Latest 100 Heat Transfer Objective Questions
1.The time constant of a thermocouple is
the time taken to attain the final temperature to be measured
the time taken to attain 50% of the value of initial temperature difference
the time taken to attain 63.2% of the value of initial temperature difference
determined by the time taken to reach 100�C from 0�C
none of the above.
ANSWER : : the time taken to attain 63.2% of the value of initial temperature difference
2.Thermal conductivity of water in general with rise in temperature
increases
decreases
remains constant
may increase or decrease depending on temperature
none of the above.
ANSWER : : may increase or decrease depending on temperature
3.Which of the following is the case of heat transfer by radiation
blast furnace
heating of building
cooling of parts in furnace
heat received by a person from fireplace
all of the above.
ANSWER : : heat received by a person from fireplace
4.Heat transfer takes place as per –
zeroth law of thermodynamics
first law of thermodynamic
second law of the thermodynamics
Kirchhoff law (e) Stefan’s law.
opt 5
ANSWER : : second law of the thermodynamics
5.Thermal conductivity of air with rise in temperature
increases
decreases
remains constant
may increase or decrease depending on temperature
none of the above.
ANSWER : : increases
6.Thermal conductivity of air at room temperature in kcal/m hr �C is of the order of
0.002
0.02
0.01
0.1
0.5.
ANSWER : : 0.02
7.Thermal conductivity of water at 20�C is of the order of
0.1
0.23
0.42
0.51
0.64.
ANSWER : : 0.51
8.Temperature of steam at around 540�C canbe measured by
thermometer
radiatiouv pyrometer
thermistor
thermocouple
thermopile.
ANSWER : : thermocouple
9.Unit of thermal conductivity in S.I. units is
J/m2 sec
J/m �K sec
W/m �K
(a) and (c) above
(b) and (c) above.
ANSWER : : (b) and (c) above.
10.Cork is a good insulator because it has
free electrons
atoms colliding frequency
low density
porous body
all of the above.
ANSWER : : porous body
11.Thermal conductivity of non-metallic amorphous solids with decrease in temperature
increases
decreases
remains constant
may increase or decrease depending on temperature
unpredictable.
ANSWER : : decreases
12.Sensible heat is the heat required to
change vapour into liquid
change liquid into vapour
increase the temperature of a liquid of vapour
convert water into steam and superheat it
convert saturated steam into dry steam.
ANSWER : : increase the temperature of a liquid of vapour
13.Heat is closely related with
liquids
energy
temperature
entropy
enthalpy.
ANSWER : : temperature
14.When heat is transferred from one particle of hot body to another by actual motion of the heated particles, it is referred to as heat transfer by
conduction
convection
radiation
conduction and convection
convection and radiation.
ANSWER : : conduction
15.When heat is Transferred by molecular collision, it is referred to as heat transfer by
conduction
convection
radiation
scattering
convection and radiation.
ANSWER : : convection
16.When heat is transferred form hot body to cold body, in a straight line, without affecting the intervening medium, it is referred as heat transfer by
conduction
convection
radiation
conduction and convection
convection and radiation.
ANSWER : : radiation
17.Which of the following is a case of steady state heat transfer
I.C. engine
air preheaters
heating of building in winter
all of the above
none of the above.
ANSWER : : none of the above.
18.Total heat is the heat required to
change vapour into liquid
change liquid into vapour
increase the temperature of a liquid or vapour
convert water into steam and superheat it
convert saturated steam into dry steam.
ANSWER : : convert water into steam and superheat it
19.Thermal conductivity of solid metals with rise in temperature normally
increases
decreases
remains constant
may increase or decrease depending on temperature
unpredictable.
ANSWER : : decreases
20.Heat transfer in liquid and gases takes place by
conduction
convection
radiation
conduction and convection
convection and radiation.
ANSWER : : convection
21.Metals are good conductors of heat because
their atoms collide frequently
thier atoms-are relatively far apart
they contain free electrons
they have high density
all of the above.
ANSWER : : their atoms collide frequently
22.Heat flows from one body to other when they have
different heat contents
different specific heat
different atomic structure
different temperatures
none of the above.
ANSWER : : different temperatures
23.Pick up tne wrong case. Heat flowing from one side to other depends directly on
face area
time
thickness
temperature difference
thermal conductivity.
ANSWER : : thickness
24.The insulation ability of an insulator with the presence of moisture would
increase
decrease
remain unaffected
may increase/decrease depending on temperature and thickness of insulation
none of the above.
