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

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 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.

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