu _{x}/ (Re _{x}) (Pr _{x}) = 5 St _{X} = – 2 C _{F x }

b) Nu _{x}/ 2 (Re _{x}) (Pr _{x}) = 4 St _{X} = – C _{F x }/3

c) Nu _{x}/ (Re _{x}) (Pr _{x}) = St _{X} = – ½ C _{F x }

d) Nu _{x}/ (Re _{x}) (Pr _{x}) = 2 St _{X} = – C _{F x }/4

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3. The average drag coefficient for turbulent boundary layer flow past a thin plate is given by

C _{f }= 0.455/ (log _{10} R _{el})^{ 2.58}

Where R _{el }is the Reynolds number based on plate length. A plate 50 cm wide and 5 m long is kept parallel to the flow of water with free stream velocity 3 m/s. Calculate the drag force on both sides of the plate. For water, kinematic viscosity = 0.01 stokes

a) 53.38 N

b) 63.38 N

c) 73.38 N

d) 83.38 N

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_{f}(p U

_{INFINITY}/2) (l) (b) = 25.42 N per unit width.

4. Consider the above problem, estimate the value of Reynolds number

a) 0.12

b) 0.13

c) 0.14

d) 0.15

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_{INFINITY}/v = 0.15.

5. During test-run, air flows at 215 m/s velocity and 25 degree Celsius temperature past a smooth thin model airfoil which can be idealized as a flat plate. If the chord length of the airfoil is 15 cm, find drag per unit width. The relevant physical properties of air are

p = 1.82 kg/m^{3}

v = 15.53 * 10 ^{-6} m^{2}/s

a) 25.42 N per unit width

b) 35.42 N per unit width

c) 45.42 N per unit width

d) 55.42 N per unit width

### View Answer

_{f}(p U

_{INFINITY}/2) (l) (b) = 25.42 N per unit width.

6. A flat plate was positioned at zero incidence in a uniform flow stream of air. Assuming boundary layer to be turbulent over the entire plate, workout the ratio of skin-friction forces on the front and rear half part of the plate

a) 1.557

b) 1.447

c) 1.347

d) 1.247

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_{1}/F

_{ 2}= 0.574/1 – 0.574 = 1.347.

7. For a particular engine, the underside of the crankcase can be idealized as a flat plate measuring 80 cm by 20 cm. The engine runs at 80 km/hr and the crankcase is cooled by the air flowing past it at the same speed. Find loss of the heat from the crank case surface (t _{S} = 25 degree Celsius). Assume the boundary layer to be turbulent

a) 465.04 W

b) 565.04 W

c) 665.04 W

d) 765.04 W

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8. With respect to above problem, find the value of Nusselt number

a) 2000.89

b) 3000.89

c) 4000.89

d) 5000.89

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^{0.8 }(Pr)

^{ 0.33 }= 2000.89.

9. A flat plate 1 m by 1 m is placed in a wind tunnel. The velocity and temperature of free stream air are 80 m/s and 10 degree Celsius. The flow over the whole length of the plate is made turbulent by turbulizing grid placed upstream of the plate. Find the thickness of hydrodynamic boundary layer at trailing edge of the plate

a) 19.55 mm

b) 18.55 mm

c) 17.55 mm

d) 16.55 mm

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^{0.2}= 0.01655 m.

10. A flat plate 1 m by 1 m is placed in a wind tunnel. The velocity and temperature of free stream air are 80 m/s and 10 degree Celsius. The flow over the whole length of the plate is made turbulent by turbulizing grid placed upstream of the plate. Find the heat flow from the surface of the plate

a) 9424.5 W

b) 8424.5 W

c) 7424.5 W

d) 6424.5 W

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## Set 4

1. The ratio of inertia force to viscous force is known as

a) Grashof number

b) Reynolds number

c) Fourier number

d) Nusselt number

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2. Velocity within the given fields would be similar in magnitude, direction and turbulence pattern when

a) Nusselt number are different

b) Nusselt number are same

c) Reynolds number are different

d) Reynolds number are same

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3. Reynolds number is given by the quantity

a) p V l/δ

b) 2 p V l/δ

c) 3 p V l/δ

d) 4 p V l/δ

Where,

p is density

δ is viscosity

V is volume

l is length

### View Answer

^{2}l

^{2}/ δ V l.

