Convection is the transfer of internal energy

into or out of an object due to the bulk or macroscopic motion of surrounding a

fluid. This heat transfer mechanism can be spontaneously forced by the

generation of convection cells or by the propulsion of the fluid across the

object or by the propulsion of object through the fluid 11. The heat transfer due to the forced convection

is given by the following equation 11:The convective heat transfer coefficient is

dependent on the physical properties of the fluid and the physical situation,

for example, the coefficient for turbulent flow is larger than that for laminar

flow.The flow region adjacent to the wall in which the

viscous effects are significant is called the boundary layer. The boundary

layers play important roles in the study of convective heat transfer and mass

transfer between a surface and a fluid. In a thin layer, when the moving fluid particles

approach contact with the solid surface, with decreasing distance

from the

surface, the velocity of fluid

varies from a

value approximately equal to the free stream value

to zero (Figure 2.3).

The boundary layer thickness

is normally

defined as the value of distance

from the solid

body to the point at which the viscous flow velocity

.

Strictly speaking, the value of

is an arbitrary

value, which depends on the friction force of the molecular interactions between

fluid and solid surface. 12

The fundamental concept of the boundary layer was

first defined by L. Prandtl1.

In the boundary layer region, velocity gradients and shear stresses

are both large,

conversely, outside of this boundary layer, in what is called irrotational flow

region, velocity gradients and shear stresses can be ignored. These two regions

are divided by the hypothetical line of

. 11

1 The aerodynamic boundary layer was first

defined by Ludwig Prandtl in a paper presented on August 12, 1904 at the third

International Congress of Mathematicians in Heidelberg, Germany.