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

effective thermal conductivity and mass weighted thermal conductivity

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   June 18, 2020, 05:53
Default effective thermal conductivity and mass weighted thermal conductivity
  #1
Senior Member
 
Weiqiang Liu
Join Date: Feb 2018
Posts: 278
Rep Power: 9
Weiqiang Liu is on a distinguished road
Hi all,

I am modeling a hydrogen combustion in porous zone with fluent. I used a UDS to solve solid energy equation and also the energy equation of fluid is modified.

For fluid energy equation, I modified thermal conductivity of fluid to an effective thermal conductivity. Specificly, effective thermal conductivity is mass weighted thermal conductivity added with an additional term.

Then I need to define source term for both solid and fluid energy equation and fluid thermal conductivity will be used in the source terms as well.

My question is: for the thermal conductivity emerged in source term, should I should use the value return by DEFINE_PROPERTY which is effective thermal conductivity or the original mass weighted averaged velocity? According to the symbols emerged in paper, it seems like original mass weighted thermal conductivity should be used. However, I am not very sure.

Can anybody give me some suggestions?

Best regards

Weiqiang
Weiqiang Liu is offline   Reply With Quote

Old   June 18, 2020, 06:37
Default Thermal Conductivity
  #2
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,946
Blog Entries: 1
Rep Power: 36
vinerm will become famous soon enough
DEFINE_PROPERTY returns value to Fluent, not to UDF. If you want to fetch value in UDF, use C_K_L(c,t). Which value to use depends on the source term. Fluent uses averaged properties only with equilibrium model. With non-equilibrium model, individual material properties are used. So, if you have different source terms for fluid and solid, you should use fluid and solid thermal conductivity, resp., not averaged. But, as mentioned above, it depends on source term formulation; the phenomenon which is the cause for the source.
__________________
Regards,
Vinerm

PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.
vinerm is offline   Reply With Quote

Old   June 18, 2020, 06:42
Default
  #3
Senior Member
 
Weiqiang Liu
Join Date: Feb 2018
Posts: 278
Rep Power: 9
Weiqiang Liu is on a distinguished road
Quote:
Originally Posted by vinerm View Post
DEFINE_PROPERTY returns value to Fluent, not to UDF. If you want to fetch value in UDF, use C_K_L(c,t). Which value to use depends on the source term. Fluent uses averaged properties only with equilibrium model. With non-equilibrium model, individual material properties are used. So, if you have different source terms for fluid and solid, you should use fluid and solid thermal conductivity, resp., not averaged. But, as mentioned above, it depends on source term formulation; the phenomenon which is the cause for the source.
Do you mean C_K_L(c,k) access volume averaged conductivity not fluid conductivity?

I tried to compute non equilibrium phenomenon with my own UDF and still kept the in built non equilibrium model in fluent disabled
Weiqiang Liu is offline   Reply With Quote

Old   June 18, 2020, 06:48
Default Macro
  #4
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,946
Blog Entries: 1
Rep Power: 36
vinerm will become famous soon enough
C_K_L returns property being applied to the cell. If you have equilibrium model enabled, then it will return averaged property since there is only one cell zone. If you enable non-equilibrium model, then C_K_L will return property depending upon the cell zone that it addresses. Do note that by disabling the NET model, ET is still enabled and thermal equation is still based on porosity averaged properties of fluid and solid. So, your fluid thermal field is not really fluid thermal field, but thermal field based on fluid-solid averaged zone. If you really want only UDS to be active for solid, then set the porosity to 1. It will have no effect on momentum since porosity does not play a role there, but it will affect superficial velocities and reactions.
__________________
Regards,
Vinerm

PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.
vinerm is offline   Reply With Quote

