Page 142 - Industrial Ventilation Design Guidebook

I 04 CHAPTER 4 PHYSICAL FUNDAMENTALS

FIGURE 4.23 One-dimensional heat flow.

Integration of Eq. (4.154) with constant heat conductivity gives

One-dimensional heat conduction means that the heat flow is in one di-
rection only, and one coordinate is required to represent the case. For exam-
ple, in the case of a cylinder it is parallel with the radius.

4.3.1.2 Convection
Convection occurs in a moving fluid, generally from the fluid to a solid
surface or vice versa. Although heat transfer between single particles is by con-
duction, it is the energy transfer with the matter that governs the heat transfer.
The basic laws of heat and mass transfer have to be considered in order to de-
scribe convection mathematically.
Natural convection is self-induced and is created by the density dif-
ferences, which are temperature related; the boiling of water in a kettle is
an example of free convection. Forced convection is caused by an exter-
nal force being applied by mechanical means such as a fan or pump; the
cooling of a warm bottle in cool flowing water is an example of forced
convection.
Convection is influenced by the fluid flow adjacent to the solid surface. To
appreciate the mechanics of this mode of heat transfer, the nature of the fluid
flow in relation to the particular flow process must be known. Consideration
of the flow structure created by the passage of a turbulent fluid over a smooth
solid surface shows (see Fig. 4.24)

1. The determining factor in convection is the flow boundary layer.
Outside the boundary layer, the fluid is considered to have achieved a
maximum velocity at an infinite distance from the surface.
2. In a laminar boundary layer, no mixing takes place and the flow is
parallel. In this case the heat transfer occurs mainly by conduction
through the boundary layer.