Learning Forum

[Amir]

Hello all, I have been working on a simulation where humid air passes through a pipe at lower temperature. At the most recent case, contour of film temperature is showing below zero temperature at some areas. My question is that what does this number mean? there is ice being formed? or some thing is wrong?

INLET: HUMID AIR WITH H2O MASS FRACTION OF 0.01771, AT 27C and 5m/s. Flow enters the pipe where the walls boundary condition is set to be 15C fixed.

Thanks.

[Rob]

If the wall is fixed at 15C what's stopping all/most of the liquid condensing instantly? What time step did you use?

[Amir]

Rob, the time step size for this case was 0.01s, and total physical time of 150s and it was a transient simulation. This contour is for the last step.

About the first part of your reply, I am not sure if I understood what you meant. But looking at the animation of "film thickness", the liquid film starts forming at other areas as well, and moves to the point that is observable at the attached contour as time goes forward. And H2O mass fraction is 0.01771 at inlet and 0.01100 at the outlet.

 

[Rob]

Try with a smaller time step or more film sub steps. With too much evaporation/condensation the amount of latent heat added into the near wall region can make the model a little unstable. 

[Amir]

Thanks Rob, reducing time step size resolved the issue.

[Amir]

Rob, too much latent heat making the solution unstable, could it also be in case when increasing physical time of the simulation?

Using EWF for physical time of 50s, solution converged and residulas looked stable for both time step sizes of 0.005 and 0.001 seconds. Results were pretty much the same.

When increasig physical time to 150s with 0.005s for time step size, residuals are fluctuating at the end and I am looking to see if that would make the results unaccountable?

 

[Rob]

It depends on what's happening in the flow & film. Is the film getting very thin/thick? 

[Amir]

Regarding the flow (and also your comment on my other post for the same case with shorter physical time 50s), it seems we reach the near steady state solution. The fluctuations are happening when I run it for longer physical time(150s).

 

About the film, film is growing and getting thicker, but it is still below the "maximum film thickness" value specified under EWF tab [default value is 5mm]. I also tried increasing max film thickness value under EWF tab from 5mm to 1cm, but it did not seem to be the issue.

 

 

[Rob]

Interesting. If you look at the flow and film thickness is anything weird going on? 

Changing the max thickness only helps if you're getting near to that value. The default is just that, and "thin" is very dependent on the domain scale.