## Fluids

#### 2D axisymmetric mixing tank with air sparging

• krndv
Subscriber

Hello,

I am trying to simulate a mixing tank with air sparging from inlet. The problem is set up as 2D- axisymmetric, Turbulent, Multiphase (Simple VOF) case. The system has an impeller for mixing, the phenomena is trying to capture using MRF technique.

More Details

2. Inlet velocity is defined, Pure air is coming in so volume fraction of air kept as 1

3. MRF (around impeller) region is given -100 RPM using frame motion option.

4. The impeller wall is given moving wall (rotational) BC, with relative rpm with respect to adjacent zone (MRF zone) as zero.

5. Standard K-Epsilon Turbulence used.

6. Mesh quality - Minimum Orthogonal Quality = 9.98567e-01, Maximum Aspect Ratio = 1.487

7. Domian lenghth (along X axis) is 355 mm, in which up to 180mm is filled with water and above that air.

8. Interface defined between stagnant and moving zone

ISSUES

1. The problem is giving solution when solved in 3D.

2. Not converging with 2D axisymmetric set-up.

3. The problem has to be set up as 2D-axisymmetric or Axisymmetric swirl for capturing impeller rotation?

4. MRF technique valid with 2D Axisymmetric, multiphase case?

5. Possibility of using sliding mesh? (Tried and failed)

Hope some one can help me with answers for above question. Thanks in advance

• krndv
Subscriber

• Rob
Ansys Employee

You'll need 2d-axi with swirl if you want the impeller to rotate, but have a think about the direction the blade will push the fluid. Also read up on the VOF model (including the theory) then explain why you chose it.

Bear in mind the difference between a 2d and 2d-axi case.  Have you used the transient solver?

• krndv
Subscriber

I have used Transient solver.Tried steady state, just got an oscillating residual, no stable result.

I will try with 2D-Axi swirl.

I was interested in getting just the volume fraction of air inside or gas hold up. So I thought simple VOF will be computationally less intensive as it solves one momentum equation for both the phases.

• Rob
Ansys Employee

VOF is a free surface model: it's perfect for tracking the water surface at the head space, but not designed for spargers. Read up on Eulerian and DPM then ask/explain your next steps.

• krndv
Subscriber

What about carrying out the simulation with only water and degassing BC on top and air sparging from inlet? WIth Euler-Euler Multiphase model.

valid case set up?

• krndv
Subscriber

• Rob
Ansys Employee

Should be: I'd initialise with an estimated hold up volume fraction to help the solver.

• krndv
Subscriber

Ok, Thank you.

Could you please tell how to initialise hold up volume fraction?

Currently Im following the below given step

1. Steady state run for few iterations (10^-3 convergence of residuals) with no air input by giving sparger wall B.C and swintching off volume fraction equation.

2. After getting steady velocity of water domai - Switch on sparger(Mass flow rate inlet), volume fraction equation and transient run with time step of 0.01 secwith 20iterations per time step.

• DrAmine
Ansys Employee

You can initialize with certain volume fraction (standard) or use patch function to patch volume fraction in certain region of the domain. Please consider that degassing BC is only suitable for dilute dispersed flow.

• krndv
Subscriber

Thank you. Solution Doesn't seem like converging. Oscillating residuals and effect of impeller is not seen in volume fraction of air, the flow is straight not disturbed.

• krndv
Subscriber

1mm mesh used, quad dominant, inflation layer near walls.

METHODS- Pressure: PRESTO, Vol fraction: QUICK, First order upwind for all others.

• DrAmine
Ansys Employee
What about double checking the case and scrutinizibg velocity and relative velocity contours you can still model a sector of the whole tank with periodic conditions ?
• krndv
Subscriber

Thank you Amine.

You are saying about a 3D sector (Like 60 degree one)  with periodic boundary condition?

• DrAmine
Ansys Employee

Yes if you want to save some cells.

• krndv
Subscriber

Thank You Amine.

Tried and seems to be working.

• DrAmine
Ansys Employee

Great seems 2d axisymmetric has some issues here. Mark this as solved.

• krndv
Subscriber

I have one more doubt, while simulation a 60 degree sector I should use inlet flow rate (in kg/s) as actual one or 1/6th of actual flow rate.

• DrAmine
Ansys Employee

For the sector modeled. I would rather use velocity inlet if you know the density at inlet.

• krndv
Subscriber

Yes its pure air coming in.

But the sparger got 7 distinct holes (per sector, 42 in full geometry) of 2.5mm dia, instead of drawing separate holes i made a slit of width 2.5mm with mass flow rate inlet B.C.

I will try with velocity inlet.

Thank you.

• DrAmine
Ansys Employee

Mass flow is always per sector in Fluent.

Yes. Have a fun. Please mark this as "Is Solved".

• krndv
Subscriber

Hello,

What is the technical difference between 'degassing' and 'pressure outlet (with back flow fraction = 0)' at the outlet boundary on a tank?

Thanks

• DrAmine
Ansys Employee

Open new thread and I will tell you what is the difference if the documentation is not sufficient.