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turbulent mixing controlled reaction vs. chemical kinetics controlled reaction

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Old   October 31, 1999, 21:46
Default turbulent mixing controlled reaction vs. chemical kinetics controlled reaction
  #1
Hyeryung NA
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In general, turbulent-mixing(or diffusion) controlled reaction mechanism is treated as 'more important' in the high temperature reactor including combustor. So, sometimes, chemical kinetics controlled reaction mechanism(Arrhenius type reaction mechanism) can be neglected. And mixture fraction approach with prescribed chmistry, or similar concept, can be used to predict the flow field with combustion.

Would you let me know, or introduce reference(s) for the guideline of high temperature range where chemically controlled reaction mechanism can be generally neglected ? I am only interested in the gaseous combustion for this question.

Sincerely

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Old   November 1, 1999, 07:36
Default Re: turbulent mixing controlled reaction vs. chemical kinetics controlled reaction
  #2
Jason Norman
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There is a rule that kinetics can be ignored with Damkohler numbers greater than ~1000. However, it does depend somewhat on your particular problem.

For info Damkohler no. can be calculated as:

Da = mixing time scale / chemical time scale

= [k/epsilon]/[(adiabatic flame density)/ (slowest reaction rate @adiabatic Temp & stoich conc)]

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Old   November 1, 1999, 12:03
Default Re: turbulent mixing controlled reaction vs. chemical kinetics controlled reaction
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Jean-Yves
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Hello,

I invite you to visit our internet web site where you can simulate reaction on-line with NetReaction. And generate meshes for CFX or Fluent.

Regards

jy
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Old   November 1, 1999, 12:14
Default Re: turbulent mixing controlled reaction vs. chemical kinetics controlled reaction
  #4
Jean-Yves
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Hello

I invite you to visit our internet web-site www.pfd.ie to simulate chemical reaction on-line.

You may also generate meshes and ask news about PFDReaction, incorporated in CFD4.3

Regards

jy
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Old   November 2, 1999, 10:22
Default Re: turbulent mixing controlled reaction vs. chemical kinetics controlled reaction
  #5
Dobrin
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Hello, I use the Eddy Dissipation Combustion Model by Magnussen, (Magnussen,B.F., Hjertager,B.H.(1976).16th Symp.on Combustion. The Combustion Institute,Pitsburg,pp719-729) where the kinetiks are presumed as one step global reaction mechanism and the combustion is controlled only by the turbulent mixing. If you are interested in more details, please write me. However, I can advise you to use some approaches which use reduced kinetics in order to simulate the pollutants. For example the Eddy dissipation Concept model by the same Magnussen or the Eddy dissipation Kinetics Model by Rasmussen are very advanced turbulent combustion models and easy to implement into the CFD codes.

Regards

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Old   November 2, 1999, 19:29
Default Re: turbulent mixing controlled reaction vs. chemical kinetics controlled reaction
  #6
Hyeryung NA
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Thank you very much, Dobrin.

For the time being, I use commercial CFD package Fluent so that, I do not have plan to write my own code in short future. At this moment, I would like to know the APPROXIMATE VALUE of temperatue(say, 1600K or 1770K or...) above which chemical kinetics can be neglected in engineering aspect. It of course might be dependent of each system which has its own mixing or diffusion characteristics. But I do not mean exact value or universally accepted theory. I mean APPROXIMATE value within reasonable error range(maybe 10-20% or ...). Anyway thank you for your advice and I myself will study, too. And if you have more advice for me, please let me know.

Sincerely, Hyeryung

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Old   November 3, 1999, 06:41
Default Re: turbulent mixing controlled reaction vs. chemical kinetics controlled reaction
  #7
Jason Norman
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I'll comment further to my earlier message:

'Turning off kinetics' is dependent upon the fuel / oxidant mixture and the scale of your flame.

If you were modelling a methane / pure oxygen flame, then you can be fairly confident that the kinetic reaction rate is very fast with a corresponding high temperature. Therefore the controlling step will be the fuel and oxidant mixing. Such a system could be modelled using eddy-breakup alone although the preferred method would be to use a mixture fraction approach.

If you were modelling an ammonia / air flame, then the kinetics are relatively slow. Therefore, although you could model it with just eddy-breakup, you wouldn't represent the slow reaction rates and the solution would be wrong (even though it may 'look good'). It would therefore be advisable to include kinetics in this simulation.

