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Mechanical Engineering
Heat And Mass Transfer
Quiz 5
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Q.1
Free convection flow depends on
Density
Coefficient of viscosity
Gravitational force
All of these
Q.2
The critical thickness of insulation for a sphere is
k/h₀
2k/h₀
h₀/k
h₀/2k
Q.3
Unit of thermal conductivity in M.K.S. units is
K cal/kg m² °C
K cal m/hr m² °C
K cal/hr m² °C
K calm/hr °C
Q.4
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
Q.5
Wien’s law states that the wave length corresponding to ________ is proportional to the absolute temperature.
Minimum energy
Maximum energy
Both (A) and (B)
None of these
Q.6
The amount of radiation mainly depends on
Nature of body
Temperature of body
Type of surface of body
All of the above
Q.7
The heat transfer by conduction through a thick cylinder (Q) is given by (where T₁ = Higher temperature, T₂ = Lower temperature, r₁ = Inside radius, r₂ = Outside radius, l = Length of cylinder, and k = Thermal conductivity)
Q = [2πlk (T₁ - T₂)]/2.3 log (r₂/r₁)
Q = 2.3 log (r₂/r₁)/[2πlk (T₁ - T₂)]
Q = [2π (T₁ - T₂)]/2.3 lk log (r₂/r₁)
Q = = 2πlk/2.3 (T₁ - T₂) log (r₂/r₁)
Q.8
Thermal diffusivity of a substance is given by (where h = Thermal diffusivity, ρ = Density of substance, S = Specific heat, and k = Thermal conductivity)
h = k/ ρS
h = ρS/k
h = S/ρk
h = kρ/S
Q.9
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
Q.10
The heat transfer takes place according to
Zeroth law of thermodynamics
First law of thermodynamics
Second law of thermodynamics
Kirchhoff's law
Q.11
In case of liquids and gases, the heat transfer takes place according to
Conduction
Convection
Radiation
None of these
Q.12
Depending on the radiating properties, a body will be opaque when
P = 0, x = 0 and a = 1
P=1, x = 0 and a = 0
P = 0, x = 1 and a = 0
X = 0, a + p = 1
Q.13
The process of heat transfer from one particle of the body to another is called conduction, when the particles of the body
Move actually
Do not move actually
Affect the intervening medium
Does not affect the intervening medium
Q.14
The thermal diffusivities for solids are generally
Less than those for gases
Less than those for liquids
More than those for liquids and gases
More or less same as for liquids and gases
Q.15
Log mean temperature difference in case of counter flow compared to parallel flow will be
Same
More
Less
Depends on other factors
Q.16
Heat transfer by radiation mainly depends upon
Its temperature
Nature of the body
Kind and extent of its surface
All of the above
Q.17
The transfer of heat by molecular collision is known as
Conduction
Convection
Radiation
None of these
Q.18
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
Q.19
Sensible heat factor is given by (where S.H. = Sensible heat, and L.H. = Latent heat)
S.H/(S.H + L.H)
(S.H + L.H) /S.H
(L.H - S.H)/S.H
S.H/(L.H - S.H)
Q.20
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
Q.21
The automobile radiator is a heat exchanger of
Parallel flow type
Counter flow type
Cross flow type
Regenerator type
Q.22
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
Q.23
In free convection heat transfer transition from laminar to turbulent flow is governed by the critical value of the
Reynold's number
Grashoff's number
Reynold's number, Grashoff's number
Prandtl number, Grashoff's number
Q.24
According of Kirchhoff's 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 for all bodies is same and is equal to the emissive power of a perfectly black body
Q.25
Thermal conductivity of wood depends on
Moisture
Density
Temperature
All of the above
Q.26
The rate of energy emission from unit surface area through unit solid angle, along a normal to the surface, is known as
Emissivity
Transmissivity
Reflectivity
Intensity of radiation
Q.27
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
Q.28
The ratio of the energy absorbed by the body to total energy falling on it is called
Absorptive power
Emissive power
Absorptivity
Emissivity
Q.29
Absorptivity of a body will be equal to its emissivity
At all temperatures
At one particular temperature
When system is under thermal equilibrium
At critical temperature
Q.30
In a heat exchanger with one fluid evaporating or condensing, the surface area required is least in
Parallel flow
Counter flow
Cross flow
All of these
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