[joygiak]

Hello,

I am working in a project, which requires modelling a rotating fan inside of an assembly. I've already complete the mesh in ICEM. My mesh is divided in three domains as you can see in picture 1,2 (2 structured meshes for the ducts and 1 unstructured mesh for the fan and it's casing). I merged these three domains in ICEM and with that way, I have one hybrid mesh in one file (picture 3,4). I would be really greatful, if you can give me an advice for my following questions.

1) When I import the mesh in CFX and trying to make a domain, I can select only Fluid and Assembly ( I can't choose the fan for example, so I can make a separate domain). Is this fact a problem for the rotation setup?

2) Is it better to import my three different meshes in CFX and work with interfaces? (Is it wrong to have the fan and it's casing in one domain?)

3) Which is the proper way to simulate the rotation of the fan, in case I have the whole assembly geometry and just one mesh. (I read about fluid rotation or solid rotation with static fluid. The bad thing is that i can't make domains in CFX with whole geometry mesh).

I just need an initial thought or advice, so I can then work and search it on my own.

Thank you in advance.

[ghorrocks]

You will need to put the rotating stuff such as the blades into another domain. This domain will need to have a interface with the stationary domain surrounding it. This needs to be done in the geometry and mesher, not CFX-Pre.

Is it better to import my three different meshes in CFX and work with interfaces? (Is it wrong to have the fan and it's casing in one domain?)

Interfaces are OK, but in this case you can probably avoid interfaces in the stationary domains by careful meshing. You will need an interface to connect the stationary and rotating domain.

[joygiak]

Thank you very much, i will work on it !

[ghorrocks]

Make sure you do the CFX tutorials on rotating machinery so you know how to set it up. They are available on the ANSYS Customer webpage, or if you are student on the ANSYS Academic webpage (the academic page has less examples and does not cover advanced topics - but hopefully it has some rotating machinery examples).

[joygiak]

Dear Glenn,

I've completed the setup of the ducted fan with rotation inside of the assembly (with 4 domains and using interfaces). Also, I start running some cases with changing the boundary conditions, to see which is the better choice. I tried some cases of the following boundary conditions (inlet and outlet are changing, all other surfaces are walls) :

1) Inlet boundary condition of speed or mass flow with outlet boundary condition of static pressure.
2) Inlet Total pressure with outlet mass flow
3) Inlet Total pressure with outlet static pressure

Cases 1) , 2) are working well with only problem not very good convergence of velocity in the direction of main flow.

About case 3) the weird thing is that it creates recirculations not only in the outlet region after the fan but also in the inlet region before the fan, which makes no sense. So solver tells me to put openings in both inlet and outlet (I understand the meaning of openings). Is this case logic? I use Pressure reference of air equal to 1 atm . As total pressure in inlet I use Prelative=Po-Pstatic=(1/2)*density*V^2 with V=constant, and since Pstatic=Preference=1atm. In outlet, also i know that the pressure will be aprox 1atm, so i put the relative value to zero ( the duct after the fan has length about 10 times the fan diameter).

Are there any better boundary conditions for my geometry ( fan with casing inside of closed ducts) that you can suggest me ? To mention the domains are inlet duct before fan/rotating part of the fan/static part of the fan/outlet duct after the fan.

Any help would be appreciated,
Thanks in advance,

[ghorrocks]

You should select boundary conditions which match the conditions you know are applied to the device. Does it operate at a known flow rate, pressure change or some other operating conditions?

[joygiak]

Yes i know the mass flow of the specific fan (not the pressure drop), but also in my problem i have an extra forced mass flow in inlet surface which is independent from the fan's flow. In other words, I have a constant flow that is starting in inlet surface, then decelerates because of the duct walls and then meets the fan which has it's own mass flow from the data sheet

[ghorrocks]

You have to put the boundary conditions at locations where you can specify the flow. If you can't specify the flow conditions then you can't put a boundary condition there.

But it sounds like you know the mass flow rate. Then you should use a boundary which specifies the mass flow rate, either by defining the mass flow rate directly or indirectly by defining the velocity. The other boundary should be zero pressure, either as total pressure or static pressure depending on how you set it up.

CFX-Solver Modelling guide has a best practises guide for rotating machinery, that is worth reading. There is also advice on selecting boundary conditions which you should be aware of.