CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM

reactingFoam constant/reactions file

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

Reply
 
LinkBack Thread Tools Display Modes
Old   July 26, 2011, 11:45
Default reactingFoam constant/reactions file
  #1
New Member
 
Join Date: Jun 2011
Posts: 29
Rep Power: 5
Cyberholmes is on a distinguished road
Hello,

I wanted to check to make sure I was using the correct units for my Arrhenius values in the reactions file for reactingFoam. Apparently, back when CHEMKIN was used, the prefactor A was given in units of (cm^3/mol)^(r-1) s^-1, and activation energy (given as Ta??) was expressed in units of cal/mol. These seem like odd units to use. Are these still the units used in the reactions file? Thank you.
Cyberholmes is offline   Reply With Quote

Old   July 26, 2011, 11:59
Default
  #2
Senior Member
 
Laurence R. McGlashan
Join Date: Mar 2009
Posts: 367
Rep Power: 13
l_r_mcglashan will become famous soon enough
If there are no units specified after REACTIONS then the Act. energy is in cal/mole. A is in CGS units.

The whole chemkin format is a pain. Make sure when you read in a mechanism to check it has been read in properly. You can use chemkinToFoam to check it.
__________________
Laurence R. McGlashan :: Website
l_r_mcglashan is offline   Reply With Quote

Old   July 26, 2011, 12:02
Default
  #3
New Member
 
Join Date: Jun 2011
Posts: 29
Rep Power: 5
Cyberholmes is on a distinguished road
I'm not actually using chemkin, I'm using the new format that is not chemkin (I'm using OpenFOAM 2.0.0). I just wanted to check if the units chemkin used, which I mentioned in the above post, are still the same ones used in the new format, in the reactions file in the constant directory.

Thanks.
Cyberholmes is offline   Reply With Quote

Old   July 27, 2011, 11:24
Default
  #4
New Member
 
Join Date: Jun 2011
Posts: 29
Rep Power: 5
Cyberholmes is on a distinguished road
Does anyone know the units of the reactions file?
Cyberholmes is offline   Reply With Quote

Old   July 27, 2011, 11:49
Default
  #5
Senior Member
 
Laurence R. McGlashan
Join Date: Mar 2009
Posts: 367
Rep Power: 13
l_r_mcglashan will become famous soon enough
You can either ask them directly or look through the code, which is will be somewhere in src/thermophysicalModels/, probably in reactionThermo or specie.

I would *assume* it's chemkin standard (looks like it), so cal/mole for act energy, but most of my assumptions are usually incorrect.

As I said earlier, an easy way of checking would be to use chemkinToFoam on your chemkin files and see if the numbers that are in the foam files match.
__________________
Laurence R. McGlashan :: Website
l_r_mcglashan is offline   Reply With Quote

Old   July 28, 2011, 10:00
Default
  #6
Member
 
Tibo
Join Date: Jun 2011
Posts: 68
Rep Power: 5
megacrout is on a distinguished road
Quote:
Originally Posted by l_r_mcglashan View Post
I would *assume* it's chemkin standard (looks like it), so cal/mole for act energy, but most of my assumptions are usually incorrect.
Aha, I really like this one!

There is an answer here: http://www.openfoamworkshop.org/6th_...ist_slides.pdf
on slide 27. It is a bit confusing though as he made a typo while writing the last unit (twice K instead of K, kJ or K, J) and he gave 2 different values of A even though they should be equal except for the order of magnitude.

So this is no absolute answer and
Quote:
Originally Posted by l_r_mcglashan View Post
ask them directly or look through the code, which will be somewhere in src/thermophysicalModels/, probably in reactionThermo or specie.
is probably a better option. IŽll give it a shot right now.
Thanks for the advice Laurence.

Tibo
megacrout is offline   Reply With Quote

Old   July 28, 2011, 13:20
Default
  #7
New Member
 
Join Date: Jun 2011
Posts: 29
Rep Power: 5
Cyberholmes is on a distinguished road
Ah, I figured out something at least! The K is not a typo in the slides you posted, because the third value in the native format is the activation energy divided by the gas constant, so the units are (cal/mol)/(cal/(mol * K)) = K. I checked the units using chemkinToFoam and they work out.

What I can't figure out is what the deal is with A, the first value. It says the units in chemkin are (cm^3/mol)^(r-1) s^-1, and the units in native are (m^3/kmol)^(r-1) s^-1, and so the numbers in that field should differ between the two formats only by a factor of a power of ten. However, they don't. In the slides, they differ by a factor of ~31.6, and in the reactinFoam tutorial on the wiki, they differ by a factor of 177.8.

What does this mean?
Cyberholmes is offline   Reply With Quote

Old   October 10, 2011, 05:06
Default
  #8
Member
 
Tibo
Join Date: Jun 2011
Posts: 68
Rep Power: 5
megacrout is on a distinguished road
They differ by those amounts because the guys who did those computations did them wrong. It is pretty easy to check with chemkinToFoam (thanks to Laurence for the advice, 2nd post of this thread).

A factor 31.6 results from converting units of a reaction of the type A + 2B -> products while considering it as a reaction of the type 0.5A + B -> products. Instead of multiplying the power of ten by 3 (1+2 from 1A + 2B) while converting the units from mol to kmol or conversely, they used a power of 1.5 (0.5 + 1 from 0.5A + 1B). The difference is a factor of 10^1.5 = 31.62.

From there on, I tried to understand where the factor 177.8 you found could come from. You solve 10^x = 177.8 and get x = 2.250. It might thus come from a reaction of the type 2A + 2.5B -> products being considered as A + 1.25B -> products. Difference is (2+2.5)-(1+1.25) = 2.25. I donŽt know what the reaction you looked at was, though.

Hope that helps.

Tibo
megacrout is offline   Reply With Quote

Reply

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
GroovyBC the dynamic cousin of funkySetFields that lives on the suburb of the mesh gschaider OpenFOAM 297 August 12, 2014 06:51
channelFoam for a 3D pipe AlmostSurelyRob OpenFOAM 3 June 24, 2011 13:06
1.7.x Environment Variables on Linux 10.04 rasma OpenFOAM Installation 9 July 30, 2010 04:43
BlockMesh FOAM warning gaottino OpenFOAM Native Meshers: blockMesh 7 July 19, 2010 14:11
ParaView Compilation jakaranda OpenFOAM Installation 3 October 27, 2008 12:46


All times are GMT -4. The time now is 17:10.