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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

View Answer

Answer: b [Reason:] It is total energy reflected by the body. Its value depends upon the nature of the surface of the body, its temperature and wavelength of incident radiations.

2. Of the radiant energy 350W/m2 incident upon a surface 250W/m2 is absorbed, 60W/m2 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

View Answer

Answer: b [Reason:] Reflectivity = fraction of total energy reflected by the body = 60/350 = 0.171.

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

View Answer

Answer: c [Reason:] Heat loss by convection from both sides of the plates = 2 h A d t = 1.1 W. Energy lost by reflection = 2.0 – 1.1 = 0.9 W.

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

View Answer

Answer: d [Reason:] It should be wavelength of incident radiation. It is the fraction of total energy reflected by the body.

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

View Answer

Answer: a [Reason:] It is absorbing a certain fixed amount of radiations.

6. Which one is true regarding gases?
a) Reflectivity is unity
b) Absorptivity is zero
c) Reflectivity is zero
d) Transmissivity is zero

View Answer

Answer: c [Reason:] Gases are known to reflect very little of the radiation incident on their surface.

7. Thin glass plate is an example of
a) Transparent body
b) Opaque body
c) Black body
d) White body

View Answer

Answer: a [Reason:] For transparent body, transmissivity is zero.

8. Radiant energy with an intensity of 800 W/m2 strikes a flat plate normally. The absorptivity is thrice the reflectivity and twice the transmissivity. Determine the rate of reflection
a) 518.20 W/m2
b) 418.20 W/m2
c) 318.20 W/m2
d) 218.20 W/m2

View Answer

Answer: d [Reason:] Rate of reflection = p Q 0 = 218.20 W/m2.

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
heat-transfer-questions-answers-reflectivity-q9
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

View Answer

Answer: b [Reason:] A = B + C + D.

10. Which one is true for opaque body?
a) Transmissivity is zero
b) Reflectivity is zero
c) Absorptivity is zero
d) Reflectivity is unity

View Answer

Answer: a [Reason:] For opaque body, sum of absorptivity and reflectivity is unity.

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

View Answer

Answer: b [Reason:] It is the source temperature.

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

View Answer

Answer: c [Reason:] The time constant of a thermocouple represents the time required to attain 63.2% value.

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

View Answer

Answer: a [Reason:] Depending upon the type of fluid used, the response time for different sizes and materials of thermocouple wires usually lie between o.o4 to 2.5 seconds.

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/m2 K
c = 0.4 k J/kg K
p = 8000 kg/m3
a) 1.094 seconds
b) 2.094 seconds
c) 3.094 seconds
d) 4.094 seconds

View Answer

Answer: d [Reason:] t – t 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/m2 K
c = 2 k J/kg K
p = 1250 kg/m3
a) 251.49 seconds
b) 261.49 seconds
c) 271.49 seconds
d) 281.49 seconds

View Answer

Answer: c [Reason:] T 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/m2 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

Answer: d [Reason:] T 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/m3, specific heat = 325 J/kg K, thermal conductivity = 40 W/m K and the heat transfer coefficient between the junction and gas = 215 W/m2 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

Answer: a [Reason:] t – t 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/m2 K. Find out the time constant of the thermocouple. Assume the thermos-physical properties as given below
h = 37.5 W/m2 K
p = 7500 kg/m3
c = 400 J/kg K
a) 6.67 seconds
b) 106.67 seconds
c) 206.67 seconds
d) 306.67 seconds

View Answer

Answer: b [Reason:] T = p V c/h A = p r c/3 h = 106.67 seconds.

9. What percentage of water an average human body can have?
a) 52%
b) 62%
c) 72%
d) 82%

View Answer

Answer: c [Reason:] Average human body is 72% water by mass.

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

View Answer

Answer: c [Reason:] The solution to the transient heat flow in infinite flat plates are available in the form of these charts.

Set 3

1. Temperature and velocity profiles are identical when the dimensionless Prandtl number is
a) 1
b) 2
c) 3
d) 4

View Answer

Answer: a [Reason:] They are identical when Prandtl number is unity.

