Heat Transfer MCQs | Heat Transfer Short Questions and Answers

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Questions
41 Absorptivity of a body will be equal to its emissivity
A at all temperatures
B at one particular temperature
C when system is under thermal equi-librium
D at critical temperature

Answer: when system is under thermal equi-librium
42 Planck’s law holds good for
A Black Bodies
B Polished Bodies
C All Coloured Bodies
D All Of The Above

Answer: Black Bodies
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43 A grey body is one whose absorptivity
A varies with temperature
B varies with the wave length of incident ray
C varies with both
D does not vary with temperature and wave length of the incident ray

Answer: does not vary with temperature and wave length of the incident ray
44 The time constant of a thermocouple is
A the time taken to attain the final temperature to be measured
B the time taken to attain 50% of the value of initial temperature difference
C the time taken to attain 63.2% of the value of initial temperature difference
D determined by the time taken to reach 100°C from 0°C

Answer: the time taken to attain 63.2% of the value of initial temperature difference
45 The concept of overall coefficient of heat transfer is used in heat transfer problems of
A conduction
B convection
C radiation
D conduction and comte_ction

Answer: conduction and comte_ction
46 In heat transfer, conductance equals conductivity (kcal/hr/sqm/°C/cm) divided by
A hr (time)
B sqm (area)
C °C (temperature)
D cm (thickness)

Answer: Planck’s constant
47 Thermal Conductivity Of Glass-Wool Varies From Sample To Sample Because Of Variation In
A Composition
B Density
C Porosity
D All Of The Above

Answer: All Of The Above
48 Moisture would find its way into insulation by vapour pressure unless it is prevented by
A high thickness of insulation
B high vapour pressure
C less thermal conductivity insulator
D a vapour seal

Answer: a vapour seal
49 Thermal diffusivity is
A a dimensionless parameter
B function of temperature
C used as mathematical model
D a physical property of the material

Answer: a physical property of the material
50 Thermal diffusivity of a substance is .
A proportional of thermal conductivity
B inversely proportional to k
C proportional to (k)
D inversely proportional to k2

Answer: proportional of thermal conductivity
51 Thermal Conductivity Of Wood Depends On
A Moisture
B Density
C Temperature
D All Of The Above

Answer: All Of The Above
52 Heat conducted througfi unit area and unit thick face per unit time when temperature difference between opposite faces is unity,is called
A thermal resistance
B thermal coefficient
C temperature gradient
D thermal conductivity

Answer: thermal conductivity
53 A grey body is one whose absorptivity
A varies with temperature
B varies with wavelength of the incident ray
C is equal to its emissivity
D does not vary with temperature and. wavelength of the incident ray

Answer: is equal to its emissivity
54 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
A 1 :1
B 2: 1
C 1 : 2
D 4 : 1

Answer: 1 : 2
55 In heat exchangers, degree of approach is defined as the difference between temperatures of
A cold water inlet and outlet
B hot medium inlet and outlet
C hot medium outlet and cold water inlet
D hot medium outlet and cold water outlet

Answer: hot medium outlet and cold water outlet
56 According of Kirchhoff’s law,
A radiant heat is proportional to fourth power of absolute temperature
B emissive power depends on temperature
C emissive power and absorptivity are constant for all bodies
D 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.
57 According to Kirchoff’s law, the ratio of emissive power to absorptivity for all bodies is equal to the emissive power of a
A grey body
B brilliant white polished body
C red hot body
D black body

Answer: black body
58 The concept of overall coefficient of heat transfer is used in case of heat transfer by
A conduction
B convection
C radiation
D conduction and convection

Answer: conduction and convection
59 The value of Prandtl number for air is about
A 0.1
B 0.3
C 0.7
D 1.7

Answer: 0.7
60 Log mean temperature difference in case of counter flow compared to parallel flow will be
A same
B more
C less
D depends on other factors

Answer: more

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