Free convection vs Forced convection [with Pdf]

The main difference between Free convection vs forced convection is that in the case of free convection the fluid molecules move due to the temperature and density difference while in the case of forced convection the motion in fluid molecules is generated by an external source.

What is free convection?

Free convection is also known as Natural convection is the type of convection heat transfer in which the fluid molecules move due to the density and temperature gradient to transfer the heat.

Eg: For the boiling of water, the molecules in the bottom layer gain heat from the heat source. therefore the water molecules in this layer become less dense in comparison with the upper layer.

Boiling of water

Therefore a buoyant force is created on the molecules in the lower layer and hence these molecules move toward the upper direction. While the less dense upper molecules move in a downward direction due to the higher density.

Therefore for the natural convection, no external equipment is required to generate motion into the molecule.

What is forced convection?

Forced convection is the type of convective heat transfer in which the motion in molecules is generated by the use of an external source for the transfer of heat. E.g. Exhaust fan or a ceiling fan.

The exhaust fan helps to eject the heat inside of the room by generating motion into the air molecules.

The rate of heat transfer in forced convection is higher which can be controlled by external equipment.

Therefore the system with forced convection has a higher overall heat transfer coefficient than free convection.

Free convection vs forced convection

Sr. No.Free convectionForced convection
1In free convection, the molecules move due to density and temperature variation.In forced convection, the fluid molecules are forced to move by an external source.
2The rate of heat transfer is lower.The rate of heat transfer is higher.
3No external equipment is required.External equipment is necessary for convective heat transfer eg. Pump, Blower, fan, etc.
4The motion of molecules is comparatively slower.Molecules of fluid are forced to move faster.
5Equipment based on natural convection is larger in size.The equipment based on forced convection is compact in size.
6The flow of molecules cannot be controlled.The flow of molecules can be controlled by controlling the fan, pump, or blower.
7It has less overall heat transfer coefficient.The overall heat transfer coefficient is higher.
8 E.g. movement of water molecules while boiling.E.g. Movement of molecules due to the fan or blower.

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