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How exactly does SW Flow predict fan swirling, and How does the user replicate this
HKHayden Krause13/11/2013

I am trying to estimate the swirling effect of an internal fan.  I have questions on two aspects of this:

  1. What is the free body diagram of a particle of air leaving the fan when swirl is defined?
  2. How does one predict the fans swirling effect without experimental or complex CDF analysis?

I have answered both, however my answers are incomplete and/or may be incorrect and need checking.  So below is what I know for each question – please comment below, provide links, share experience. 

  1.   What is the free body diagram of a particle of air leaving the fan when swirl is defined?

The image below defines the velocity components acting on one particle of air leaving the face of the fan (defined as a boundary condition).  Relating this image to SW Flow sim.  The fluid exiting the fan will have a normal component (defined by the fan curve definition).  If a user wants to add swirling effects he/she can do so by adding the angular velocity (omega) and radial velocity (Vr) components.  Angular velocity can be converted from tangential by Vt = rw.

FBD.png

2. How does one predict the fans swirling effect without experimental or complex CDF analysis?

So far I have a simple method for calculating swirling but I don’t think it is very good.  As far as I am aware, SWFlow Sim makes its swirling predictions using this method (You can get SWFlow to make the swirl predictions for you if you setup your fan using “Axial” definition instead of “Fan Curve” definition - though precicely what is happening is hidden from the user).  The following equations are used to calculate Angular Velocity (Omega).

EQ.png

I have attached a spreadsheet with this calculation made for a 40x40x10 DC fan.  All information is provided except efficiency data for the fan.  This can be estimated to be between 10% and 30% accoring to sources I have found (but I am not sure). 

I am not sure what efficiency this relates to (electrical, mechanical ....).

I am sure this is not the complete picture for a good estimation of swirling because this does not predict radial velocities.  If anyone can elaborate on what I have got - that would be great.  Or indeed if they can correct me - then please feel free!  I realise that this must be a crude method of estimating fan swirl, but this coupled with a sensitivity analysis should be enough for most people. 

Any papours on this subject would also be good if any1 knows of any. 

Hayden