Learning Forum

[Ali Qasim]

Hi everyone,
I am modeling a membrane energy exchanger for dehumidification of the air in a hot, humid climate. In this energy exchanger system, the air flows on one side of the membrane, and on the other, the liquid desiccant solution flows. Both fluids are separated by the semi-permeable membrane, which allows only moisture transfer but not liquid. I am modeling this system through the ANSYS FLUENT and UDF is used to model moisture transfer through the membrane.
As the moisture transfers through the membrane, it changes phase at the air-liquid solution interface. In ANSYS Fluent, the effect of this temperature change is not considered while solving the energy equation.
How can I include this effect of phase change of vapor (absorption on the solution-air interface) phenomenon on the performance of the system in terms of temperature change of solution and air?
Can I include this effect using Eulerian Wall Film?
or
Is there any way to modify the energy equation using UDF?

Thanks and Regards,
Ali

[SRP]

Hi,

you can indeed modify the energy equation using User Defined Functions (UDF) to include the effect of phase change of vapor on the performance of the system.

You can also check forum discussion: Multiphase flow properties (ansys.com)

Eulerian model for evaporation and heat exchange (ansys.com)

Hope this helps you.

[Ali Qasim]

Hi,
Thank you for your response.
Could you please point me to some specific resources to include this effect of phase change in the energy equation using UDF?
Thank you very much.

[Rob]

If you're using the Eulerian model phase change options then Fluent will account for the latent heat assuming you have the correct values in the liquid & gas formation enthalpies. Otherwise you may need a sink/source term for the latent heat. 

[Ali Qasim]

Thanks for the prompt response. I am using the Species Transport Model (and Laminar, Energy Equation). I am using UDF for mass transfer through the membrane. How can I consider energy source term using UDF (or any other convenient way) for this problem?

Thank you.

[Rob]

So, mass sink & mass source? You need to account for the energy that's added & removed with the mass (Fluent assumes sources are added/removed at 298.15K, it's in the manual somewhere, unless something has been changed) and unless you explicitly source energy to account for latent heat it's not included.