Heat energy transferred between a surface and a moving fluid at different temperatures is known as convection.
In reality this is a combination of diffusion and bulk motion of molecules. Near the surface the fluid velocity is low, and diffusion dominates. Away from the surface, bulk motion increase the influence and dominates.
Convective heat transfer may take the form of either
Forced convection occurs when a fluid flow is induced by an external force, such as a pump, fan or a mixer.
Natural convection is caused by buoyancy forces due to dens
ity differences caused by temperature variations in the fluid. At heating the density change in the boundary layer will cause the fluid to rise and be replaced by cooler fluid that also will heat and rise. This continues phenomena is called free or natural convection.
Boiling or condensing processes are also referred as a convective heat transfer processes.
The equation for convection can be expressed as:
q = hc A dT (1)
q = heat transferred per unit time (W)
A = heat transfer area of the surface (m2)
hc= convective heat transfer coefficient of the process (W/m2K or W/m2oC)
dT = temperature difference between the surface and the bulk fluid (K or oC)
The convective heat transfer coefficient - hc - is dependent on the type of media, gas or liquid, the flow properties such as velocity, viscosity and other flow and temperature dependent properties.
In general the convective heat transfer coefficient for some common fluids is within the ranges:
- Free Convection - Air : 5 - 25 (W/m2K)
- Free Convection - Water: 20 - 100 (W/m2K)
- Forced Convection - Air: 10 - 200 (W/m2K)
- Forced Convection - Water: 50 - 10.000 (W/m2K)
- Boiling Water : 3.000 - 100.000 (W/m2K)
- Condensing Water Vapor: 5.000 - 100.000 (W/m2K)
A fluid flows over a plane surface 1 m by 1 m with a bulk temperature of 50oC. The temperature of the surface is 20oC. The convective heat transfer coefficient is 2,000 W/m2oC.
q = 2,000 (W/m2oC) (1 (m) 1 (m)) (50 (oC) - 20 (oC))
= 60,000 (W)
= 60 (kW)