Combustion of octane

[Alan Thomason]

Hello All..

I am trying to produce a plot of species versus equivalence ratio for the combustion of octane (as a start...methanol/ethanol and other motor fuels in the future).  I am not a combustion specialist, I am doing this to try to help improve the methodology used by my university program in teaching introductory combustion to students including me in a program which specializes in internal combustion engines.

There are a lot of examples using the GRI3.0 dataset, and a very nice example program is available in multiple areas which through Matlab produces this graph for CH4.

This dataset is not intended for octane, but after some fumbling around I found a pair of datasets which do through this site:

http://www.cerfacs.fr/cantera/mechanisms/iso-oct.php

When I use either of those datasets however, the NO is not showing as present.  For my own sake, I wanted to reproduce a graph shown in Heywood (p. 93 in the 1988ed.)

Other than the absence of NO, I feel like I am getting really close.  Should I be evaluating the equilibrium of N2, O2 and NO for the same temperature and pressure conditions using another dataset once the equilibrium has been established for these other constituents.  If so, why? 

All help is appreciated, and many thanks to all of those whose work I have used knowingly or unknowingly to get this far.  This includes creating the combustion datasets, founding/improving/maintaining Cantera, providing the example Matlab file (which I have changed only to suit my needs).  It is really quite amazing how far you can get on such a complicated problem with the voluntary assistance of others.

Best Regards and Thanks for your help...

[Ray Speth]

Alan,

Those mechanisms do not contain any of the nitrogen compounds except for N2, so it can't show up in the equilibrium composition. You need to create a modified phase definition which includes any nitrogen-containing species of interest, along with their thermodynamic data. You can create a CTI file that combines the species of the octane mechanism with nitrogen-containing species from the GRI 3.0 mechanism as follows:

ideal_gas(

    name = "gas",

    elements = " C  H  N  O  Ar  He ",

    species = ['curran: all',

               '''gri30: N NH NH2 NH3 NNH NO NO2 N2O HNO

                  CN HCN H2CN HCNN HCNO HOCN HNCO NCO'''],

    reactions = "curran: all",

    initial_state = state(temperature=300.0, pressure=OneAtm)

)

If you want to include the reactions as well, that's a little more complicated. In that case, you would probably just want to create a copy of the mechanism file and add the species and reaction definitions for the NOx chemistry from a different mechanism.

Regards,

Ray

On Friday, June 12, 2015 at 12:07:04 PM UTC-4, Alan Thomason wrote:

Hello All..

I am trying to produce a plot of species versus equivalence ratio for the combustion of octane (as a start...methanol/ethanol and other motor fuels in the future).  I am not a combustion specialist, I am doing this to try to help improve the methodology used by my university program in teaching introductory combustion to students including me in a program which specializes in internal combustion engines.

There are a lot of examples using the GRI3.0 dataset, and a very nice example program is available in multiple areas which through Matlab produces this graph for CH4.

This dataset is not intended for octane, but after some fumbling around I found a pair of datasets which do through this site:

http://www.cerfacs.fr/cantera/mechanisms/iso-oct.php

When I use either of those datasets however, the NO is not showing as present.  For my own sake, I wanted to reproduce a graph shown in Heywood (p. 93 in the 1988ed.)

...

[Alan Thomason]

Hi Ray...Thanks very much for the fast response.  I am going to try your suggestions, but I have reservations.  There is so much work in preparing these combustion mechanisms, and the need for the nitrogen reactions seemingly so apparent that I am keen to know why they were not included in the first place.  There must be a reason.  I will feel like a 'hack' simply shoving them in without a very good understanding of combustion chemistry.  Perhaps you or someone else can provide me a bit more background as to why this would be necessary?

Thanks very much.

[Ray Speth]

Alan,

As far as predicting the equilibrium concentration goes, all you're relying on is the species thermochemical data. The only concern here is whether the data for the common species between the two mechanisms (i.e. N2 and O2) is consistent. In this case, while the NASA polynomials for these species are different, the computed enthalpies and entropies seem to be in good agreement. You could try both sets to see if it makes any difference in the results, but I'd be surprised if it did.

When we start talking about combining reaction mechanisms, then generally yes, more caution is warranted. In the case of nitrogen chemistry though, it's not unreasonable to consider it separately from the hydrocarbon mechanism, since there's not a lot of feedback from the nitrogen chemistry to the hydrocarbon chemistry -- adding the nitrogen chemistry will not appreciably affect computed flame speeds or ignition delay times. Furthermore, to the extent that the most important nitrogen chemistry involves only simple hydrocarbon compounds, it is possible to develop a generic NOx sub-mechanism which can be used with different fuel molecules.

Regards,

Ray

[Alan Thomason]

Hello...

I started down the path of trying to combine the reaction files.  I liken this to a gorilla trying to defuse a nuclear bomb, but I might be optimistic there.  The first problem is that the formatting of the gri30.cti file I have makes it difficult to see how I will understand the data.  This is a working *.cti file (I checked that I was using this file), so I don't think it is corrupt.  You have been so helpful already and I don't want to impose, but I'm not sure where else to find this information. 

Warm Regards,

Alan Thomason

On Friday, June 12, 2015 at 5:04:11 PM UTC-4, Ray Speth wrote:

[Ray Speth]

Alan,

The files you posted are the same as the ones at https://github.com/Cantera/cantera/blob/master/data/inputs/gri30.cti and https://github.com/Cantera/cantera/blob/master/data/inputs/gri30_highT.cti, right? If you're trying to use Windows Notepad to edit the file, then the formatting will be messed up. I would recommend using a better editor such as Notepad2 or Notepad++ which handles Unix-style line endings correctly.

Regards,

Ray

[Alan Thomason]

Hi Ray...

I used Notepad++ and that worked perfectly!  Thanks so much, it was just what I needed.

Best Regards,

Alan