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[2247269037]

Hello everyone,

I am building a 2D axisymmetric CFD model of a hydrogen-based shaft furnace in ANSYS Fluent. The model is a counter-current gas-solid moving bed.

The gas phase enters from the side/lower reduction zone and flows upward. The solid phase, iron ore pellets, enters from the furnace top and moves downward. Therefore, the top boundary has two physical functions:

Gas phase: top gas outlet
Solid phase: pellet inlet

I am using the Eulerian multiphase model with one gas phase and one granular solid phase. The pellets are treated as a pseudo-fluid / granular phase.

My question is about the top boundary condition. In Fluent, one boundary zone usually has only one boundary type, such as pressure-outlet, velocity-inlet, or mass-flow-inlet. However, in my physical model, the same furnace top location should allow gas to leave the domain while solids enter the domain.

I have considered three possible methods:

1. Split the top boundary into two adjacent zones. One zone is defined as pressure-outlet for the gas phase, and the other zone is defined as velocity-inlet or mass-flow-inlet for the solid phase.
2. Use one pressure-outlet boundary and set the backflow volume fraction of the solid phase. However, I understand that backflow settings are only used when reverse flow actually occurs at the pressure outlet, so this may not be able to force a continuous solid feed.

My questions are:

1. In the Fluent Eulerian granular model, can one boundary be used as a gas pressure outlet and a solid inlet at the same time?
2. Is it valid to use pressure-outlet plus backflow volume fraction to represent continuous pellet feeding?
3. If backflow volume fraction cannot force a solid inlet, is splitting the top boundary into a gas outlet zone and a solid inlet zone the recommended method?
4. For a counter-current gas-solid moving bed or shaft furnace model, what is the best practice for this type of boundary condition?
5. Are there any Fluent tutorials or examples involving gas outlet and solid inlet at the same physical boundary location?

Any suggestions would be greatly appreciated.

Thank you very much.

 

[Rob]

One point of pedantry with the Eulerian model. You can only add particles below the packing limit, so technically gas will also enter the top boundary. You've also missed source terms, which will reduce complexity somewhat. 

So, at the top the easiest option is a pressure outlet, with gas being the return (backflow) material. Take care to set a suitable backflow temperature. You may lose some solids if you're near the fluidisation velocity. Use a source term below that to add solids & momentum, and ensure you're well below the packing limit. 

At the bottom you've got a solid outlet. Your problem is going to be setting a suitable pressure to hold up the moving solids. In reality you'll have a hopper, conveyor or other restriction that's intended to force a particle residence time to ensure the reaction has time to complete. Above that you can either use a gas source term, or ports/sparger ring depending on the geometry. 

 

How are you accounting for the gas-solid reactions? I assume you've found the phase interaction mass transfer (chemistry) options? 

 

[Rob]

Note this isn't a trivial problem so expect to do some thinking! 2d-axi symmetric is a very good starting point as it'll work (or otherwise) with much less cpu load.