Heat Transfer of Convective

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 or assisted convection
• natural or free convection

Forced or Assisted Convection

Forced convection occurs when a fluid flow is induced by an external force, such as a pump, fan or a mixer.

Natural or Free Convection

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 heat transfer per unit surface through convection was first described by Newton and the relation is known as the Newton's Law of Cooling.

The equation for convection can be expressed as:

q = k A dT         (1)
where
q = heat transferred per unit time (W)
A = heat transfer area of the surface (m^o)
k = convection heat transfer coefficient of the process (W/m²K or W/m^2oC)
dT = temperature difference between the surface and the bulk fluid (K or ^oC)

Heat Transfer Coefficients - Units

• 1 W/m²K = 0.85984 kcal/h m^{2 o}C = 0.1761 Btu/ ft² h ^oF
• 1 Btu/ft² h ^oF = 5.678 W/m² K = 4.882 kcal/h m^{2 o}C
• 1 kcal/h m^{2 o}C = 1.163 W/m²K = 0.205 Btu/ ft² h ^oF

Convective Heat Transfer Coefficients

The convection heat transfer coefficient - k - 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:

• Air : 10 - 100 (W/m²K)
• Water : 500 - 10,000 (W/m²K)

Example - Convective Heat Transfer

A fluid flows over a plane surface 1 m by 1 m with a bulk temperature of 50^oC. The temperature of the surface is 20^oC. The convective heat transfer coefficient is 2,000 W/m^2oC.

q = (2,000 W/m^2oC) ((1 m) (1 m)) ((50^oC) - (20^oC))
    = 60,000 (W)
    = 60 (kW)

 Convective Heat Transfer Chart