Convection 

The transfer of thermal energy in  fluids in motion by either free convection or forced convection.

Example:

Free convection - When a hot apple pie is left to cool on a table. Convection occurs because of the temperature difference between the pie and surrounding air.  Heated air  is transported upward and replaced with cooler air.  The movement of the air is driven by buoyancy forces, which are temperature dependent.

Forced convection - When a hot apple pie is cooled using a fan blowing air across it.  Heated air  is transported upward and replaced with cooler air.  The movement of the air is driven by deliberate forces (i.e. fans, blowers, pumps).  

The governing equation for convection is Newton's Law of Cooling:

= Heat Transfer (W)

h = heat transfer coefficient (W/m2K)

Tw = Temperature at the wall (K)

 T= Temperature of surroundings (K)

 

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