When a fluid moves past a surface at a different temperature, heat is transferred through a combination of conduction within the fluid near the wall and bulk fluid motion. This process is called convection.

Newton's law of cooling

Convective heat transfer is commonly modelled as

$$\dot Q=hA(T_s-T_\infty),$$

where $T_s$ is surface temperature, $T_\infty$ is a representative fluid temperature and $h$ is the convective heat-transfer coefficient.

What the coefficient represents

Unlike thermal conductivity, $h$ is not a property of the fluid alone. It depends on the flow speed and geometry, fluid properties, boundary-layer behaviour and whether the flow is laminar or turbulent.

Forced and natural convection

In forced convection, pumps, fans or external motion drive the flow.

In natural convection, temperature-induced density differences combine with gravity to create buoyant motion.

Boundary layers

The thermal and velocity boundary layers near a surface strongly influence convection. Faster mixing or thinner boundary layers generally increase the rate at which heat can be carried away from the wall.

Convection therefore links heat transfer directly to fluid mechanics.