How to use
Please tell us what you learn, and about your problems
Some cautionary notes
GRI-Mech is an optimized detailed chemical reaction mechanism
capable of the best representation of natural gas flames and ignition that we are
able to provide at this time. Our ongoing program
comprises development of extensions to include more chemistry, more robust
parameterizations, and additional tools and documentation to make working with
GRI-Mech more effective for everybody.
In order to use the input files directly you need the
Sandia National Laboratory
Chemkin-II programs. Ignition profiles calculated with the GRI-Mech
reaction mechanism and thermochemical data should be independent of the program
used to compute them; noticeable small differences in flame profiles should be
expected, however, if you are using a flame code other than
because the transport calculation, the numerical method of solving the
differential equations, and so on, differ from one flame modeling program to
Before telling you more we are obliged to say:
|LEGAL NOTICE These files, both the ones intended for use
as computer input as well as those comprising documentation,
were prepared by The University of California, Berkeley,
Stanford University, The University of Texas at Austin,
and SRI International as a result of work sponsored
by the Gas Research Institute (GRI). Neither GRI,
members of GRI, nor any person acting on behalf of either:|
GRI-Mech. A paper document giving a more detailed account of the way
GRI-Mechis developed and maintained is available. Send us an e-mail or regular mail request to get a copy of this document.
GRI-Mech is essentially a list of elementary chemical reactions
and associated rate constant expressions. Most of
the reactions listed have been studied one way or another in the laboratory, and so
the rate constant parameters mostly have more or less direct measurements behind
them. In order to get successive releases out in a timely manner we do not
continually prepare comprehensive hard copy documents explaining the sources of
information that were consulted and evaluated, the rate constant theory that went
into constructing the set, and the details of each step of the optimization
process. We do update the documentation provided on this Web site with such
information regularly. You are welcome to make whatever use you like of this
information and have our permission to cite this Web documentation if you so
desire. (But see the foregoing disclaimer.)
Once we have a starting set of rate constant parameters, we undertake extensive sensitivity tests on a variety of experimental data related to natural gas ignition and flames. These tests tell us which rate parameters should be considered closely to 'tune' the set so as to get an optimum representation of the data. We then go through a long process of simultaneous parameter optimization, most of it done automatically---i.e., by a computer adjusting the parameters rather than one of us---to get the parameter set for each successive release. This is carried out with strict constraints to keep the rate parameters being optimized within predetermined bounds that we set on the basis of evaluations of the uncertainties in measurements of the rates of elementary reactions and by applications of conventional reaction rate theory. Once an optimal parameter set is found, it is checked against the literature in an extensive 'validation round'.
In addition to reviewing the literature rate constant data we also examine the
thermochemistry of the free radicals in the mechanism. Some of the thermochemical
GRI-Mech differ from those in the
Sandia data base. For this reason we provide
a file of the complete
represented as the coefficients
of NASA polynomials.
Our mechanism files are specifically
use with the
Chemkin modeling programs. For the flame calculations
in the optimization and validation process we use the
Sandia transport package and database.
(R.J. Kee, J. Warnatz and J. A. Miller, Sandia Report SAND83-8209)
(Note: The symbol CH2* used for singlet methylene in
was changed to CH2(S), as in the original Sandia notation, in later releases.)
GRI-Mechfor combustion modeling is to just do it! If you have
Chemkinup and running, and have tested it using the sample files provided with the
Chemkindistribution, all you need to do is substitute the
GRI-Mechinput files for the
Chemkinsamples. That's it! Whatever works for you to make the
Chemkinsample files run will also run the
GRI-Mechworks in their own labs as well as from others who may see things that should be done from very different perspectives. Our project has benefitted over and over again from comments by GRI-Mech users; we look forward to more of them!
It makes us feel good to hear about successful applications of
GRI-Mech, but it helps us more in our development work, and
consequently all users of later releases, to hear about failures.
We welcome suggestions of any kind.
Please be as specific as you can in telling us about your results and your problems. We will be happy to include the results of your tests, with appropriate citation to you, in our Web and printed materials. Just ask.
We try to keep our list of interested parties up to date so that we can quickly communicate changes, problems, and mechanism updates. Since our ftp daemon only records logins, not actual user names, we ask that people who set out to use GRI-Mech let us know their e-mail addresses. Please mail to email@example.com.
|PLEASE DO NOT MAKE ANY SUBSTITUTIONS|
GRI-Mechhas been optimized as a whole, and should be used just as you get it if you want to exploit its ability to model natural gas combustion. You will not surpass the overall performance we get for natural gas combustion by adjusting any 'sensitive' reaction rate constant expressions. Any substitution of 'better' rate constant expressions or thermochemical or transport coefficient values, or outright removal of species or reactions, will put you at risk of getting significantly deteriorated performance of the mechanism when tested against the available spectrum of natural gas combustion data.
We recognize that GRI-Mech users are human and will be tweaking
away on this or that aspect of our product anyhow, or making major
changes for specific purposes, like doing sensitivity analyses of one
kind or another. Keep in mind in doing so that we do not claim
GRI-Mech is a suitable starting point for
mechanism building by patchwork means. You can use it that way
if you like, but we cannot predict what the consequences may be.
GRI-Mechcontains numerous entries that are 'unimportant' for natural gas combustion.
There are several reasons why we have them there anyway. One is that
there are special purposes (like models of flame radiation or ionization)
where elementary reactions that are otherwise negligible become very
important, and we want to have these reactions on hand for occasions
where somebody is checking these aspects. A second reason is that the
combustion of some other fuels (methanol, acetylene, ...) can be modeled
GRI-Mech as a subset, with the knowledge
that the part of the mechanism relevant
to natural gas has been optimized in the manner described above.
There are techniques for shortening reaction and species lists, which
produce what are called reduced mechanisms. You can find several of such
reduced mechanisms at this site. We
invite others to 'submit' their reaction mechanisms reduced from
GRI-Mech for inclusion on this Web document.
GRI-Mechas presented in this directory will require additional calculations to find out which of them are really needed for your application.
GRI-Mechwhich really do not need to be included for modeling natural gas combustion increase the demand on computer resources for doing the chemical part of the model by a large factor. We accept this in order to avoid coping with the numerous problems that arise in streamlining such computations.
Computer time has not been a problem for us even when using
GRI-Mech on relatively
small workstations. If you are using an older PC, then you may encounter
longer integration times than you want to live with. We are thinking
about ways to deal with this and will welcome suggestions about how
this could best be done.