ANSWER : : decrease
25.Film coefficient is defined as Inside diameter of tube
Equivalent thickness of film
Thermal conductivity Equivalent thickness of film Specific heat x Viscosity
Thermal conductivity Molecular diffusivity of momentum Thermal diffusivity
Film coefficient x Inside diameter Thermal conductivity
none of the above.
ANSWER : : Thermal conductivity Equivalent thickness of film Specific heat x Viscosity
26.Thermal conductivity of wood depends on
moisture
density
temperature
all of the above
none of the above.
ANSWER : : all of the above
27.Which of the following is expected to have highest thermal conductivity
steam
solid ice
melting ice
water
boiling water.
ANSWER : : solid ice
28.A grey body is one whose absorptivity
varies with temperature
varies with wavelength of the incident ray
is equal to its emissivity
does not vary with temperature and. wavelength of the incident ray
none of the above.
ANSWER : : is equal to its emissivity
29.Emissivity of a white polished body in comparison to a black body is
higher
lower
same
depends upon the shape of body
none of the above.
ANSWER : : lower
30.The rate of energy emission from unit surface area through unit solid angle, along a normal to the surface, is known as
emissivity
transmissive
reflectivity
intensity of radiation
absorptivity.
ANSWER : : intensity of radiation
31.Which of the following has least value of conductivity
glass
water
plastic
rubber
air.
ANSWER : : air.
32.Which of the following has maximum value of thermal conductivity
aluminium
steel
brass
copper
lead.
ANSWER : : aluminium
33.According to Prevost theory of heat exchange
it is impossible to transfer heat from low temperature source to t high temperature source
heat transfer by radiation requires no medium
all bodies above absolute zero emit radiation
heat transfer in most of the cases takes place by combination of conduction, convection and radiation
rate of heat transfer depends on thermal conductivity and temperature difference.
ANSWER : : all bodies above absolute zero emit radiation
34.A non-dimensional number generally associated with natural convection heat transfer is
Grashoff number
Nusselt number
Weber number
Prandtl number
Reynold number.
ANSWER : : Grashoff number
35.Heat conducted through unit area and unit thick face per unit time when temperature difference between opposite faces is unity,is called
thermal resistance
thermal coefficient
temperature gradient
thermal conductivity
heat-transfer.
ANSWER : : thermal conductivity
36.Heat is transferred by all three modes of transfer, viz, conduction, convection and radiation in
electric heater
steam condenser
melting of ice
refrigerator condenser coils
boiler.
ANSWER : : boiler.
37.LMTD in case of counter flow heat exchanger as compared-to parallel flow heat exchanger is
higher
lower
same
depends on the area of heat exchanger
depends on temperature conditions.
ANSWER : : higher
38.Thermal diffusivity is
a dimensionless parameter
function of temperature
used as mathematical model
a physical property of the material
useful in case of heat transfer by radiation.
ANSWER : : a physical property of the material
39.Unit of thermal diffusivity is
m2/hr
m2/hr�C
kcal/m2 hr
kcal/m.hr�C
kcal/m2 hr�C.
ANSWER : : m2/hr
40.The ratio of heat flow Q1/Q2 from two walls of same thickness having their thermal conductivities as ATj – 2K2 will be
I
0.5
2
0.25
4.0
ANSWER : : 2
41.Thermal conductivity of a material may be defined as the
quantity of heat flowing in one second through one cm cube of material when opposite faces ^re maintained at a temperature difference of 1�C
quantity of heat flowing in one second through a slab of the material of area one cm square, thickness 1 cm when its faces differ in temperature by 1�C
heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces
all of the above
none of the above.
ANSWER : s: d
42.In convection heat transfer from hot flue gases to water tube, even though flow may be turbulent, a laminar flow region (boundary layer of film) exists close to the tube. The heat transfer through this film takes place by
convection
radiation
conduction
both convection and conduction
none of the above.
ANSWER : : conduction
43.Two balls of same material and finish have their diameters in the ratio of 2 : 1 and both are heated to same temperature and allowed to cool by radiation. Rate of cooling by big ball as compared to smaller one will be in the ratio of
1 :1
2: 1
1 : 2
4 : 1
1 : 4.
ANSWER : : 1 : 2
44.Heat transfer by radiation mainly depends upon
its temperature
nature of the body
kind and extent of its surface
all of the above
none of the above.
ANSWER : : all of the above
45.In heat transfer, conductance equals conductivity (kcal/hr/sqm/�C/cm) divided by
hr (time)
sqm (area)
�C (temperature)
cm (thickness)
kcal (heat).
ANSWER : : cm (thickness)
46.Thermal conductivity of glass-wool varies from sample to sample because of variation in
composition
density
porosity
structure
all of the above.
ANSWER : : all of the above.
