CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

DO Radiation Model Boundary Condition

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree2Likes
  • 1 Post By LuckyTran
  • 1 Post By LuckyTran

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   April 22, 2016, 17:42
Default DO Radiation Model Boundary Condition
  #1
New Member
 
Sbhat
Join Date: Apr 2016
Posts: 6
Rep Power: 10
SBhat is on a distinguished road
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?
SBhat is offline   Reply With Quote

Old   April 24, 2016, 12:26
Default
  #2
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,741
Rep Power: 66
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
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.
Oula likes this.
LuckyTran is offline   Reply With Quote

Old   April 24, 2016, 23:31
Default
  #3
New Member
 
Sbhat
Join Date: Apr 2016
Posts: 6
Rep Power: 10
SBhat is on a distinguished road
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?
SBhat is offline   Reply With Quote

Old   April 25, 2016, 01:47
Default
  #4
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,741
Rep Power: 66
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
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.
LuckyTran is offline   Reply With Quote

Old   April 26, 2016, 17:23
Default
  #5
New Member
 
Sbhat
Join Date: Apr 2016
Posts: 6
Rep Power: 10
SBhat is on a distinguished road
Thank you very much.
SBhat is offline   Reply With Quote

Old   February 25, 2019, 11:03
Default
  #6
Member
 
Oula
Join Date: Apr 2015
Location: United Kingdom
Posts: 81
Rep Power: 11
Oula is on a distinguished road
Quote:
Originally Posted by LuckyTran View Post
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.
Oula is offline   Reply With Quote

Old   February 25, 2019, 11:38
Default
  #7
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,741
Rep Power: 66
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
Quote:
Originally Posted by Oula View Post
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.
Oula likes this.
LuckyTran is offline   Reply With Quote

Reply

Tags
boundaries condition, do radiation model, emissivity

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
sliding mesh problem in CFX Saima CFX 46 September 11, 2021 07:38
Problem in setting Boundary Condition Madhatter92 CFX 12 January 12, 2016 04:39
Error finding variable "THERMX" sunilpatil CFX 8 April 26, 2013 07:00
New topic on same subject - Flow around race car Tudor Miron CFX 15 April 2, 2004 06:18
Please help with flow around car modelling! Tudor Miron CFX 17 March 19, 2004 19:23


All times are GMT -4. The time now is 05:51.