Learning Forum

[valeria.boffula]

Hello, I have two main questions about the setup of mass flow outlet condition in Fluent. I have a domain in which two phases are present and the liquid is in the upper part of the tank (see fig.below) and an acceleration field will move the liquid to the outlet in a certain time.  A multiphase VOF model is used and I use a profile to set the liquid mass flow rate ( it is zero at the simulation start and after 30s it is constant and equal to a certain value). I notice that, since the beginning of the simulation, even if at the outlet there is only gas, Fluent sets liquid; and, even if I force the intialization patching that zone with only gas on the suface outlet I continue having liquid. Because of the aim of my analysis is to check the % of gas at the outlet, having always liquid will forge my results. How can this condition be fixed? The second question is: why I can not put a total mass flow (gas+liquid) and let Fluent to choose the correct mass flow according to the phase that it sees at the outlet? I mean, I expect that if I have 20% of gas  and 80% of liquid at the outlet the mass flow at the outlet should be corrected consequentl. 

[Rob]

I don't follow the bit about Fluent setting liquid even if you have gas. Pictures may help. 

The mass flow outlet was removed for some multiphase several versions back as it wasn't stable. The issue is that if we remove x kg/s and there is a variable amount of a denser phase that the pressure on the boundary becomes unstable. A volume flow boundary (velocity) would be numerically better and potentially physically more sensible as pumps etc suck volume. 

[valeria.boffula]

Thank you for the answer.

I can not post the immage I have this error: 

"Image upload failed due to a XHR Transport error. Code: 0"

Can you help me? 

[Rob]

No, but I can harass someone who can fix that. 

[valeria.boffula]

Thanks, let me know so we can discuss about the problem

[Rob]

Someone has posted images into another thread so please try again. If you need one image per post that's fine - the rules are there for people needlessly bumping threads.