4. Air enters a rectangular duct measuring 30 cm by 40 cm with a velocity of 8.5 m/s and a temperature of 40 degree Celsius. The flowing air has a thermal conductivity 0.028 W/m K, kinematic viscosity 16.95 * 10 ^{-6} m^{2}/s and from empirical correlations the Nusselt number has been approximated to be 425. Find out the flow Reynolds number

a) 1.1719 * 10 ^{6}

b) 2.1719 * 10 ^{6}

c) 0.1719 * 10 ^{6}

d) 4.1719 * 10 ^{6}

### View Answer

_{e}= V d

_{e}/v.

5. Consider the above problem, find the convective heat flow coefficient

a) 24.71 W/m^{2} K

b) 34.71 W/m^{2} K

c) 44.71 W/m^{2} K

d) 54.71 W/m^{2} K

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_{U}k/d

_{e}.

6. For laminar flow, Reynolds number should be

a) Less than 2300

b) Equal to 2300

c) Greater than 2300

d) Less than 4300

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7. For turbulent flow, Reynolds number must be

a) Less than 5000

b) Equal to 6000

c) Less than 6000

d) Greater than 6000

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8. What is the value of convective coefficient of air and superheated steam in case of forced convection?

a) 30-900 W/m^{2} K

b) 30-700 W/m^{2} K

c) 30-300 W/m^{2} K

d) 30-400 W/m^{2} K

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9. Heat is being transferred by convection from water at 48 degree Celsius to glass plate whose surface is exposed to water at 40 degree Celsius. The thermal conductivity of water is 0.6 W/m K and the thermal conductivity of glass is 1.2 W/m K. The spectral gradient of temperature in the water at the water glass interface is 10 ^{-4} K/m. The heat transfer coefficient in W/m^{2 }K is

a) 0.0

b) 750

c) 6.0

d) 4.8

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10. For transient flow, the value of Reynolds number may vary between

a) 1450-9870

b) 1200-4500

c) 2300-6000

d) 6000-9000

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## Set 5

1. “All the factors relating to geometry of the sections are grouped together into a multiple constant called the shape factor” True or false

a) True

b) False

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2. Shape factor for plane wall is equal to

a) A/δ

b) 2A/δ

c) 3A/δ

d) 4A/δ

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3. For a prescribed temperature difference, bodies with the same shape factor will allow heat conduction proportional to

a) k/2

b) 2k

c) k

d) k/4

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4. Shape factor for cylinder is

a) 6 π l/log _{e} (r _{2}/r _{1})

b) 4 π l/log _{e} (r _{2}/r _{1})

c) π l/log _{e} (r _{2}/r _{1})

d) 2 π l/log _{e} (r _{2}/r _{1})

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5. The annealing furnace for continuous bar stock is open at the ends and has interior dimensions of 0.6 m * 0.6 m * 1.5 m long with a wall 0.3 m thick all around. Calculate the shape factor for the furnace?

a) 15.24 m

b) 16.34 m

c) 14.54 m

d) 13.76 m

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6. Shape factor for sphere is

a) 4 π r _{1} r _{2}

b) 4 π r _{1} r _{2}/r _{2 }– r _{1}

c) 4 π /r _{2 }– r _{1}

d) r _{1} r _{2}/r _{2 }– r _{1}

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^{2}.

7. Which is true regarding a complete rectangular furnace?

a) 6 walls, 12 edges and 6 corners

b) 0 walls, 2 edges and 4 corners

c) 6 walls, 12 edges and 8 corners

d) 2 walls, 6 edges and 8 corners

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8. The shape factor for complete rectangular furnace is

Where a, b and c are the inside dimensions and d x is the wall thickness

a) 2/ d x (a b + b c + c a) + 4 (0.64) (a + b +c) + 8 (0.45) d x

b) 2/ d x (a b + b c + c a) + 4 (0.44) (a + b +c) + 8 (0.35) d x

c) 2/ d x (a b + b c + c a) + 4 (0.34) (a + b +c) + 8 (0.25) d x

d) 2/ d x (a b + b c + c a) + 4 (0.54) (a + b +c) + 8 (0.15) d x

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_{edge}= 0.54 times length of edge. S

_{corner }= 0.15 d x.

9. For the same material and same temperature difference, the heat flow in terms of shape factor is given by

a) S k d t

b) k d t/S

c) 2S k/d t

d) 2S/3

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10. For the same amount of fabrication material and same inside capacity, the heat loss is lowest in

a) Sphere

b) Cylinder

c) Rectangle

d) Cube