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March 23, 2000, 10:47 
enthalpy of Formation

#1 
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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 preexponential 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 

March 23, 2000, 22:56 
Re: enthalpy of Formation

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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. Greg 

March 24, 2000, 10:26 
Re: enthalpy of Formation

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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.
Joakim 

March 24, 2000, 12:46 
Re: enthalpy of Formation

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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. 

March 24, 2000, 12:52 
Re: enthalpy of Formation

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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. 

March 25, 2000, 23:19 
Re: enthalpy of Formation

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Sorry my mistake, I was referring to the segregated solver not the coupled one  which I'm not using at the moment.


March 27, 2000, 11:38 
Re: enthalpy of Formation

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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. Joakim 

March 29, 2000, 13:48 
Re: enthalpy of Formation

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But if you set the preexponential 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. 

March 30, 2000, 12:02 
Re: enthalpy of Formation

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As far as I know also in Fluent enthalpy in a system including reactions is defined by:
H =Hformation(Tref=298K)+ integral(cp dT) from Tref 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 preexp. factor to zero. Turning volumetric reactions off should prevent the solution from being affected. 

March 30, 2000, 12:47 
Re: enthalpy of Formation

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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. 

March 31, 2000, 06:19 
Re: enthalpy of Formation

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Yes, indeed there seems to be nearly nothing in the documentation. Just on small hint on page 1126 and Eq. 11.215 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. 

June 25, 2000, 12:28 
Re: enthalpy of Formation

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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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