1. Thermal Radiation
If there is a temperature difference between two media, heat transfer will inevitably occur between them. Heat transfer exists in three basic forms: heat conduction, heat convection, and thermal radiation. Thermal radiation is a phenomenon in which an object radiates electromagnetic waves due to its temperature. Objects with a temperature above absolute zero can produce thermal radiation. The higher the temperature, the greater the total energy radiated. While an object emits radiation energy outward, it also absorbs radiation energy emitted by other objects and converts it into thermal energy. This heat transfer process of mutual emission and absorption of radiation energy between objects is radiative heat transfer.
A common situation in engineering is the radiative heat transfer between the surface of an object and the surrounding environment. If the surface area of this object is S, the surface temperature is T1, and the external temperature is T2, the radiative heat transfer rate can be calculated using the Stefan-Boltzmann law:
Q=σεS(T1⁴-T2⁴), where σ is a constant term, and ε is the emissivity. Emissivity is the ratio of the radiation power emitted by an object to the radiation power emitted by a blackbody at the same temperature. It is a value less than 1, and the surface emissivity of different materials varies.
2. External Emissivity
In Fluent, if the thermal radiation model is not enabled, we will find that the wall boundary of the model still has a Radiation function. In Radiation, you can set the External emissivity, which defines the emissivity of this wall surface to the outside. Why is thermal radiation calculated on the wall surface even when the radiation model is not enabled?
In the case where thermal radiation is not enabled, it is still possible to calculate the radiation energy loss emitted from the wall surface to the outside. Only the surface temperature is needed. Knowing the surface temperature, the surface radiation emission can be calculated using the Stefan-Boltzmann law. This allows for a quick calculation of the radiation energy lost from the surface. It is a simple mathematical formula calculation that only computes the energy lost due to radiation emitted to the outside.
3. Internal Emissivity
When we enable the thermal radiation model in Fluent, Fluent provides many radiation models, such as P1, S2S, DO models, etc. These models can calculate the reflection, absorption, transmission, and attenuation of radiation energy, which is a relatively complex phenomenon.
After enabling the radiation model, in the wall boundary conditions, you can find that in the Radiation tab, Internal emissivity appears. This internal emissivity is a key parameter for energy exchange within the computational domain of the radiation model, used to handle the absorption and re-emission behavior of the wall surface. This calculation is relatively large, as it computes the complex internal radiation heat transfer process.
4. Examples (1) Simulating heat transfer between the outer wall of a furnace and the environment: involving thermal radiation between the furnace outer wall and the environment, External Emissivity can be used to define the emissivity. (2) Simulating radiation heat transfer between the inner wall of a furnace and flue gas: involving thermal radiation between the inner surface of the furnace and the internal fluid domain, the thermal radiation model needs to be enabled, such as the DO radiation model, and Internal Emissivity is used to define the emissivity. Combustion and Radiation Case Sharing 1. Fluent Combustion Model and Non-Premixed Combustion Model Application Case 2. Fluent Non-Premixed Combustion Case 3. Fluent Solar Radiation Model Video Case Sharing Welcome to leave comments!
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