CFB+wall BC for solid vel and gran temp
 

Hello,

I am trying to run a multiphase flow inside a high flux high density circulating fluidized bed. I am very new to this multiphase modelling and I need a help if anybody can. I apreciate your time in advance.

Highlight of my problem and question are as follows:

Case: gas-solid two phase flow inside a riser section of Circulating Fluidized Bed with 400kg/m2-s flux and 7 m/s gas velocity.

Model chosen: Eulerian multiphase model(Kinetic theory with granular model)

Boundary conditions: I am able to set all the boundary conditions except shear stress and granular temperature conditions at the wall of the riser.

Question: At riser walls, I have set no-slip condition for gas phase.

For solid, normal velocity is zero. Tangential velocity is from a certain relation.(Tangential velocity as a function of its gradient at wall)

** HOW CAN I SET SUCH VELOCITY FOR SOLIDS AT WALLS?

Granular temp., At wall again I want to specify it as a function of its gradient such as, Granular temp at wall = A.(gradient of Gran. temp at wall)+B where A and B are cerntain constants based on simulation results.

** HOW CAN I SET SUCH GRANULAR TEMP AT RISER WALLS?

Thank you very much for your time and I hope for your sincere help.

Umesh

Re: CFB+wall BC for solid vel and gran temp
 

In Fluent 6.1 you only have the algebraic balance of granular temperature developed by Syamlal, which neglects the convective and diffusive terms for the granular temperature.

This is an assumption which can considered valid in bubbling fluidized beds and in all situations where the volume fraction of the solid phase is high, while it's not strictly correct for a diluite flow.

However, I used the model of Fluent 6.1 and I obtained quite interesting results.

You can specify the velocity B.C. using a UDF, with a DEFINE_PROFILE, and hooking it to the wall.

FLUENT 6.1 doesn't allow you to specify the granular temperature B.C. at the wall. No PDE is solved for the granular temperature, so such a condition is not required.

If you need the complete partial differential equation balance for the granular temperature, you have to switch to FLUENT 6.2, which implements it. In FLUENT 6.2 also the Johnson and Jackson boundary conditions are available for the solid phase.

To implement your B.C. in FLUENT 6.2, you need an UDF, which has to be hooked in the FLUENT B.C. panel.

Best regards, ap

I have the same problem than Umesh but using Ansys Fluent 12.1:

Hello,

I am trying to run a multiphase flow inside a high flux high density circulating fluidized bed. I am very new to this multiphase modelling and I need a help if anybody can. I apreciate your time in advance.

Model chosen: Eulerian multiphase model(Kinetic theory with granular model)

Boundary conditions: I am able to set all the boundary conditions except shear stress and granular temperature conditions at the wall of the riser.

Question: At riser walls, I have set no-slip condition for gas phase.

For solid, normal velocity is zero. Tangential velocity is from a certain relation.(Tangential velocity as a function of its gradient at wall)

** HOW CAN I SET SUCH VELOCITY FOR SOLIDS AT WALLS?

Granular temp., At wall again I want to specify it as a function of its gradient such as, Granular temp at wall = A.(gradient of Gran. temp at wall)+B where A and B are cerntain constants based on simulation results.

** HOW CAN I SET SUCH GRANULAR TEMP AT RISER WALLS?

Thank you very much for your time and I hope for your sincere help.

Ignacio Paz