47.Moisture would find its way into insulation by vapour pressure unless it is prevented by
high thickness of insulation
high vapour pressure
less thermal conductivity insulator
a vapour seal
all of the above.
ANSWER : : a vapour seal
48.The amount of heat flow through a body by conduction is
directly proportional to the surface area of the body
directly proportional to the temperature difference on the two faces of the body
dependent upon the material of the body
inversely proportional to the thickness of the body
all of the above.
ANSWER : : all of the above.
49.Thermal diffusivity of a substance is .
proportional of thermal conductivity
inversely proportional to k
proportional to (kf
inversely proportional to k2
none of the above.
ANSWER : : proportional of thermal conductivity
50.Which of the following property of air does not increase with rise in temperature
thermal conductivity
thermal diffusivity
density
dynamic viscosity
kuiematic viscosity.
ANSWER : : density
51.The concept of overall coefficient of heat transfer is used in case of heat transfer by
conduction
convection
radiation
conduction and convection
convection and radiation.
ANSWER : : conduction and convection
52.In free con-vection heat transfer, Nusselt number is function of
Grashoff no. and Reynold no.
Grashoff no. and Prandtl no.
Prandtl no. and Reynold no.
Grashoff no., Prandtl no. and Reynold no.
none of the above.
ANSWER : : Grashoff no. and Prandtl no.
53.The unit of overall coefficient of heat transfer is
kcal/m2
kcal/hr �C
kcal/m2 hr �C
acl/m hr �C
kcal/m3 hr �C.
ANSWER : : kcal/m2 hr �C
54.According to Kirchoff’s law, the ratio of emissive power to absorptivity for all bodies is equal to the emissive power of a
grey body
brilliant white polished body
red hot body
black body
none of the above.
ANSWER : : black body
55.Thermal diffusivity of a substance is
directly proportional to thermal conductivity
inversely proportional to density of substance
inversely proportional to specific heat
all of the above
none of the above.
ANSWER : : all of the above
56.The thermal diffusivities for gases are generally
more than those for liquids
less than those for liquids
more than those for solids
dependent on the viscosity
same as for the liquids.
ANSWER : : more than those for liquids
57.Log mean temperature difference in case of counter flow compared to parallel flow will be
same
more
less
depends on other factors
none of the above.
ANSWER : : more
58.According of Kirchhoff law,
radiant heat is proportional to fourth power of absolute temperature
emissive power depends on temperature
emissive power and absorptivity are constant for all bodies
ratio of emissive power to absorptive power is maximum for perfectly black body
ratio of emissive power to absorptive power for all bodies is same and is equal to the emissive power of a perfectly black body.
ANSWER : : ratio of emissive power to absorptive power for all bodies is same and is equal to the emissive power of a perfectly black body.
59.Joule sec is the unit of
universal gas constant
kinematic viscosity
thermal conductivity
Planck’s constant
none of the above.
ANSWER : : Planck’s constant
60.According to Stefan’s law, the total radiation from a black body per second per unit area is proportional to
absolute temperature
T2
T5
t
l/T.
ANSWER : : t
61.The unit of Stefan Boltzmann constant is
watt/cm2 �K
watt/cm4 �K
watt2/cm �K4
watt/cm2 �K4
watt/cm2 �K2.
ANSWER : : watt/cm2 �K4
62.Total emissivity of polished silver compared to black body is
same
higher
more or less same
very much lower
very much higher.
ANSWER : : very much lower
63.A steam pipe is to be insulated by two insulating materials put over each other. For best results
better insulation should be put over pipe and better one over it
inferior insulation should be put over pipe and better one over it
both may be put in any order
whether to put inferior OIL over pipe or the better one would depend on steam temperature
unpredictable.
ANSWER : : better insulation should be put over pipe and better one over it
64.The energy distribution of an ideal reflector at higher temperatures is largely in the range of
shorter wavelength
longer wavelength
remains same at all wavelengths
wavelength has nothing to do with it
none of the above.
ANSWER : : shorter wavelength
65.The value of Portland number for air is about
0.1
0.3
0.7
1.7
10.5.
ANSWER : : 0.7
66.The value of the wavelength for maximum emissive power is given by
Wien’s law
Planck’s law
Stefan’s law
Fourier’s law
Kirchhoff’s law.
ANSWER : : Wien’s law
67.All radiations in a black body are
reflected
refracted
transmitted
absorbed
partly reflected and partly absorbed.
ANSWER : : absorbed
68.The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body. This statement is known as
Krichoff’s law
Stefan’s law
Wien’ law
Planck’s law
Black body law.
