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Mechanical Engineering Objective Questions { Heat Transfer }

85. The total emissivity power is defined as the total amount of radiation emitted by a black body per unit
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86. 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
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87. The emissive power of a body depends upon its
B.Wave length
C.Physical nature
D.All of the above
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88. In regenerator type heat exchanger, heat transfer takes place by
A.Direct mixing of hot and cold fluids
B.A complete separation between hot and cold fluids
C.Flow of hot and cold fluids alternately over a surface
D.Generation of heat again and again
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89. Planck's law holds good for
A.Black bodies
B.Polished bodies
C.All coloured bodies
D.All of the above
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90. 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
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91. Depending on the radiating properties, a body will be opaque when
A.ρ = 0, τ = 0 and α = 1
B.ρ = 1, τ = 0 and α = 0
C.ρ = 0, τ = 1 and α = 0
D.τ = 0, α+ρ = 1
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