Page 143 - Industrial Ventilation Design Guidebook

4 i HF:AT AND MASS TRANSFER 105

FIGURE 4.24 Laminar and turbulent boundary layers and temperature distribution inside the
boundary layer.

3. In a turbulent boundary layer, flow takes place in the direction
perpendicular to the surface over which the flow occurs.
A heat transfer factor (a) between the fluid and surface is defined as

where 0 is the temperature difference between the surface and the fluid at a
long distance from the surface.
When heat transfer occurs by conduction through the boundary layer,

where 8 is the thickness of the boundary layer, and the unit of a is W m " K .
The heat transfer factor a thus decreases as the boundary layer thickness in-
creases. The following discussion gives some indication of the range of the
heat transfer values obtained due to the different modes of convective heat
transfer.
Next we give some values of a to give an idea of the magnitude of the heat
transfer:

2
a, W m- K- 1
Free convection 3.5-50
Forced convection, air 10-500
Forced convection, liquid 100-5000

A liquid has a higher rate of conductivity than a gas.
In boiling convection, liquid motion is created by steam bubbles breaking
loose from the surface.
If steam condenses on a surface, there is no boundary layer; the resis-
tance to heat flow is due to scale, metal thickness, and the condensed liq-
uid layer, resulting in a high heat transfer factor. A thin layer of air or
other noncondensing gas forms at the surface through which the steam dif-
fuses. The heat transfer factor diminishes rapidly but is considerably
higher than in dry convection.