ANSWER : : Krichoff’s law
69.According to Stefan-Boltzmann law, ideal radiators emit radiant energy at a rate proportional to
absolute temperature
square of temperature
fourth power of absolute temperature
fourth power of temperature
cube of absolute temperature.
ANSWER : : fourth power of absolute temperature
70.Fourier’s law of heat conduction is valid for
one dimensional cases only
two dimensional cases only
three dimensional cases only
regular surfaces having non-uniform temperature gradients
irregular surfaces.
ANSWER : : one dimensional cases only
71.According to Wien’s law, the wavelength corresponding to maximum energy is proportion to
absolute temperature (T)
I2
f
t
1/r.
ANSWER : : absolute temperature (T)
72.The thermal diffusivities for solids are generally
less than those for gases
jess than those for liquids
more than those for liquids and gases
more or less same as for liquids and gases
zerci.
ANSWER : s: c
73.In counter flow heat exchangers
both the fluids at inlet (of heat exchanger where hot fluid enters) are in their coldest state
both the fluids at inlet are in their hottest state
both the fluids .at exit are in their hottest state
one fluid is in hottest state and other in coldest state at inlet
any combination is possible depending on design of heat exchanger.
ANSWER : : both the fluids at inlet are in their hottest state
74.Stefan Boltzmann law is applicable for heat transfer by
conduction
convection
radiation
conduction and radiation combined
convection and radiation combined.
ANSWER : : radiation
75.A perfect black body is one which
is black in colour
reflects all heat
transmits all heat radiations
abslprbs heat radiations of all wave lengths falling on it
fully opaque.
ANSWER : : abslprbs heat radiations of all wave lengths falling on it
76. 40% of incident radiant energy on the surface of a thermally transparent body is reflected back. If the transmissivity of the body be 0.15, then the emissivity of surface is
0.45
0.55
0.40
0.75
0.60.
ANSWER : : 0.45
77.Depending on the radiating properties, a body will be black when
p = 0, x = 0 and a = 1
p= l,T = 0anda = 0
p = 0, x = 1 and a = 0
x = 0, a + p = 0
a = 0,x + p= 1.
ANSWER : re a = absorptivity, p == reflectivity, X = transmissivity.
78.Planck’s law holds good for
black bodies
polished bodies
all colored bodies
all of the above
none of the above.
ANSWER : : black bodies
79.The total emissivity power is .defined as the total amount of radiation emitted by a black body per unit
temperature
thickness
area
time
area and time.
ANSWER : : time
80.Two plates spaced 150 mm apart are maintained at 1000�C and 70�C. The heat transfer will take place mainly by
convection
free convection
forced convection
radiation
radiation and convection.
ANSWER : : radiation
81.The ratio of the energy absorbed by the body to total energy falling on it is called
absorptive power
emissive power
absorptivity
emissivity
none of the above.
ANSWER : : absorptive power
82.Depending on the radiating properties, a body will be opaque when
p = 0, x = 0 and a = 1
p=l,x = 0anda = 0
p = 0, x = 1 and a = 0
x – 0, a + p = 1
a=0,x + p= 1.
ANSWER : re a = absorptivity, p = reflectivity, X = transmissivity.
83.In re generator type heat exchanger, heat transfer takes place by
direct mixing of hot and cold fluids
a complete separation between hot and cold fluids
flow of hot and cold fluids alternately over a surface
generation of heat again and again
indirect transfer.
ANSWER : : flow of hot and cold fluids alternately over a surface
84.The amount of radiation mainly depends on
nature of body
temperature of body
type of surface of body
all of the above
none of the above.
ANSWER : : all of the above
85.A grey body is one whose absorptivity
varies with temperature
varies with the wave length of incident ray
varies with both
does not vary with temperature and wave length of the incident ray
there is no such criterion.
ANSWER : : does not vary with temperature and wave length of the incident ray
86.The emissive power of a body depends upon its
temperature
wave length
physical nature
all of the above
none of the above.
ANSWER : s: d
87.Depending on the radiating properties, body will be transparent when
p = 0, x = 0 and a = 1
p=l,x = 0, and a = 0
p = 0, T= l, and a = 0
X = 0, a + p = 1
a = 0,x + p= 1.
ANSWER : : p = 0, T= l, and a = 0
88.If the temperature of a solid surface changes form 27�C to 627�C, then its emissive power changes in the ratio of
3
6
9
27
81.
ANSWER : : 81.
89.Absorptivity of a body will be equal to its emissivity
at all temperatures
at one particular temperature
when system is under thermal equiplibrium
at critical temperature
for a polished body.
ANSWER : : when system is under thermal equiplibrium