Convection

Convective heat transfer is the transport of thermal energy due to bulk motion of a fluid (liquid or gas) over a surface, and is based on Newton's Law of Cooling:

• \( q_{1-2} [W] \) = rate of heat flow from 1 (hotter) to 2 (colder)
• \( A [m^2] \) = surface area normal to the direction of heat flow
• \( T_1 [K] \) = temperature of 1 (e.g., system)
• \( T_2 [K] \) = temperature of 2 (e.g., surroundings)
• \( h [\frac{W}{m^2 K}] \) = convective heat transfer coefficient (which always includes the effect of conduction in the fluid)

The convective heat transfer coefficient h is a proportionality constant between the temperature difference and heat flux. Note that it is not a material property, but depends on geometry, flow conditions, and fluid properties, which can all be measured experimentally.

Convection can be free (driven by differences in density) or forced (driven by external forces such as pumps and fans).