3 Convection

Convection is a heat transport process in a fluid involving the combined action of heat conduction and energy storage when the fluid is undergoing a mixing motion. Since convection heat transfer is closely related to fluid motion, prior knowledge of the flow field is prerequisite before the mechanism of heat transfer in the fluid can be investigated. Generally, the motion of a fluid, either a gas or a liquid, can be divided into two basic flow conditions, namely, laminar and turbulent.

In laminar flow, the fluid particles follow smooth and continuous paths in an orderly fashion. The fluid particles in any one layer do not move across layers, and particles remain in an orderly sequence within each layer without passing one another. Therefore, heat is transferred purely by molecular conduction among the particles through direct contact. Heat is also exchanged through the interface between the fluid and the surface of the walls. On the other hand, turbulent flow is irregular, and the fluid velocity components are random, varying with time and space. Turbulent eddies of the fluid move across the streamlines. Energy is transferred by mixing the fluid particles in motion. Thus, increasing the mixing rate will increase the heat transfer rate.

Convection heat transfer can be classified into forced and free (or natural) convection. In the former process, the fluid motion is induced by some external means such as a pump, blower, fan, or wind. In the latter process, the fluid motion is generated from the density difference caused by a temperature variation in the fluid.