15. 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
A.Convection
B.Radiation
C.Conduction
D.Both convection and conduction

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16. 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

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17. The value of Prandtl number for air is about
A.0.1
B.0.3
C.0.7
D.1.7

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18. Thermal radiation extends over the range of
A.0.01 to 0.1 μ
B.0.1 to 100 μ
C.100 to 250 μ
D.250 to 1000 μ

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19. Which of the following is the case of heat transfer by radiation
A.Blast furnace
B.Heating of building
C.Cooling of parts in furnace
D.Heat received by a person from fireplace

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20. When heat is transferred from one particle of hot body to another by actual motion of the heated particles, it is referred to as heat transfer by
A.Conduction
B.Convection
C.Radiation
D.Convection and radiation

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21. Thermal conductivity of air at room temperature in kcal/m hr °C is of the order of
A.0.002
B.0.02
C.0.01
D.0.1

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