Heat Transfer Mechanisms

1 Heat Transfer Mechanisms

Heat is transferred from a high temperature zone towards a low temperature zone via three different modes of heat transfer.

1.1 Conduction

Conduction depends on molecular activity and requires the presence of matter. Example is heat transfer through wall.

Figure 1 Heat Conduction

Heat conduction heat transfer rate (Qk) through a body, as shown in Figure 1, at steady state is governed by Fourier's Law which is expressed as

(1)

1.2 Convection

Convection takes place by bulk transport and the mixing of macroscopic parts of hot and cold elements of a fluid. This also includes the transfer of heat between a solid surface and a fluid. Convection heat transfer is of two types:

(a) Natural/Free Convection

(b) Forced Convection

The basic equation for convection heat transfer is Newton's Law of Cooling. The convection heat transfer Qc is expressed as:

(2)

1.3 Radiation

The governing equation for radiation heat transfer (Qr) from a black body is Stenfan-Boltzmann Law:

(3)

(5)

For simplicity, Eqn. (5) is re-written as:

(6)