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J. McKelliget March 23, 2000 10:47

enthalpy of Formation
Can anyone elucidate the role of enthalpy of formation of a species.

From reading the manuals it figures into the reaction energy source term, is used in the calculation of reverse reactions, and for the PDF model it figures into the enthalpy.

I have a finite rate model using the coupled solver, I turn off the reactions from the solve/controls menu, set the pre-exponential factor for the reactions to zero, I have no reverse reactions. Yet the value I set for enthalpy of reaction affects my solution - usually preventing convergence. There must be some other way it enters the model.

Any ideas?.

J. McKelliget

Greg Perkins March 23, 2000 22:56

Re: enthalpy of Formation
I assume you are using Fluent 5 in which case you turn off the reactions under Define/Models/Species/Volumetric Reactions.

The Solve/Controls/Solution option should turn on/off the solving of the entire equations, eg. species transport.

I'd use Volumetric Reactions and turn off reactions and see what happens. If this is what you've done or still produces the same result, it does seem weird.

Let me know what the result is, since I'm calculating gas phase reactions too.


Joakim Majander March 24, 2000 10:26

Re: enthalpy of Formation
Heat of formation is included in the enthalpy. Thus changing it effects "heat of reaction" and will give different temperatures. Actually in Fluent 5 there is no "heat of reaction". The enthalpy does not change due to reactions, but the teamperature does.


J. McKelliget March 24, 2000 12:46

Re: enthalpy of Formation
What you are saying makes sense, but is in direct contradiction with what it says in the manuals.

In the manuals the only time heat of reaction appears in the enthalpy is in the PDF model.

J. McKelliget March 24, 2000 12:52

Re: enthalpy of Formation
No, if you use the coupled solver the species equations are solved in with flow. There is a separate line where you can turn off just the reactions.

Yes, if I turn of the reactions through through the define/solver menu then enth. of form. has no effect, but it should also have no effect if I turn of the reactions through the Solve/Controls menu.

Greg Perkins March 25, 2000 23:19

Re: enthalpy of Formation
Sorry my mistake, I was referring to the segregated solver not the coupled one - which I'm not using at the moment.

Joakim Majander March 27, 2000 10:38

Re: enthalpy of Formation
Look at "Energy sources due to reactions" from the manuals. Heat of formation doesn't seem to be included in enthalpy as I claimed, but heat of reaction (Sh) is calculated from heat of formation. This is what you have to do, when heat of formation is not included in enthalpy.

So you should give the correct heat of formations.


J. McKelliget March 29, 2000 12:48

Re: enthalpy of Formation
But if you set the pre-exponential factor in the reactions to zero then the heat of formation should not affect the model results.

I guess we shall just have to consider this as one of the unexplained mysteries of the universe.

Volker Pawlik March 30, 2000 11:02

Re: enthalpy of Formation
As far as I know also in Fluent enthalpy in a system including reactions is defined by:

H =H-formation(T-ref=298K)+ integral(cp dT) from T-ref to T. The second term is also called sensible enthaply.

Therefore if you turned reactions on there will be an influence on your result due to the enthalpy relation above even if you set the pre-exp. factor to zero. Turning volumetric reactions off should prevent the solution from being affected.

J. McKelliget March 30, 2000 11:47

Re: enthalpy of Formation
Is there anything in the documentation that says that this equation applies to finite rate (non PDF) chemistry?

If there is, then I can't find it.

Volker Pawlik March 31, 2000 05:19

Re: enthalpy of Formation
Yes, indeed there seems to be nearly nothing in the documentation. Just on small hint on page 11-26 and Eq. 11.2-15 where they refer to heat of formation in connection with finite rate. What do the Fluent Guys say?

But as longer as I think about it, there should not be no influence on temperature (as you get?) because the reaction rate should be zero, hence heat of reaction too. Maybe the reason is that handling a numerical term which is close to zero causes the problem.

Nicole Phyfe June 25, 2000 11:28

Re: enthalpy of Formation
The enthalpy of formation for a species appears in the energy equation when the finite rate model is used. See equations 8.3.1 and 8.3.10.

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