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# Multiple choice question for engineering

## Set 1

1. The convective coefficient in boilers and condenser is
a) Low
b) High
c) Moderate
d) Depends on time and temperature

Answer: b [Reason:] For low heat transfer rate it is high.

2. In thermometer heat is dissipated from which end?
a) Back side
b) Front side
c) Middle
d) No heat is dissipated

Answer: a [Reason:] In thermometric well heat dissipation is from the end.

3. The thermometric well is treated as a fin of
a) It is not treated as a fin
b) Infinite length
c) Finite length
d) It depends on the amount of heat dissipation

Answer: c [Reason:] Generally for maximum heat dissipation it is treated as a fin of finite length.

4. Thermometric well apparatus doesn’t consists of
a) Thermometer
b) Pipe line
c) Oil
d) Ammeter

Answer: d [Reason:] It doesn’t consist of an ammeter.

5. For what purpose we use thermometric well?
a) To measure current
b) To measure resistance
c) To measure temperature of gas
d) To stop heat dissipation

Answer: c [Reason:] We use this apparatus for the measurement of temperature of gas flowing through pipeline.

6. In thermometric well, the error can be minimized by
(i) The value of parameter ml slows down by some quantity.
(ii) Lagging of the tube so that conduction of heat along its length is arrested
Identify the correct option
a) True
b) False

Answer: b [Reason:] Here, we increase the value of parameter ml.

7. Suppose there are two reservoirs A and B. Let TA and TB be the temperatures of respective reservoirs. At any instant say T A = TB. In this case maximum temperature occurs at
a) Middle
b) Left
c) Right
d) All over the rod

Answer: a [Reason:] Here temperatures are same so x = ½ i.e. middle of the rod.

8. The relevant boundary conditions in case of heat transfer from a bar connected to two heat sources at different temperatures are
a) α = α 1 at x = 0 and α = α 2 at x = infinity
b) α = α 1 at x = 1 and α = α 2 at x = 2L
c) α = α 1 at x = infinity and α = α 2 at x = 1
d) α = α 1 at x = 0 and α = α 2 at x = L

Answer: d [Reason:] It should be at x = o and x = L.

9. “The tube that is used in thermometric well is considered as a hollow fin and the temperature distribution is obtained by using the relation applicable to infinitely long fin”. Choose the correct option
a) True
b) False

Answer: b [Reason:] The relation applicable to fin with insulated tip is used here.

10. “The diameter of well does not have any effect on temperature measurement by the thermometer”. Choose the correct option
a) True
b) False

Answer: a [Reason:] Here, there is no relation between well diameter and temperature.

## Set 2

1. The time constant of a thermocouple is the time taken to
a) Minimum time taken to record a temperature reading
b) Attain 50% of initial temperature difference
c) Attain the final value to be measured
d) Attain 63.2% of the value of initial temperature difference

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

2. 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 junction diameter needed for the thermocouple to have thermal time constant of one second. 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) 0.556 mm
b) 0.656 mm
c) 0.756 mm
d) 0.856 mm

Answer: b [Reason:] T = p V c/h A = p r c/3h. So, r = 3 h T/p c = 0.000328 m = 0.328 m.

3. A low value of time constant can be achieved for a thermocouple by
(i) Increasing the wire diameter
(ii) Increasing the value heat transfer coefficient
(iii) Use light metals of low density and low specific heat
Identify the correct statements
a) 2 and 3
b) 1 and 3
c) 1 and 2
d) 1, 2 and 3

Answer: a [Reason:] Diameter of wire should be less.

4. Which of the following has units of time constant?
a) p V/h A
b) p c/h A
c) p V c/h A
d) V c/h A
Where,
P is density, A is area, c is specific heat and V is volume

Answer: c [Reason:] It has the unit of time and is time constant of the system.

5. “Thermal radiation suffers no attenuation in a vacuum”. Choose the correct answer
a) True
b) False

Answer: a [Reason:] It is gradual loss of intensity of any kind of flux.

6. How does the body temperature falls or rises with time?
a) Logarithmic
b) Parabolic
c) Linear
d) Exponentially

Answer: d [Reason:] The rate depends on the parameter h A/p V c.

7. The lumped parameter solution for transient conduction can be conveniently stated as
a) t – t a/t I – t a = 2 exponential (- B I F 0)
b) t – t a/t I – t a = exponential (- B I F 0)
c) t – t a/t I – t a = 3 exponential (- B I F 0)
d) t – t a/t I – t a = 6 exponential (- B I F 0)

Answer: b [Reason:] This is the general solution for lumped system parameter.

8. An iron billet (k = 65 W/m K) measuring 20 * 15 * 80 cm is exposed to a convective flow resulting in convection coefficient of 11.5 W/m2 K. Determine the Biot number
a) 0.02376
b) 0.008974
c) 0.004563
d) 0.006846

Answer: d [Reason:] B = h L C/k = 0.006846.

