Learning Forum

[Maryam Aboulfotouh]

Hello All,
I'm Simulating PCM using Ansys Fluent, but as the solution progresses and after the transition phase while the liquid fraction is increasing, I find that the program is not converging with the time step I have calculated using the courant number (0.01 sec) I start to decrease the time step, However again as the solution progresses I need to lower the time step even more right now it is at (0.0005 sec) just for converging.
This is a problem for me as I need 10,000 sec and it will take at this rate approx. 1 year for 1 run to complete!!
 
I don't know if that is a computational problem or something in the program.
Please advise how I can fix it.
I really appreciate any help you can provide. 

[Rob]

What is the flow doing? Ie what happens as the PCM melts? 

[Maryam Aboulfotouh]

the liquid fraction is increasing also the temperature is somehow starting to steady .. is that what you meant by your question ?

[Rob]

Yes, thank you.  So, as the liquid fraction increases is it starting to move? In a solid you're just considering thermal conduction; once it's a liquid you can add convection and diffusion into mix. With more fluid, higher temperatures and potentially flow pathways opening up it's very possible something happens at a faster rate. Plot some velocity and solid/fluid contours to get an idea of what's going on.