Heat and Mass Transfer
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

Radiation
Both convection and conduction
Convection
Conduction

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Heat and Mass Transfer
According of Kirchhoff's law

Ratio of emissive power to absorptive power for all bodies is same and is equal to the emissive power of a perfectly black body
Emissive power and absorptivity are constant for all bodies
Emissive power depends on temperature
Radiant heat is proportional to fourth power of absolute temperature

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