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Mechanical Engineering
Heat And Mass Transfer
Quiz 4
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Q.1
The emissivity for a black body is
0
0.5
0.75
1
Q.2
Heat is closely related with
Energy
Liquids
Temperature
Entropy
Q.3
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
Q.4
Two long parallel surfaces each of emissivity 0.7 are maintained at different temperatures and accordingly have radiation heat exchange between them. It is desired to reduce 75% of the radiant heat transfer by inserting thin parallel shields of emissivity 1 on both sides. The number of shields should be
One
Two
Three
Four
Q.5
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
Q.6
A heat exchanger with heat transfer surface area A and overall heat transfer coefficient U handles two fluids of heat capacities Cmax and Cmin. The number of transfer units (NTU) used in the analysis of heat exchanger is specified as
A.Cmin/U
U/A.Cmin
A.U.Cmin
A.U/Cmin
Q.7
Reynolds number is the ratio of
Energy transferred by convection to that by conduction
Kinematic viscosity to thermal diffusivity
Inertia force to viscous force
None of the above
Q.8
According to Stefan's law, the total radiation from a black body per second per unit area is proportional to
Absolute temperature
T
2
T
5
T
Q.9
The critical temperature is the temperature
Below which a gas does not obey gas laws
Above which a gas may explode
Below which a gas is always liquefied
Above which a gas will never liquefied
Q.10
Thermal conductivity of non-metallic amorphous solids with decrease in temperature
Increases
Decreases
Remain constant
May increase or decrease depending on temperature
Q.11
If the temperature of a solid surface changes form 27°C to 627°C, then its emissive power changes in the ratio of
3
9
270
81
Q.12
Kirchhoff's law states that
The total radiation from a black body per second per unit area is directly proportional to the fourth power of the absolute temperature
The wave length corresponding to the maximum energy is proportional to the absolute temperature
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
None of the above
Q.13
Metals are good conductors of heat because
Their atoms collide frequently
Their atoms are relatively far apart
They contain free electrons
They have high density
Q.14
The transfer of heat by molecular collision is smallest in
Solids
Liquids
Gases
None of these
Q.15
According to Stefan Boltzmann law, the total radiation from a black body per second per unit area is directly proportional to the
Absolute temperature
Square of the absolute temperature
Cube of the absolute temperature
Fourth power of the absolute temperature
Q.16
Cork is a good insulator because it has
Free electrons
Atoms colliding frequency
Low density
Porous body
Q.17
Pick up the wrong case. Heat flowing from one side to other depends directly on
Face area
Time
Thickness
Temperature difference
Q.18
Thermal conductivity of solid metals with rise in temperature normally
Increases
Decreases
Remain constant
May increase or decrease depending on temperature
Q.19
The logarithmic mean temperature difference (tm) is given by (where Δt1 and Δt2 are temperature differences between the hot and cold fluids at entrance and exit)
tm = (Δt1 - Δt2)/ loge (Δt1/Δt2)
tm = loge (Δt1/Δt2)/ (Δt1 - Δt2)
tm = tm = (Δt1 - Δt2) loge (Δt1/Δt2)
tm = loge (Δt1 - Δt2)/ Δt1/Δt2
Q.20
Emissivity of a white polished body in comparison to a black body is
Higher
Lower
Same
Depends upon the shape of body
Q.21
The heat of sun reaches to us according to
Conduction
Convection
Radiation
None of these
Q.22
Thermal diffusivity of a substance is
Directly proportional to the thermal conductivity
Inversely proportional to density of substance
Inversely proportional to specific heat
All of the above
Q.23
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
Q.24
In heat exchangers, degree of approach is defined as the difference between temperatures of
Cold water inlet and outlet
Hot medium inlet and outlet
Hot medium outlet and cold water inlet
Hot medium outlet and cold water outlet
Q.25
Fouling factor is used
In heat exchanger design as a safety factor
In case of Newtonian fluids
When a liquid exchanges heat with a gas
None of the above
Q.26
The natural convection air cooled condensers are used in
Domestic refrigerators
Water coolers
Room air conditioners
All of these
Q.27
The concept of overall coefficient of heat transfer is used in heat transfer problems of
Conduction
Convection
Radiation
Conduction and convection
Q.28
An electric cable of aluminium conductor (k = 240 W/mK) is to be insulated with rubber (k = 0.15 W/mK). The cable is to be located in air (h = 6 W/m²). The critical thickness of insulation will be
25 mm
40 mm
160 mm
800 mm
Q.29
Which of the following is expected to have highest thermal conductivity?
Steam
Solid ice
Melting ice
Water
Q.30
Which of the following has maximum value of thermal conductivity?
Aluminium
Steel
Brass
Copper
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