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Thermal vs. Chemico-Thermal Enthalpy

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Old   April 11, 2007, 10:12
Default Thermal vs. Chemico-Thermal Enthalpy
  #1
Ale
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Dear all,

I have some doubts about the choice between the 'thermal' and 'chemico-thermal' options for enthalpy.

- Apparently, in both cases the same balance is solved, only the definition of enthalpy being different. Therefore, I would expect to get correct results whichever option is chosen. Is that correct? In your experience, are there cases in which one option is better (or even the only correct)?

- Can I have any issue if I map boundary conditions from a model using the "thermal" option, to a model using the "chemico-thermal" option?

- What's your experience in the use of these options, from a numerical point of view (stability, convergence ...)?

Thank you very much in advance,

Ale

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Old   April 11, 2007, 20:11
Default Re: Thermal vs. Chemico-Thermal Enthalpy
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usker
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I have calculated diesel combustion changing enthalpy type (static enthalpy->thermal and then chemico-thermal). Since my problem includes chemical reaction, I believe chemico-thermal was the best choice. Static enthalpy is the sum of thermal and chemical components. If chemical reaction takes place in your model, you have to consider chemistry as well. I got convinced when I compared computation results with experiments. For example max. adiabatic flame temperature in diesel engines is about 2500-2700K. I got similar results using chemico-thermal enthalpy. But when I used only thermal option temperature was more than 3000K, which I believe is not quite right.

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Old   April 12, 2007, 03:34
Default Re: Thermal vs. Chemico-Thermal Enthalpy
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Ale
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Thank you for your reply. What you say is really interesting.

However, I still don't understand why the results should be different. The last term in equation 1-13 in the Methodology manual (Version 3.20) takes into account the chemical reaction when using the Thermal enthalpy.

It would be nice to understand more deeply how things are working in the two cases.

Thanks a lot for the support,

Ale
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Old   April 14, 2007, 16:01
Default Re: Thermal vs. Chemico-Thermal Enthalpy
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Lager
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if you have to take into account the enthalpy of formation of additional scalars for example with chemical reations you need chemico-thermal. Otherwise thermal is ok.

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Old   April 17, 2007, 03:43
Default Re: Thermal vs. Chemico-Thermal Enthalpy
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Ale
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Thanks for your contribution.

I can believe what you say, but I still would like to understand WHY it is like this.

I mean, it is clear from the manual that the equation for thermal enthalpy is obtained from the equation for chemico-thermal enthalpy by subtracting the equation for scalars multiplied by the enthalpy of formation. I would say that either solving equation A and equation B, or solving equation A and a combination of equations A and B, should lead to the same results. Of course, the numbers for enthalpy will be different, but the solution (in terms of composition and temperature) should not be different. Am I wrong?

Moreover, in the equation for thermal enthalpy there is a term (the last on the r.h.s.) that clearly takes into account the variation of enthalpy due to chemical reaction (enthalpy of formation). What makes this equation unsuitable for reacting flows?

Please, if possible support your statement with some theory.

Thanks a lot!

Ale

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