[SBhat]

Hi, I am finding it really hard to understand the difference between internal and external emissivity in the boundary condition of a opaque wall with zero thickness. I am implementing a DO radiation model in a simulation of reactor with heat flux coming in from a semi transparent wall of the reactor. For zero thickness opaque wall of the reactor, does fluent consider both emissivities for calculation or it just consider one of the emissivity? From the result that I am getting, I am seeing a big change in result ( change in internal temperature of the reactor) with the change in external emissivity of the wall but not so with the change in internal emissivity.

Could anyone please help me to understand the role of internal and external emissivity in case of zero thickness opaque wall?

[LuckyTran]

Even for zero thickness wall, both emissivities are used. The external emissivity is the emissivity associated with the external radiation (the surface associated with the radiation temperature). That is, external/internal does not refer to the external/internal surface of the wall but it refers to whether the radiation is coming from an external source or internal source.

[SBhat]

Thank you very much Lucky Tran for the reply, its really helpful. I still have a little confusion regarding the internal and external source. In my case, the reactor is cylindrical in shape. The circular wall at the front is semi transparent from where heat flux is going inside the reactor ( heat flux is applied to just this wall).The other walls are opaque. So, for these opaque walls of the reactor, the radiation heat flux coming from the semi transparent window are considered as external or internal source?

[LuckyTran]

The problem is that you are confused, it's very easy to understand (but difficult as long as you are confused). Your question actually contains its own answer.

I recommend you do some manual ray tracing to discover the answer to your question. If the ray comes from the external source (the source that is at the external radiation temperature and has a surface emissivity equal to the external emissivity) then that ray (obviously) is external radiation. On the other hand, if you can trace that ray back to any other object that is not the external source then it's not external radiation.

[SBhat]

Thank you very much.

[Oula]

Originally Posted by LuckyTran

Even for zero thickness wall, both emissivities are used. The external emissivity is the emissivity associated with the external radiation (the surface associated with the radiation temperature). That is, external/internal does not refer to the external/internal surface of the wall but it refers to whether the radiation is coming from an external source or internal source.

Hi LuckTran

The walls in my model is radiating heat to the inside of the domain so everything inside the domain is receiving radiation from the walls, I applied the "Radiation" thermal BC on these walls and therefore I need to set both emissivities (i.e. internal and external). If I set values for both, does that mean that part of the heat will emit to the outside of the domain?

Your help will be greatly appreciated.

[LuckyTran]

Originally Posted by Oula

Hi LuckTran

The walls in my model is radiating heat to the inside of the domain so everything inside the domain is receiving radiation from the walls, I applied the "Radiation" thermal BC on these walls and therefore I need to set both emissivities (i.e. internal and external). If I set values for both, does that mean that part of the heat will emit to the outside of the domain?

Your help will be greatly appreciated.

You only use the radiation thermal BC if you have external radiation. If you have only radiation between walls and inside the domain, then you use the heat flux or temperature condition. The "Radiation" thermal condition is only for external radiation.


These are boundary conditions. Radiation is a physical process, not a boundary condition.


The convection and radiation boundary conditions exist for when you want to model some stuff happening outside the domain. They do not say anything, or limit any physics happening inside your model.