[siw]

Hi,

I'm simulating a filtration unit which includes a centrifugal fan. The unit draws air in from the atmosphere and the fan drives the air through the unit (which contains filter media) to the outlet. I do not have the rotating frames module in CFX because I only have CFD-Flo so I am having to model the fan pressure rise by defining a volume momentum source in the domain where the fan would be. I do not have a choice about getting the velocity flowfield correct so I am only working to get the fan pressure rise correct in the CFD model.

Using a subdomain I have input the fan static pressure rise vs. volumetric flow rate curve into an expression. But the momentum source requires a pressure gradient. Therefore, I need a length to convert the pressure change to pressure gradient. There is no suitable length, as say with an axial-flow fan, so I tried a mean radius of the centrifugal fan but the pressure rise was less than half what was expected. So my only option is trial and error of trying different length values, a lengthy process as the simulation are not quick.

Has anyone tried this approach before and recommend the settings/approach?

Thanks

[ghorrocks]

The length you should use is the length of your momentum source box.

[siw]

But in the case of a centrifugal fan the domain is cylindrical, with a non-constant radius and the inlet is a circular face at the top and the outlet is a rectangular face at the side, so there is not an obvious length of the source box. I can see what you say for an axial-flow fan, as I mentioned.

[ghorrocks]

Whatever the length you have used for whatever fan it is, that is the length you use. Axial or radial makes no difference.

If this does not make sense please post an image of the region you want to apply the momentum source to.