Convection

Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion. The faster the fluid motion, the greater the convection heat transfer. In the absence of any bulk fluid motion, heat transfer between a solid surface and the adjacent fluid is by pure conduction.

Convection is governed by Newton’s Law of cooling, which states:

$${\dot{Q}}_{\text{conv}}^{\prime \prime }=\bar{h}(T_s-T_{\infty })$$

where:

• $\bar{h}$ is the average heat transfer coefficient $[\text{W}/{\text{m}}^2\cdot \text{K}]$
• $T_s$ is the surface temperature
• $T_{\infty }$ is the temperature of the fluid sufficiently far from the surface.
  • Note that at the surface, the fluid temperature equals the surface temperature of the solid.

The convection heat transfer coefficient $h$ is not a property of the fluid. It is an experimentally determined parameter whose value depends on all the variables influencing convection such as the surface geometry, the nature of fluid motion, the properties of the fluid, and the bulk fluid velocity.

Thus, the coefficient $h$ is highly dependent on various conditions.