Learning Forum

[mdfaizan444]

i am doing steady state nanofluid flow simulation inside tube in two phase mixture model. I am getting this type of residual (below figure) dont know whether it is correct or not..

[Rob]

Check the flow field. The flow may be transient or you've achieved boiling or some other unstable flow regime. 

[mdfaizan444]

No, flow is steady only and maximum temperature is achieving in the system is 35 degree only (for water+Al2O3 nanofluid) so i dont think these two factor is affecting 

Actually i am doing in two phase mixture model with slip velocity between the phase. In slip velocity gravitational acceleration term is there so i have to on it. But when i off the gravity then residual graph is coming decreasing and convergence is occurring easily .

But when i on the gravity then residual is coming like that only (above figure) and also convergence is not occuring.

[Rob]

Sounds like gravity is having significant effect on the solution.  Try switching to transient and see how it behaves. What size is the second phase? 

Multiphase models often give a transient solution as local flow is very unsteady: bulk flow may still be fairly stable. 

[mdfaizan444]

Second phase size is 100 nm. 

 

[Rob]

That should follow the water pretty well. What does the velocity field look like?

[mdfaizan444]

In mixture model of nanofluid, At inlet boundary condition Velocity inlet is required for both the phase separately (Phase1 & phase2). 

i am giving same inlet velocity for both the phase that is calculated from the average mixture properties. (Mixture viscosity, mixture density), by giving the same inlet velocity will slip occur between the phase in the channel? 

[Rob]

Slip will occur as the particles try and separate: they're moving along the pipe with the liquid, but with gravity will try and settle.  What's the mesh resolution like? 

[Huuer Ginn]

I never knew about this before

drift boss

[mdfaizan444]

I am giving heat flux at the top and bottom boundary.

Also how can i calculate mixture viscosity, as secondary phase is solid?

[Rob]

You could be having problems with the second phase near the walls. What does the solution look like just before it fails? 

If the second phase is solid, why would you want mixture viscosity?

[mdfaizan444]

Solution looks good with overpredict (11%). But my concern is it is not converging. I am validating one of the paper  where they have used all three model single phase, mixture and discrete model. Thats why i am also trying to do the same.

[Rob]

Try transient. You'll probably finish up with a pseudo steady solution but that should assist with convergence. Also double check the paper settings to make sure you're modelling the same thing: there's often some ambiguity in there to hide something the author doesn't want to discuss.