Heat and Mass Transfer
In regenerator type heat exchanger, heat transfer takes place by

Direct mixing of hot and cold fluids
Flow of hot and cold fluids alternately over a surface
A complete separation between hot and cold fluids
Generation of heat again and again

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Heat and Mass Transfer
Kirchhoff's law states that

The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body
The total radiation from a black body per second per unit area is directly proportional to the fourth power of the absolute temperature
None of these
The wave length corresponding to the maximum energy is proportional to the absolute temperature

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Heat and Mass Transfer
In counter flow heat exchangers

Both the fluids at exit are in their hottest state
One fluid is in hottest state and other in coldest state at inlet
Both the fluids at inlet are in their hottest state
Both the fluids at inlet (of heat exchanger where hot fluid enters) are in their coldest state

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