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

Methane combustion using built in Arrhenius equation in ANSYS CFX

Register Blogs Community New Posts Updated Threads Search

Like Tree1Likes
  • 1 Post By Opaque

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   May 22, 2019, 10:04
Default Methane combustion using built in Arrhenius equation in ANSYS CFX
  #1
Member
 
Abdullah Arslan
Join Date: Apr 2019
Posts: 94
Rep Power: 7
Goenitz is on a distinguished road
Good day all,

My main aim is to model methane steam reforming using heat source in presence of Catalyst. For time being I am modelling methane combustion by defining my own reaction using ANSYS CFX 18.

My first problem is reaction occurs all the time even if I set activation temperature to be 3000K. Whereas in domain I assign maximum temperature of 1000K. Is there any default value which is negating my set activation temperature?

Regards

Last edited by Goenitz; May 22, 2019 at 10:05. Reason: Title change
Goenitz is offline   Reply With Quote

Old   May 22, 2019, 11:51
Default
  #2
Senior Member
 
Join Date: Jun 2009
Posts: 1,873
Rep Power: 33
Opaque will become famous soon enough
Not certain about your question.

Regardless of the value set for activation temperature, the reaction is always evaluated. The result of the reaction rate is what matters. If the rate is zero, no products are formed, and not reactants are consumed.

How did you set up the maximum temperature for the domain?

What combustion model have you selected?
Opaque is offline   Reply With Quote

Old   May 22, 2019, 12:05
Default
  #3
Member
 
Abdullah Arslan
Join Date: Apr 2019
Posts: 94
Rep Power: 7
Goenitz is on a distinguished road
I thought reaction should occur only if temperature is above activation temperature like of catalyst.

I have changed mixing rate so for zero mixing rate reaction does not proceed.

I haven't applied zero reaction rate, but reaction should occur for both +/- values which I have done. Just reaction direction is reversed by changing sign

Temperature of wall and inlet are assigned same value so domain temperature is close to that.

I have used eddy dissipation model with no limiter

The file attached is for zero reaction rate. Reaction is occurring even then.
Attached Files
File Type: txt Fluid Flow CFX_001.txt (159.2 KB, 16 views)

Last edited by Goenitz; May 23, 2019 at 08:39. Reason: File adding
Goenitz is offline   Reply With Quote

Old   May 23, 2019, 09:34
Default
  #4
Senior Member
 
Join Date: Jun 2009
Posts: 1,873
Rep Power: 33
Opaque will become famous soon enough
I think there is confusion on what the options mean:

For the Arrhenius reaction rate type:

Rate = A * T^Beta * exp (-T_activation/T)

where

A is the pre-exponential
B is the temperature exponent
T_activation is effectively E_activation / R_constant

as you can see, it is evaluated everywhere.


Now on the combustion model being used. You have selected a reaction mechanism for Methane, you have set up the details of the chemical reaction; however, at the domain level you have explicitly indicated to use mixing rate by selecting Eddy Dissipation w/o any chemical reaction considerations, i.e. the setting at the reaction definition are being ignored.

Are you trying to model Finite Rate Chemistry, or turbulent flow combustion mixing model? My advice is to check the documentation for the differences between the options, and proceed as you see fit.
Goenitz likes this.
Opaque is offline   Reply With Quote

Old   May 24, 2019, 07:32
Default
  #5
Member
 
Abdullah Arslan
Join Date: Apr 2019
Posts: 94
Rep Power: 7
Goenitz is on a distinguished road
so Rate = A * T^Beta if T_activation is equal to zero. Fair enough.
However, when I changed the Rate value to zero (both for my case and ANSYS tutorial) the reaction is still taking place. I have edited and attached the file to previous post.
Eddy dissipation model is more robust than Finite Chemistry. I have tried both anyway.

Details of chemical reaction are set at 'Reaction' tab in outline tree. The 'material' tab takes these values when user define 'material'. Then in Domain setting, when fluid is selected, that 'material' is selected comprising mixture of fluids (e.g. methane, CO, CO2,O2). Mixing rate is one of many option when @eddy dissipation model' is selected but there is none there about activation T or reaction rate. These are defned in 'Reaction' tab.
Attached Images
File Type: jpg Surface reaction.jpg (85.0 KB, 24 views)
Goenitz is offline   Reply With Quote

Old   July 5, 2021, 04:53
Default
  #6
New Member
 
rajini manoharan
Join Date: Jul 2021
Posts: 2
Rep Power: 0
arvind rajini is on a distinguished road
Are you sure , this is what activation temperature mean?"I thought reaction should occur only if temperature is above activation temperature like of catalyst."
because I have seen activation temperature 26,000 k
Can someone explain what activation temperature is?
Attached Images
File Type: jpg 1.jpg (141.9 KB, 17 views)
arvind rajini is offline   Reply With Quote

Old   July 5, 2021, 05:13
Default
  #7
New Member
 
rajini manoharan
Join Date: Jul 2021
Posts: 2
Rep Power: 0
arvind rajini is on a distinguished road
Can someone explain what activation temperature is?
arvind rajini is offline   Reply With Quote

Old   July 5, 2021, 08:01
Default
  #8
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,854
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
https://lmgtfy.app/?q=activation+temperature

Or read the CFX documentation for CFX's specific application of the concept.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
ghorrocks is offline   Reply With Quote

Reply

Tags
activation temperature, arrhenius, cfx


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
FSI simulation in ansys cfx Arash67.m CFX 1 September 29, 2017 10:52
Can you help me with a problem in ansys static structural solver? sourabh.porwal Structural Mechanics 0 March 27, 2016 18:07
Ansys CFX problem: unexpected very high temperatures in premix laminar combustion faizan_habib7 CFX 4 February 1, 2016 18:00
Coal combustion in Ansys CFX 12 kriss.dj CFX 0 March 17, 2010 17:53
CFX bought by Ansys - good or bad?! Pete CFX 38 February 21, 2003 08:34


All times are GMT -4. The time now is 21:04.