9 juli 2015

CFD simulations of a bubble screen

Tom O’Mahoney

Mike van Meerkerk

Contents of the talk

• What is a bubble screen?

• What do we eventually want to model?

• What is known from literature on bubble modelling in CFD

• Model setup

• Lab scale simulations

• Field scale simulation

• Conclusions

9 juli 2015

What is a bubble screen?

9 juli 2015

What is a bubble screen?

9 juli 2015

Air mass flow

Throughput of salt

Value of 100

corresponds to the

case without bubble

screen

What is a bubble screen?

9 juli 2015

What is a bubble screen?

9 juli 2015

What is a bubble screen?

9 juli 2015

What is known from literature

• Modelling of bubbles is a complex process

• Within literature there is no consensus about which physical

effects to model (depends on application)

• Large scale measurements of bubble screens is sparse

• Available data may not be satisfactory to validate a CFD model

Main problems identified in the literature (on bubble columns)

• The overestimation of turbulent viscosity, limiting the transient

character of the flow

• The missing 3rd dimension resulting in a higher generation of

turbulent kinetic energy

9 juli 2015

Model setup

• The Euler-Euler approach for multiphase flows is used because

volume fraction of air is large but the bubbles small

• Drag is the only interphase force of importance

• Limitations on the CFL number and mesh size

• RANS model used for turbulence

9 juli 2015

Lab scale simulations

• Based on simulations by Wen and Forrest (1987)

• Approximately 1m deep

• Different flow rates

• Velocity profile measured at different air flow rates

9 juli 2015

Lab scale simulations

9 juli 2015

Lab scale simulations

9 juli 2015

Lab scale simulations

9 juli 2015

Field scale simulation

• Field scale tests, 10 times the depth of the lab scale experiments

• Mesh is scaled accordingly but the bubbles are not scaled

• Fewer air mass flow rates measured

• Different limits of the viscosity ratio tested. Finally no limit given

9 juli 2015

Conclusions and future work

• At lab scale 3D simulations are necessary but this could be due to

the side wall effects

• Those effects seem smaller at field scale so 2D may still be

reasonable there

• High turbulent viscosity ratios make the application of RANS

modelling questionable

• Available validation data is only without salt

• The addition of the lock exchange flow is a work in progress

9 juli 2015