Multiphase convergence problem

delaneyluke · April 20, 2010, 02:26

Hi,
I'm running a liquid-gas multiphase problem with liquid as the continuous phase. I need to anlayse over a range of properties (constant) of the liquid phase. With a low viscosity (10 centipoise) i have no problems. The problem begins when i increase the viscosity to 500 cP and 1000 cP. The momentum imbalances fluctuate a lot (10-20%). I've even tried running the case as laminar (considering the higher viscosity) with the same problem.

Any suggestions.

Luke

ghorrocks · April 20, 2010, 07:38

Hopefully there is a physical explanation for it - but I can't think of one. You would have to give more details about what you are modelling.

Does some transition happen as the viscosity goes up? Maybe the bubble go from spherical cap to spherical regimes and you are using an inappropriate drag model which gives wrong drag numbers and leads to instability.

delaneyluke · April 21, 2010, 00:25

Thanks Glen,
But i tried modeling the flow as single phase (laminar, with 1000 Cp viscosity) and arrived at the same fluctuating imbalances. This is as simple as you can get. Constant property fluid, laminar flow .. i dont get why it's so tough to converge. The inlet velocity is 9.84 m/s and domain pressure 10 bar.

ghorrocks · April 21, 2010, 18:54

I repeat - does some transition happen? Maybe you go from the spherical cap regime to spherical and your drag model is not appropriate any more.

Nothing is "simple" when you are talking multiphase flow.

April 20, 2010, 02:26	Multiphase convergence problem	#1
delaneyluke Senior Member Join Date: Mar 2009 Location: Indiana, US Posts: 186 Rep Power: 17	Hi, I'm running a liquid-gas multiphase problem with liquid as the continuous phase. I need to anlayse over a range of properties (constant) of the liquid phase. With a low viscosity (10 centipoise) i have no problems. The problem begins when i increase the viscosity to 500 cP and 1000 cP. The momentum imbalances fluctuate a lot (10-20%). I've even tried running the case as laminar (considering the higher viscosity) with the same problem. Any suggestions. Luke

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April 20, 2010, 07:38		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,696 Rep Power: 143	Hopefully there is a physical explanation for it - but I can't think of one. You would have to give more details about what you are modelling. Does some transition happen as the viscosity goes up? Maybe the bubble go from spherical cap to spherical regimes and you are using an inappropriate drag model which gives wrong drag numbers and leads to instability.

April 21, 2010, 00:25		#3
delaneyluke Senior Member Join Date: Mar 2009 Location: Indiana, US Posts: 186 Rep Power: 17	Thanks Glen, But i tried modeling the flow as single phase (laminar, with 1000 Cp viscosity) and arrived at the same fluctuating imbalances. This is as simple as you can get. Constant property fluid, laminar flow .. i dont get why it's so tough to converge. The inlet velocity is 9.84 m/s and domain pressure 10 bar.

April 21, 2010, 18:54		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,696 Rep Power: 143	I repeat - does some transition happen? Maybe you go from the spherical cap regime to spherical and your drag model is not appropriate any more. Nothing is "simple" when you are talking multiphase flow.