9. A mercury thermometer with bulb idealized as a sphere of 1 mm radius is used for measuring the temperature of fluid whose temperature is varying at a fast rate. For mercury

k = 10 W/m K

α = 0.00005 m2/s

h = 10 W/m2 K

If the time for the temperature change of the fluid is 3 second, what should be the radius of thermocouple to read the temperature of the fluid?
For the thermocouple material
k = 100 W/m K
α = 0.0012 m2/s
h = 18 W/m2 K
a) .864 mm
b) .764 mm
c) .664 mm
d) .564 mm

Answer: a [Reason:] T = k l/h α. So, radius is 0.864 mm.

10. 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 junction diameter needed for the thermocouple to have thermal time constant of one second. 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) 0.456 mm
b) 0.556 mm
c) 0.656 mm
d) 0.756 mm

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

## Set 3

1. The value of transmissivity may vary from
a) 0-1
b) 1-2
c) 3-4
d) 4-5

Answer: a [Reason:] This is very small quantity. The response of the body to incident radiations is, however, completely independent of and unaffected by the simultaneous emission from the body.

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 absorptivity for the surface material
a) 0.113
b) 0.114
c) 0.115
d) 0.116

Answer: c [Reason:] Transmissivity = fraction of total energy transmitted by the body = 350 – (250 + 60)/350 = 0.115.

3. “Transmissivity can also be defined as ratio of transmitted radiation to that of incident energy flow”. True or false
a) True
b) False

Answer: b [Reason:] It is the ratio of incident energy flow to that of transmitted radiations.

4. A cavity with a small hole will always behave as a
a) White body
b) Transparent body
c) Black body
d) Opaque body

Answer: c [Reason:] No radiation passing in and out will have any effect upon the equilibrium of radiation inside the cavity.

5. What is the wavelength band of gamma rays?
a) 1 * 10 -7 to 4 * 10 -4 micron meter
b) 2 * 10 -7 to 2.4 * 10 -4 micron meter
c) 3 * 10 -7 to 3.4 * 10 -4 micron meter
d) 4 * 10 -7 to 1.4 * 10 -4 micron meter

Answer: d [Reason:] This is the maximum and minimum wavelength for gamma rays.

6. Transmissivity is defined as
a) Fraction of total energy transmitted by the body
b) Fraction of total 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

Answer: a [Reason:] It is fraction of total energy transmitted by the body.

7. A polished metal pipe 5 cm outside diameter and 370 K temperature at the outer surface is exposed to ambient conditions at 295 K temperature. The emissivity of the surface is 0.2 and the convection coefficient of heat transfer is 11.35 W/m2 degree. Calculate the overall coefficient of heat transfer by the combined mode of convection and radiation
a) 11.04 W/m2 degree
b) 12.04 W/m2 degree
c) 13.04 W/m2 degree
d) 14.04 W/m2 degree

Answer: c [Reason:] The total heat exchange can be expressed as, Q = U A d t where U is the overall coefficient of heat transfer.

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 transmission
a) 155.47 W/m2
b) 145.47 W/m2
c) 135.47 W/m2
d) 125.47 W/m2

Answer: b [Reason:] Rate of transmission = T Q 0 = 145.47 W/m2.

9. With respect to incident radiation, transmissivity varies with
a) Time
b) Temperature
c) Surface area
d) Wavelength

Answer: d [Reason:] For transparent body, transmissivity is equal to unity.

10. A body through which all the incident radiations passes, is called
a) Opaque body
b) Black body
c) Transparent body
d) White body

Answer: c [Reason:] For transparent body, transmissivity is unity.

## Set 4

1. Which of the following is true for turbulent flow?
a) G r P r > 108
b) G r P r > 109
c) G r P r > 103
d) G r P r > 1015

Answer: b [Reason:] The product G r P r is often referred to as Rayleigh number, and its value sets the criterion of turbulent character of flow.

2. Mc Adam relation is given by
a) Nu = 0.023 (Re) 0.8 (Pr) 0.4
b) Nu = 0.023 (Re) 0.8 (Pr) 0.3
c) Nu = 0.023 (Re) 0.8 (Pr) 0.2
d) Nu = 0.023 (Re) 0.8 (Pr) 0.1

Answer: a [Reason:] Here, n = 0.4 if the fluid is being heated.

3. Investigate the effect of following condition on the average value of heat transfer coefficient in flow through a tube
Two fold increases in flow velocity by varying mass flow rate
It may be presumed that there is no change in the temperature of the liquid and the tube wall, and that the flow through the tube is turbulent in character
a) 64.1% decrease
b) 64.1% increase
c) 74.1%decrease
d) 74.1% increases

Answer: d [Reason:] h 2/h 1 = (V 2/V 1) 0.8 = .0741 = 74.1%

4. Consider the above problem for two fold increase in the diameter of the tube, the flow velocity is maintained constant by change in the rate of liquid flow
a) 15%
b) 14%
c) 13%
d) 12%

Answer: c [Reason:] h 2/h 1 = (d 1/d 2) 0.2.

5. Calculate the rate of heat loss from a human body which may be considered as a vertical cylinder 30 cm in diameter, and 175 cm high while standing in a 30 km/hr wind at 15 degree Celsius. Human has a surface temperature of 35 degree Celsius
a) 588.86 W
b) 688.86 W
c) 788.86 W
d) 888.86 W

Answer: b [Reason:] Q = h A d t, h = Nu k/d.

