Convection heat transfer can be simply described as the transfer of heat along with the movement of its medium. However, convection process is a complex mechanism which involves heat conduction, energy storage, and mixing/agitation especially when the heat transfer occurs between a solid surface and fluids body. It is important to understand this method as heat transfer for gaseous and liquid phase are working this way.
In detail, convection heat transfer occurs when there is temperature difference between solid surface and fluids surrounding it (we do not discuss fluid to fluid heat transfer to simplify thing). As illustration, the surface with high temperature will transfer heat to fluids particles on the surface. This is simply called as boundary layer where heat transfer follows conduction process. This boundary layer can be assumed to stay. This particle will transfer its heat to surrounding particles. As the temperature rises, the particle will tend to move due to less atomic bonding in gaseous and liquid phase. Fluids particles will move to region where the particle has less temperature. This movement will create a mixing process, as we can observe if we boil water in a pail.
Convection heat transfer can be classified into two kinds:
- Natural/Free convection. Natural convection occurs when the heat transfer driving force is solely based on temperature difference only. There is no additional energy or work help the heat transfer. As example, boiling water in a pail can be considered as natural convection process.
- Forced Convection. In forced convection process, there is additional energy or work applied to help the circulation/movement of the fluid. If we boil a water in a pail with additional mixer, it is classified as forced convection. Wind effect can also be considered as forced convection.
Basic convection heat transfer is simple. However, convection heat transfer coefficient is complicated and may differ from one application to another application, as for example:
qc = convection heat transfer rate (Watt, Btu/h)
hk = convection heat transfer coefficient (W/m2 K, Btu/h ft2 0F)
A = surface area (m2, ft2)
Tw = wall temperature (oC, oF)
Ts = ambient temperature (oC, oF)
Gr = Grasshof Number
Pr = Prandtl Number
Re = Reynold Number
l = length (mm)
Ku = fluid coefficient (W/m.K)
D = diameter (mm)