Fluent on the whole calculates the rate according to mixing and kinetics (finite rate/EBU model)and uses which ever is the slowest. In the above example it would use kinetics.

Therefore, you can see that an answer to your question is determined by your fuel / oxidant mixture and you have to know something about the combustion properties of this. Furthermore, if you are modelling a large industrial burner then your mixing characteristics are different to those present in, say, a lab scale burner. This may also affect your decision. I described a method earlier in this post of working out whether one should use kinetics / mixing-alone using the Damkohler number. This uses both effects relating to 'flame scale' and kinetics to answer your question. I would recommend that you use this method.

Good Luck. P.S. If you don't have any joy with Da. no. why don't you model both and then compare results?
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Old   November 3, 1999, 07:47
Default Re: turbulent mixing controlled reaction vs. chemical kinetics controlled reaction
  #8
Dobrin Toporov
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Hello, Hyetyung,

Anohasimika (is my korean right?). So, Following the discussion I can say that I am fully agree with Jason Norman. Probably you have to determine which regime of combustion you are going to model (premixed ot diffusion flames). If it is premixed I can advise you to find the Borghi diagram from (Peters,N.,21thSymposium on Combustion p.1231-1256,1986). There you can see that for Da>1 regimes you can neglect the chemistry and to presume infinitely fast chemistry approach and to use EDCM by Magnussen or Flamelet Surface Density Models.

Good Luck!

Dobrin
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Old   November 5, 1999, 17:24
Default Re: turbulent mixing controlled reaction vs. chemical kinetics controlled reaction
  #9
Anthony Iannetti
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Hyeryung,

I hate to say this, and probably confuse you more, but it really depends on the TYPE of flame ou are modeling. AND what kind of ANSWER you need This is usually premixed, partially premixed, and diffusion flames. If you are modeling pollutants and don't need fine accurate results for things like instablilties and fine flow field prediction, eddy break up and the related kinetics / or mixing approach is just fine. Examples are modeling catylitic converters or other mixing control devices (because of a lack of a flame), or combustor simulations where only inlet and outlet temperature predictions are concerned. Basically what I am saying it that if you can live with a great amount of turbulence chemistry interaction being ignored, use it.

If you are modeling a combustor or any other device where you are actively pushing the limits on stabilty, trying to improve the general process,etc. I would use a PDF model, or some kind of 'tuned' magnussun model created for your case.

CFD is an assumption, anyway, but the basic limitation can be though of like this: in laminar premixed flames, the flame speed is a function of kinetics. In laminar diffusion flames, it is a function of heat transfer and mass diffucion coefficents. Here is the problem: for turbulent diffusion flames, the flame speed in increases because of the enhanced mixing of turbulent flow. But for turbulent premixed and partially premixed flames, the flame speed increases because the flame sheet 'stretching'- increasing the surface area - because of turbulence. Obviously, unless DNS was used, the magnussen type models cannot predict this. (I am a little wrong, only laminar chemistry is needed for DNS because you are not modelling ANY turbulence.)

Unfortunately, very little of this is quantified easily in book form. I would recommed Turbulence Reaction flows (1994) by Paul Libby and Forman Williams.

Hope This helps, Tony
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Old   November 9, 1999, 05:47
Default Re: turbulent mixing controlled reaction vs. chemical kinetics controlled reaction
  #10
Hyeryung NA
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O.K. I myself fully agree with, and thanks for, all posts here. Then, how about the situation (approximate temperature above which chemical kinetics can be neglected without SIGNIFICANT loss of accuracy), if I restrict the case very narrowly.

- gaseous diffusion flame with moderate or high swirl strength

- high quality gaseous fuel such as CH4 or C3H8

- air as oxidant

I guess that there might be, at least, approximate value somewhere, even though it might not be very accurate.

Sincerely, Hyeryung
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Old   November 11, 1999, 04:00
Default Re: turbulent jet mixing under confinement
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T.S.Dhanasekaran
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Dear sir

I like to discuss about the Turbulent jet mixing under confinement.Now i am Processing using Fluent.I have some list to clarify.Pls let me know your interest on that.

Thanking you Dhanasekaran
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