Learning Forum

[Ali Kandi]

Hi

I am working on a direct contact membrane simulation in which there are a saline water side (350 K temperature), a pure water side (300 K temperature), and a porous membrane. So, fisrt saline water turns into vapor at the membrane interface, and then the vapor turns into pure water at the other interface, meaning that it includes both evaporation and condensation. However, when I want to initialize the problem, an error comes up saying this:

Compute Node ID: 5
From-phase Domain ID: 1
To-phase Domain ID: 0
Cell Number: 0
Latent Heat: -2.26823e+06

ERROR: Latent heat cannot be less than zero! 

standard state enthalpy of liquids are zero, and the same for vapor (in membrane) is a positive value.

The idea is that In direct contact membrane distillation (DCMD) design, vaporization takes place at the liquid-vapor interface at the membrane pore surface in the feed side. Vapor is then permeates through the pores from the hot side to the cold side due to the difference in vapor pressure across the membrane (capillary action). The permeated vapor is then condensed in the cold stream side of the membrane.

I have attached the picture of the 2D model.

 I would appreciate it if you could help me with it.

[Rob]

Check which phase is set as "from" and which is "to". Is the reference temperature the same for both phases? 

[Ali Kandi]

Thank you for your answer. 

As you see in the below picture, and according to my explanation, phase change happens from water liquid to water vapor and vice versa in this problem.

regarding your second question, Yes reference temperature is set to 298.15 K as a constant value for both phases.

[Rob]

You don't use two mass transfer operations. Use one, and the solver will push mass one way or the other based on the temperature field. 

[Ali Kandi]

Thank you.

So you mean by selecting only one mass transfer mechanism, from phase 1 to phase 2 evaporation-condensation, both evaporation and condensation (from water vapor to water liquid and from water liquid to water vapor) will take place?

 

[Rob]

Correct, the mass transfer value has a sign so + & - mean it'll go in both directions depending on the conditions.  The From and To phases matter in the panel as there's a convention coded into Fluent as we don't tag a phase a gas or liquid anywhere in the system (the names are for our benefit, Fluent doesn't do anything with the labels in this situation).