Heat Transfer MCQs | Heat Transfer Short Questions and Answers

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