Learning Forum

[Zwernjayden]

How do I remove reactions and species to simplify my flamelet? I don't need a model wtih 300 reactions

 

I am also having trouble finding a CHEMKIN file that has ethanol and oxygen combustion c2h5oh+o2->h2o+co2, even though this is the default reaction for the ethyl-alcohol-air reaction template

[jcooper]

Hi Jayden:

'

The best way to remove reactions and species is to do a formal mechanism reduction with Chemkin. 

For mechanism reduction, a small sample problem that is similar to the case is run with Chemkin using the full mechanism.  That becomes the baseline for reduction.

Reduction is then done based on some targets you specify, such as maximum temperature and maximum CO  (for hydrocarbon combustion).  You should have no more than 2 strong targets for the reduction, and they should relate to what you are hoping to use the mechanism to predict.

You can take a look at the following links for an overview of how it works:

https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v242/en/chemkin_bp/chemkin_bp_practices.html

This link goes though the steps of setting up a reduction and outlines some rules of thumb for setting targets.  The process describes a quite complicated selection of mechanism reduction steps, but I have had pretty good results by just using the DRG reduction method.  (If one or several of the reduction methods in the sequence keep failing, just eliminate them from your list of methods. It is usually the more aggressive ones that fail, so you will be erring on the side of caution and not removing too many species to leave an accurate mechanism.)

https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v242/en/chemkin_bp/ckbp_wbredoper.html

 

You won't find a mechanism with the c2h5oh+o2->h2o+co2 reaction, because it doesn't occur in real life. This is a global, one-step reaction and only accurate for very fast combustion processes with a good balance of fuel to oxidizer. These generally don't require detailed mechanisms to be solved because there are very few intermediates.

For ethanol combustion, the C2 mechanism that comes with Chemkin might work.  You can find that in the Chemkin installation here:

The SRK mechanism activates real gas properties for the species.  If you search for C2H5OH in these mechanisms, you will see that the combustion of C2H5OH is a multistep process involving multiple radical species, and that it breaks down into smaller CHx molecules first before the CO/CO2 and H2 products form.

I hope this helps.

Best Regards,

Judy