Learning Forum

[Hamza]

Hello Community,

i'm running a Two-Phases Simulation, where in a vertical tube water flows through the pipe with an inlet temperature of 383,15K and Mass heat Flux of 400 kg/s(m^2). A Heat-Flux of 150kW/m^2  is given in the outer Wall of the pipe. At the end of the pipe water should have boiled and occurs the formation of vapor but in very small proportions.

The Problem is that the Enthalpies aren't accurate, what lieds to a superheated Steam (the back condesation dont occur) and the Wall-Temperatures just keep increasing at higher values.

How can i fix the Problem with the Entalpies, it seems that they arent been calculated correctly.

Thank you for your support.#

Case-Setup:  https://ruhr-uni-bochum.sciebo.de/s/kBt3KyKA1vuvtNz

[SRP]

Hi,

The evaporation-condensation model in Fluent can be used for simulating water steam.

For more information please refer theory guide: 14.7.5. Evaporation-Condensation Model (ansys.com)

If you are unable to access the link, please refer to this forum discussion:Using Help with links (ansys.com)

Thank you 

[DrAmine]

Do you expect 100% evaporation of all water? It reminds of typical scenarios for deploying RPI Boiling Model (even there 100% evaporation was never the purpose of the model).

[Hamza]

 

No, a 100% evaporation should not occur, i have a subcooled boiling water. The problem that i am facing is that i receive an anaccurate results for the Heat Trannsfer Coefficient compared to the results from the experiment. And i found out that is because the enthalpies for both water and steam aren’t correct, i would expected that the Steam-enthalpie increase in the area where steam ist, but it is always less than the Water-Enthalpie. In addition, the wall temperature in the steam region rises strongly, resulting in overheated steam. I have a pressure of 4.8MPa and Saturation Temperature at 543.5 K.

 

[DrAmine]

Enthlpy of Steam should be greater than the one of the liquid. Check your material properties and if the latent heat, in case constant and not using RGP, is properly provided. I think that is correct otherwise Ansys Fluent will write a message that Latent Heat is negative.

For heat Transfer Coeffient: always correct the reference value of Tref to whatever you want to compare with.

[Hamza]

Thank you DrAmine for your reply.

I double checked all my material properties and they are all correct. I have put a link for my case-setup in the first message, so if u can take a look on it, and tell me what's incorrect, cause I haven't found the error.

Can you please explain more about how to correct the Tref. In the material properties I set the Tref as temperature of saturation (534.55 K) for both water and steam.

[DrAmine]

I won't take a look into your files. I hope othe non Ansys Employees can chime in here and look into you case.

Yes that is typically what one will do. You can however use Real Gas Property Tables and with that the latent heat as well as all other material properties (and energy) will be a function of pressure and temperature.

 

Now regarding the Tref for Heat Transfer Calculation: that has nothing do with Saturation. Generally from the experiments or corrleations you will know about the reference: generally taken as inlet temperature or bulk temperature or temperature at certain distance from the wall.