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

View Answer

Answer: c [Reason:] It is an excellent example of the similar nature of energy and momentum transfer.

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

View Answer

Answer: b [Reason:] Drag force = 2 C 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

View Answer

Answer: d [Reason:] Re = l U 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/m3
v = 15.53 * 10 -6 m2/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

Answer: a [Reason:] Drag force = 2 C 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

View Answer

Answer: c [Reason:] F 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

View Answer

Answer: b [Reason:] Heat loss by crankcase = h A d t = 565.4 W.

8. With respect to above problem, find the value of Nusselt number
a) 2000.89
b) 3000.89
c) 4000.89
d) 5000.89

View Answer

Answer: a [Reason:] Nusselt number = 0.036 (Re) 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

View Answer

Answer: d [Reason:] Thickness = (l) (0.371)/ (Re) 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

View Answer

Answer: c [Reason:] Heat flow from the plate = h A d t = 7424.5 W.

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

View Answer

Answer: b [Reason:] Reynolds number is indicative of the relative importance of inertial and viscous effects in a fluid motion.

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

View Answer

Answer: d [Reason:] Reynolds number constitutes an important criterion of kinematic and dynamic similarity in forced convection heat transfer.

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

Answer: a [Reason:] p V 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 m2/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

Answer: c [Reason:] R e = V d e/v.

5. Consider the above problem, find the convective heat flow coefficient
a) 24.71 W/m2 K
b) 34.71 W/m2 K
c) 44.71 W/m2 K
d) 54.71 W/m2 K

View Answer

Answer: b [Reason:] h = N 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

View Answer

Answer: a [Reason:] In laminar flow, the fluid particles move in flat or curved un-mixing layers or streams and follow a smooth continuous path.

7. For turbulent flow, Reynolds number must be
a) Less than 5000
b) Equal to 6000
c) Less than 6000
d) Greater than 6000

View Answer

Answer: d [Reason:] In turbulent flow, the motion of fluid particles is irregular, and it proceeds along erratic and unpredictable paths.

8. What is the value of convective coefficient of air and superheated steam in case of forced convection?
a) 30-900 W/m2 K
b) 30-700 W/m2 K
c) 30-300 W/m2 K
d) 30-400 W/m2 K

View Answer

Answer: c [Reason:] In forced convection, flow of fluid is caused by a pump, a fan or by the atmospheric winds. These mechanical devices provide a definite circuit for the circulating currents.

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/m2 K is
heat-transfer-questions-answers-reynolds-number-q9
a) 0.0
b) 750
c) 6.0
d) 4.8

View Answer

Answer: b [Reason:] q = h d t = 6000. Therefore, h = 6000/48 – 40.

10. For transient flow, the value of Reynolds number may vary between
a) 1450-9870
b) 1200-4500
c) 2300-6000
d) 6000-9000

View Answer

Answer: c [Reason:] The term transient designates a phenomenon which is time dependent.

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

View Answer

Answer: b [Reason:] They are grouped together into a single constant instead of multiple one.

2. Shape factor for plane wall is equal to
a) A/δ
b) 2A/δ
c) 3A/δ
d) 4A/δ

View Answer

Answer: a [Reason:] It should be obtained by dividing area with respect to length.

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

View Answer

Answer: c [Reason:] It is proportional to material thermal conductivity irrespective of size and configuration.

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)

View Answer

Answer: d [Reason:] It is two times the rest of the product.

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

View Answer

Answer: a [Reason:] Shape factor for 4 walls = 4 (area of wall)/ (thickness) = 4 (1.5) (0.6)/0.3 = 12 m. Shape factor for 4 edges = 4 (0.54) (1.5) = 3.24 m. Total = 15.24 m.

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

View Answer

Answer: b [Reason:] In case of sphere it is given in option b. Area of sphere is 4 π r 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

View Answer

Answer: c [Reason:] It has 6 walls, 12 edges and 8 corners.

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

View Answer

Answer: d [Reason:] S 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

View Answer

Answer: a [Reason:] The heat flow is proportional to shape factor.

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

View Answer

Answer: b [Reason:] For cylinder it is 0.439 i.e. lowest.