Radiation Coefficient
For a number of purposes where heat transfer occurs by a combination of mechanisms such as radiation + convection, it is convenient to deal with heat transfer coefficient for radiation. An example is the heat loss from the walls of the furnace. In this case heat losses occur both by convection and radiation to the surrounding. It is convenient to define a radiation heat transfer coefficient similar to convective heat transfer. Radiation heat transfer coefficient  is defined as
 |
(15) |
By equations 14 and 15 we get for small temperature difference between  and  ,
 |
(16) |
When both convection and radiation are involved in transfer of heat, for example heat loss from the external wall of the furnace. Total heat loss can be evaluated from
 . |
(17) |
Thermal resistance by radiation  . If heat flow path involves the circuit of conduction to a surface and convection and radiation from the surface, with the overall temperature difference 
 |
(18) |
Illustration on heat transfer coefficient-1
A round duct 0.6 m diameter carries  , of preheated air at about  the inside surface temp of the duct is  . Estimate the a) heat loss/ m
(b) Estimates the drop-in air temp for 30 m length of the pipe
Thermal properties of air
| |
500 K |
600 K |
mean |
|
0.696 |
0.58 |
0.638 |
|
40.76*10-3 |
46.9*10-3 |
43.8*10-3 |
|
38.8*10-6 |
52.7*10-6 |
46*10-6 |
|
Use of 10 given 
The heat loss 827  is loss of sensible heat of air
For 30 m long pipe the temp drop would be 
|