6. A heat treat steel plate measures 3 m by 1 m and is initially at 30 degree Celsius. It is cooled by blowing air parallel to 1 m edge at 9 km/hr. If the air is at 10 degree Celsius. Estimate the convection heat transfer from both sides of the plate
a) 477.7 W
b) 547.7 W
c) 647.7 W
d) 747.6 W

Answer: d [Reason:] Q = h A d t, h = Nu k/d.

7. Consider the above problem, find the value of Reynolds number
a) 166003
b) 177003
c) 188003
d) 199003

Answer: a [Reason:] Re = p V l/µ = 16603

8. The oil pan of 1.6 engine approximates a flat plate 0.3 m wide by 0.45 m long and protrudes below the framework of the automobile. The engine oil is at 95 degree Celsius and the ambient air temperature is 35 degree Celsius. If the automobile runs at 36 km/hr, make calculations for the rate of heat transfer from the oil-pan surface. Assume negligible resistance to conduction through the oil pan
a) 190.37 W
b) 180.37 W
c) 170.37 W
d) 160.37 W

Answer: c [Reason:] Q = h A d t, where h = (Nu) (k)/l and Nu = 0.664 (Re) 0.5 (Pr).33.

9. Air flows through a 10 cm internal diameter tube at the rate of 75 kg/hr. Measurements indicate that at a particular point in the tube, the pressure and temperature of air are 1.5 bar and 325 K whilst the tube wall temperature is 375 K. Find heat transfer rate from one meter length in the region of this point
The general non-dimensional correlation for turbulent flow in the tube is
Nu = 0.023 (Re) 0.8 (Pr) 0.4
Where the fluid properties are evaluated at the bulk temperature
a) 147.35 W
b) 157.35 W
c) 167.35 W
d) 177.35 W

Answer: d [Reason:] Q = h A d t, where h = (Nu) (k)/l = 177.35 W.

10. Consider the above problem, find the value of Nusselt number
a) 40.46
b) 50.56
c) 60.66
d) 70.76

Answer: a [Reason:] Nu = 0.023 (Re) 0.8 (Pr) 0.4 = 40.46.

## Set 5

1. Conduction plus fluid flow in motion is known as
b) Conduction
c) Convection
d) Heat exchanger

Answer: c [Reason:] It is convection i.e. conduction plus some velocity. It is the process of energy transport affected by the mixing of a fluid medium.

2. How many types of convection are there?
a) 4
b) 3
c) 2
d) 1

Answer: b [Reason:] It is of three types i.e. forced convection, natural convection and mixed convection.

3. Which of the following heat flow situations pertains to free or natural convection?
a) Air conditioning installations and nuclear reactors
b) Flow of water inside the condenser tubes
c) Cooling of internal combustion engine
d) Cooling of billets in atmosphere

Answer: d [Reason:] Cooling of billets in atmosphere is both free and natural convection.

4. Mark the system where heat transfer is given by forced convection
a) Chilling effect of cold wind on warm body
b) Fluid passing through the tubes of a condenser and other heat exchange equipment
c) Heat flow from a hot pavement to surrounding atmosphere
d) Heat exchange on the outside of cold and warm pipes

Answer: b [Reason:] If the fluid motion involve in the process is induced by some external means then it is called forced convection.

5. Forced convection in a liquid bath is caused by
a) Intense stirring by an external agency
b) Molecular energy interactions
c) Density difference brought about by temperature gradients
d) Flow of electrons in a random fashion

Answer: a [Reason:] If the fluid motion involve in the process is induced by some external means then it is called forced convection.

6. A finned tube hot water radiator with a fan blowing air over it is kept in rooms during winter. The major portion of the heat transfer from the radiation is due to
a) Combined conduction and radiation
b) Radiation to the surroundings
c) Better conduction
d) Convection to the air

Answer: d [Reason:] Convection is a process in which thermal energy is transferred between solid and fluid flowing through it.

7. A body cooling from 80 degree Celsius to 70 degree Celsius takes 10 minutes when left exposed to environmental conditions. If the body is to cool further from 70 degree Celsius to 60 degree Celsius under the same external conditions, it will take
a) Same time of 10 minutes
b) More than 10 minutes
c) Less than 10 minutes
d) Time will depend upon the environmental conditions

Answer: b [Reason:] Q = h A (t b – t a). Apparently the cooling depends upon the same temperature difference.

8. On a summer day, a scooter rider feels more comfortable while on the move than while at a stop light because
a) An object in motion captures less radiation
b) Air has a low specific heat and hence it is cooler
c) More heat is loss by convection and radiation while in motion
d) Air is transparent to radiation and hence it is cooler than the body

Answer: d [Reason:] The situation corresponds to forced convection when the scooter is in motion and the convective heat transfer coefficient for forced convection is greater than that for free convection.

9. What is the value of convective coefficient of oil in case of forced convection?
a) 1460-3000 W/m2 K
b) 460-3000 W/m2 K
c) 60-3000 W/m2 K
d) 160-3000 W/m2 K