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

Both convection and conduction
Conduction
Radiation
Convection

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Heat and Mass Transfer
According to Prevost theory of heat exchange

Heat transfer in most of the cases takes place by combination of conduction, convection and radiation
It is impossible to transfer heat from low temperature source to t high temperature source
Heat transfer by radiation requires no medium
All bodies above absolute zero emit radiation

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