Old   June 18, 2020, 06:59
Default
  #5
Senior Member
 
Weiqiang Liu
Join Date: Feb 2018
Posts: 278
Rep Power: 9
Weiqiang Liu is on a distinguished road
Quote:
Originally Posted by vinerm View Post
C_K_L returns property being applied to the cell. If you have equilibrium model enabled, then it will return averaged property since there is only one cell zone. If you enable non-equilibrium model, then C_K_L will return property depending upon the cell zone that it addresses. Do note that by disabling the NET model, ET is still enabled and thermal equation is still based on porosity averaged properties of fluid and solid. So, your fluid thermal field is not really fluid thermal field, but thermal field based on fluid-solid averaged zone. If you really want only UDS to be active for solid, then set the porosity to 1. It will have no effect on momentum since porosity does not play a role there, but it will affect superficial velocities and reactions.
what if I use C_K_L to access volume averaged thermal conductivity and then compute fluid thermal conductivity by myself according to the volume averaged equation since I know solid thermal conductivity as a constant.
Weiqiang Liu is offline   Reply With Quote

Old   June 18, 2020, 07:30
Default Fluid Thermal Conductivity
  #6
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,946
Blog Entries: 1
Rep Power: 36
vinerm will become famous soon enough
You can do that; however, fluid's thermal conductivity is a user-specified property. So, until and unless this is a very complicated function requiring gradients of fields, you can directly calculate the thermal conductivity in your UDF.
__________________
Regards,
Vinerm

PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.
vinerm is offline   Reply With Quote

Old   June 18, 2020, 08:32
Default
  #7
Senior Member
 
Weiqiang Liu
Join Date: Feb 2018
Posts: 278
Rep Power: 9
Weiqiang Liu is on a distinguished road
Quote:
Originally Posted by vinerm View Post
You can do that; however, fluid's thermal conductivity is a user-specified property. So, until and unless this is a very complicated function requiring gradients of fields, you can directly calculate the thermal conductivity in your UDF.
Do you mean I can just calculate thermal conductivity in DEFINE_SOURCE where I need to use fluid thermal conductivity?
Weiqiang Liu is offline   Reply With Quote

Old   June 18, 2020, 12:37
Default Thermal Conductivity
  #8
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,946
Blog Entries: 1
Rep Power: 36
vinerm will become famous soon enough
If you have provided constant thermal conductivity for fluid, then you don't need to fetch or calculate. However, if you are using some profile or UDF, then you already know the equation and you can calculate it within UDF.
__________________
Regards,
Vinerm

PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.
vinerm is offline   Reply With Quote

Old   June 19, 2020, 00:08
Default
  #9
Senior Member
 
Weiqiang Liu
Join Date: Feb 2018
Posts: 278
Rep Power: 9
Weiqiang Liu is on a distinguished road
Quote:
Originally Posted by vinerm View Post
If you have provided constant thermal conductivity for fluid, then you don't need to fetch or calculate. However, if you are using some profile or UDF, then you already know the equation and you can calculate it within UDF.
yes, I know the equation. it's just a mass weighted average plus some additional terms. However, I think I can still use C_K_L to access thermal conductivity of cell and then use volume averaged equation to get gas conductivity. I mean I just need to calculate thermal conductivity in DEFINE_PROPERTY macro for one time. For other macros in which I need to use fluid thermal conductivity. I just need to use C_K_L(c,t) to fetch the value, right?
Weiqiang Liu is offline   Reply With Quote

Reply

Tags
define_property, define_source, effective conductivity, mass weighted

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
Setting the height of the stream in the free channel kevinmccartin CFX 12 October 13, 2022 21:43
Domain Reference Pressure and mass flow inlet boundary AdidaKK CFX 75 August 20, 2018 05:37
mass flow in is not equal to mass flow out saii CFX 12 March 19, 2018 05:21
Ansys CFX problem: unexpected very high temperatures in premix laminar combustion faizan_habib7 CFX 4 February 1, 2016 17:00
Constant velocity of the material Sas CFX 15 July 13, 2010 08:56


All times are GMT -4. The time now is 09:22.