This is the mechanism by which heat is transferred by motion of a fluid material.

**Natural convection:** The flow of the fluid occurs because of the difference in density caused by thermal expansion.

**Forced convection:** The flow of the fluid occurs because it is forced by a blower or pump.

There is no simple equation for calculating the amount of heat transferred by convection. The heat lost or gained by a surface at one temperature in contact with a fluid at another temperature depends on many factors, such as the shape and orientation of the surface, the mechanical and thermal properties of the fluid, and the nature of fluid flow (laminar or turbulent), and whether it is by natural convection of forced convection.

For engineering calculation of heat loss by convection, the following equation is often used,

$$\frac{dQ}{dt} = h A \, \Delta T$$

,where

$\frac{dQ}{dt}$ = rate of heat transfer,

A = surface area,

$\Delta T$ = temperature difference between the surface and the fluid,

h = convection coefficient

Convection coefficient is temperature dependent and needs to be determined empirically.

Next: Convective Heat Transfer