Modeling Heat Transfer by Radiation
This section summarizes the functionalities available within the Heat Transfer Module for the modeling of heat transfer by radiation.
Radiation is a heat transfer mode that should be considered in addition to conduction and convection in the following cases:
•
In a transparent or semi-transparent medium.
•
For a surface with a large emissivity (close to 1). Whereas most polished metals have an emissivity below 0.1, other materials such as concrete, black painting, glass, plastic, skin, paper, and ceramic have an emissivity over 0.8 for infrared radiation.
•
When the temperature difference between a surface and the surrounding surfaces or the ambient is large.
•
When conduction and convection are small, as radiation becomes the predominant transfer mode.
•
At small time scales, for which conduction and convection effects are too slow and have a less significant effect.
As an example, consider a plate of concrete with a cold bottom wall at temperature
T
0
, placed in a hot environment with an air flux at temperature
T
amb
:
The figures below illustrate the relative importance of heat transfer by radiation and by convection (due to the air flux), when the emissivity
ε
is 0.1 (left) and 0.9 (right).
Figure 3-12:
Integral of convective and radiative heat fluxes (W/m) along the top boundary, for two values of emissivity, as a function of the temperature difference.
Whereas the boundary radiative and convective heat fluxes are of the same order for temperature difference up to 500K when the emissivity is low (
ε
=0.1), radiation becomes the dominant mode of heat transfer even for a small temperature difference when the emissivity is high (
ε
=0.9). Note that the convective heat flux decrease observed for high temperature gradients is related to the fact that the velocity, not the mass flow rate, is prescribed at the air inlet.
See
Heat Transfer Variables
for the definition of the variables
q0
and
rflux
giving access to the convective and radiative heat fluxes on boundaries.
Two configurations are considered regarding the functionalities available for the modeling of radiative heat transfer:
•
Radiation in transparent media
•
Radiation in participating media
