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How can I model H2-AIR combustion

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Old   August 6, 2007, 12:59
Default How can I model H2-AIR combustion
  #1
popi
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Hi everybody

I'm trying to model a H2-AIR combustion in a combustion chamber. After a lot of unsuccessful tries, I think I'm able to model a H2-AIR combustion. In fact, I reach a successful result one week ago ( that's what i think). However I'm modelling the combustion whithout any idea of the next parameters: -Chemical timescale -Extinction temperature -Mixing rate limit -Eddy dissipation model coefficient A -Eddy dissipation model coefficient B

I don't know what do some of these parameters mean. And neither what could be the correct value of them in this case ( H2-AIR combustion ). Maybe this is not a suitable question for this forum but.... if anybody knows something about this, please reply me. However I'm also tryng whith a chemisty expert.

Thank you all

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Old   August 6, 2007, 16:08
Default Re: How can I model H2-AIR combustion
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Joey2007
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Have a look at CFD-Wiki combustion model ( http://www.cfd-online.com/Wiki/Combu...Break-Up_model ). Formulation of CFX is similar. A and B are clear from this.

CFX formulation: Mixing rate limit: upper limit for ratio of epsilon and k. Helpfull for fixing model failures in wall regions.

Extinction temperature: Lower temperature limit for burnable mixture

Chemical time scale: parameter for simple turbulent quenching model.

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Old   August 12, 2007, 04:51
Default Re: How can I model H2-AIR combustion
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Harry
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Hi popi,

Could you introduce some of your successful experience? I have tried a lot of times, but the results are not good as I expect, especially the flame shape and pressure variation in the flame propagating process. I am so depressed.

Thank you
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Old   August 13, 2007, 06:38
Default Re: How can I model H2-AIR combustion
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popi
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Hi Harry;

My succesfull experiences, may not be OK. However I'll explain you. My first Try was using "Hydrogen-Air FLL STP and NO PDF" library and Flamelet model. But I realize that I couldn't create my own initial mixture. This library only contains burned mixtures, and I wanted an unburned mixture.

So I try with the simpliest library: "Hidrogen-Air". The real problem of this library is that you can't describe a Flamelet model, and the default model for combustion is the EDDY DISSIPATION model. There is not flame, and flamefront, but...It's the nearest aproach that I reach. The results are quite well.

I Start the reaction process with a Source Point of Energy. Once JOEY told me, that this wasn't possible; but I think it is. With this Temperature rising (due to the energy source point), I only start the reaction process. It's only the beginning, after this the combustion process continuous by its own. Maybe this is a bad modelling... I don't really know.

However, I reach my succesful experience with this model. I simulate a combustion chamber from the Top position and combustion happenned with this mass fractions and conditions: H2=0.02 T=750K O2=0.228 P=13Bar N2=constraint I reach a 84% of the combustion, almost everything.

Now I'm trying to simulate this case from the beginning(from the bottom position) but it's imposible. I reach the same conditions at the top position, but combustion process only happens in a 40%. I don't understand nothing.

Maybe you could help me
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Old   August 13, 2007, 07:36
Default Re: How can I model H2-AIR combustion
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Harry
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Hello, Popi

Thanks for your explanation.

My simulation is premixed methane air flame, propagating in a tube, just like the tunnel in coal mines. I consider the flow is laminar and use the FRC model.

I adopt your ingnition method(appreciate your email). The mixture was ignited, but the flame shape I concerned didn't develop as that in my experiments,__) to __(. I was so disappointed. If anyone knows why, please tell me.

Your case is IC engine? I can not give you useful advice. Maybe we can discuss sth. else in future. Keep in tough!

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Old   August 13, 2007, 12:40
Default Re: How can I model H2-AIR combustion
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popi
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Hi Harry;

I forgot to tell you somethg. When you model a combustion process with any model different from "flamelet" or "Premixed and partially premixed", there is not flame creation. I mean,the combustion process ocurrs but there's not flame. Combustion ocurrs like a chemical "normal" reaction, whithout a flamefront.The reaction is quite fast, and could seem a flame, but there's not. Maybe this is the reason your experiments don't match the simulation. Could be?

Thanks for all best regards Popi
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Old   August 13, 2007, 16:16
Default Re: How can I model H2-AIR combustion
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Joey2007
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The library contains unburnt mixtures. Have a look into the file its ascii. You can read it. If you use diffusion flamelet only the burnt mixtures are used in the solver.

Eddy dissipation. I do not understand how you managed it to ignite it in the way you describbed. Have a look at here: http://www.cfd-online.com/Wiki/Combu...Break-Up_model . The Eddy break up model source term does not feel heat! That's one difference between model and reality. With pure eddy dissipation model for premixed flame you have to introduce products to 'ignite the model'. You may introduce the products by an additional reaction or a source or initialisation.

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Old   August 14, 2007, 21:41
Default Re: How can I model H2-AIR combustion
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Harry
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Do you mean I cann't get a flame shape variation process without "flamelet" or "Premixed and partially premixed" model?
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Old   August 16, 2007, 05:32
Default Re: How can I model H2-AIR combustion
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popi
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No. I think, You can't get a flame whithout a flamelet model(premixed or not premixed), but the model is simplier and the results could help you. In a flamelet model, combustion happens in a thin flamefront, that separates the burnt mixture and the unburnt mixture. If you are trying whith a No flamelet model, results are differents. There's not a thin flamefront, but there's a zone (not as thinner as in the flamelet model) where the reaction is happenning and that separates the burnt and unburnt mixtures. Generally, this results should not be too far from those reach whith the flamelet model, but if you're researching the flame behavior may be this approach could not be good.

best regards
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