Re: 'getDeltaEnthalpy'

[Ray Speth]

Yuruan,

getDeltaEnthalpy calculates the enthalpy change for the reaction at the current temperature and pressure. If you want the standard enthalpy of reaction, just set the mixture to the standard state temperature first.

Regards,

Ray

On Friday, June 29, 2012 6:06:12 AM UTC-4, Yuruan wrote:

Hello again!


I have been looking up the exact meaning of the function 'getDeltaEnthalpy' in Cantera's GasKinetics.cpp file. I haven't found much information.

I would like to know if it refers to the differences in the enthalpies of formation of products and reactants in a given reaction. Is this the standard enthalpy of the reaction (i.e. the standard enthalpy change of the reaction, 'Δ_rH°')?

Thank you in advance!


Yuruan

[Ray Speth]

Thomas, the getDeltaEnthalpy / delta_H method returns the enthalpy change (in J) per kmol of reaction. To get the heat release rate, you need to scale these by the rates of progress for all the reactions:

   sum(gas.deltaH() * gas.netRatesOfProgress())

This should give more or less the same value as the first expression you gave.

Regards,

Ray

On Monday, July 9, 2012 4:56:58 AM UTC-4, Thomas Fiala wrote:

Ray,

which reference is getDeltaEnthalpy based on, and what units does it have?

The background is that we want to calculate the heat of reaction in a counterflow flame and the influence of the various reactions on it. We calculate the total volumetric heat of reaction (in Python) using
 sum(gas.enthalpies_RT()*gas.netProductionRates())*ct.GasConstant*gas.temperature()
which is in J m^-3.
Ideally, the sum of the enthalpy changes of all reactions should yield the same result:
 sum(gas.delta_H())
(getDeltaEnthalpy is apparently wrapped as delta_H() in Python)
However, this gives a quite different number. Is it just a matter of reference, or is it not possible to use getDeltaEnthalpy to calculate the total volumetric heat of reaction?

Thanks a lot!

Thomas

[Thomas Fiala]

Thanks a lot, Ray! That was exactly what we were looking for.

Regards,

Thomas

[Mehdi Baniasadi]

Hi Ray,

You said that "the getDeltaEnthalpy / delta_H method returns the enthalpy change (in J) per kmol of reaction" however, I think that enthalpy change should be kmol of reactants or products.
Am I right?

Best

[Ray Speth]

Medhi,

No, I don't think it would be useful to report these properties per kmol reactants or products. For one, it would introduce an arbitrary choice in deciding whether to use the reactant or product moles as the basis. Second, deciding how many moles of reactants or products are present in a reaction is not always trivial once you start thinking about things like third bodies or falloff reactions. How many moles of reactants do the following reactions have?

    O + H + M <=> OH + M

    2 H + H2 <=> 2 H2

    O + CO (+ M) <=> CO2 (+ M)

Regards,

Ray