Very low steam fraction RPI boiling
 

Dear All,

I am currently working on my first CFD boiling simulation using RPI model and I face an issue I can not fix. I have started with a very straightforward steady state situation, vertical tube 20mm diameter and 2000mm length. Water inlet at the bottom at 20°C, 1 bar and 1m/s.
I have imposed a heat flux on the wall which i increase gradually simulation after simulation. It appears that the monophasic situation works very well. I activate the multiphase model when the temperature on the wall is about to reach the saturation temperature and that's where the problem start. Indeed, whatever the heat flux imposed, the steam volume fraction remains very low (i can not reach more than e-5). I have tried to increase a lot the heat flux but the solver diverge then...
Briefly, my simulation is working very well in monophasic state (even when the multiphase option is activated when the heat flux is low enough) but when the heat flux is high enough to create steam, the fraction remains very low whatever the flux.
As it is my first RPI boiling simulation, i am using the "default" values for all the RPI model parameters adviced by Fluent Manual.
I precise that I have activated the gravity...

https://zupimages.net/up/23/21/7pd3.png
Any idea ?
https://zupimages.net/viewer.php?id=23/21/7pd3.pngThank you for your help !


Pierre