Radiation is defined as the transference of energy using electromagnetic waves. When these waves impact on any type of matter, they become heat. All bodies with a temperature greater than absolute zero emit radiation, and in contrast to conduction or convection, this phenomenon requires no medium.
In SimScale, radiation is modelled using a Diffuse View Factors Model. This approximation implies several assumptions that can be applied to most of the engineering problems:
The different surfaces of the model will exchange heat between them, and they will also provide or subtract heat from the adjacent fluid. Coupling of both convection and radiation will be achieved once the simulation converges.
In SimScale, Radiation is available in Convective Heat Transfer simulations. If we switch the radiation interruptor on, two additional fields appear when assigning the Boundary Conditions: the Radiative Behavior and Additional Radiative Source.
The Radiative Behavior specifies the relationship between the Net Radiative Heat per Surface Unit Qr [W/m²] and the Temperature of every surface Ts [K]:
Qr(S→S’) = F𝝐 𝝈(Ts-Ts’)⁴
F is the View Factor between S and S’, 𝝈 is the Stefan-Boltzmann constant (𝝈 = 5.6696 ·10-8 W /m²K), and 𝝐 is the emissivity of the surface S. This value depends on the material of the surface, and it measures its capability to emit radiation. The default value that appears is 0.9, which is a good approximation for walls made of brick or concrete.
Apart from the radiative heat interchange that the surfaces will perform, we can set an additional radiative source. This source represents any additional (mainly external) source of radiation that goes through the surface and it will not heat it up. The best example is the solar radiation getting into the domain through an open window (Radiative behavior: Transparent; Additional radiative source different from zero).