Difference between natural and forced convection

Definition of Convection (Convective heat transfer)

• Convection is the transfer of the heat from one point to another point within a fluid by mixing of hot and cold portions of the fluid.
• Convection is restricted to the flow of heat in fluids.
• Example: the heating of water in cooking pans
• Heat transfer by convection occurs due to the movement of fluid on a microscopic scale in the form of circulating currents.
• This circulating current may arise by heat transfer process itself or by some external agency.
• According to this, there are two types of convection 

1.     Free or natural convection

2.     Forced convection

Free or natural convection

• In convective heat transfer when the circulating currents arise from the heat transfer process itself due to the density differences arising from temperature differences within the fluid mass, is called natural or free convection.

Example:-

• Heat flows from the radiator to the adjacent air in the room.

        The heated air becomes lighter and flows along with the ceiling, to which it         transfers heat and then back to the lower part of the room to recirculated and     circulating current set up.

• Heating of vessel containing liquid by means of a gas flame

           The liquid at the bottom of the vessel gets heated, expands and rises as it becomes lighter. Then the remaining cold liquid of higher density takes its place and circulating current is set up. 

Forced convection

• In convective heat transfer when the circulating currents are produced by an external agency such as an agitator in a reaction vessel, pump, fan or blower, is called as forced convection.

Example:-

• Heating of vessel containing liquid by means of a gas flame and agitator
• Air is forced by a fan carrying with it heat from the wall if the wall temperature is higher or giving heat to the wall if the wall temperature is lower than the air temperature.
• Heat flow to the pumped fluid through the heated pipe