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Why a different enthalpy of water is used in the tutorials?

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Old   November 2, 2017, 05:35
Default Why a different enthalpy of water is used in the tutorials?
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Yun MO
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Hi, Fluenters,

When I mimic the Fluent Tutorial: Modeling Nucleate Boiling Using Ansys Fluent and the Tutorial : Heat and Mass Transfer with Mixture Model and Evaporation-Condensation model, I found that I could not find a reason why the latent heat (= enthalpy of saturated vapor - enthalpy of saturated liquid)  of water is different to that in the thermal property tables of saturated water what we are using.

In the Fluent Tutorials, saturated water and vapor are set as follows.

Reference temperature:298.15K
For saturated water: h = 0 J/kg.mol
For saturated vapor: h_g = 2.992325e7 J/kg.mol


However, I found they are as follow from the handbook, for an example, 2001 ASHRAE Fundamentals Handbook (SI),

T=25℃(298.15K)
h_f = 104.83 kJ/kg, 9
h_g=2546.5 kJ/kg
Latent =h_g h_f = 2526.5 104.83 = 2441.67 kJ/kg = 2441.67 * 18.015(water molecular weight, kg/kmol) =43986KJ/kmol = 4.399e7 J/kmol.

If we set h_f= 0 J/kmol, h_g = h_f + L = 0 + 4.399e7 J/kmol = 4.399e7 J/kmol.


It can see that it totally differs to that in the tutorials.

On the other hand, the saturated properties of water and vapor can be directly obtained from the library in Fluent as follows:
M= 18.015 Kg/kmol
T= 298 K
h_f= -2.858412E+08 j/kmol
h_g= -2.418379E+08 j/kmol

Latent heat =h_g h_f=4.400330E+07j/kmol
Latent hat =4.400e7 J/kmol / 18.015kg/kmol = 2.442592E+06 j/kg

It agrees to the one that I can find in the handbook.

And then why the latent heat of water at 298.15K in the tutorials is 2.992325e7J/kg.mol (that is, 1661KJ/kg, but not 2443KJ/kg)?

And since there is a water library in Fluent, why the tutorials intently set h_f = 0 j/kmol, h_g = 2.992325e7 j/kg.mol?

Why? Where the latent heat (or enthalpy) of water in the tutorials was got?

I asked the support of Ansys Japan but they could not tell the reason.

Could anyone explain why the latent heat in the tutorials is different to the one in the handbook?
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