Convection takes place when energy is transferred from a surface to a fluid flowing over it as a result of a difference between the temperatures of the surface and the fluid. Convection heat transfer mode is comprised of two mechanisms
Energy transfer due to random molecular motion (diffusion)
Energy transferred by the bulk or macroscopic motion of the fluid ( advection)
This fluid motion is associated with the aggregate or collective movement of the large number of molecules. Such motion, in the presence of temperature gradient, contributes to the heat transfer. Because the molecules in the aggregate retain their random motion, the total heat transfer is then due to a superposition of energy transport by the random motion of the molecules and by the bulk motion of the fluid.
Convection heat transfer may be classified according to the nature of the flow.
Forced convection takes place when the flow is caused by an external agent such as fan, pump or atmospheric winds. For example, consider the use of a fan to provide forced convection air cooling of hot electrical components on a stack of printed circuit boards.
Natural convection takes place when the flow is induced by density differences caused by the temperature variations in the fluid. For example, consider heat transfer that occurs from hot components on a vertical array of circuit boards in still air.
The rate equation for convection is known as Newton's law of cooling. This is given by
q" is the convective heat flux (W/m2). Convective heat flux is proportional to the difference between the surface and temperatures, Tsand
, respectively. The proportionality constant is termed the convection heat transfer coefficient. It depends on the surface geometry, the nature of the fluid motion, and the fluid involved. Any study of convection ultimately reduces to a study of the means by which h may be determined. Although consideration of these means is postponed to Chapter 6, convection heat transfer will frequently appear as a boundary condition in the solution of conduction problems. In the solution of such problems we presume h to be known, using typical values gven in Table.
Table1.1:Typical values of the convection heat transfer coefficient