Learning Forum

[samiasadaf_uetian]

Hi Fluent Experts.

I am simulating water flow boiling through microchannel using Multiphase VOF model on Ansys Fluent

I have a question

Why Ansys Fluent in Material properties takes reference Enthalpy of liquid- water is -2.858e+08 J/kgmol with molecular weight of 18.0152 kg/kmol at reference temperature of 298 K

Similarly Ansys takes enthalpy of vapor-water -2.418e+08 J/kgmol with molecular weight of 18.01534 kg/kmol as at reference temperature of 298.15 K

I have calculated and understand the latent heat of vaporization i.e. the difference of both enthalpies (h_fg) is same as that the difference in enthalpies values for liquid-water and vapor taken from steam table at 298.15 K

But still why fluent is using these huge reference values with -ve signs.

Thanks

[jcooper]

Hi:

These are the real formation enthalpies.  Watch out for the units….

The standard enthalpy of formation of water at 25 C and 1 atm  is -285.8 kJ/mol. This is because the reaction (of O2 and H2) that creates a molecule of water releases heat, so water actually ends up at a lower energy state than its components. 

The units of the Fluent enthalpy of formation are in [J/kgmol], however, so we must convert:

Converting the units: 

Hf = -285.8 kJ/mol  * 1000 kJ/J  *1000 mol/kmol = -2.85e+08 J/kmol

 

Regards,

Judy

 

 

[samiasadaf_uetian]

Thank you Judy for your kind explanation

As I am simulating water flow boiling through microchannel at inlet temperature of 372.15K and Saturation temperature is 373.15K using Lee phase change model,

I was wondering why enthalpy of formation is needed in boiling ( conversion of liquid phase to vapor phase)? There is no chemical reaction taking place.

 

[jcooper]

 

The difference in the enthalpies of formation is used to calculate the latent heat of evaporation/condensation.  (One way to understand this is to consider steam burns at STP:  These are so terrible because the steam condenses on the skin as it makes contact, releasing additional heat in the process. This heat release will be the heat of phase change from vapour to liquid -2.418e+08 J/kgmol – (-2.858e+08 J/kgmol) = +0.44e+08 J/mol of steam condensed)